Earthquake
An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves.
Tsunami
A tsunami also called a tsunami wave train, and at one time incorrectly referred to as a tidal wave, is a series of water waves caused by the displacement of a large volume of a body of water, usually an ocean, though it can occur in large lakes.
Tornado
A tornado (often referred to as a twister or, erroneously, a cyclone) is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud.
Floods
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water
Volcanic Eruptions
Volcanoes can cause widespread destruction and consequent disaster through several ways. The effects include the volcanic eruption itself that may cause harm following the explosion of the volcano or the fall of rock.
Classification and Terminology of Avalanche
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All avalanches share common elements: a trigger which causes the avalanche, a start zone from which the avalanche originates, a slide path along which the avalanche flows, a run out where the avalanche comes to rest, and a debris deposit which is the accumulated mass of the avalanched snow once it has come to rest. As well avalanches have a failure layer that propagates the failure and the bed surface along which the snow initially slides, in most avalanches the failure layer and the bed surface are the same. Additionally slab avalanches have a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a shallow staunch fracture at the bottom of the start zone. The crown and flank fractures are vertical walls in the snow delineating the snow that was entrained in the avalanche from the snow that remained on the slope.
The nature of the failure of the snow pack is used to morphologically classify the avalanche. Slab avalanches are generated when an additional load causes a brittle failure of a slab that is bridging a weak snow layer; this failure is propagated through fracture formation in the bridging slab. Loose snow, point release, and isothermal avalanches are generated when a stress causes a shear failure in a weak interface, either within the snow pack, or at the base. When the failure occurs at the base they are known as full depth avalanches. Spin drift avalanches occur when wind lifted snow is funneled into a steep drainage from above the drainage.
Loose snow avalanches occur in freshly fallen snow that has a lower density and are most common on steeper terrain. In fresh, loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usually forming a teardrop appearance. This is in contrast to a slab avalanche.
Slab avalanches account for around 90% of avalanche-related fatalities, and occur when there is a strong, cohesive layer of snow known as a slab. These are usually formed when falling snow is deposited by the wind on a lee slope, or when loose ground snow is transported elsewhere. When there is a failure in a weak layer, a fracture very rapidly propagates so that a large area, that can be hundreds of meters in extent and several meters thick, starts moving almost instantaneously.
A third starting type is a wet snow avalanche or isothermal avalanche, which occurs when the snow pack becomes saturated by water. These tend to also start and spread out from a point. When the percentage of water is very high they are known as slush flows and they can move on very shallow slopes.
Among the largest and most powerful of avalanches, powder snow avalanches can exceed speeds of 300 km/h, and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even up hill for short distances. A powder snow avalanche is a powder cloud that forms when an avalanche accelerates over an abrupt change in slope, such as a cliff band, causing the snow to mix with air. This turbulent suspension of snow particles then flows as a gravity current.
