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WHAT CAUSES LANDSLIDES?

Landslides often strike after heavy rain or snow has

fallen. Loose soil and rocks begin to trickle downhill,

then gravity takes over, and the whole hillside slips

away. Landslides are common on slopes where the

vegetation has been removed by tree-felling or

farming. Without tree or plant roots to anchor the

soil, heavy rain easily washes it away. Landslides,

mudflows, and avalanches are called mass wasting.

Mass wasting is sometimes triggered by a volcanic

eruption or by an earthquake.

Erosion is the wearing down and the carrying away

of the Earth’s rock by the action of wind and moving

water. Erosion happens fastest on steep hillsides after

heavy rain, when. LANDSLIDES sometimes strike.

HOW DOES EROSION HAPPEN?

Rocks are gradually broken down into smaller pieces

by the wind, rain, snow, and frost. For example, when

water freezes in rock cracks, the water expands,

slowly widening the cracks and fracturing the rock.

The fragments of weathered rock are then blown away

by the wind or carried away by water in the form

of streams and rivers, ice in glaciers, or waves

pounding coasts.

Weathering and erosion usually wear away

landscapes fairly slowly. A landslide occurs

when a huge mass of rock and soil

suddenly breaks off a hillside, engulfing

everything in its path. Landslides can cause

great destruction, and can even wipe out

whole towns.

WIND EROSION 3

Top-heavy pinnacles called hoodoos

are created in deserts where sand-

filled winds scour rocks. Wind is the

main cause of erosion in dry, desert

areas. Extreme desert temperatures,

with scorching days and freezing nights,

also cause rocks to crack. If heavy rain

falls after a long drought, flash floods

then sweep away loose soil and rock.

1 GLACIAL EROSION
Wide U-shaped valleys were formed by glaciers during past ice ages,
when ice covered much of Northern Europe. Glaciers are huge, slow-
moving rivers of ice, which carry boulders and stones on their sides and
bases. This load scrapes away at the rock below, shaping the landscape.

4 DESTRUCTIVE SLIDE

The landslide shown here caused

a roaring tide of earth and rocks

to bury several streets in the

town of Santa Tecla, El Salvador,

in Central America. The

landslide was set off by a minor

earthquake. In 1970, a landslide

set off by a major earthquake

buried the town of Yungay in

Peru, killing 18,000 people.

1 WATER EROSION

The Colorado River, in the United States, has worn away the rock,

creating narrow gorges and steep-sided canyons. Erosion is greatest

after heavy rain. The eroded rock, gravel, and silt are carried along in

the river water then deposited in lakes, at deltas, or in the sea.

Erosion

LANDSLIDES

Trickling earth and pebbles

may dislodge large boulders,

causing further destruction

Forested hillside was swept

away as the landslide

gathered force

Rocks and earth finally come to

rest on flatter ground

FIND OUT MORE. Coasts 59 • Ice 58 • Rivers 56 • Rocks 46–47

erosion