CAUSES OF DISASTER MANAGEMENT

A volcano forms when magma beneath the Earth's crust forces its way to the surface. Alternating layers of solidified lava and pyroclastic materials (ash and cinders) build up the typical cone shape of a stratovolcano as they are ejected through the central vent during eruptions. Vent in the crust of the Earth from which molten rock, hot rock fragments, ash, gas, and steam issue. A volcano forms when magma beneath the Earth's crust forces its way to the surface. Alternating …
Most volcanoes are found on the boundaries of the enormous plates that make up the Earth's surface. Some of the most violent eruptions take place along convergent boundaries where one plate margin is forced beneath another. The most famous such boundary is the circum-Pacific belt bordering the Pacific Ocean; the island arcs and mountain ranges of this “Ring of Fire” have seen gigantic explosions, among them the eruptions of Mount Pinatubo in the Philippines in 1991, Mount Saint Helens in the U.S. state of Washington in 1980, and Krakatoa (Krakatau) in Indonesia in 1883. Volcanic activity is also common at divergent boundaries, where two plates slowly pull apart and allow molten rock to escape to the surface; the most prominent example is the Mid-Atlantic Ridge, site of volcanic islands such as Iceland, the Azores, Ascension, Saint Helena, and Tristan da Cunha. Yet another type of volcanic activity is found on the island of Hawaii, located on a “hot spot” near the middle of the Pacific Plate where plumes of partially melted rock rise from below the Earth's crust; there the volcanoes Kilauea and Mauna Loa frequently eject streams and fountains of lava. Some of the best-known volcanism takes place around the Mediterranean Sea, where the eruptions of Mount Etna, Vesuvius, the islands of Stromboli and Volcano, and other volcanoes have been observed for millennia. Some volcanoes have cultural or religious significance for the peoples around them; these include Misti Volcano in Peru, Mount Fuji in Japan, and Ol Doinyo Lengai in Tanzania.
 









EARTHQUAKE

Sudden shaking of the ground caused by a disturbance deeper within the crust of the Earth.
Most earthquakes occur when masses of rock straining against one another along fault lines suddenly fracture and slip. The Earth's major earthquakes occur mainly in belts coinciding with the margins of tectonic plates. These include the Circum-Pacific Belt, which affects New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America; the Alpide Belt, which passes through the Mediterranean region eastward through Asia; oceanic ridges in the Arctic, Atlantic, and western Indian oceans; and the rift valleys of East Africa. The “size,” or magnitude, of earthquakes is usually expressed in terms of the Richter scale, which assigns levels from 1.0 or lower to 8.0 or higher. The largest quake ever recorded (Richter magnitude 9.5) occurred off the coast of Chile in 1960. The “strength” of an earthquake is rated in intensity scales such as the Mercalli scale, which assigns qualitative measures of damage to terrain and structures that range from “not felt” to “damage nearly total.” The most destructive quake of modern times occurred in 1976, when the city of Tangshan, China, was leveled and more than 250,000 people killed. See also seismic wave; seismology.

CLYCLONE



Any large system of winds that circulates about a centre of low atmospheric pressure in a counterclockwise direction north of the Equator and in a clockwise direction south of it.
Cyclones that occur in the mid- and high latitudes are known as extratropical cyclones; they are frequently preceded by thickening and lowering clouds, followed by precipitation. Cyclones that form in the lower latitudes are known as tropical cyclones; smaller than extratropical cyclones, they tend to be more violent and can cause considerable damage (see tropical cyclone). Wind systems that circulate around a high-pressure centre in directions opposite to that of cyclones are known as anticyclones.
Severe atmospheric disturbance in tropical oceans. Cross section of a tropical cyclone. A cyclone derives its power from the warm air and water found …
Tropical cyclones have very low atmospheric pressures in the calm, clear centre (the eye) of a circular structure of rain, cloud, and very high winds. In the Atlantic and Caribbean they are called hurricanes; in the Pacific they are known as typhoons. Because of the Earth's rotation, tropical cyclones rotate clockwise in the Southern Hemisphere and counterclockwise in the Northern. They may be 50–500 mi (80–800 km) in diameter, and sustained winds in excess of 100 mph (160 kph) are common. In the eye, however, the winds drop abruptly to light breezes or even complete calm. The lowest sea-level pressures on Earth occur in or near the eye.


LANDSLIDE



The movement of a mass of rock or soil down a slope.
The term is used to describe a variety of phenomena, from rock falls to the gradual downhill flow of soil. Landslides occur when the force of gravity acting on the materials within a slope overcomes the material's resistance to shearing. Among the processes that can lead to a landslide are the steepening of a slope by natural erosion or excavation, the overloading of the slope by an inflow of water, and the motion caused by an earthquake.
FLOOD


High-water stage in which water overflows its natural or artificial banks onto normally dry land, such as a river inundating its floodplain.
Uncontrollable floods likely to cause considerable damage commonly result from excessive rainfall in a brief period, but they may also result from ice jams during the spring rise in rivers, and from tsunamis. Common measures of flood control include improving channels, constructing protective levees and storage reservoirs, and implementing programs of soil and forest conservation to retard and absorb runoff from storms. seismic sea wave tidal wave 
Catastrophic ocean wave, usually caused by a submarine earthquake.
Underwater or coastal landslides or volcanic eruptions also may cause tsunamis. The term tsunami is Japanese for “harbour wave.” The term tidal wave is a misnomer, because the wave has no connection with the tides. Perhaps the most destructive tsunami ever occurred in 2004 in the Indian Ocean, after an earthquake struck the seafloor off the Indonesian island of Sumatra. More than 200,000 people were killed in Indonesia, Thailand, India, Sri Lanka and other countries as far away as Somalia on the Horn of Africa. Propagation of disturbances from place to place in a regular and organized way.
Most familiar are surface waves that travel on water, but sound, light, and the motion of subatomic particles all exhibit wavelike properties. In the simplest waves, the disturbance oscillates periodically (see periodic motion) with a fixed frequency and wavelength. Mechanical waves, such as sound, require a medium through which to travel, while electromagnetic waves (see electromagnetic radiation) do not require a medium and can be propagated through a vacuum. Propagation of a wave through a medium depends on the medium's properties. See also seismic wave.