GLACIERS AND GLACIATION
of the Cryosphere
surface at Dome C Station, Antarctica. Image taken by Stephen Hudson (GNU).
cryosphere is part of the hydrosphere and includes glaciers and frozen
sea ice. It is located mostly at the north and south poles.
cryosphere is an effective agent of erosion and deposition. It can
significantly modify landscapes.
cryosphere impacts Earth’s climate by cooling water and air, and setting up
oceanic and atmospheric circulation patterns.
cryosphere impacts the biosphere because water cooled by polar ice has the
largest concentration of dissolved oxygen and supports a diverse fauna
(oxygen gas is most soluble in cold water).
Glacier- A large, long-lasting mass of ice that forms on land and
moves downhill under its own weight.
general categories of glaciers:
glaciers – glaciers formed in mountainous regions of high elevation
(Example: Glacier National Park in Montana).
They are confined to a
valley and flow downhill due to gravity.
St. Mary Lake, Glacier National Park. Public Domain Image taken by Ken
sheet glaciers– glaciers that exist over a large part of a continent (>
50,000 km2) in regions near the earth’s north or south poles
(Examples: Antarctica, Greenland).
Antarctica has 85% of
glacier ice currently on earth, and Greenland has 10%.
If Antarctica’s ice were to
melt, the sea level would rise 70 m or 230 ft.
In Antarctica, two ice
sheets (west and east) flow downward and outward from a central point: the
Transantarctic Mountains. The East Antarctic Ice Sheet = 2700 m thick and
the West Antarctic Ice Sheet = 4800 m thick.
Public Domain Image (generated by NASA's Blue Marble data set).
and Growth of Glaciers
Transition from snow to glacier ice: Powdery snow → Granules → Firn
(somewhat compacted)→ Glacier ice.
Domain Image by Luis María Benítez.
Glacier ice moves downhill and is eventually lost (ablated) by
melting, evaporation, or calving of icebergs.
calving off the coast of Greenland. Photo by Mila Zinkova (GNU).
upper part of a glacier where snowfall is collected.
of ablation- The lower part of a glacier where ice is lost.
Equilibrium line- The boundary between these zones. It marks the
highest point at which the glaciers snow cover is lost during the warm
equilibrium line may migrate from year to year depending upon where there
has been more accumulation or more ablation.
If there is
more accumulation than
ablation→ the equilibrium line moves down the glacier (positive budget).
If there is
more ablation than
accumulation→ the equilibrium line moves up the glacier (negative budget).
Terminus- The lower edge of a glacier (the position depends on the
velocity of ice movement varies across the glacier with velocities being
highest in the thick center and lowest at the thin edges (due to drag).
Movement of Alpine Glaciers
Glacier speed varies from
millimeters to tens of meters per day (depends on slope).
The base of a glacier slides over
underlying rock on a thin layer of meltwater (basal sliding).
The thicker parts of glaciers (the
centers) move faster than thinner parts of glaciers (the edges) which
experience more frictional drag.
The lower part of glaciers
flows plastically as individual ice grains move relative to each other (the
zone of plastic flow). Here the ice grains deform to accommodate
upper part of the glacier moves rigidly. Here fractures called crevasses
may develop here.
Meltwater at the base of a glacier may work its way into cracks and freeze.
Pieces of bedrock may be worked loose and picked up by the moving glacier in
a process is called plucking.
Luis María Benítez (GNU).
carried along at the base of the glacier may grind and polish the bedrock (abrasion).
Large rocks may produce glacial striations in the bedrock as they move over
Erosional Features of Alpine Glaciation (Angular Topography):
Glaciation is characterized by the following types of erosional features:
Public Domain Image by Luis María Benítez.
U-shaped valley – The characteristic cross section profile of a valley
carved by glacial erosion.
