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Question:1

(a) Al-Beruni

Al-Biruni (973-1050?), Arab scientist, who wrote on a wide variety of scientific subjects. His most important contributions as a scientist were his keen observations of natural phenomena, rather than theories. Sometimes called “the master,” he became one of the best-known Muslim scientists of his time.

Al-Biruni was born in what is now Uzbekistan. At the time, it was part of a vast region called Persia. Al-Biruni's records show that he wrote 113 works, but most of them have been lost. His subjects included astronomy, astrology, chronology, geography, mathematics, mechanics, medicine, pharmacology, meteorology, mineralogy, history, religion, philosophy, literature, and magic. One or more books on most of these subjects have survived. Al-Biruni's important works include Canon, his most comprehensive study of astronomy; Densities, which records specific gravities of various metals, liquids, and gems; Astrolabe, one of the most valuable descriptions of that astronomical instrument; Pharmacology, which contains more than 700 descriptions of drugs; and India, his best-known work, in which he used his knowledge of Sanskrit to describe Indian customs, languages, science, and geography.

(b) Water Pollution

I INTRODUCTION
Water Pollution, contamination of streams, lakes, underground water, bays, or oceans by substances harmful to living things. Water is necessary to life on earth. All organisms contain it; some live in it; some drink it. Plants and animals require water that is moderately pure, and they cannot survive if their water is loaded with toxic chemicals or harmful microorganisms. If severe, water pollution can kill large numbers of fish, birds, and other animals, in some cases killing all members of a species in an affected area. Pollution makes streams, lakes, and coastal waters unpleasant to look at, to smell, and to swim in. Fish and shellfish harvested from polluted waters may be unsafe to eat. People who ingest polluted water can become ill, and, with prolonged exposure, may develop cancers or bear children with birth defects.

II MAJOR TYPES OF POLLUTANTS
The major water pollutants are chemical, biological, or physical materials that degrade water quality. Pollutants can be classed into eight categories, each of which presents its own set of hazards.

A Petroleum Products
Oil and chemicals derived from oil are used for fuel, lubrication, plastics manufacturing, and many other purposes. These petroleum products get into water mainly by means of accidental spills from ships, tanker trucks, pipelines, and leaky underground storage tanks. Many petroleum products are poisonous if ingested by animals, and spilled oil damages the feathers of birds or the fur of animals, often causing death. In addition, spilled oil may be contaminated with other harmful substances, such as polychlorinated biphenyls (PCBs).

B Pesticides and Herbicides
Chemicals used to kill unwanted animals and plants, for instance on farms or in suburban yards, may be collected by rainwater runoff and carried into streams, especially if these substances are applied too lavishly. Some of these chemicals are biodegradable and quickly decay into harmless or less harmful forms, while others are nonbiodegradable and remain dangerous for a long time.

When animals consume plants that have been treated with certain nonbiodegradable chemicals, such as chlordane and dichlorodiphenyltrichloroethane (DDT), these chemicals are absorbed into the tissues or organs of the animals. When other animals feed on these contaminated animals, the chemicals are passed up the food chain. With each step up the food chain, the concentration of the pollutant increases. In one study, DDT levels in ospreys (a family of fish-eating birds) were found to be 10 to 50 times higher than in the fish that they ate, 600 times the level in the plankton that the fish ate, and 10 million times higher than in the water. Animals at the top of food chains may, as a result of these chemical concentrations, suffer cancers, reproductive problems, and death.
Many drinking water supplies are contaminated with pesticides from widespread agricultural use. More than 14 million Americans drink water contaminated with pesticides, and the Environmental Protection Agency (EPA) estimates that 10 percent of wells contain pesticides. Nitrates, a pollutant often derived from fertilizer runoff, can cause methemoglobinemia in infants, a potentially lethal form of anemia that is also called blue baby syndrome.

C Heavy Metals
Heavy metals, such as copper, lead, mercury, and selenium, get into water from many sources, including industries, automobile exhaust, mines, and even natural soil. Like pesticides, heavy metals become more concentrated as animals feed on plants and are consumed in turn by other animals. When they reach high levels in the body, heavy metals can be immediately poisonous, or can result in long-term health problems similar to those caused by pesticides and herbicides. For example, cadmium in fertilizer derived from sewage sludge can be absorbed by crops. If these crops are eaten by humans in sufficient amounts, the metal can cause diarrhea and, over time, liver and kidney damage. Lead can get into water from lead pipes and solder in older water systems; children exposed to lead in water can suffer mental retardation.

D Hazardous Wastes
Hazardous wastes are chemical wastes that are either toxic (poisonous), reactive (capable of producing explosive or toxic gases), corrosive (capable of corroding steel), or ignitable (flammable). If improperly treated or stored, hazardous wastes can pollute water supplies. In 1969 the Cuyahoga River in Cleveland, Ohio, was so polluted with hazardous wastes that it caught fire and burned. PCBs, a class of chemicals once widely used in electrical equipment such as transformers, can get into the environment through oil spills and can reach toxic levels as organisms eat one another.

E Excess Organic Matter
Fertilizers and other nutrients used to promote plant growth on farms and in gardens may find their way into water. At first, these nutrients encourage the growth of plants and algae in water. However, when the plant matter and algae die and settle underwater, microorganisms decompose them. In the process of decomposition, these microorganisms consume oxygen that is dissolved in the water. Oxygen levels in the water may drop to such dangerously low levels that oxygen-dependent animals in the water, such as fish, die. This process of depleting oxygen to deadly levels is called eutrophication.

F Sediment
Sediment, soil particles carried to a streambed, lake, or ocean, can also be a pollutant if it is present in large enough amounts. Soil erosion produced by the removal of soil-trapping trees near waterways, or carried by rainwater and floodwater from croplands, strip mines, and roads, can damage a stream or lake by introducing too much nutrient matter. This leads to eutrophication. Sedimentation can also cover streambed gravel in which many fish, such as salmon and trout, lay their eggs.

G Infectious Organisms
A 1994 study by the Centers for Disease Control and Prevention (CDC) estimated that about 900,000 people get sick annually in the United States because of organisms in their drinking water, and around 900 people die. Many disease-causing organisms that are present in small numbers in most natural waters are considered pollutants when found in drinking water. Such parasites as Giardia lamblia and Cryptosporidium parvum occasionally turn up in urban water supplies. These parasites can cause illness, especially in people who are very old or very young, and in people who are already suffering from other diseases. In 1993 an outbreak of Cryptosporidium in the water supply of Milwaukee, Wisconsin, sickened more than 400,000 people and killed more than 100.

H Thermal Pollution
Water is often drawn from rivers, lakes, or the ocean for use as a coolant in factories and power plants. The water is usually returned to the source warmer than when it was taken. Even small temperature changes in a body of water can drive away the fish and other species that were originally present, and attract other species in place of them. Thermal pollution can accelerate biological processes in plants and animals or deplete oxygen levels in water. The result may be fish and other wildlife deaths near the discharge source. Thermal pollution can also be caused by the removal of trees and vegetation that shade and cool streams.

III SOURCES OF WATER POLLUTANTS
Water pollutants result from many human activities. Pollutants from industrial sources may pour out from the outfall pipes of factories or may leak from pipelines and underground storage tanks. Polluted water may flow from mines where the water has leached through mineral-rich rocks or has been contaminated by the chemicals used in processing the ores. Cities and other residential communities contribute mostly sewage, with traces of household chemicals mixed in. Sometimes industries discharge pollutants into city sewers, increasing the variety of pollutants in municipal areas. Pollutants from such agricultural sources as farms, pastures, feedlots, and ranches contribute animal wastes, agricultural chemicals, and sediment from erosion.

The oceans, vast as they are, are not invulnerable to pollution. Pollutants reach the sea from adjacent shorelines, from ships, and from offshore oil platforms. Sewage and food waste discarded from ships on the open sea do little harm, but plastics thrown overboard can kill birds or marine animals by entangling them, choking them, or blocking their digestive tracts if swallowed.

