Geology Reference
In-Depth Information
these conditions can also occur at low latitudes (closer to the equator) in mountain
ranges with high elevations.
Scientists categorize the ice on earth into the following types:
Valley glaciers: Valley or alpine glaciers are the most common glaciers and the
smallest. Thousands of valley glaciers exist in mountain ranges all over the world
at high and low latitudes. Valley glaciers low in a path similar to streams —
through mountain valleys toward lower elevations and (eventually) the sea.
Ice sheets: Only two large, continent-covering ice sheets exist today: one covering
Greenland, and one covering Antarctica. Ice sheets are not contained by valleys
and flow out in all directions from a central point. When ice sheets flow out over
the sea, they form ice shelves.
Ice caps: Ice caps cover areas of a continent, such as portions of Iceland, on a much
smaller scale than ice sheets. Like ice sheets, ice caps flow out in all directions
(The Arctic ice cap is technically not a cap because it covers only ocean. Instead,
it is a mass of sea ice. )
Sometimes an ice sheet or ice cap has valley glaciers flowing from its main body into
surrounding valleys. These are called outlet glaciers because they are outlets for the
main ice sheet body. When many alpine or valley glaciers flow together and form a large,
broad glacier that spreads out in the lowlands at the base of a mountain, it is called a
piedmont glacier.
Understanding Ice as a Geologic Force
Glaciers combine the flowing movement of a liquid with the strength of a solid, and the
result is an incredibly powerful force for moving rocks and sediment on the earth's sur-
face. In this section, I describe how scientists track the growth and melting of glaciers,
and I explain the basic mechanics of how glacial ice flows.
Transforming snow into ice
When snow falls on a glacier, it becomes packed down by the pressure of added snow on
top of it. Over time this pressure and compaction force the snowflakes closely together,
reducing air space between them and creating a tightly packed, dense snow called firn.
Firn resists completely melting in the summer and refreezes each winter. The annual
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