Glaciers and Glaciation - The Changing Earth: Exploring Geology and Evolution - page 360

cover nearly 15 million km 2 , or about 10% of Earth's land
surface. As a matter of fact, if all glacial ice were in the
United States and Canada, it would form a continuous ice
cover about 1.5 km thick!
At fi rst glance, glaciers appear static. Even briefl y visiting
a glacier may not dispel this impression because, although
glaciers move, they usually do so slowly. Nevertheless, they
do move and just like other geologic agents such as running
water, glaciers are dynamic systems that continuously adjust
to changes. For example, a glacier may fl ow slower or more
rapidly depending on decreased or increased amounts of
snow or the absence or presence of water at its base. And gla-
ciers may expand or contract depending on climatic changes.

◗ Figure 14.4 Glacial Ice

Snowflakes

Glacial ice

Glaciers—Part of the Hydrologic Cycle
In Chapter 1, you learned that one of Earth's systems, the
hydrosphere, consists of all surface water, including water
frozen in glaciers. So glaciers make up one reservoir in the
hydrologic cycle where it is stored for long periods, but even
this water eventually returns to its original source, the oceans
(see Figure 12.3). Glaciers at high latitudes, as in Alaska, north-
ern Canada, and Scandinavia, fl ow directly into the oceans
where they melt, or icebergs break off (a process known as
calving) and drift out to sea where they eventually melt. At
low latitudes or areas remote from the oceans, glaciers fl ow to
lower elevations where they melt and the liquid water enters
the groundwater system (another reservoir in the hydrologic
cycle) or it returns to the seas by surface runoff.
In addition to melting, glaciers lose water by sublimation ,
when ice changes to water vapor without an intermediate liq-
uid phase. Sublimation is not an exotic process; it occurs in the
freezer compartment of a refrigerator. Because of sublimation,
the older ice cubes at the bottom of the container are much
smaller than the more recently formed ones. In any case, the
water vapor so derived enters the atmosphere where it may
condense and fall as rain or snow, but in the long run, this
water also returns to the oceans.

Granular
snow

Firn

The conversion of freshly fallen snow to fi rn and then to glacial ice.

a

b This iceberg in Portage Lake in Alaska shows the blue color of
glacial ice. The longer wavelengths of white light are absorbed by
the ice, but blue (short wavelength) is transmitted into the ice and
scattered, accounting for the blue color.

How Do Glaciers Originate and Move?
Ice is a crystalline solid with characteristic physical prop-
erties and a specifi c chemical composition, and thus is a
mineral. Accordingly, glacial ice is a type of metamorphic
rock, but one that is easily deformed. Glaciers form in any
area where more snow falls than melts during the warmer
seasons and a net accumulation takes place. Freshly fallen
snow has about 80% air-fi lled pore space and 20% solids,
but it compacts as it accumulates, partially thaws, and
refreezes, converting to a granular type of snow known as
fi rn . As more snow accumulates, the fi rn is buried and fur-
ther compacted and recrystallized until it is transformed
into glacial ice , consisting of about 90% solids and 10%
air (

volume of solids. When accumulating snow and ice reach a
critical thickness of about 40 m, the stress on the ice at depth
is great enough to induce plastic fl ow, a type of permanent
deformation involving no fracturing. Glaciers move mostly
by plastic fl ow, but they may also slide over their underlying
surface by basal slip ( Figure 14.5). Liquid water facilitates

basal slip because it reduces friction between a glacier and
the surface over which it moves.
The total movement of a glacier in a given time is a con-
sequence of plastic fl ow and basal slip, although the former
occurs continuously, whereas the latter varies depending on
the season, latitude, and elevation. Indeed, if a glacier is sol-
idly frozen to the surface below, as in the case of many polar
environments, it moves only by plastic fl ow. Furthermore,

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Figure 14.4).
Now you know how glacial ice forms, but we still have
not addressed how glaciers move. At this time, it is useful
to recall some terms from Chapter 10. Remember that stress
is force per unit area and strain is a change in the shape or

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