Earthquakes
The earth is covered in a rigid shell of tectonic plates which are between 13km (6m) and 96km (44m) thick. The point where these plates meet are called fault lines and when the plates collide along these lines it sends out a seismic shock which causes an earthquake. These in turn can trigger landslides and tsunamis.

The strength of an earthquake is measured by the Richter scale (ranging from 1 to 9), invented by seismologist Charles Richter in 1935.

2 on the Richter scale is the lowest level that you would be able to feel an earthquake, of which there are about 300,000 every year. These cause little or no damage. However, an earthquake reaching 8.25 on the Richter scale, like the one which destroyed San Francisco in 1906, is classed as severe.

One of the worst in recent times occurred in Iran in December 2003 when 26,000 were killed and most of the ancient city of Bam was destroyed.

Mudslides
Mudslides are caused by a range of triggers including earthquakes, blasting operations, soil erosion as a result of over logging and heavy rain.

As an excess of water collects at the top of a slope, it puts pressure on unstable earth and rocks below. This collapses causing devastating damage to everything in its path - as earth and debris are propelled down at speeds of up to 77kmph (35mph).

People caught in the mudslide are buried under tons of mud and debris. Rocks and heavy vegetation are picked up and carried along, demolishing buildings, roads and power lines leaving survivors without homes, water and electricity. Rescue efforts are often hampered by the destruction of road and rail links and many people die from disease and starvation in the aftermath.

One of the worst natural catastrophes of the century happened in 1999 when a mudslide in La Guaira, Venezuela, killed an estimated 35,000 people.

Avalanches
As snow falls on a mountain it forms layers. When these layers are disturbed and one slides away from the rest, a surface avalanche occurs. A full-depth avalanche happens when a number of layers separate at the same time.

Avalanches are caused by a number of things. If there has been heavy, continuous snowfall it will make the layers top heavy and so become more susceptible to breaking away. This is why 90% of avalanches happen during snowstorms. If there is a rise in temperature this will also make the snow unstable, as melting within the snowpack can seriously weaken some of the upper layers of snow.

Avalanches can be triggered by a loud noise or the weight of a person on the snow. The warning sound is the snow cracking, which often sounds like a gun shot. Most of the 100 avalanche deaths a year happen when skiers decide to go off-piste - and out-skiing an avalanche isn’t easy, as they can travel approximately 440kmph (200mph)!

Tips for avalanche survival include yelling and discarding equipment to make yourself lighter, as well as using swimming motions to try and reach the surface. When the avalanche is finally over, the snow can set as hard as cement and victims are usually so well insulated that rescuers do often not hear their cries for help until they are standing very close. Off-piste skiers are advised to carry and activate avalanche beacons.

Blights
Blights are spread through the earth and air, destroying crops on a vast scale. If a community’s livelihood relies on the production of one type of crop or that crop is the main source of food, blights can by devastating. Potato blight rots the crop making it inedible.

During the potato famine in Ireland, of 1845-47, one million people died of starvation and about the same amount emigrated. In the 1860’s another form of blight destroyed two million hectares of grape vines in Europe, almost wiping out the wine industry.

Blight on the rye harvest in Salem, USA, in the 17th Century didn’t wipe out the crop. Instead it infected it with a poisonous fungus that could survive the heat of the baking process. It had hallucinogenic effects and it’s believed that many of the witches who were put on trial were actually driven mad because of the infected rye in their bread.