Glaciers

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Glacier: a Flowing Stream of Ice

• Mountain
• 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

Abrasion

• Polish
• Striations
• Chatter Marks
• Crescentic Gouges
• Bedrock Scour

Deposition

• Till
• Outwash
• Varved Clays

Meltwater Erosion

Glacial Landforms

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.

Glacial Chronology

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.

Ice Ages

• Pleistocene 3 M.y.
• Permian 250-220 M.y.
• Ordovician 450 M.y.
• Precambrian
  • 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 be frozen.
• 90% of the earth's natural greenhouse effect is due to water vapor.
  • 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 effect.
  • 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 greenhouse effect.
• We can't do much about evaporation from the oceans, but we have been adding to the atmospheric load of carbon dioxide.
• Other important greenhouse gases are methane and oxides of nitrogen.
  • Molecule for molecule, methane is twenty times more powerful than carbon dioxide as a greenhouse gas.
  • Landfilling garbage (which releases methane) instead of burning it may not always be the best idea.
  • 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 rocks
• 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 atmosphere
• 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 water.
• 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

Evidence: 

• Glacial deposits on all continents, even at low latitudes 
• Glacial deposits immediately succeeded by thick deposits of carbonate rocks 

Possible reasons: 

• 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 at present.
• 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?

Milankovich Cycles

Cool Summers More Important Than Cold Winters

Axis Tilt

• Small Axis Tilt: Mild winters but cool summers. Favors Ice Age
• Large Axis Tilt: Cold winters but hot summers. Favors Interglacial

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

• 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. 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.

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.

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Created 20 January 1997; Last Update November 2, 1999
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