Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Glacier: a Flowing Stream of Ice
- Continental (Greenland, Antarctica)
Snowfall vs Melting & Evaporation (Ablation)
Zone of Accumulation
- Snowfall Exceeds Melting & Evaporation
- Excess Snow Turns to Ice & Flows Out
Zone of Melting or Ablation
- Melting & Evaporation Exceeds Snowfall
- Melting Excess Made up by Ice Flowing in
Terminus of Glacier
Snowfall & Inflow = Melting & Evaporation (Ablation)
Results of Glaciation
- Chatter Marks
- Crescentic Gouges
- Bedrock Scour
- Varved Clays
Mountain Glacier Landforms
Evolution of a typical mountain glacier landscape.
Continental Glacier Landforms
Two stages in the retreat of a typical continental glacier.
||The Greenland Ice Cap shows the dome-like
form of a typical continental glacier.
The table below gives an idea how complex the Pleistocene
really was. Ice advances and retreats in different areas are
given different names because it is not always certain that they
began at the same time everywhere.
|Time (1000 Years)
|2000 (2 M.Y.)
|4000 (4 M.Y.)
still in Antarctica
Glaciation in Antarctica
- Pleistocene 3 M.y.
- Permian 250-220 M.y.
- Ordovician 450 M.y.
- 900-650 M.y. (Snowball Earth)
- 2300 M.y.
- Earth seems to have alternated between "icehouse"
and "greenhouse" episodes.
The Greenhouse Effect
- "A little greenhouse effect is a good thing" (Carl
Sagan). Without a natural greenhouse effect, earth would
- 90% of the earth's natural greenhouse effect is due to
- Ever notice how hot nights are sticky? It's no
accident - it's hot because it's sticky.
High humidity results in a water-vapor greenhouse
- In summer, both New Orleans and Phoenix might hit
100 F, but by midnight Phoenix could be down to
60, whereas New Orleans might still be 90. The
difference is due to humidity and a water-vapor
- We can't do much about evaporation from the oceans, but
we have been adding to the atmospheric load of
- Other important greenhouse gases are methane and oxides
- Molecule for molecule, methane is twenty times
more powerful than carbon dioxide as a greenhouse
- Landfilling garbage (which releases methane)
instead of burning it may not always be the best
- Some scientists have proposed that abrupt warming
episodes in earth's past may have been triggered
by releases of methane.
The Carbonate-Silicate Cycle
- Earth has almost as much carbon dioxide as Venus
- Volcanoes add carbon dioxide to the atmosphere
- Carbon dioxide is removed from the air to make carbonate
- Mountain-building favors cooling
- Uplift exposes rocks to weathering
- Calcium silicates (plagioclase, amphiboles,
pyroxenes) are chemically weathered
- Calcium is carried to the sea where organisms
bind it into carbonate minerals
- Creation of carbonates removes carbon dioxide
from the atmosphere
- Weathering of carbonates returns carbon dioxide to the
- Plate tectonics carries some carbonates into the earth
- Heat liberates carbon dioxide
- Carbon dioxide returns to the atmosphere
- The cycle does not require life but does require liquid
- Uplift of the Himalayas resulted in delivery of huge
volumes of sediment to the oceans, perhaps lowering
carbon dioxide and helping to trigger cooling.
The Snowball Earth
Between 900 and 600 m.y. ago, Earth froze completely (or
almost) about four times. Global freezing alternated with
extremely rapid sea-level rise and global warming
- Glacial deposits on all continents, even at low latitudes
- Glacial deposits immediately succeeded by thick deposits
of carbonate rocks
- Fainter early sun - the early sun was perhaps 30 per cent
less bright than today. At the time of the snowball earth
it was perhaps 5 per cent less bright. Over most of earth's
history temperatures have been above freezing, implying
that the atmosphere had a stronger greenhouse effect than
- Biological changes? Possibly appearance of new types of
organisms took more CO2 out of the atmosphere
Probable Sequence of Events
- Global ice cover. The more ice, the more sunlight
reflected to space.
