ROCKS
Majority of the continental crust is composed of rocks with high content of silicon and aluminum. These rocks are low in density and light in color. The oceanic crust in contrast, consists of denser dark colored rock containing a great abundance of iron, calcium and magnesium.

IGNEOUS ROCKS

Introduction
Igneous rock is formed from magma or lava. Magma is found deep inside the earth. Lava is magma that reaches outside the earth’s surface. When magma and lava cool, they harden into igneous rocks.

Types of Igneous Rocks
Most igneous rocks have a crystalline structure. Their textures, however vary. By looking at the textures, there are two main types of igneous rocks. One type has coarser textures than other. The textures of each type result from the way the rocks were formed.

Intrusive Rocks
Intrusive rock is igneous rock formed from magma. Magma cools and hardens underground. Magma can intrude, or push its way into underground cracks and openings where it becomes trapped. Surrounded by rock, the magma cools slowly. Some trapped magma cools only a few degrees per century. As a result, the magma takes thousands of years to harden into rock. Granite and Gabbro are examples of intrusive rocks.
Since intrusive rocks cool slowly, their crystals have time to grow. The large crystals give intrusive rocks a coarse grained texture.

Extrusive Rocks
Extrusive rock is igneous rock formed from lava. When lava is extruded or pushed out from a volcano, it is instantly exposed to cooler temperatures. The lava cools quickly, hardening into rock. Basalt and Rhyolite are extrusive rocks. Basalt is commonly found in Hawaiian Islands and Colombia Plateau.
Since extrusive rocks cool quickly, their crystals are small. As a result, extrusive rocks have a fine grained texture. Some extrusive rocks cool so rapidly that crystals do not have time to form. These rocks have a glossy texture. Obsidian is a rock formed in this way. Extrusive igneous rocks are more commonly associated with oceanic crust than with the continental crust.

Igneous Rock Composition
A rock’s color depends on the color of its minerals. Thus, the color of an igneous rock gives a clue to its composition. Light-colored igneous rocks are rich in quartz, feldspar and other light colored minerals. Dark colored igneous rocks are rich in Biotite Mica, Olivine and other dark colored minerals.

Some Common Igneous Rocks
Of all the rocks, Granite is the most common intrusive rock. Basalt is the most common extrusive rock. Pumice and Scoria are rocks formed while gases were escaping from lava. As a result, both are full of holes and very light weight. Pumice is so light that it floats on water.

SEDIMENTARY ROCKS

Introduction
Sedimentary rock is formed from sediments. Sediments are small bits of matter deposited by water, ice or wind. They can be bits of rock, shell, or the remains of plant or animal. A sedimentary rock is formed when sediments are cemented together or hardened by other processes. Sedimentary rocks unlock clues to a region’s past. If sedimentary rocks are common in a region, it was probably under water at one time. Sedimentary rocks cover nearly all of the ocean floor and about 2/3rd of the continents, making them the most widely distributed of the three rock categories at the earth’s surface.
How Sedimentary rocks are formed
Sediment is deposited when it is dropped by water, wind, or whatever is carrying it. Deposited sediment tends to build up in layers. This is the first step in changing sediments to rock. As sediment is buried more deeply, it is subjected to higher temperatures and pressures. Under such conditions, changes may occur that transform sediments into rock. Two processes that help transform sediments into rock are compaction and cementation.

Compaction
Compaction is the decrease in volume brought about by pressure. Sediment builds up into layers. The top layers pushed down on to the bottom layers. When squeezed, fine sediments like clay and sand stick together to form a rock.

Cementation
Coarse sediments do not stick together as well as fine sediments. However, they will stick with one another if they are cemented together. Cementation is the process by which sediments are joined together to form rock. Cementation occurs as minerals are deposited between sediments. When these minerals harden, the sediments are cemented together and a solid rock is formed.

Other ways
Sediments may harden into rock in other ways. Shell bones, and other plant and animal remains undergo chemical changes as they decay. Some of these changes may cause such sediments to harden into rock. Some sediments when dissolved in water crystallize as water evaporates. The crystals form a solid mass of rock.

Characteristics of Sedimentary Rocks
Many sedimentary rocks have visible layering. Layers form when different sediments pile on top of the other. These layers can often be seen even after the sediments change into rock. In addition, many sedimentary rocks contain fossils.
Ripple marks and mud cracks can also be found in some sedimentary rocks. Ripple marks are formed by water or wind moving over loose sediments. Cracks are often formed on the surface of wet mud as it dries. Sometimes these ripple marks and mud cracks are buried under new layers of sediment. When this happens they may be preserved in rocks.

Types of sedimentary rocks
Sedimentary rock is divided into three different types. Each type is formed from a different kind of sediment.

Clastic Rock
Clastic rock is a sedimentary rock formed from sediments that have been cemented or compacted together. These sediments are often broken bits of rocks and minerals. They range in size from very large to very small. The texture of clastic rocks is usually fragmental. Examples are conglomerate, sand stone, silt stone, shale, etc.

Chemical Rock
Chemical rock is a sedimentary rock made of minerals that were once dissolved in water. Some chemical rocks are formed when water evaporates and leaves mineral deposits behind. Rock salt is an example of a chemical rock formed in this way.
Chemical rocks may form when chemical changes cause minerals dissolved in water to crystallize. The crystals settle to the bottom, forming layers. Some lime stones form this way. Examples are rock salt, rock gypsum, etc.

