1. Intro Material

• largest reservoir of fresh H2O, covers ~10% of total land surface.
• glacier dynamics very closely linked to climate, both past and present.
• mountain and high-latitude landscape development

Two basic types: (based on size and shape)

• Valley (or alpine)
• Continental (or ice sheets)

• Two major requirements:
• LOW TEMPS- temperature must be low enough to have snow an the ground all year (ie. doesn't all melt during the summer). High elevations or high latitudes.
• PRECIPITATION - in the form of snow (duh!); Some cold climates are too arid and thus glaciers will not form.
• Metamorphism of snow into glacial ice:
• process of burial, compaction and loss of porosity (air space); when ice reaches ~20 to 100 m thick it begins to move downhill.
• Glacier Mass Balance (simple box model).  Zone of accumulation; zone of ablation; equilibrium line (snowline); very closely linked to variations in climate. Many glaciers show trends related to climate variations over periods of decades; need monitoring?.

• Two primary mechanisms:
• Plastic Flow- internal deformation of individual ice crystals; adds up to net flow; very analogous to ductile deformation of deeply buried rocks.
• Basal Slip- meltwater at base "lubricates" ice mass; caused by special property of H2O (lower melting point at higher pressure; ice skate analogy).
• Crevasses (large cracks) from by brittle deformation in the upper 50 m. Below that, deformation (flow) is plastic.
• Velocity Patterns
• Valley glaciers- max. velocity in the top centerline; friction causes slower velocity near base and walls. If basal slip is dominant, then ice my move all together as one mass. Typical velocity ~10-100 m/yr.  Surges- sudden period of rapid movement lasting 1-3 years; velocities up to 6000 m/yr. Enhanced basal slip by pressurized water.
• Continental glaciers - max velocity in center near base where ice is thickest and under greatest pressure. Typical velocity ~1-10 m/yr.  Ice Streams- rapidly moving parts of the ice sheet. velocities up to 750 m/yr. Basal slip mechanism

• Erosional Features:

rock flour, striations, roches moutonées, cirques, arêtes, U-shaped valleys, hanging valleys, fjords.
 
• Depositional Features:
• all glacial related sediment is called drift; Three basic types of drift are:
• till: ice deposits, unstratified and poorly sorted, contains erratics.
• outwash: meltwater deposits, stratified and well sorted.
• loess: wind deposited rock flour
• Ice-related deposits (composed of till)
• moraines (terminal, end, lateral, medial, ground)
• drumlins
• Water-related deposits (composed of stratified sediments)
• kames (deltas that built out into glacial lakes)
• eskers

• Main Evidence for Ice Ages:
• glacial deposits interbedded with soils containing warm-climate plant material.
• oxygen isotope record of marine microfossils.
• oxygen isotope, CO₂, and dust records in Antarctic and Greenland ice cores.

• Long-term, steady global cooling during the Cenozoic Era.
Preferred explanation: Tectonics - positions of the continents controls ocean circulation and the distribution of heat on Earth. Tectonic uplift of Tibetan Plateau, Andes, Rockies, Alps) affect atmospheric circulation and weathering rates.
 
• Alternating glacial and interglacial cycles.
Preferred explanation: Milankovitch Astronomical Cycles - controls the pattern of incident solar energy to Earth. Effects seasonal T differences.
 
Three Orbital Parameters:
• Eccentricity = shape of Earth's orbit; 100,000 yr. cycle.
• Tilt = tilt of Earth's axis of rotation ranges from 21.5? to 24.5?; 41,000 yr. cycle.
• Precession = wobble of rotation axis; 23,000 yr. cycle.

• Major problems that are not yet resolved:
• Why are Ice Ages relatively rare in the geologic record?
• What controls the atmospheric abundance of CO2? cause or an effect of Ice Ages?
• Many think that the oceans hold the key (60x larger reservoir than atmosphere)