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58 Earth

FIND OUT MORE. Antarctica 276 • Erosion 55 • Mountains 45 • Prehistoric Life 77

At the poles and on high mountains,

vast areas are covered in ice – in

rivers of ice called glaciers and in

layers of ice called ice sheets or ice

caps. Ice is a major force of erosion

on land, where glaciers gouge deep

valleys in the landscape.

HOW DO GLACIERS FORM?

Glaciers form when more snow falls each winter than

melts each summer. As the snow builds up, the top

layers press down on the layers below and turn them

to ice. When enough ice has formed, the glacier’s

great weight and gravity set it moving slowly

downhill, at a rate of about 1 m (3.3 ft) a day. The

glacier continues to advance as long as more snow

is falling at its top than ice is melting at its tip.

WHAT ARE ICE CAPS?

Ice caps are vast, domed sheets of ice that cover the

land in the polar regions. They are formed as snow

builds up year after year, creating a thick layer of ice.

Three-quarters of the world’s fresh water is locked up

in the polar ice caps. The ice cap covering Antarctica

at the South Pole is over 4 km (2.5 miles) deep.

4 ICEBERG

Icebergs form where glaciers

and the ice caps meet the ocean.

Huge chunks of ice calve, or

break off, and fall into the water.

Ocean currents may then carry

the floating ice to warmer waters,

where it can endanger shipping.

In 1912, the luxury liner Titanic

was sunk by an iceberg, and

over 1,500 people drowned.

GLACIAL EROSION 3

Over thousands of years, as the

base of the glacier scrapes over

rock, it gouges out a wide,

U-shaped valley. It may also carve

sharp peaks called horns on

mountains, and scoop rounded

hollows called cirques or corries

out of rock. Small lakes or tarns

are often left in cirques when the

glacial ice has melted.

Ice

Waves and ocean currents

sculpt the iceberg into pinnacles

and crags above sea level and

into deep caves below

Around one-ninth

of an iceberg shows above

the sea’s surface; the rest

lies underwater

The largest icebergs are the

size of small countries and can

tower up to 160 m (525 ft)

above sea level

A lateral moraine, or heap

of rock and debris, is pushed

along at the glacier’s sides

The sea’s surface may freeze if

the water temperature drops to

–2.2 ̊C (28 ̊F), forming pack ice

Deep crevices, or fissures,

form where the ice cracks

as it moves

Glacial ice can be more than

1 km (0.6 miles) deep

Meltwater trickles from the

tip, or snout, of the glacier

ice