Sea ice
is always in motion. The winds above and ocean currents below can
cause it to drift great distances. Sea ice in the Northern Hemisphere,
for example, may drift for several years and travel thousands of kilometers
as it floats across the Arctic Ocean.
Icebergs should not be confused with sea ice. Icebergs are made of
land ice that has fallen into the ocean from the glaciers flowing out of
mountainous terrain. Icebergs are much thicker and less common than
chunks of sea ice. Icebergs are rare in the Arctic region, whereas
sea ice maintains a large and persistent concentration. The
image to the right offers a comparison of sea ice and land ice in an Antarctic
sea. Sea ice is usually only 2-3 meters thick while icebergs
may range up to 100's of meters thick.
When
winds and ocean currents cause ice to drift, they can travel 20 km (12.4
miles) in a single day. Ice exiting the Arctic Ocean through Fram
Strait has been observed traveling more than 25 km/day (almost 1 ft/sec
!). It is, in fact, unusual for sea ice not to be moving.
Although it is possible for free-floating sea ice to remain motionless
for periods up to three weeks, it is an exception to the general rule.
Near land, sea ice may be frozen to the shoreline or seafloor. It remains
fixed in place as a result. Ice that is attached to the land is known
as "fast" ice, because it is 'fastened' to it. Fast ice, ironically,
is the slowest of all the types of sea ice.
The constant
motion of sea ice keeps it broken into large fragments called floes.
Floes can be less than a meter to over a kilometer wide and are usually
only 2-3 meters thick.
C.
Friedrich, The Polar Sea, 1824.
Winds and waves, however,
may cause ice floes to collide sharply with one another. This causes
floes to pile up or become deformed at the edges. Sea ice can become
significantly thicker as a result, sometimes as much as 20 or more meters
thick. The process by which floes collide forcefully with one another,
causing their edges to crumple together and thicken, is called ridging.
When a sheet of sea ice simply overrides another, it is called rafting.
Ridging and rafting are two ways that sea ice thicknesses typically increase
to greater than 3 meters.
Have you
ever heard of polar ice caps? "Ice cap" is the popular way in which
many refer to the concentrations of ice in the polar regions. This
term, however, creates an image of a non-moving single mass of ice...and
is best reserved for the large amount of land ice resting on the
Antarctic continent. Visualizing polar sea ice as a "cap"
is somewhat inappropriate given the constant state of motion and fragmented
nature that sea ice maintains. Scientists that study sea ice phenomena
refer to sea ice as "ice cover" or as an "ice pack" rather than as an ice
"cap" to better reflect the dynamic nature of sea ice. This is especially
appropriate for the North Polar Region where floating sea ice dominates.
Q: Scientists refer
to the concentrations of sea ice at the Earth's poles as:
A. polar ice caps
B. polar ice cubes
C. polar ice cover
D. cool
Q: Name the two usual
ways by which sea ice thickness increases to be greater than 3 meters:
A. wrinkling and rafting
B. crunching and smashing
C. ridging and rafting
D. piling and crashing
Q: What is an ice
floe?
A. a fragment of sheetlike sea ice
B. glacial ice flowing out of mountainous terrain
C. a frosty dessert
D. sea ice that has melted