Organic Rock
Organic rock is a sedimentary rock made of substances that were once part of, or made by living things. Some lime stones are formed by once-living things. For example, when animals with shells die, their shells sink to the ocean floor. As layers of shells build up, they may harden into rock. Coal is also an organic rock. It has been formed from dead plants buried in swamps. Other examples are coquina, organic lime stone (CaCO3), etc.

METAMORPHIC ROCKS

Introduction
Metamorphic rock is formed when already existing rock is changed by great heat, great pressure, or chemical action. Metamorphic rocks are formed when igneous rocks, sedimentary rocks, or other metamorphic rocks are changed.

How Metamorphic Rocks are formed
Great heat, great pressure and chemical action can cause a rock to change. Metamorphism is the changing of one type of rock to another, as a result of great heat, pressure and chemical change. Most metamorphism occurs underground. Rocks underground are subjected to high pressures and temperatures.
Metamorphism occurs at temperatures between 100oC to 800oC. Within this temperature range, the rock is still solid but softened. In this softened state, minerals in a rock may rearrange themselves. Their crystals may separate into layers or may change in shape or size. At these high temperatures chemical reactions may occur. Major changes in a rock’s composition may result. Furthermore, pressure can squeeze a rock’s minerals’ crystals. The crystals may become flattened or elongated in shape.
The two ways in which Metamorphism takes place are:

Regional Metamorphism
Regional metamorphism occurs when rocks over a large region are exposed to great heat and pressure. Mostly, regional metamorphism is due to the deep burial or to the movements of rocks in the earth’s crust.

Contact Metamorphism
Metamorphic rocks are often found around the edges of igneous rock formations. These rocks result from contact metamorphism. Contact metamorphism occurs when rocks are heated as they come into contact with Magma or Lava.

Characteristics of Metamorphic Rocks

Metamorphic rocks have certain distinct characteristics. These set them apart from other rocks. For instance, all metamorphic rocks have a crystalline structure. Many have flaky, or needle-like crystals arranged in parallel layers. Many metamorphic rocks contain unusual minerals that only form under high temperatures and pressures. In addition, metamorphic rock formed from sedimentary rock, may have distorted layers. Finally, some metamorphic rocks become more dense as a result of great pressure exerted upon them. The pressure squeezes the molecules in the rock closer together. The rock’s mass is forced into smaller volume.

Types
The metamorphic are by far the least abundant of the three major surface rock categories. They are much more common deep within the crust, where they have been produced by the tectonic activity. Scientists classify metamorphic rocks into two types:

Foliated Rock
Foliated rock is metamorphic rock whose minerals are arranged in parallel layers. Foliation results when minerals recrystallize, or are flattened under pressure. Foliation also results when minerals of different densities separate into layers behaving much like a mixture of oil and water. This separation results in a series of alternating light and dark bands. Many foliated rocks break into thin sheets. Slate, Schist and Gneiss are examples of foliated rocks.

Non-foliated Rock
Non-foliated rock is metamorphic rock without layers. This type of rock does not break up into flat sheets. Marble and Quartzite are the examples of non-foliated rocks.

METAMORPHIC ROCK TYPE--------PARENT ROCK
Quartzite--------------------Sand Stone or Silt Stone
Slate -----------------------Shale
Phyllite ---------------------Shale, Slate
Schist ----------------------Shale, Slate, Phyllite
Gneiss ----------------------Granite, Diorite
Marble ----------------------Lime Stone, Dolomite
Anthracite coal --------------Bituminous Coal


ROCK CYCLE

Introduction

The concept of the Rock Cycle, which may be considered as a basic outline of physical geology, was initially proposed in the late 18th century by James Hutton, a founding father of modern geology. This rock cycle shown in the figure uses arrows to indicate chemical and physical processes, and boxes to represent earth materials.

Crystallization
The first rock type, Igneous Rock, originates when molten material called magma cools and solidifies. This process, called crystallization, may occur either beneath the earth’s surface, or following a volcanic eruption at the surface. Initially or shortly after forming, the earth’s outer shell is believed to have been molten. As this molten material is gradually cooled and crystallized, it generates a primitive crust that consists entirely of igneous rock.

Weathering, Transportation and Deposition
If igneous rocks are exposed at the earth’s surface, they will undergo weathering, in which the day-in and day-out influences of the atmosphere slowly disintegrate and decompose the rock. The resulting material will be picked up, transported and deposited by any of a number of erosion agents – gravity, running water, glaciers, winds or waves.

Lithification
Once this material called sediment, is deposited usually as horizontal beds in the ocean, it will undergo Lithification a term meaning “conversion into rock”. Sediment is lithified when compacted by the weight of overlying layers or when cemented as percolating groundwater fills the pores with mineral matter.

Metamorphism
If the resulting sedimentary rock is buried deep within the earth or involved in the dynamics of mountain building, it will be subject to great pressures and heat. The sedimentary rock will react to the changing environment and turn into third rock type, Metamorphic Rock.

Melting
When metamorphic rock is subjected to still greater heat and pressure, it will melt to create magma, which will eventually solidifies as Igneous Rock.

Shortcuts
The full cycle just described does not always take place. Shortcuts in the cycle are indicated by dashed lines in the figure. Igneous rocks, for example rather than being exposed to weathering and erosion at the earth’s surface, may be subjected to heat and pressure found far below and change to metamorphic rock. On the other hand, metamorphic and sedimentary rocks, as well as sediment, may be exposed at the surface and turned into new raw materials for sedimentary rock.

Identify Different Rocks from Photos

Attached Thumbnails

         

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