Natural hazards

What is a natural hazard?

An extreme natural event or process that causes loss of life and/or extreme damage to property and creates severe disruption to human activities. 

They include earthquakes, tropical storms, floods and droughts

Some natural hazards occur all over the world, for exmaple floods caused by heavy rain. Others are limited, e.g. landslides or tornados.

How often hazards occur can depend on the location of the hazard, for exmaple, Bangladesh regularly suffers form river flooding as a result of the heavy rains caused by the monsoon.

Hurricanes and typhoons only occur during a certain period of the year when weather conditions are suitbale for their formation in the tropical oceans.

How do hazards affect people?

In many developed countries, loss of life is low but economic costs are extremely high. In devleoping countries, loss of life tends to be high while economic costs are usually lower.

The risk of natural disasters is increasing as a result of population growth, urbanisation and alteration of the natural environment.

What causes earthquakes and volcanoes?

• Movemements beneath the earth's surface. 
• Scientists cannot yet predict accurately when they will occur or how severe they will be.

The earth is made up of several concentric layers of rocks of different densities and temperatures. If you were to slice the earth in half, there would be 3 major zones.

The core is the central part and it is made of 2 sections, the inner core, which is solid, and the semi liquid outer core. Around the core is the mantle, where temperatures are so high that rocks exist in a semi-molten state. 

It is the thickest layer and makes up more than 80% of the earths volume. The outermost layer is the curst, the thinnest layer that varies between 5km and 90km thick.

The crust is broken into 7 large and 12 smaller sections known as plates, which float like rafts on the less dense material of the mantle below. These plates fit together like a pieces of a jigsaw.

They move relative to one another as a result of convection fluids in the earths mantle. This movement may be up several centimetres in a year.

The movement of these crustal plates is known as plate tectonics

Plates are made up of oceanic and continental crust. Continetal crust is mainly composed of granite and is less dense than oceanic crust. It does not sink and is permanent.

Oceanic crust is mainly made of igneous rock known as basalt. It is constantly being destoryed and replaced and can sink beneath the continental crust.

Plates meet at plate boundaries and it is in narrow belts along these boundaries where the major centres of volcanic and earthquake acitvity occur. Also where high mountain ranges are located.

What happens when the plates meet?

Convergent boundaries:

• Where plates collide with one another

• Collions are very slow 

• When an oceanic plate collides with a continental plate the oceanic plate is forced downwards into the mantle forming a subduction zone.

• The edge of the continental…