Natural Hazards

Tropical storms are the correct name for hurricanes, cyclones and typhoons. They have their source over warm tropical seas heated to at least 27�C. Tropical storms develop in three stages:

• birth
• development
• death

Air above the sea heated to 27�C becomes warm and moist. It rises rapidly, cools and forms heavy cloud and rain. Air rushes into the area and fuels the updraft of warm air. The rotating Earth starts to spin the whole system. The system reaches land. It loses its sources of heat and moisture and slowly begins to burn out.

Density of population - If the earthquake happens in an urban area, far more people would be killed or made homeless than if it occurred in a less densely populated rural area. Some very powerful earthquakes happen in regions with hardly any people living there, such as Alaska, and so no-one is even injured.

Building quality - LEDCs have less money to spend on buildings which are made to withstand the earthquake. Towns in LEDCs often erect buildings which are flimsy or may have inadequate foundations. When an earthquake strikes, destruction can be total. Wealthier, MEDC countries which are susceptible to earthquakes, such as Japan, can afford to have buildings reinforced with cross bracings or flexible gas mains can be used to reduce the risk of fire. Some buildings have alarms that cut off the gas supply automatically and windows and furniture may be adapted to reduce injuries

Time of earthquake - Some of the most devastating earthquakes occur during the day when most people are up and about and so there are more casualties. The massive earthquake in Mexico City in 1985 occurred during the morning rush hour when children were on their way to school and the roads were full of traffic.

Distance from the epicentre - The epicentre is the centre of the earthquake on the surface, from which the shock or seismic waves radiate out. The seismic waves decrease in strength away from the epicentre, and so the area of greatest devastation is usually at this epicentre.

Nature of the underlying rock - Where the plates are being subducted into the mantle, at a subduction zone, the huge slabs of rock are put under great stress. This builds up until it is greater than the strength of the rock and so the rock 'fails' or breaks and an earthquake results. If the rock is very strong and resistant to failure, the pressure build up will be very high. When failure eventually occurs, the earthquake would be more powerful.

Earthquake strength - This is usually measured on the Richter scale. On this scale, a weak earthquake measuring only 1 or 2 on the scale may result in only rattling windows and books falling off shelves. A powerful earthquake, measuring perhaps 7 or 8 on the scale, can cause total devastation.

Planning and preparation for earthquakes 
Most MEDCs in areas of tectonic activity plan and prepare for earthquakes. When warnings can be given, usually in more economically developed regions such as California, governments and industry can prepare and organise rescue efforts. Although some hazards can be predicted, some more accurately than others, specialists are reluctant to give too many false alarms as people may ignore the warnings. Some hazards can be modified to reduce the amount of destruction caused. Scientists have experimented with "lubricating" the San Andreas Fault in California. They inject sand or liquids into the fault so that movement occurs without sudden jolts. Evacuation is often the most important priority, but it may create more problems:

• If the earthquake occurs near an international boundary, it is difficult for people to evacuate.
• Refugee camps are difficult to organise. After Mount Pinatubo erupted in the Philippines, more than 400 people died due to disease outbreaks in the camps.
• After the danger has passed it is difficult to move people back into the area with no services or infrastructure.

Different responses

Countries differ in their responses to a hazard event. MEDCs have more resources to use for disaster relief, for fire control, for organising emergency procedures, for hospitals, for emergency water and food, for temporary accommodation such as tents and for special teams equipped to search for survivors. These teams may have infrared heat seeking devices to search for survivors in the rubble. LEDCs will have fewer, if any, of these facilities and sometimes valuable days can be lost whilst the army and police try to deal with the unexpected event. It is difficult to issue emergency warnings and evacuation orders as few people have TV, radio or telephones. Buildings in LEDCs are likely to be more flimsy and not designed to withstand the earthquake as in richer countries.

core...The inner layer of the Earth

crust ...The hard outer layer of the Earth that is made up of continental and oceanic plates

LEDC ...Economically Less Developed Country (poor)

low pressure ...An area of rising air

MEDC ...Economically More Developed Country (rich)

mantle ...The molten rock that the Earth's crust lies on

plate margins ....Where plates meet

subduction ...When one tectonic plate is forced under another

tectonic plates ...A large area of the Earth's crust

tropical storm ...A severe storm of high winds and heavy rain powered by energy from warm tropical seas. Also known as hurricanes, cyclones and typhoons .

The location of continents today of far removed from what it was millions of years ago, when it is believed that all continents were joined to one land mass. Alfred Wegener put forward a theory regards their movement in 1912 with his theory of Continental Drift. His theory was based on observations such as:

Biological: Coal is found in UK but needs warm, wet, humid conditions to form.

