Natural Hazards in Detail
 

[B]Define Natural Hazards[/B]
A natural hazard is a natural phenomenon that might have a negative effect on humans and other animals, or the environment.

[B]Classification of Natural Hazards[/B]
Natural hazard events can be classified into two broad categories:
a. Geophysical
b. Biological

[B]Difference between Hazard and Disaster[/B]
An example of the distinction between a natural hazard and a disaster is that an earthquake is the hazard which caused the disaster. Natural hazards can be provoked or affected by anthropogenic processes, e.g. land-use change, drainage and construction.

Earthquake
 

[B]What is an earthquake?[/B]
An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane.

[B]Epicenter and Hypocenter[/B]
The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

[B]Intensity of Earthquake[/B]
Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the main shock. Main shocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!

[B]What causes earthquakes and where do they happen?[/B]
The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

[B]Why does the earth shake when there is an earthquake?[/B]
While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!

Volcanoes
 

[B]What is Volcano?[/B]
Volcanoes are openings, or vents where lava, tephra (small rocks), and steam erupt on to the Earth's surface. Many mountains form by folding, faulting, uplift, and erosion of the Earth's crust. Volcanic terrain, however, is built by the slow accumulation of erupted lava. The vent may be visible as a small bowl shaped depression at the summit of a cone or shield-shaped mountain. Through a series of cracks within and beneath the volcano, the vent connects to one or more linked storage areas of molten or partially molten rock (magma). This connection to fresh magma allows the volcano to erupt over and over again in the same location. In this way, the volcano grows ever larger, until it is no longer stable. Pieces of the volcano collapse as rock falls or as landslides.

[B]How do volcanoes erupt?[/B]
Molten rock below the surface of the Earth that rises in volcanic vents is known as magma, but after it erupts from a volcano it is called lava. Magma is made of molten rock, crystals, and dissolved gas—imagine an unopened bottle of soda with grains of sand inside. The molten rock is made of the chemicals oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. After cooling, liquid magma may form crystals of various minerals until it becomes completely solid and forms an igneous or magmatic rock. magma formation, movement to the surface, and eruption through a volcanic vent.

Originating many tens of miles beneath the ground, magma is lighter than surrounding solid rock. It is driven towards Earth's surface by buoyancy; it is
lighter than the surrounding rock, and by pressure from gas within
it. Magma forces its way upward and may ultimately break though weak areas in the Earth's crust. If so, [B]an eruption begins[/B]. Magma can be erupted in a variety of ways. Sometimes molten rock simply pours from the vent as fluid lava flows. It can also shoot violently into the air as dense clouds of rock shards (tephra) and gas. Larger fragments fall back around the vent, and clouds of tephra may move down the slope of the volcano under the force of gravity. Ash, tiny pieces of [B]tephra [/B]the thickness of a strand of hair, may be carried by the wind only to fall to the ground many miles away. The smallest ash particles may be erupted miles into the sky and carried many times around the world by winds high in the atmosphere before they fall to the ground.

A volcanic eruption is an awesome display of Earth’s power. Yet, while eruptions are spectacular to watch, they can cause disastrous loss of life and property, especially in densely populated regions of the world.

Some volcanic eruptions are explosive and others are not. The explosivity of an eruption depends on the composition of the magma.

a. If [B]magma is thin and runny[/B], gases can escape easily from it. When this type of magma erupts, it flows out of the volcano. A good example is the eruptions at H[I]awaii’s volcanoe[/I]s.
Lava flows rarely kill people because they move slowly enough for people to get out of their way.

b. If [B]magma is thick and sticky[/B], gases cannot escape easily. Pressure builds up until the gases escape violently and explode. A good example is the eruption of [I]Washington’s Mount St. Helens[/I].
In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra. Tephra can range in size from tiny particles of ash to house-size boulders.

[B]Explosive volcanic eruptions[/B] can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountain sides destroying almost everything in their path. Ash erupted into the sky falls back to Earth like powdery snow. If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes.


[B]Volcano as window of earth:[/B]
Today geologists agree that volcanism is a profound process resulting from the thermal evolution of planetary bodies. Heat does not easily escape from large bodies such as Earth by the processes of conduction or radiation. Instead, heat is transferred from Earth’s interior largely by convection—that is, the partial melting of Earth’s crust and mantle and the buoyant rise of magma to the surface. Volcanoes are the surface sign of this thermal process. Their roots reach deep inside Earth, and their fruits are hurled high into the atmosphere.