Glaciers and Glaciation - The Changing Earth: Exploring Geology and Evolution - page 359

Continental Glaciers
Continental glaciers, also known as ice sheets , are vast, cov-
ering at least 50,000 km 2 , and they are unconfi ned by topo-
graphy; that is, their shape and movement are not controlled
by the underlying landscape. Valley glaciers are long, narrow
tongues of ice that conform to the shape of the valley they
occupy, and the existing slope determines their direction
of fl ow. In contrast, continental glaciers fl ow outward in all
directions from a central area or areas of accumulation in
response to variations in ice thickness.
In Earth's two areas of continental glaciation, Greenland
and Antarctica, the ice is more than 3000 m thick and covers
all but the highest mountains (

merge to form a continuous ice sheet covering more than
12,650,000 km 2 . The glaciers in Antarctica fl ow into the sea,
where the buoyant effect of water causes the ice to fl oat in
vast ice shelves ; the Ross Ice Shelf alone covers more than
547,000 km 2 (Figure 14.3a).
Although valley glaciers and continental glaciers are eas-
ily differentiated by size and location, geologists also recognize
an intermediate variety called an ice cap . Ice caps are similar
to, but smaller than, continental glaciers, covering less than
50,000 km 2 . The 6000-km 2 Penny Ice Cap on Baffin Island,
Canada (Figure 14.3c), and the Juneau Icefi eld in Alaska and
Canada at about 3900 km 2 are good examples. Some ice caps
form when valley glaciers grow and overtop the divides and
passes between adjacent valleys and coalesce to form a contin-
uous ice cover. They also form on fairly fl at terrain in Iceland
and some of the islands in the Canadian Arctic.

Figure 14.3a, b). The con-
tinental glacier in Greenland covers about 1,800,000 km 2 ,
and in Antarctica the East and West Antarctic Glaciers

◗

◗

Figure 14.3 Continental Glaciers and Ice Caps

GLACIERS—MOVING BODIES
OF ICE ON LAND
We use the term glaciation to indicate all glacial activity,
including the origin, expansion, and retreat of glaciers, as
well as their impact on Earth's surface. Presently, glaciers

0 600 Km

East Antarctic
ice sheet

70 °

75 °

Weddell Sea

80 °

Filchner
Ice
Shelf

Amery Ice
Shelf

85 °

South
Pole

70 ° 75 ° 80 ° 85 ° 85 ° 80 ° 75 ° 70 ° 65 °

West
Antarctic
ice
sheet

Ross
Ice
Shelf

80 °

Ross Sea

75 °

Unglaciated surface
Land ice
Ice shelf

70

°

a The West and East Antarctic ice sheets merge to form a nearly
continuous ice cover that averages 2160 m thick. The blue lines are
lines of equal thickness.

b View of part of the Antarctic ice sheet. Notice the nunatak, which
is a peak extending above the glacial ice.

c View of the Penny Ice Cap on Baffi n Island, Canada. It covers
about 6000 km 2 .