Oil spills often occur through accidents, such as the wrecks of the tanker Amoco Cadiz off the French coast in 1978 and the Exxon Valdez in Alaska in 1992. Routine and deliberate discharges, when tanks are flushed out with seawater, also add a lot of oil to the oceans. Offshore oil platforms also produce spills: The second largest oil spill on record was in the Gulf of Mexico in 1979 when the Ixtoc 1 well spilled 530 million liters (140 million gallons). The largest oil spill ever was the result of an act of war. During the Gulf War of 1991, Iraqi forces destroyed eight tankers and onshore terminals in Kuwait, releasing a record 910 million liters (240 million gallons). An oil spill has its worst effects when the oil slick encounters a shoreline. Oil in coastal waters kills tidepool life and harms birds and marine mammals by causing feathers and fur to lose their natural waterproof quality, which causes the animals to drown or die of cold. Additionally, these animals can become sick or poisoned when they swallow the oil while preening (grooming their feathers or fur).

Water pollution can also be caused by other types of pollution. For example, sulfur dioxide from a power plant’s chimney begins as air pollution. The polluted air mixes with atmospheric moisture to produce airborne sulfuric acid, which falls to the earth as acid rain. In turn, the acid rain can be carried into a stream or lake, becoming a form of water pollution that can harm or even eliminate wildlife. Similarly, the garbage in a landfill can create water pollution if rainwater percolating through the garbage absorbs toxins before it sinks into the soil and contaminates the underlying groundwater (water that is naturally stored underground in beds of gravel and sand, called aquifers).

Pollution may reach natural waters at spots we can easily identify, known as point sources, such as waste pipes or mine shafts. Nonpoint sources are more difficult to recognize. Pollutants from these sources may appear a little at a time from large areas, carried along by rainfall or snowmelt. For instance, the small oil leaks from automobiles that produce discolored spots on the asphalt of parking lots become nonpoint sources of water pollution when rain carries the oil into local waters. Most agricultural pollution is nonpoint since it typically originates from many fields.

IV CONTROLS
In the United States, the serious campaign against water pollution began in 1972, when Congress passed the Clean Water Act. This law initiated a national goal to end all pollution discharges into surface waters, such as lakes, rivers, streams, wetlands, and coastal waters. The law required those who discharge pollutants into waterways to apply for federal permits and to be responsible for reducing the amount of pollution over time. The law also authorized generous federal grants to help states build water treatment plants that remove pollutants, principally sewage, from wastewater before it is discharged.

Since the passage of the Clean Water Act in 1972, most of the obvious point sources of pollution in the United States have been substantially cleaned up. Municipal sewage plants in many areas are now yielding water so clean that it can be used again. Industries are treating their waste and also changing their manufacturing processes so that less waste is produced. As a result, surface waters are far cleaner than they were in 1972. In 1990 a survey of rivers and streams found that three-quarters of these waters were clean enough for swimming and fishing. Cleaning up the remainder of these rivers and streams will require tackling the more difficult problems of diffuse, nonpoint source pollution.

Congress first took up the nonpoint source problem in 1987, requiring the states to develop programs to combat this kind of pollution. Since interception and treatment of nonpoint pollution is very difficult, the prime strategy is to prevent it.

In urban areas, one obvious sign of the campaign against nonpoint pollution is the presence of stenciled notices often seen beside storm drains: Drains To Bay, Drains To Creek, or Drains To Lake. These signs discourage people from dumping contaminants, such as used engine oil, down grates because the material will likely pollute nearby waterways. Householders are urged to be sparing in their use of garden pesticides and fertilizers in order to reduce contaminated runoff and eutrophication. At construction sites, builders are required to fight soil erosion by laying down tarps, building sediment traps, and seeding grasses.

In the countryside, efforts are underway to reduce pollution from agricultural wastes, fertilizers, and pesticides, and from erosion caused by logging and farming. Farmers and foresters are encouraged to protect streams by leaving streamside trees and vegetation undisturbed; this practice stabilizes banks and traps sediment coming down the slope, preventing sediment buildup in water. Hillside fields are commonly plowed on the contour of the land, rather than up and down the incline, to reduce erosion and to discourage the formation of gullies. Cows are kept away from streamsides and housed in barns where their waste can be gathered and treated. Increasingly, governments are protecting wetlands, which are valuable pollution traps because their plants absorb excess nutrients and their fine sediments absorb other pollutants. In some places, lost wetlands are being restored. Despite these steps, a great deal remains to be done.
In the United States, the EPA is in overall charge of antipollution efforts. The EPA sets standards, approves state control plans, and steps in (if necessary) to enforce its own rules. Under the Safe Drinking Water Act (SDWA), passed in 1974 and amended in 1986 and 1996, the EPA sets standards for drinking water. Among other provisions, the SWDA requires that all water drawn from surface water supplies must be filtered to remove Cryptosporidium bacteria by the year 2000. The law also requires that states map the watersheds from which drinking water comes and identify sources of pollution within those watersheds. While America’s drinking water is among the safest in the world, and has been improving since passage of the SDWA, many water utilities that serve millions of Americans provide tap water that fails to meet the EPA standards.

The EPA has equivalents in many countries, although details of responsibilities vary. For instance, the federal governments may have a larger role in pollution control, as in France, or more of this responsibility may be shifted to the state and provincial governments, as in Canada. Because many rivers, lakes, and ocean shorelines are shared by several nations, many international treaties also address water pollution. For example, the governments of Canada and the United States have negotiated at least nine treaties or agreements, starting with the Canada-U.S. Boundary Waters Treaty of 1909, governing water pollution of the many rivers and lakes that flow along or across their common border.
Several major treaties deal with oceanic pollution, including the 1972 Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter and the 1973 International Convention for the Prevention of Pollution from Ships (known as MARPOL). International controls and enforcement, however, are generally weak.

Contributed By:
John Hart
Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.

(c) Semi Conductors

I INTRODUCTION
Semiconductor, solid or liquid material, able to conduct electricity at room temperature more readily than an insulator, but less easily than a metal. Electrical conductivity, which is the ability to conduct electrical current under the application of a voltage, has one of the widest ranges of values of any physical property of matter. Such metals as copper, silver, and aluminum are excellent conductors, but such insulators as diamond and glass are very poor conductors (see Conductor, electrical; Insulation; Metals). At low temperatures, pure semiconductors behave like insulators. Under higher temperatures or light or with the addition of impurities, however, the conductivity of semiconductors can be increased dramatically, reaching levels that may approach those of metals. The physical properties of semiconductors are studied in solid-state physics. See Physics.

II CONDUCTION ELECTRONS AND HOLES
The common semiconductors include chemical elements and compounds such as silicon, germanium; selenium, gallium arsenide, zinc selenide, and lead telluride. The increase in conductivity with temperature, light, or impurities arises from an increase in the number of conduction electrons, which are the carriers of the electrical current See Electricity; Electron. In a pure, or intrinsic, semiconductor such as silicon, the valence electrons, or outer electrons, of an atom are paired and shared between atoms to make a covalent bond that holds the crystal together See Chemical Reaction; see Crystal). These valence electrons are not free to carry electrical current. To produce conduction electrons, temperature or light is used to excite the valence electrons out of their bonds, leaving them free to conduct current. Deficiencies, or “holes,” are left behind that contribute to the flow of electricity. (These holes are said to be carriers of positive electricity.) This is the physical origin of the increase in the electrical conductivity of semiconductors with temperature. The energy required to excite the electron and hole is called the energy gap.

III DOPING
Another method to produce free carriers of electricity is to add impurities to, or to “dope,” the semiconductor. The difference in the number of valence electrons between the doping material, or dopant (either donors or acceptors of electrons), and host gives rise to negative (n-type) or positive (p-type) carriers of electricity. This concept is illustrated in the accompanying diagram of a doped silicon (Si) crystal. Each silicon atom has four valence electrons (represented by dots); two are required to form a covalent bond. In n- type silicon, atoms such as phosphorus (P) with five valence electrons replace some silicon and provide extra negative electrons. In p-type silicon, atoms with three valence electrons such as aluminum (Al) lead to a deficiency of electrons, or to holes, which act as positive electrons. The extra electrons or holes can conduct electricity.

