Disaster Dossier

Learn more about the forces of nature from these handy overviews, adapted from The Adventure of Geography (National Geographic Society, 1993).

DROUGHT

A drought is a prolonged period of greatly reduced precipitation. Droughts can last a few weeks (in which case they are called dry spells) or months, or even years. They strike more people than any other natural disaster.

EARTHQUAKE

An earthquake is a shaking of the Earth caused by the release of energy as rock suddenly breaks or shifts under stress. Most quakes are associated with faults—fractures in the Earth's crust along which great masses of rock move. Not all faults are visible at the Earth's surface.

Movement along a fault is often so gradual that only sensitive scientific instruments can detect it. When the movement of rock under stress is sudden, however, it causes an earthquake. Energy is released as seismic waves, or vibrations. The waves move through the Earth, causing a quake.

Most earthquakes occur along the edges of the Earth's major plates. The largest earthquake belt is where plates underlying the Pacific Ocean come in contact with plates underlying continents surrounding the Pacific.

FLOOD

A flood is the rising and overflowing of a body of water onto land that is not normally covered with water. One type of flood is the overflowing of inland streams or lakes caused by seasonal events or severe weather—including spring rains, melting snows, monsoons, hurricanes, and cloudbursts. These floods occur primarily along streams or rivers and their floodplains. Another type is coastal flooding caused by high winds and tides or by seismic sea waves called tsunami.

HURRICANE

"Hurricane" is one of the names for a rotating tropical storm with winds of at least 74 miles (119 kilometers) per hour. Such storms are called hurricanes when they develop over the Atlantic Ocean or the eastern Pacific Ocean .

Hurricanes are identical to cyclones that form over the Bay of Bengal and the northern Indian Ocean , and to typhoons in the western Pacific Ocean . These storms occur most frequently in summer and autumn when ocean temperatures are warmest.

Most Atlantic hurricanes are born off Africa 's west coast in a region lying between 5 and 25 degrees north of the Equator. There, the trade winds of the Northern and Southern Hemispheres meet, often causing disturbances.

TORNADO

A tornado, also called a twister, is a violently rotating column of air that descends to the ground during intense thunderstorm activity.

Argentina , Australia , India , and the nations of the former Soviet Union are among the many countries that experience tornadoes, but more form in the United States than in any other place on Earth. Between 700 and 1,000 are reported there each year.

Some tornadoes have one giant whirlwind; others have several smaller ones within a larger storm. They all start when cold, dry air and warm, moist air collide, producing a strong updraft. Denser cold air is forced over warm air, usually producing thunderstorms.

VOLCANO

A volcano is a vent in Earth's crust. Steam, ash, and molten rock may gush out through the volcano. The term "volcano" can also refer to the mountains, usually cone-shaped, formed by eruptions.

The Earth's rigid shell, or lithosphere, is made up of numerous slabs of rock called plates. The movement and interaction of these plates are responsible for most volcanic activity. The plates drift on the zone in the Earth's mantle, or middle layer; where rocks are near their melting point. The rocks are so hot that they will move or bend like red-hot iron. Pockets of molten rock called magma are also located there.

Scientists speculate that movement within this extremely hot zone may cause the plates to shift. As the plates move, their edges collide, slide under or past each other, or pull apart.

Where the edge of one of the huge plates slides under or pulls away from the edge of another, magma may rise toward the surface of the Earth. When the molten material reaches the surface, it is called lava. It often comes out through volcanoes.

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