- Weathering and erosion shut down, so the removal of
carbon dioxide to make carbonate rocks ceases
- Volcanoes continue to erupt CO2
- At 10% CO2, abrupt warming begins
- There is rapid melting, abrupt sea level rise, and rapid
deposition of carbonate rocks as excess carbon dioxide
reacts with calcium in the oceans. Some lines of evidence
suggest temperatures went from 50 C to +50 C in 10,000
years or less.
- Implications for life? How did anything survive? Were
there ice-free places? Did life survive at submarine hot
springs? Was there a mass extinction, clearing the way
for the abrupt appearance of Cambrian life forms?
Cool Summers More Important Than Cold Winters
- Small Axis Tilt: Mild winters but cool summers. Favors
- Large Axis Tilt: Cold winters but hot summers. Favors
Shape of Orbit + Precession
- Summer at Aphelion (Eccentric Orbit)
- Near-circular Orbit
- Mild winters but cool summers. Favors Ice Age
- Summer at Perihelion (Eccentric Orbit)
- Cold winters but hot summers. Favors Interglacial
Where We Stand Now, And a Mystery
- Earth's axial tilt is decreasing. It was about 24.5
degrees 9,000 years ago and will reach a minimum of
amount 22.5 degrees in about 10,000 years. The tropics
are retreating toward the equator at about 1.7 mm/hour.
During a typical class period, the tropics move about
twice the diameter of a pencil lead. The decreasing axial
tilt favors an ice age.
- We reach aphelion in July. That favors cool summers,
hence an ice age.
- The shape of the earth's orbit favors an interglacial.
The earth's axial tilt reached a maximum about 9,000 years ago
and was increasing for 20,000 years before that. The increasing
tilt would have favored hotter summers, hence ice retreat. Just
like sunlight reaches maximum in June, but summer is hottest in
August, there's a time lag.
The mystery. The roughly 100,000 year period of ice ages
corresponds most closely to a 100,000 year cycle in the shape of
the earth's orbit. Yet the variation in sunlight between January
(perihelion) and July (aphelion) is trivial compared to the
seasonal variations due to the tilt of the earth's axis, and the
changes in distance from the sun are tinier yet. Nobody has yet
figured out how the shape of the earth's orbit translates into
What Causes Ice Ages?
Within Earth (Endogenic)
- Carbonate-Silicate Cycle
- Volcanic Eruptions - Sudden output of CO2 (warming)
or particulates (cooling)
- Mountain Building - Changes in atmospheric circulation
- Continent-Ocean configuration
Outside Earth (Exogenic)
- Changes in Sun (faint early sun)
- Variations in Earth Orbit (Milankovitch Cycles)
Don't Really Know
Are We Headed For Another Ice Age?
- Heating & Cooling in Historic Times
- Smoke, Haze, CO2 May Alter Climate
Don't Really Know
Global warming due to fossil fuels may be catastrophic in many
ways, but will probably not much affect these longer-term cycles.
We will have run out of fossil fuels long before the duration of
a typical interglacial.
- " Little Ice Age" 500-700 years BP (1300-1500
- Medithermal 2500 BP
- Altithermal 5000 BP
Last dwarf mammoths die out - Wrangel Island, Siberia
- Cochrane Advance 8000 BP - Northern Ontario. Last major
Late Pleistocene in the Midwest
- Valders-great Lake Advances 11000 BP
- Two Creeks Retreat 12000 BP
- Woodfordian Advance
- Mankato (Minn.) 14000 BP
- Cary (Wis.)
- Farmdalian Retreat 28000 BP
- Altonian Advance(s) 67000 BP
Man in The Great Lakes
- European Contact 1700-present
- Early Historic Tribes 1600-1750
- Winnebago, Menominee, Sauk, Fox, Huron, Chippewa,
- Woodland 800-1600
- Hopewell 100 BC-800 AD
- Large Mounds
- Transcontinental Trade
- Early Woodland Mound Builders 500-100 BC
- Burial Mounds
- Some Copper
- Old Copper Culture 5000-1000 BC
- Hammered Copper Artifacts
- Boreal Archaic 5000-100 BC
- Ground Axes
- " Bannerstones"
- Paleo-indians 7000-5000 BC
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Created 20 January 1997; Last Update November 2, 1999
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