When p-type and n-type semiconductor regions are adjacent to each other, they form a semiconductor diode, and the region of contact is called a p-n junction. (A diode is a two-terminal device that has a high resistance to electric current in one direction but a low resistance in the other direction.) The conductance properties of the p-n junction depend on the direction of the voltage, which can, in turn, be used to control the electrical nature of the device. Series of such junctions are used to make transistors and other semiconductor devices such as solar cells, p-n junction lasers, rectifiers, and many others.

Semiconductor devices have many varied applications in electrical engineering. Recent engineering developments have yielded small semiconductor chips containing hundreds of thousands of transistors. These chips have made possible great miniaturization of electronic devices. More efficient use of such chips has been developed through what is called complementary metal-oxide semiconductor circuitry, or CMOS, which consists of pairs of p- and n-channel transistors controlled by a single circuit. In addition, extremely small devices are being made using the technique of molecular-beam epitaxy.

Contributed By:
Marvin L. Cohen
Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.

Question:2 Movements of Earth

Earth (planet)

I INTRODUCTION
Earth (planet), one of nine planets in the solar system, the only planet known to harbor life, and the “home” of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans.

Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.

Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets—Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth’s atmosphere includes just the right amount of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth’s atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth and is also neither too near nor too far from the Sun. But because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot—462°C (864°F)—hot enough to melt lead and too hot for life to exist.

Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, indicating that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.

For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas—for example, that the Earth was a sphere and that it traveled around the Sun—were based on brilliant reasoning. However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system. By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. And with the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.

In the second half of the 20th century, more advances in the study of Earth and the solar system occurred due to the development of rockets that could send spacecraft beyond Earth. Human beings were able to study and observe Earth from space with satellites equipped with scientific instruments. Astronauts landed on the Moon and gathered ancient rocks that revealed much about the early solar system. During this remarkable advancement in human history, humans also sent unmanned spacecraft to the other planets and their moons. Spacecraft have now visited all of the planets except Pluto. The study of other planets and moons has provided new insights about Earth, just as the study of the Sun and other stars like it has helped shape new theories about how Earth and the rest of the solar system formed.

As a result of this recent space exploration, we now know that Earth is one of the most geologically active of all the planets and moons in the solar system. Earth is constantly changing. Over long periods of time land is built up and worn away, oceans are formed and re-formed, and continents move around, break up, and merge.

Life itself contributes to changes on Earth, especially in the way living things can alter Earth’s atmosphere. For example, Earth at one time had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth’s atmosphere and made it possible for animal life to evolve on land.

A variety of scientific fields have broadened our knowledge about Earth, including biogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, and zoogeography. Collectively, these fields are known as Earth science. By studying Earth’s atmosphere, its surface, and its interior and by studying the Sun and the rest of the solar system, scientists have learned much about how Earth came into existence, how it changed, and why it continues to change.

II EARTH, THE SOLAR SYSTEM, AND THE GALAXY
Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earth travels at a velocity of about 107,000 km/h (about 67,000 mph) as it orbits the Sun. All but one of the planets orbit the Sun in the same plane—that is, if an imaginary line were extended from the center of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit.
Earth’s orbital path is not quite a perfect circle but instead is slightly elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km (about 95 million mi) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi) from the Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun.

The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of stars bound together by gravity. The Milky Way has armlike discs of stars that spiral out from its center. The solar system is located in one of these spiral arms, known as the Orion arm, which is about two-thirds of the way from the center of the Galaxy. In most parts of the Northern Hemisphere, this disc of stars is visible on a summer night as a dense band of light known as the Milky Way.

Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles. Its polar diameter, measured from the North Pole to the South Pole, is somewhat less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets—Mercury, Venus, Earth, and Mars—that make up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune. For example, the largest planet, Jupiter, has a diameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. A famous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earths would fit inside it.

Earth has one natural satellite, the Moon. The Moon orbits the Earth, completing one revolution in an elliptical path in 27 days 7 hr 43 min 11.5 sec. The Moon orbits the Earth because of the force of Earth’s gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon’s gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than 4 billion years ago when a large meteorite or small planet struck the Earth.

As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min 4.1 sec. The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr 48 min 46 sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It takes more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy’s center.

Earth’s axis of rotation is inclined (tilted) 23.5° relative to its plane of revolution around the Sun. This inclination of the axis creates the seasons and causes the height of the Sun in the sky at noon to increase and decrease as the seasons change. The Northern Hemisphere receives the most energy from the Sun when it is tilted toward the Sun. This orientation corresponds to summer in the Northern Hemisphere and winter in the Southern Hemisphere. The Southern Hemisphere receives maximum energy when it is tilted toward the Sun, corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere. Fall and spring occur in between these orientations.

III EARTH’S ATMOSPHERE
The atmosphere is a layer of different gases that extends from Earth’s surface to the exosphere, the outer limit of the atmosphere, about 9,600 km (6,000 mi) above the surface. Near Earth’s surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent). The remaining 1 percent of atmospheric gases consists of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapor; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.

A Layers of the Atmosphere
The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi) above sea level at the equator. Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi) is called the mesosphere. At an altitude of about 90 km, temperatures begin to rise. The layer that begins at this altitude is called the thermosphere because of the high temperatures that can be reached in this layer (about 1200°C, or about 2200°F). The region beyond the thermosphere is called the exosphere. The thermosphere and the exosphere overlap with another region of the atmosphere known as the ionosphere, a layer or layers of ionized air extending from almost 60 km (about 50 mi) above Earth’s surface to altitudes of 1,000 km (600 mi) and more.

Earth’s atmosphere and the way it interacts with the oceans and radiation from the Sun are responsible for the planet’s climate and weather. The atmosphere plays a key role in supporting life. Almost all life on Earth uses atmospheric oxygen for energy in a process known as cellular respiration, which is essential to life. The atmosphere also helps moderate Earth’s climate by trapping radiation from the Sun that is reflected from Earth’s surface. Water vapor, carbon dioxide, methane, and nitrous oxide in the atmosphere act as “greenhouse gases.” Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun in the lower atmosphere and thereby help warm Earth’s surface. Without this greenhouse effect, heat radiation would escape into space, and Earth would be too cold to support most forms of life.

Other gases in the atmosphere are also essential to life. The trace amount of ozone found in Earth’s stratosphere blocks harmful ultraviolet radiation from the Sun. Without the ozone layer, life as we know it could not survive on land. Earth’s atmosphere is also an important part of a phenomenon known as the water cycle or the hydrologic cycle. See also Atmosphere.

B The Atmosphere and the Water Cycle
The water cycle simply means that Earth’s water is continually recycled between the oceans, the atmosphere, and the land. All of the water that exists on Earth today has been used and reused for billions of years. Very little water has been created or lost during this period of time. Water is constantly moving on Earth’s surface and changing back and forth between ice, liquid water, and water vapor.

The water cycle begins when the Sun heats the water in the oceans and causes it to evaporate and enter the atmosphere as water vapor. Some of this water vapor falls as precipitation directly back into the oceans, completing a short cycle. Some of the water vapor, however, reaches land, where it may fall as snow or rain. Melted snow or rain enters rivers or lakes on the land. Due to the force of gravity, the water in the rivers eventually empties back into the oceans. Melted snow or rain also may enter the ground. Groundwater may be stored for hundreds or thousands of years, but it will eventually reach the surface as springs or small pools known as seeps. Even snow that forms glacial ice or becomes part of the polar caps and is kept out of the cycle for thousands of years eventually melts or is warmed by the Sun and turned into water vapor, entering the atmosphere and falling again as precipitation. All water that falls on land eventually returns to the ocean, completing the water cycle.

IV EARTH’S SURFACE
Earth’s surface is the outermost layer of the planet. It includes the hydrosphere, the crust, and the biosphere.