Observational: The shapes of countries appear to 'fit' one another, for example, S. America and Africa.

In the 1940's-1960's the theory was revised and Plate Tectonic theory emerged. It is based on the premise that the lithosphere (crust and rigid upper mantle) is divided into plates, which are moved by convection currents coming from the earth's core.

late movement is either towards, away, or alongside adjacent plates. Crust is continually being created or destroyed and a variety of landforms are found at plate margins.

There are 2 main types of crust that lie on plates and each has its own characteristics:

Oceanic Crust (Sima) Young (under 200 million years), thin, dense and heavy, will sink. Always being created and destroyed. For example, Basalt. Continental Crust (Sial) Old, light, thick (up to 150km under mountains) permanent, does not sink. For example, Granite.

Movement of plates is in one of 3 ways:

Towards each other:Convergent (destructive or collision).

Away from each other:Divergent.

Alongside each other:Transform or transcurrent.

There is a further subdivision for Convergent plates, which is dependent on the properties of the plates moving towards each other (oceanic / continental crust).

Convergent plate boundaries (together)

The two types of margins that occur here are Destructive Margins or collision Zones.The chart and the diagrams below outline their main features:

Destructive margins Oceanic crust moves towards continental crust. Oceanic crust sinks, and slowly destroyed. Results in deep ocean trenches, island arcs and fold mountains. Can be referred to as subduction zones. Collision Zones Two continental crusts meet, neither sinks, fold mountains result. For example, the Alps.

Divergent plate boundaries (away)

Also known as a Constructive margin. Plates move away from each other, for example, N. American and Eurasian plates, cresting mid-ocean ridges such as the Mid Atlantic Ridge. New material appears at the ocean ridge.

This occurs when magma from the earth's interior is able to make its way to the surface, via a vent. This is usually associated with plate boundaries. Volcanoes vary greatly in their shape, as does the type of material emitted during an eruption. Volcanoes range from steep to gently sided.

Types of volcanoes

wo contrasting types exist; those dominated by lava, usually found at diverging boundaries, where basalt can rise freely to the surface (volcanoes of Iceland) and those dominated by Ash, usually found along subduction zones where large amounts of pyroclastic material is ejected (Japan - Fuji Yama)

The cone shapes of volcanoes are summarised in the table below:

Cone Shape:Characteristics: Fissure Very gentle slope, found at diverging ocean plates, basaltic lava, can flow over large distances. Basic / Shield Have gentle slopes, steeper than fissure due to repeated explosions and subsequent build up of basalt based lava (Mauna Loa Hawaii). Cone Symmetrical in shape, A) acid where thick viscous lava, rapidly cools, B) Ash / Cinder. Composite Very large old volcanoes. Both ash and lava are deposited (Mt. St. Helens). Crater / Caldera Form when a very violent eruption occurs after a build up of gas beneath the volcano. Can destroy the magma chamber leaving a large crater.

Several basic conditions are necessary for hurricane formation:

A Warm oceans with surface temperatures in excess of 27 degrees, and a deep layer of water to 60m B A location between 5degrees north and south of the equator. (Without this the corriolis force is not sufficient to create the spinning motion characteristics of hurricanes.) C Relatively stable and uniform atmospheric conditions of temperature, humidity and pressure. In the upper troposphere air drawn in at lower altitudes must be able to escape. D Relative humidity of over 60% to provide sufficient energy to power the hurricane. E Little change of horizontal wind with height. F Existing cyclonic spinning of winds in the lower troposphere.

Formation

Different air types meet and due to their different properties, do not mix. In the USA warm moist air with its origins in the Caribbean meets colder air that has originated in the northern Rockies. There is a vast contrast between the two, and massive instability results. A tornado can form if:

A The two meeting air masses are conditionally unstable. B Low level air has a relative humidity of over 65%. C A low level southerly jet stream exists in the humid air.

Associated hazards - Earthquakes usually kill far more people due to the secondary effects of fires, floods, starvation and disease, (especially water-related diseases) than by collapsing buildings. In the very long term, whole communities can die as people are afraid to return to the area and harvests are disrupted. Earthquakes at sea can cause large tsunamis or giant waves, which drown thousands and destroy harvests. The waves can travel thousands of kilometres before reaching land and flooding coastal areas. Rockfalls and landslides may be triggered, causing more devastation. In mountainous or cold regions, avalanches may be caused which can bury villages and towns.