Zealand's Franz Joseph glacier. Copyright, Matthew Cox
Hanging valley – A small valley that terminates abruptly high above a
Veil Falls in Yosemite National Park flowing from a hanging valley. (GNU
Truncated spur – The triangular facet produced where the lower end of a
ridge has been eroded by glacial ice.
lake – A lake occupying a depression caused by glacial erosion of
– A steep-sided, amphitheater-like hollow carved into a mountain at the head
of a glacial valley.
Horn – A sharp peak formed where cirques cut back into a mountain on
Arètes – A sharp ridge that separates adjacent glacial valleys.
arète in the
Fairweather Range, Alaska. © Michael Collier
Erosional Features of
Glaciation (Rounded Topography):
weight of the moving ice sheet produces rounded knobs and striated bedrock.
The orientation of the striations can be used to interpret the direction of
movement of the ice sheet.
Luis María Benítez (GNU).
ice sheets may bury mountain ranges, rounding and streamlining them in the
direction of ice movement.
Glaciers pick up rock fragments from the underlying bedrock and from the
the debris without tumbling or sorting. The deposited debris is called
glacial till (angular, poorly sorted, and unlayered). When till becomes
lithified, it is called tillite.
boulders transported large distances by glaciers are called erratics.
Rock, a 400 ton erratic on the Waterville Plateau, Washington. Wikipedia
Public Domain Image.
An elongate deposit of glacial till. There are several types:
moraine, Sierra Nevada, California. © Marli Miller, University of Oregon
Lateral moraine- Forms along the sides of a valley glacier as rocks fall
from the steep cliffs of glacial valleys.
Medial moraine- Form where two tributary glaciers come together and
adjacent lateral moraines join.
Recessional moraine- Deposits produced as glaciers recede during
End moraine- A type of recessional moraine that forms at the terminus of
a glacier during glacial recession as debris piles up long the front of the
ice. They tend to be crescent shaped.
Terminal moraine- Special type of end moraine that marks the farthest
extent of the glacier.
Drumlin- A streamlined hill of till produced by continental glaciers
(the origin is not entirely understood). Drumlins point in the direction of
Luis María Benítez (Creative Commons Attribution ShareAlike 2.5).
glaciers melt, braided streams develop on the surface of land and carry away
material known as outwash.
Under the ice, meltwater moves in tunnels and carries sediments that can be
deposited in sinuous ridges of sediment called eskers (well-sorted
Sims Corner Eskers and Kames National Natural Landmark. Public Domain Image.
Kettle- A small depression in outwash formed when a block of glacial ice
gets buried by sediment. When the ice melts, a depression remain. These
may fill with water creating small lakes (glaciated Minnesota is the land of
in a braided depost of Alaska's Bering Glacier, © Bruce Molnia, Terra
Varves- Seasonal deposits of silt and clay that form alternating light
and dark layers.
silt-clay set represents one year:
-The silt is deposited during the warmer part of the year when more melting
and sediment transport occurs.
clay is deposited during the colder part of the year when the lake is frozen
and the clay can settle out of suspension.
can be used to determine how long a glacial lake lasted.
the Sanpoil River Valley, Washington. USGS Image.
Glaciation in the Past
times in the past, colder climates resulted in more extensive glaciation
than the present.
last glacial period began ~2,000,000 years ago and ended ~10,000 years ago.
North American ice sheet extended as far south as Ohio and Illinois.
till was deposited over the Midwestern U.S., providing good soil for
Great Lakes occupy basins that were formed by heavy lobes of ice. These
basins are undergoing isostatic crustal rebound.
Level was ~ 130 m lower than it currently is today because sea water was
locked up in large ice sheets at the poles. Submerged stream channels and
land mammal fossils on the continental shelf support lower sea level.
the end of last glaciation, the climate was more humid than today. In the
western US, there was significant rainfall and numerous lakes (called
pluvial lakes) once existed in Nevada, Utah, and California (Example: Utah's
Great Salt Lake).
is some evidence for glaciations occurring all the way back