A Hydrosphere
The hydrosphere consists of the bodies of water that cover 71 percent of Earth’s surface. The largest of these are the oceans, which contain over 97 percent of all water on Earth. Glaciers and the polar ice caps contain just over 2 percent of Earth’s water in the form of solid ice. Only about 0.6 percent is under the surface as groundwater. Nevertheless, groundwater is 36 times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere as water vapor. Only 0.017 percent of all the water on Earth is found in lakes and rivers. And a mere 0.001 percent is found in the atmosphere as water vapor. Most of the water in glaciers, lakes, inland seas, rivers, and groundwater is fresh and can be used for drinking and agriculture. Dissolved salts compose about 3.5 percent of the water in the oceans, however, making it unsuitable for drinking or agriculture unless it is treated to remove the salts.

B Crust
The crust consists of the continents, other land areas, and the basins, or floors, of the oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km (9 to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km (3 to 6 mi). The crust has a definite boundary called the Mohorovičić discontinuity, or simply the Moho. The boundary separates the crust from the underlying mantle, which is much thicker and is part of Earth’s interior.

Oceanic crust and continental crust differ in the type of rocks they contain. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock, called magma, cools and solidifies. Sedimentary rocks are usually created by the breakdown of igneous rocks. They tend to form in layers as small particles of other rocks or as the mineralized remains of dead animals and plants that have fused together over time. The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks are altered by heat and pressure deep underground.

Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continental crust consists of lighter-colored, less dense igneous rocks, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks.

C Biosphere
The biosphere includes all the areas of Earth capable of supporting life. The biosphere ranges from about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor. For a long time, scientists believed that all life depended on energy from the Sun and consequently could only exist where sunlight penetrated. In the 1970s, however, scientists discovered various forms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlight penetrated. They learned that primitive bacteria formed the basis of this living community and that the bacteria derived their energy from a process called chemosynthesis that did not depend on sunlight. Some scientists believe that the biosphere may extend relatively deep into Earth’s crust. They have recovered what they believe are primitive bacteria from deeply drilled holes below the surface.

D Changes to Earth’s Surface
Earth’s surface has been constantly changing ever since the planet formed. Most of these changes have been gradual, taking place over millions of years. Nevertheless, these gradual changes have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of huge supercontinents, and the breakup of supercontinents into smaller continents.

The weathering and erosion that result from the water cycle are among the principal factors responsible for changes to Earth’s surface. Another principal factor is the movement of Earth’s continents and seafloors and the buildup of mountain ranges due to a phenomenon known as plate tectonics. Heat is the basis for all of these changes. Heat in Earth’s interior is believed to be responsible for continental movement, mountain building, and the creation of new seafloor in ocean basins. Heat from the Sun is responsible for the evaporation of ocean water and the resulting precipitation that causes weathering and erosion. In effect, heat in Earth’s interior helps build up Earth’s surface while heat from the Sun helps wear down the surface.

D1 Weathering
Weathering is the breakdown of rock at and near the surface of Earth. Most rocks originally formed in a hot, high-pressure environment below the surface where there was little exposure to water. Once the rocks reached Earth’s surface, however, they were subjected to temperature changes and exposed to water. When rocks are subjected to these kinds of surface conditions, the minerals they contain tend to change. These changes constitute the process of weathering. There are two types of weathering: physical weathering and chemical weathering.

Physical weathering involves a decrease in the size of rock material. Freezing and thawing of water in rock cavities, for example, splits rock into small pieces because water expands when it freezes.

Chemical weathering involves a chemical change in the composition of rock. For example, feldspar, a common mineral in granite and other rocks, reacts with water to form clay minerals, resulting in a new substance with totally different properties than the parent feldspar. Chemical weathering is of significance to humans because it creates the clay minerals that are important components of soil, the basis of agriculture. Chemical weathering also causes the release of dissolved forms of sodium, calcium, potassium, magnesium, and other chemical elements into surface water and groundwater. These elements are carried by surface water and groundwater to the sea and are the sources of dissolved salts in the sea.

D2 Erosion
Erosion is the process that removes loose and weathered rock and carries it to a new site. Water, wind, and glacial ice combined with the force of gravity can cause erosion.

Erosion by running water is by far the most common process of erosion. It takes place over a longer period of time than other forms of erosion. When water from rain or melted snow moves downhill, it can carry loose rock or soil with it. Erosion by running water forms the familiar gullies and V-shaped valleys that cut into most landscapes. The force of the running water removes loose particles formed by weathering. In the process, gullies and valleys are lengthened, widened, and deepened. Often, water overflows the banks of the gullies or river channels, resulting in floods. Each new flood carries more material away to increase the size of the valley. Meanwhile, weathering loosens more and more material so the process continues.

Erosion by glacial ice is less common, but it can cause the greatest landscape changes in the shortest amount of time. Glacial ice forms in a region where snow fails to melt in the spring and summer and instead builds up as ice. For major glaciers to form, this lack of snowmelt has to occur for a number of years in areas with high precipitation. As ice accumulates and thickens, it flows as a solid mass. As it flows, it has a tremendous capacity to erode soil and even solid rock. Ice is a major factor in shaping some landscapes, especially mountainous regions. Glacial ice provides much of the spectacular scenery in these regions. Features such as horns (sharp mountain peaks), arêtes (sharp ridges), glacially formed lakes, and U-shaped valleys are all the result of glacial erosion.

