received as snow on the surface. There are several major forms that snow and ice
can take on Earth’s surface.
Snow cover is present in some places year round and seasonally in many places,
including middle latitude lowlands and tropical mountains. The ground surface is
blanketed and, ironically, this greatly moderates ground temperatures so that life
can survive underneath. Snow can be considered to be a reservoir of fresh water.
For instance, human activities in the U.S. Southwest are dependent on stream flow
that emanates from the summer melt of the snowpack in the mountains.
Glaciers are large bodies of ice that have formed because season after season of
snowfall has been greater than the melting and sublimation subtracting from the
mass. The snow becomes denser over time, translating into glacial ice. Ice is inter-
esting because it seems so brittle in our common experience, but it is capable of
bending and flowing when subjected to slow, steady pressure. Once ice is more
than a few tens of meters thick it will move under its own weight and gouges
and deposits materials on a grand scale.
Earth “recently” emerged from the Pleistocene Epoch (the “Ice Age”), which
ended 12,000 years ago. Winters were noticeably longer and colder than today
and about a third of Earth’s continental surface was covered by ice. A concomitant
effect was the dramatic lowering of sea level on the order of 100 m caused by the
storage of the water in glaciers. The glaciers that remain provide excellent guid-
ance to glacial land shaping processes more widespread during the Pleistocene.
Ice caps are large areas hosting thick ice coverings sometimes in excess of
3 km. Ice caps are not limited to land and where the glacial ice extends over the
ocean that part is known as an ice shelf. Today, major glacial ice caps cover
Antarctica and Greenland but sometimes the sea ice of the Arctic and Antarctic
are included in the discussion of ice caps. Another distinction is that over land,
ice sheets are masses of ice covering less than 50,000 km^2.
Sea ice is a common feature of the world’s cold oceans. Sea ice is found in the
north and south polar seas and fluctuates seasonally. Averaging 25 million square
miles in extent (the approximate size of North America), sea ice expands and con-
tracts according to season. Scientists divide sea ice into first-year and multiyear
varieties. Multiyear ice is 2- to 4-m thick and has survived at least one summer
melt. Rather than being smooth, sea ice frequently has significant creases, cracks,
and mounds due to drift caused by currents and winds.
Approximately equal in area to sea ice is frozen ground also known as permafrost.
Permafrost is more widespread in the Northern Hemisphere than in the Southern
Hemisphere because of the much greater area of unglaciated higher latitudes in the
formerregion. The parts of the planet subject to a cold winter frequently experience
frozen ground. However, permafrost is soil material frozen for more than a year at a
time. Permafrost has variants with more than 30 percent of the volume in ice to
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