Natural Disasters

Natural Disasters  Earthquakes    

Floods Droughts Volcanoes Tsunamis

Earthquakes When the Earth moves Earthquakes are caused by the motion of tectonic plates individual sections that make up the Earth's surface like panels on a football. Immense strain accumulates along fault lines where adjacent plates meet. When the rock separating the plates give way, sudden seismic groundshaking movement occurs.

Earthquakes Of course, if we want to know why earthquakes happen, we need to dig a little deeper.

Centre of the Earth The Earth is made up of three main layers:  The core is at the centre of the Earth  The mantle is a mobile semi-molten layer around the core  The outer-shell of the Earth is called the crust. Scientists call this the lithosphere - it's the part we're on now

Earthquake s tectonic plates. Below the sea, they can measure The crust is made up of 12 individual three to six miles (4km-9.6km) thick and under land this increases to 20-44 miles (32km70.8km). Below the crust, radiation from the Earth's core heats the semi-molten mantle to temperatures of over 5000°C.

All fluids when heated - even molten rock - are affected by a process called convection. The makes hot liquid rise to displace cooler liquid, creating a current. Tectonic plates effectively float on the mantle, like croutons in a bowl of super-heated soup. But these plates and constantly moving due to the convection current.

Earthquakes Of course, they creep along very slowly - roughly the same speed your fingernails grow. Even at this sub snail-pace, the effects can be devastating. The combined annual force of earthquakes is equal to 100,000 times the power of the atomic bombs that flattened Hiroshima.

The point where the seismic activity occurs is the epicentre, where the earthquake is strongest. But it doesn't always end there, seismic waves travel out from the epicentre, sometimes creating widespread destruction as they pass.

Floods When rain starts to fall, it drains down from the hillsides into streams, along rivers and out into the sea. That's under normal circumstances anyway. But when rain pours for weeks at a time, the land becomes saturated. It is called flood

Floods

Stormy weather Low pressure develops on the boundary of warm and cold air - this is known to meteorologists as a 'weather front'. The air here has a natural tendency to rise, and as air rises, it cools. Any water vapour present in the air condenses to form clouds. Consequently, low pressure is generally associated with wet and windy weather.

Floods The upper reaches of rivers quickly fill and force the excess water downstream. In the lower reaches, water flows slower. Here, the river swells and begins to break its banks. This is entirely normal - flood plains are part of the river's natural defense mechanism.

These low-lying, wide flat areas in the lower reaches of a river provide relief and take up the excess water. It's best not to build on them, though sometimes people do.

All kinds of debris gets caught up in a flood - dead cattle, sheep, trees, gravel. If any of this flotsam gets lodged under a bridge, it creates a dam and backs up the whole system

Droughts During a drought, the land can become starved of nourishment or contaminated with mineral salts and even when it does rain, often the ground cannot support growth. The basic cause of all drought is insufficient rainfall. This can be due to global patterns of air circulation, like the Sahara - or, it can be created by man.

Droughts The effects of dams and irrigation in hot climates can have a dramatic effect on the environment. The dams and water irrigation preventing rivers from running their natural course can dry the land. This prevents water vapour from rising to create rain clouds.

In a natural desert like the Sahara, dry air descends over the region bringing clear skies and hot sunshine. These hot deserts get little rain at any season. The problem often lasts for many years with rainy seasons failing to show.

Short term droughts can also be caused when high pressure systems persist over an area of land for several weeks. This blocks the passage of low pressure systems that bring rain.

Volcanos Without doubt, erupting volcanoes are the most awesome and terrifying sights in nature. In fact, their untamed destructive firepower has shaped and influenced many ancient cultures from Pompeii, to Japan.

Volcanoes How volcanoes occurs Like earthquakes, volcanoes form at weak-points in the Earth's crust, known as 'fault-lines'.

When two tectonic plates collide, the event can provide the catalyst for volcanic activity. As one section slides on top of the other, the one beneath is pushed down into the mantle. Water trapped in the crust can cause reactions within the mantle. Why? Well, it's thought that when the water mixes with the mantle, it lowers the mantle's melting point, and the solid mantle melts to form a liquid, known as 'magma'.

Volcanoes Under increasing pressure Since liquid rock is less dense than solid rock, magma begins to rise through the Earth's crust. It forces its way up, melting surrounding rock and increasing the amount of magma.

Magma only stops rising when the pressure from the rock layer above it becomes too great. It gathers below the Earth's surface in a 'magma chamber'.

When the pressure increases in the chamber, the crust finally gives way and magma spews out onto the Earth's surface forming a volcano. When it reaches the surface, magma becomes known as 'lava'.

Volcanoes Volcanic eruptions vary in intensity and appearance depending on two factors: 1. The amount of gas contained in the magma its viscosity - how runny it is 2. In general, the explosive eruptions come from high gas levels and high viscosity in the magma. Whilst lava floes result from magmas with low gas levels and low viscosity.

Tsunamis Tsunamis A tsunami (pronounced tsoo-nahmee) is a chain of fast moving waves caused by sudden trauma in the ocean. They can be generated by earthquakes, volcanic eruptions, or even the impact of meteorites. Tsunami are also incorrectly known as tidal waves, but unlike tidal waves they are not caused by changes in the tides.

Tsunamis They are most common around the edge of the Pacific, where more than half of the world's volcanoes are found. These seismic surges can assault coastlines, often with little or no warning. Rocks weighing as much as 20 metric tonnes have been plucked from sea walls and carried 180m inland.

Tsunamis aren't like wind-generated waves that rhythmically roll onto a beach. A tsunami can have a wavelength (ie distance between wave crests) in excess of 100km (60 miles) and there may be an hour between them. They travel at great speeds across an ocean with hardly any energy losses and are barely noticeable out at sea.

Tsunamis Over the deep Pacific Ocean, a tsunami travels at about 800kph (500mph).

As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it behaves like a normal wave - only with more muscle.

Shallow water slows the tsunami and its height grows. Tsunamis batter the coast with tremendous amounts of energy. They can strip sand from beaches, tearing up trees, and even obliterating whole towns. Some have been known to reach as much as 30m above sea level.

Created By ::

10+2 (a)

1. Jaspreet 2. Pawan 3. Riddu Chhabra

Guided By :: Mr. Parmeet Singh Miss Poonam Rajpal