Wind is an important cause of erosion only in arid (dry) regions. Wind carries sand and dust, which can scour even solid rock.
Many factors determine the rate and kind of erosion that occurs in a given area. The climate of an area determines the distribution, amount, and kind of precipitation that the area receives and thus the type and rate of weathering. An area with an arid climate erodes differently than an area with a humid climate. The elevation of an area also plays a role by determining the potential energy of running water. The higher the elevation the more energetically water will flow due to the force of gravity. The type of bedrock in an area (sandstone, granite, or shale) can determine the shapes of valleys and slopes, and the depth of streams.
A landscape’s geologic age—that is, how long current conditions of weathering and erosion have affected the area—determines its overall appearance. Relatively young landscapes tend to be more rugged and angular in appearance. Older landscapes tend to have more rounded slopes and hills. The oldest landscapes tend to be low-lying with broad, open river valleys and low, rounded hills. The overall effect of the wearing down of an area is to level the land; the tendency is toward the reduction of all land surfaces to sea level.
D3 Plate Tectonics
Opposing this tendency toward leveling is a force responsible for raising mountains and plateaus and for creating new landmasses. These changes to Earth’s surface occur in the outermost solid portion of Earth, known as the lithosphere. The lithosphere consists of the crust and another region known as the upper mantle and is approximately 65 to 100 km (40 to 60 mi) thick. Compared with the interior of the Earth, however, this region is relatively thin. The lithosphere is thinner in proportion to the whole Earth than the skin of an apple is to the whole apple.
Scientists believe that the lithosphere is broken into a series of plates, or segments. According to the theory of plate tectonics, these plates move around on Earth’s surface over long periods of time. Tectonics comes from the Greek word, tektonikos, which means “builder.”
According to the theory, the lithosphere is divided into large and small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, the Indo-Australian plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Philippine plate, and the Caribbean plate. Plate sizes vary a great deal. The Cocos plate is 2,000 km (1,000 mi) wide, while the Pacific plate is nearly 14,000 km (nearly 9,000 mi) wide.
These plates move in three different ways in relation to each other. They pull apart or move away from each other, they collide or move against each other, or they slide past each other as they move sideways. The movement of these plates helps explain many geological events, such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.
D3a When Plates Pull Apart
When the plates pull apart, two types of phenomena occur depending on whether the movement takes place in the oceans or on land. When plates pull apart on land, deep valleys known as rift valleys form. An example of a rift valley is the Great Rift Valley that extends from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the oceans, long, sinuous chains of volcanic mountains called mid-ocean ridges form, and new seafloor is created at the site of these ridges. Rift valleys are also present along the crests of the mid-ocean ridges.
Most scientists believe that gravity and heat from the interior of the Earth cause the plates to move apart and to create new seafloor. According to this explanation, molten rock known as magma rises from Earth’s interior to form hot spots beneath the ocean floor. As two oceanic plates pull apart from each other in the middle of the oceans, a crack, or rupture, appears and forms the mid-ocean ridges. These ridges exist in all the world’s ocean basins and resemble the seams of a baseball. The molten rock rises through these cracks and creates new seafloor.
D3b When Plates Collide
When plates collide or push against each other, regions called convergent plate margins form. Along these margins, one plate is usually forced to dive below the other. As that plate dives, it triggers the melting of the surrounding lithosphere and a region just below it known as the asthenosphere. These pockets of molten crust rise behind the margin through the overlying plate, creating curved chains of volcanoes known as arcs. This process is called subduction.
If one plate consists of oceanic crust and the other consists of continental crust, the denser oceanic crust will dive below the continental crust. If both plates are oceanic crust, then either may be subducted. If both are continental crust, subduction can continue for a while but will eventually end because continental crust is not dense enough to be forced very far into the upper mantle.
The results of this subduction process are readily visible on a map showing that 80 percent of the world’s volcanoes rim the Pacific Ocean where plates are colliding against each other. The subduction zone created by the collision of two oceanic plates—the Pacific plate and the Philippine plate—can also create a trench. Such a trench resulted in the formation of the deepest point on Earth, the Mariana Trench, which is estimated to be 11,033 m (36,198 ft) below sea level.
On the other hand, when two continental plates collide, mountain building occurs. The collision of the Indo-Australian plate with the Eurasian plate has produced the Himalayan Mountains. This collision resulted in the highest point of Earth, Mount Everest, which is 8,850 m (29,035 ft) above sea level.
D3c When Plates Slide Past Each Other
Finally, some of Earth’s plates neither collide nor pull apart but instead slide past each other. These regions are called transform margins. Few volcanoes occur in these areas because neither plate is forced down into Earth’s interior and little melting occurs. Earthquakes, however, are abundant as the two rigid plates slide past each other. The San Andreas Fault in California is a well-known example of a transform margin.
The movement of plates occurs at a slow pace, at an average rate of only 2.5 cm (1 in) per year. But over millions of years this gradual movement results in radical changes. Current plate movement is making the Pacific Ocean and Mediterranean Sea smaller, the Atlantic Ocean larger, and the Himalayan Mountains higher.
V EARTH’S INTERIOR
The interior of Earth plays an important role in plate tectonics. Scientists believe it is also responsible for Earth’s magnetic field. This field is vital to life because it shields the planet’s surface from harmful cosmic rays and from a steady stream of energetic particles from the Sun known as the solar wind.
A Composition of the Interior
Earth’s interior consists of the mantle and the core. The mantle and core make up by far the largest part of Earth’s mass. The distance from the base of the crust to the center of the core is about 6,400 km (about 4,000 mi).
Scientists have learned about Earth’s interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth’s interior. Finally, seismic waves generated by earthquakes provide geophysicists with information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth in the form of waves. The way these waves travel through the interior of Earth reveals the nature of materials inside the planet.
The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km (about 1,800 mi). Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock. The term asthenosphere is derived from Greek and means “weak layer.” The asthenosphere’s soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere’s plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.
The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a “sticky” liquid when it is subjected to enough heat and pressure.
The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km (about 1,404 mi) thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km (about 758 mi) thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one of the sources of heat in Earth’s interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C (12,000°F).
B The Core and Earth’s Magnetism
Scientists believe that Earth’s liquid iron core is instrumental in creating a magnetic field that surrounds Earth and shields the planet from harmful cosmic rays and the Sun’s solar wind. The idea that Earth is like a giant magnet was first proposed in 1600 by English physician and natural philosopher William Gilbert. Gilbert proposed the idea to explain why the magnetized needle in a compass points north. According to Gilbert, Earth’s magnetic field creates a magnetic north pole and a magnetic south pole. The magnetic poles do not correspond to the geographic North and South poles, however. Moreover, the magnetic poles wander and are not always in the same place. The north magnetic pole is currently close to Ellef Ringnes Island in the Queen Elizabeth Islands near the boundary of Canada’s Northwest Territories with Nunavut. The south magnetic pole lies just off the coast of Wilkes Land, Antarctica.
Not only do the magnetic poles wander, but they also reverse their polarity—that is, the north magnetic pole becomes the south magnetic pole and vice versa. Magnetic reversals have occurred at least 170 times over the past 100 million years. The reversals occur on average about every 200,000 years and take place gradually over a period of several thousand years. Scientists still do not understand why these magnetic reversals occur but think they may be related to Earth’s rotation and changes in the flow of liquid iron in the outer core.
Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth’s magnetic field. Known as the dynamo theory, this theory appears to be the best explanation yet for the origin of the magnetic field. Earth’s magnetic field operates in a region above Earth’s surface known as the magnetosphere. The magnetosphere is shaped somewhat like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.
Inside the magnetosphere are the Van Allen radiation belts, named for the American physicist James A. Van Allen who discovered them in 1958. The Van Allen belts are regions where charged particles from the Sun and from cosmic rays are trapped and sent into spiral paths along the lines of Earth’s magnetic field. The radiation belts thereby shield Earth’s surface from these highly energetic particles. Occasionally, however, due to extremely strong magnetic fields on the Sun’s surface, which are visible as sunspots, a brief burst of highly energetic particles streams along with the solar wind. Because Earth’s magnetic field lines converge and are closest to the surface at the poles, some of these energetic particles sneak through and interact with Earth’s atmosphere, creating the phenomenon known as an aurora.
VI EARTH’S PAST
A Origin of Earth
Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system formed about 4.6 billion years ago from a giant cloud of gas and dust known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in a violent explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.
Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the atoms of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the Sun. The fusion of hydrogen atoms in the Sun is the source of its energy.
Many scientists favor the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except for Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.
According to this theory, temperatures decreased with increasing distance from the center of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense out—that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came together with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets—Jupiter and Saturn—are mostly made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.
B The Early Earth
Within the planetesimal Earth, heavier matter sank to the center and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the center contained radioactive material, an important source of Earth’s internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.
Not long after they formed, more than 4 billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon’s surface, which is unchanged. Earth’s craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.
Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By 4 billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so it was impossible for life to exist.
C Geologic Time
Geologists divide the history of the Earth into three eons: the Archean Eon, which lasted from around 4 billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.
The Archean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archean is generally dated as the age of the oldest terrestrial rocks, which are about 4 billion years old. The Archean Eon ended 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.
Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Until the late 1800s scientists could only determine the relative ages of rock strata. They knew that in general the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.
The study of fossils also enabled geologists to determine the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.
Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could determine an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists were able to assemble Earth’s geologic time scale from the Archean Eon to the present. See also Geologic Time.
C1 Precambrian
The Precambrian is a time span that includes the Archean and Proterozoic eons and began about 4 billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth’s history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about 4 billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to 4.2 billion years.
The Precambrian is also the longest chapter in Earth’s history, spanning a period of about 3.5 billion years. During this timeframe, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet as a result of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapor. As Earth continued to cool, the water vapor condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth’s water vapor originally came from comets containing frozen water that struck Earth during the period of meteorite bombardment.
By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth’s surface as early as the middle Precambrian. About a billion years ago, the Earth’s plates were centered around the South Pole and formed a supercontinent called Rodinia. Slowly, pieces of this supercontinent broke away from the central continent and traveled north, forming smaller continents.
Life originated during the Precambrian. The earliest fossil evidence of life consists of prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote’s hereditary material was copied unchanged. The first prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, but certainly by about 3.5 billion years ago, and were anaerobic—that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.
Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as cyanobacteria or blue-green algae. These cyanobacteria gradually introduced oxygen in the atmosphere as a result of photosynthesis. In shallow tropical waters, cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old.
For billions of years, life existed only in the simple form of prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduce by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The very latest part of Precambrian time some 560 million to 545 million years ago saw the appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils appear to be the precursors of many of the fossil groups that were to explode in Earth's oceans in the Paleozoic Era. See also Evolution; Natural Selection.
C2 Paleozoic Era
At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Almost all of the major groups of animals we know today made their first appearance during the Cambrian explosion. Almost all of the different “body plans” found in animals today—that is, the way an animal’s body is designed, with heads, legs, rear ends, claws, tentacles, or antennae—also originated during this period.
Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species were able to flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.
Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the supercontinent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some of the continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built a number of mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.
Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. Their slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the supercontinent Pangaea and resulted in some of the grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented over 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouthlike opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today’s Mediterranean Sea.
The Paleozoic came to an end with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.
C3 Mesozoic Era
The Mesozoic Era, beginning 248 million years ago, is often characterized as the Age of Reptiles because reptiles were the dominant life forms during this era. Reptiles dominated not only on land, as dinosaurs, but also in the sea, in the form of the plesiosaurs and ichthyosaurs, and in the air, as pterosaurs, which were flying reptiles. See also Dinosaur; Plesiosaur; Ichthyosaur; Pterosaur.
The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and were one of the most successful animals in Earth’s history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first birds and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.
The Mesozoic was an eventful era geologically with many changes to Earth’s surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America began splitting from what is now Africa, and in the process the South Atlantic Ocean formed. As the landmass that became North America drifted away from Pangaea and moved westward, a long subduction zone extended along North America’s western margin. This subduction zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. Much of this feature, called the American Cordillera, exists today as the eastern margin of the Pacific Ring of Fire.
During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.
C4 Cenozoic Era
The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.
A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continues to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller microcontinents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that seafloor spreading is still causing the country to grow.
Late in the Tertiary Period, about 6 million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between 2 million and 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anatomically modern human beings, known as Homo sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that anatomically modern humans originated only recently in Earth’s 4.6-billion-year history, within the past 200,000 years. See also Human Evolution.
VII EARTH’S FUTURE
With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid-1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth’s immediate future depends to a great extent on the behavior of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet’s surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide that was removed from Earth’s early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things. See also Global Warming.
Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the seafloor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.
In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun’s mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about 3 billion years from now, when it will be hot enough to boil Earth’s oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about 7 billion years from now. As a red giant the Sun’s outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth’s rocks. Earth will end its existence as a burnt cinder. See also Sun.
Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey beyond the solar system to colonize other planets in the Milky Way Galaxy and find another place to call “home.”


Reviewed By:
Alan V. Morgan
Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.


Question No:3
(a)
Endocrine System
I INTRODUCTION
Endocrine System, group of specialized organs and body tissues that produce, store, and secrete chemical substances known as hormones. As the body's chemical messengers, hormones transfer information and instructions from one set of cells to another. Because of the hormones they produce, endocrine organs have a great deal of influence over the body. Among their many jobs are regulating the body's growth and development, controlling the function of various tissues, supporting pregnancy and other reproductive functions, and regulating metabolism.
Endocrine organs are sometimes called ductless glands because they have no ducts connecting them to specific body parts. The hormones they secrete are released directly into the bloodstream. In contrast, the exocrine glands, such as the sweat glands or the salivary glands, release their secretions directly to target areas—for example, the skin or the inside of the mouth. Some of the body's glands are described as endo-exocrine glands because they secrete hormones as well as other types of substances. Even some nonglandular tissues produce hormone-like substances—nerve cells produce chemical messengers called neurotransmitters, for example.
The earliest reference to the endocrine system comes from ancient Greece, in about 400 BC. However, it was not until the 16th century that accurate anatomical descriptions of many of the endocrine organs were published. Research during the 20th century has vastly improved our understanding of hormones and how they function in the body. Today, endocrinology, the study of the endocrine glands, is an important branch of modern medicine. Endocrinologists are medical doctors who specialize in researching and treating disorders and diseases of the endocrine system.
II COMPONENTS OF THE ENDOCRINE SYSTEM
The primary glands that make up the human endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal body, and reproductive glands—the ovary and testis. The pancreas, an organ often associated with the digestive system, is also considered part of the endocrine system. In addition, some nonendocrine organs are known to actively secrete hormones. These include the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta. Almost all body cells can either produce or convert hormones, and some secrete hormones. For example, glucagon, a hormone that raises glucose levels in the blood when the body needs extra energy, is made in the pancreas but also in the wall of the gastrointestinal tract. However, it is the endocrine glands that are specialized for hormone production. They efficiently manufacture chemically complex hormones from simple chemical substances—for example, amino acids and carbohydrates—and they regulate their secretion more efficiently than any other tissues.
The hypothalamus, found deep within the brain, directly controls the pituitary gland. It is sometimes described as the coordinator of the endocrine system. When information reaching the brain indicates that changes are needed somewhere in the body, nerve cells in the hypothalamus secrete body chemicals that either stimulate or suppress hormone secretions from the pituitary gland. Acting as liaison between the brain and the pituitary gland, the hypothalamus is the primary link between the endocrine and nervous systems.
Located in a bony cavity just below the base of the brain is one of the endocrine system's most important members: the pituitary gland. Often described as the body’s master gland, the pituitary secretes several hormones that regulate the function of the other endocrine glands. Structurally, the pituitary gland is divided into two parts, the anterior and posterior lobes, each having separate functions. The anterior lobe regulates the activity of the thyroid and adrenal glands as well as the reproductive glands. It also regulates the body's growth and stimulates milk production in women who are breast-feeding. Hormones secreted by the anterior lobe include adrenocorticotropic hormone (ACTH), thyrotropic hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), and prolactin. The anterior lobe also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain.
The posterior lobe of the pituitary gland contains the nerve endings (axons) from the hypothalamus, which stimulate or suppress hormone production. This lobe secretes antidiuretic hormones (ADH), which control water balance in the body, and oxytocin, which controls muscle contractions in the uterus.
The thyroid gland, located in the neck, secretes hormones in response to stimulation by TSH from the pituitary gland. The thyroid secretes hormones—for example, thyroxine and three-iodothyronine—that regulate growth and metabolism, and play a role in brain development during childhood.
The parathyroid glands are four small glands located at the four corners of the thyroid gland. The hormone they secrete, parathyroid hormone, regulates the level of calcium in the blood.
Located on top of the kidneys, the adrenal glands have two distinct parts. The outer part, called the adrenal cortex, produces a variety of hormones called corticosteroids, which include cortisol. These hormones regulate salt and water balance in the body, prepare the body for stress, regulate metabolism, interact with the immune system, and influence sexual function. The inner part, the adrenal medulla, produces catecholamines, such as epinephrine, also called adrenaline, which increase the blood pressure and heart rate during times of stress.
The reproductive components of the endocrine system, called the gonads, secrete sex hormones in response to stimulation from the pituitary gland. Located in the pelvis, the female gonads, the ovaries, produce eggs. They also secrete a number of female sex hormones, including estrogen and progesterone, which control development of the reproductive organs, stimulate the appearance of female secondary sex characteristics, and regulate menstruation and pregnancy.
Located in the scrotum, the male gonads, the testes, produce sperm and also secrete a number of male sex hormones, or androgens. The androgens, the most important of which is testosterone, regulate development of the reproductive organs, stimulate male secondary sex characteristics, and stimulate muscle growth.
The pancreas is positioned in the upper abdomen, just under the stomach. The major part of the pancreas, called the exocrine pancreas, functions as an exocrine gland, secreting digestive enzymes into the gastrointestinal tract. Distributed through the pancreas are clusters of endocrine cells that secrete insulin, glucagon, and somastatin. These hormones all participate in regulating energy and metabolism in the body.
The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin, a hormone that may help regulate the wake-sleep cycle. Research has shown that disturbances in the secretion of melatonin are responsible, in part, for the jet lag associated with long-distance air travel.
III HOW THE ENDOCRINE SYSTEM WORKS
Hormones from the endocrine organs are secreted directly into the bloodstream, where special proteins usually bind to them, helping to keep the hormones intact as they travel throughout the body. The proteins also act as a reservoir, allowing only a small fraction of the hormone circulating in the blood to affect the target tissue. Specialized proteins in the target tissue, called receptors, bind with the hormones in the bloodstream, inducing chemical changes in response to the body’s needs. Typically, only minute concentrations of a hormone are needed to achieve the desired effect.
Too much or too little hormone can be harmful to the body, so hormone levels are regulated by a feedback mechanism. Feedback works something like a household thermostat. When the heat in a house falls, the thermostat responds by switching the furnace on, and when the temperature is too warm, the thermostat switches the furnace off. Usually, the change that a hormone produces also serves to regulate that hormone's secretion. For example, parathyroid hormone causes the body to increase the level of calcium in the blood. As calcium levels rise, the secretion of parathyroid hormone then decreases. This feedback mechanism allows for tight control over hormone levels, which is essential for ideal body function. Other mechanisms may also influence feedback relationships. For example, if an individual becomes ill, the adrenal glands increase the secretions of certain hormones that help the body deal with the stress of illness. The adrenal glands work in concert with the pituitary gland and the brain to increase the body’s tolerance of these hormones in the blood, preventing the normal feedback mechanism from decreasing secretion levels until the illness is gone.
Long-term changes in hormone levels can influence the endocrine glands themselves. For example, if hormone secretion is chronically low, the increased stimulation by the feedback mechanism leads to growth of the gland. This can occur in the thyroid if a person's diet has insufficient iodine, which is essential for thyroid hormone production. Constant stimulation from the pituitary gland to produce the needed hormone causes the thyroid to grow, eventually producing a medical condition known as goiter.
IV DISEASES OF THE ENDOCRINE SYSTEM
Endocrine disorders are classified in two ways: disturbances in the production of hormones, and the inability of tissues to respond to hormones. The first type, called production disorders, are divided into hypofunction (insufficient activity) and hyperfunction (excess activity). Hypofunction disorders can have a variety of causes, including malformations in the gland itself. Sometimes one of the enzymes essential for hormone production is missing, or the hormone produced is abnormal. More commonly, hypofunction is caused by disease or injury. Tuberculosis can appear in the adrenal glands, autoimmune diseases can affect the thyroid, and treatments for cancer—such as radiation therapy and chemotherapy—can damage any of the endocrine organs. Hypofunction can also result when target tissue is unable to respond to hormones. In many cases, the cause of a hypofunction disorder is unknown.
Hyperfunction can be caused by glandular tumors that secrete hormone without responding to feedback controls. In addition, some autoimmune conditions create antibodies that have the side effect of stimulating hormone production. Infection of an endocrine gland can have the same result.
Accurately diagnosing an endocrine disorder can be extremely challenging, even for an astute physician. Many diseases of the endocrine system develop over time, and clear, identifying symptoms may not appear for many months or even years. An endocrinologist evaluating a patient for a possible endocrine disorder relies on the patient's history of signs and symptoms, a physical examination, and the family history—that is, whether any endocrine disorders have been diagnosed in other relatives. A variety of laboratory tests—for example, a radioimmunoassay—are used to measure hormone levels. Tests that directly stimulate or suppress hormone production are also sometimes used, and genetic testing for deoxyribonucleic acid (DNA) mutations affecting endocrine function can be helpful in making a diagnosis. Tests based on diagnostic radiology show anatomical pictures of the gland in question. A functional image of the gland can be obtained with radioactive labeling techniques used in nuclear medicine.
One of the most common diseases of the endocrine systems is diabetes mellitus, which occurs in two forms. The first, called diabetes mellitus Type 1, is caused by inadequate secretion of insulin by the pancreas. Diabetes mellitus Type 2 is caused by the body's inability to respond to insulin. Both types have similar symptoms, including excessive thirst, hunger, and urination as well as weight loss. Laboratory tests that detect glucose in the urine and elevated levels of glucose in the blood usually confirm the diagnosis. Treatment of diabetes mellitus Type 1 requires regular injections of insulin; some patients with Type 2 can be treated with diet, exercise, or oral medication. Diabetes can cause a variety of complications, including kidney problems, pain due to nerve damage, blindness, and coronary heart disease. Recent studies have shown that controlling blood sugar levels reduces the risk of developing diabetes complications considerably.
Diabetes insipidus is caused by a deficiency of vasopressin, one of the antidiuretic hormones (ADH) secreted by the posterior lobe of the pituitary gland. Patients often experience increased thirst and urination. Treatment is with drugs, such as synthetic vasopressin, that help the body maintain water and electrolyte balance.
Hypothyroidism is caused by an underactive thyroid gland, which results in a deficiency of thyroid hormone. Hypothyroidism disorders cause myxedema and cretinism, more properly known as congenital hypothyroidism. Myxedema develops in older adults, usually after age 40, and causes lethargy, fatigue, and mental sluggishness. Congenital hypothyroidism, which is present at birth, can cause more serious complications including mental retardation if left untreated. Screening programs exist in most countries to test newborns for this disorder. By providing the body with replacement thyroid hormones, almost all of the complications are completely avoidable.
Addison's disease is caused by decreased function of the adrenal cortex. Weakness, fatigue, abdominal pains, nausea, dehydration, fever, and hyperpigmentation (tanning without sun exposure) are among the many possible symptoms. Treatment involves providing the body with replacement corticosteroid hormones as well as dietary salt.
Cushing's syndrome is caused by excessive secretion of glucocorticoids, the subgroup of corticosteroid hormones that includes hydrocortisone, by the adrenal glands. Symptoms may develop over many years prior to diagnosis and may include obesity, physical weakness, easily bruised skin, acne, hypertension, and psychological changes. Treatment may include surgery, radiation therapy, chemotherapy, or blockage of hormone production with drugs.
Thyrotoxicosis is due to excess production of thyroid hormones. The most common cause for it is Graves' disease, an autoimmune disorder in which specific antibodies are produced, stimulating the thyroid gland. Thyrotoxicosis is eight to ten times more common in women than in men. Symptoms include nervousness, sensitivity to heat, heart palpitations, and weight loss. Many patients experience protruding eyes and tremors. Drugs that inhibit thyroid activity, surgery to remove the thyroid gland, and radioactive iodine that destroys the gland are common treatments.
Acromegaly and gigantism both are caused by a pituitary tumor that stimulates production of excessive growth hormone, causing abnormal growth in particular parts of the body. Acromegaly is rare and usually develops over many years in adult subjects. Gigantism occurs when the excess of growth hormone begins in childhood.

Contributed By:
Gad B. Kletter
Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.


(b) Eco-System
Ecosystem
I INTRODUCTION
Ecosystem, organisms living in a particular environment, such as a forest or a coral reef, and the physical parts of the environment that affect them. The term ecosystem was coined in 1935 by the British ecologist Sir Arthur George Tansley, who described natural systems in “constant interchange” among their living and nonliving parts.
The ecosystem concept fits into an ordered view of nature that was developed by scientists to simplify the study of the relationships between organisms and their physical environment, a field known as ecology. At the top of the hierarchy is the planet’s entire living environment, known as the biosphere. Within this biosphere are several large categories of living communities known as biomes that are usually characterized by their dominant vegetation, such as grasslands, tropical forests, or deserts. The biomes are in turn made up of ecosystems. The living, or biotic, parts of an ecosystem, such as the plants, animals, and bacteria found in soil, are known as a community. The physical surroundings, or abiotic components, such as the minerals found in the soil, are known as the environment or habitat.
Any given place may have several different ecosystems that vary in size and complexity. A tropical island, for example, may have a rain forest ecosystem that covers hundreds of square miles, a mangrove swamp ecosystem along the coast, and an underwater coral reef ecosystem. No matter how the size or complexity of an ecosystem is characterized, all ecosystems exhibit a constant exchange of matter and energy between the biotic and abiotic community. Ecosystem components are so interconnected that a change in any one component of an ecosystem will cause subsequent changes throughout the system.
II HOW ECOSYSTEMS WORK
The living portion of an ecosystem is best described in terms of feeding levels known as trophic levels. Green plants make up the first trophic level and are known as primary producers. Plants are able to convert energy from the sun into food in a process known as photosynthesis. In the second trophic level, the primary consumers—known as herbivores—are animals and insects that obtain their energy solely by eating the green plants. The third trophic level is composed of the secondary consumers, flesh-eating or carnivorous animals that feed on herbivores. At the fourth level are the tertiary consumers, carnivores that feed on other carnivores. Finally, the fifth trophic level consists of the decomposers, organisms such as fungi and bacteria that break down dead or dying matter into nutrients that can be used again.
Some or all of these trophic levels combine to form what is known as a food web, the ecosystem’s mechanism for circulating and recycling energy and materials. For example, in an aquatic ecosystem algae and other aquatic plants use sunlight to produce energy in the form of carbohydrates. Primary consumers such as insects and small fish may feed on some of this plant matter, and are in turn eaten by secondary consumers, such as salmon. A brown bear may play the role of the tertiary consumer by catching and eating salmon. Bacteria and fungi may then feed upon and decompose the salmon carcass left behind by the bear, enabling the valuable nonliving components of the ecosystem, such as chemical nutrients, to leach back into the soil and water, where they can be absorbed by the roots of plants. In this way nutrients and the energy that green plants derive from sunlight are efficiently transferred and recycled throughout the ecosystem.
In addition to the exchange of energy, ecosystems are characterized by several other cycles. Elements such as carbon and nitrogen travel throughout the biotic and abiotic components of an ecosystem in processes known as nutrient cycles. For example, nitrogen traveling in the air may be snatched by a tree-dwelling, or epiphytic, lichen that converts it to a form useful to plants. When rain drips through the lichen and falls to the ground, or the lichen itself falls to the forest floor, the nitrogen from the raindrops or the lichen is leached into the soil to be used by plants and trees. Another process important to ecosystems is the water cycle, the movement of water from ocean to atmosphere to land and eventually back to the ocean. An ecosystem such as a forest or wetland plays a significant role in this cycle by storing, releasing, or filtering the water as it passes through the system.
Every ecosystem is also characterized by a disturbance cycle, a regular cycle of events such as fires, storms, floods, and landslides that keeps the ecosystem in a constant state of change and adaptation. Some species even depend on the disturbance cycle for survival or reproduction. For example, longleaf pine forests depend on frequent low-intensity fires for reproduction. The cones of the trees, which contain the reproductive structures, are sealed shut with a resin that melts away to release the seeds only under high heat.
III ECOSYSTEM MANAGEMENT
Humans benefit from these smooth-functioning ecosystems in many ways. Healthy forests, streams, and wetlands contribute to clean air and clean water by trapping fast-moving air and water, enabling impurities to settle out or be converted to harmless compounds by plants or soil. The diversity of organisms, or biodiversity, in an ecosystem provides essential foods, medicines, and other materials. But as human populations increase and their encroachment on natural habitats expands, humans are having detrimental effects on the very ecosystems on which they depend. The survival of natural ecosystems around the world is threatened by many human activities: bulldozing wetlands and clear-cutting forests—the systematic cutting of all trees in a specific area—to make room for new housing and agricultural land; damming rivers to harness the energy for electricity and water for irrigation; and polluting the air, soil, and water.
Many organizations and government agencies have adopted a new approach to managing natural resources—naturally occurring materials that have economic or cultural value, such as commercial fisheries, timber, and water—in order to prevent their catastrophic depletion. This strategy, known as ecosystem management, treats resources as interdependent ecosystems rather than simply commodities to be extracted. Using advances in the study of ecology to protect the biodiversity of an ecosystem, ecosystem management encourages practices that enable humans to obtain necessary resources using methods that protect the whole ecosystem. Because regional economic prosperity may be linked to ecosystem health, the needs of the human community are also considered.
Ecosystem management often requires special measures to protect threatened or endangered species that play key roles in the ecosystem. In the commercial shrimp trawling industry, for example, ecosystem management techniques protect loggerhead sea turtles. In the last thirty years, populations of loggerhead turtles on the southeastern coasts of the United States have been declining at alarming rates due to beach development and the ensuing erosion, bright lights, and traffic, which make it nearly impossible for female turtles to build nests on beaches. At sea, loggerheads are threatened by oil spills and plastic debris, offshore dredging, injury from boat propellers, and getting caught in fishing nets and equipment. In 1970 the species was listed as threatened under the Endangered Species Act.
When scientists learned that commercial shrimp trawling nets were trapping and killing between 5000 and 50,000 loggerhead sea turtles a year, they developed a large metal grid called a Turtle Excluder Device (TED) that fits into the trawl net, preventing 97 percent of trawl-related loggerhead turtle deaths while only minimally reducing the commercial shrimp harvest. In 1992 the National Marine Fisheries Service (NMFS) implemented regulations requiring commercial shrimp trawlers to use TEDs, effectively balancing the commercial demand for shrimp with the health and vitality of the loggerhead sea turtle population.

Contributed By:
Joel P. Clement
Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.

(c) Troposphere
Troposphere
Troposphere, lowest layer of the earth's atmosphere and site of all weather on the earth. The troposphere is bounded on the top by a layer of air called the tropopause, which separates the troposphere from the stratosphere, and on the bottom by the surface of the earth. The troposphere is wider at the equator (16 km/10 mi) than at the poles (8 km/5 mi).
The temperature of the troposphere is warmest in the tropical (latitude 0º to about 30º north and south) and subtropical (latitude about 30º to about 40º north and south) climatic zones (see climate) and coldest at the polar climatic zones (latitude about 70º to 90º north and south). Observations from weather balloons have shown that temperature decreases with height at an average of 6.5º C per 1000 m (3.6º F per 1000 ft), reaching about -80º C (about -110º F) above the tropical regions and about -50º C (about -60º F) above the polar regions.
The troposphere contains 75 percent of the atmosphere's mass—on an average day the weight of the molecules in air (see Pressure) is 1.03 kg/sq cm (14.7 lb/sq in)—and most of the atmosphere's water vapor. Water vapor concentration varies from trace amounts in polar regions to nearly 4 percent in the tropics. The most prevalent gases are nitrogen (78 percent) and oxygen (21 percent), with the remaining 1 percent consisting of argon (0.9 percent) and traces of hydrogen, ozone (a form of oxygen), methane, and other constituents. Carbon dioxide is present in small amounts, but its concentration has nearly doubled since 1900. Like water vapor, carbon dioxide is a greenhouse gas (see Greenhouse Effect), which traps some of the earth's heat close to the surface and prevents its release into space. Scientists fear that the increasing amounts of carbon dioxide could raise the earth's surface temperature during the next century, bringing significant changes to worldwide weather patterns. Such changes may include a shift in climatic zones and the melting of the polar ice caps, which could raise the level of the world's oceans.
The uneven heating of the regions of the troposphere by the sun (the sun warms the air at the equator more than the air at the poles) causes convection currents (see Heat Transfer), large-scale patterns of winds that move heat and moisture around the globe. In the Northern and Southern hemispheres, air rises along the equator and subpolar (latitude about 50º to about 70º north and south) climatic regions and sinks in the polar and subtropical regions. Air is deflected by the earth's rotation as it moves between the poles and equator, creating belts of surface winds moving from east to west (easterly winds) in tropical and polar regions, and winds moving from west to east (westerly winds) in the middle latitudes. This global circulation is disrupted by the circular wind patterns of migrating high and low air pressure areas, plus locally abrupt changes in wind speed and direction known as turbulence.
A common feature of the troposphere of densely populated areas is smog, which restricts visibility and is irritating to the eyes and throat. Smog is produced when pollutants accumulate close to the surface beneath an inversion layer (a layer of air in which the usual rule that temperature of air decreases with altitude does not apply), and undergo a series of chemical reactions in the presence of sunlight. Inversions suppress convection, or the normal expansion and rise of warm air, and prevent pollutants from escaping into the upper atmosphere. Convection is the mechanism responsible for the vertical transport of heat in the troposphere while horizontal heat transfer is accomplished through advection.
The exchange and movement of water between the earth and atmosphere is called the water cycle. The cycle, which occurs in the troposphere, begins as the sun evaporates large amounts of water from the earth's surface and the moisture is transported to other regions by the wind. As air rises, expands, and cools, water vapor condenses and clouds develop. Clouds cover large portions of the earth at any given time and vary from fair-weather cirrus to towering cumulus clouds (see Cloud). When liquid or solid water particles grow large enough in size, they fall toward the earth as precipitation. The type of precipitation that reaches the ground, be it rain, snow, sleet, or freezing rain, depends upon the temperature of the air through which it falls.
As sunlight enters the atmosphere, a portion is immediately reflected back to space, but