GLACIAL GEOLOGY

How important are glaciers?

• How important are glaciers today?
• Cover about 10% of present earth's surface (30% during last ice age)
• 2% of the world's water (lowers sea level ~60 m)
• How important were glaciers in the past?

Effects of the last Ice Age on North America

• Sea level >100 m lower
• Land bridges between Siberia and Alaska, England and France
• Continent depressed
• Wide continental shelves
• Hudson Bay
• Rebound (Hudson Bay, Scandinavia)
• Scoured basins - Great Lakes
• Changed courses of rivers - Missouri River
• Formed ice dammed (Lake Agassiz) and pluvial (Lake Bonneville) lakes
• Transported Canadian topsoil to the U.S.
• Glacial sediments for Cape Cod (MA) and Long Island (NY)

Glaciers

• What is a glacier? - Mass of land ice and surficial snow that persists throughout the year and is slowly moving in response to gravity or loading.
• Types of glaciers
• Alpine glaciers - constrained by topography
• Progress from mountain glacier to valley glacier to piedmont glaciers.
• Continental glaciers - not constrained by topography

Glacial Erosion

• Glacial Erosion
• Processes
• Plucking - ice wedging combined with force of glacier
• Abrasion - grinding effect of rock fragments in glacier
• Features
• Glacial grooves, glacial striations, glacial polish
• Rock flour - silt-sized sediment formed from glacial grinding

Glacial Sediments

• Glacial sedimentary deposits called drift
• Till - unstratified drift
• Moraine - drift deposited directly by glacier

Alpine Glaciers

• Anatomy of an alpine glacier
• Zone of accumulation - higher elevation zone of snow accumulation
• Zone of wastage (ablation) - lower elevation zone; more snow melts in summer than accumulates in winter
• Snowline - boundary between zone of accumulation and zone of wastage
• Glacial movement
• Rates of a few cm to meters/day
• Two mechanisms
• Plastic flow - flow within ice
• Basal slip
• Sliding of glacier
• Wet base moves faster than dry base
• Both mechanisms may operate
• Erosional Features (Fig. 12.11)
• U-shaped valley
• Hanging valley - u-shaped valley cut by a tributary valley glacier feeding a larger valley glacier
• Cirque - inclined bowl-shaped basin
• Tarn - glacial lake in a cirque
• Arête - sharp-edged ridge; intersection of two cirques
• Horn - intersection of three or more cirques
• Fjord (Fiord) - glacially deepened valley flooded by the sea; as deep as 1200 m
• Types of moraine
• Ground
• Lateral
• Medial - two alpine glaciers join, merging lateral moraines
• Terminal - carried in front of glacier; marks furthest advance
• Recessional - formed as glacier recedes in steps

Continental Glaciers (Ice Sheets)

• Not constrained by topography
• Greenland, Antarctica (80% of the world's ice)
• Average 1500 m thick
• Move in response to loading (weight and gravity)
• Depositional features common with continental glaciation
• Outwash plain - sediment deposited in front of glacier by streams formed from meltwater; stratified
• Drumlins - low, elliptical hills composed largely of till
• Eskers - long sinuous ridges of sand and gravel; deposited by subglacial stream
• Other Glacial Features
• Glacial erratics
• Kettle lakes - formed when a block of ice, which has displaced sediment, melts
• Roche moutonnée - elongate mound of bedrock worn smooth by glacial abrasion

 

• Solar intensity
• Atmospheric CO2 levels
• Albedo
• Paleogeography
• Earth’s orbit
• Solar intensity
• Increasing with time
• For example, was roughly 95% at the beginning of the Paleozoic (538 Ma)

• CO₂ is a greenhouse gas
• Greenhouse gas - absorbs infrared radiation
• Keeps energy from being lost to space
• Atmospheric carbon dioxide
• Dry air: 78% N₂, 21% O₂, 1% other gases (Ar, CO₂, etc.)
• CO₂ level increased >25% in last 150 years (from 280 to 360 ppm)

• Why wasn’t the Earth frozen over?
• Early Earth had higher atmospheric CO₂ levels
• Burial of carbon as coal and carbonate minerals decreases atmospheric CO₂ levels; keeps Earth from warming up too much

• Albedo - amount of solar radiation reflected back to space (Fig. 20-8)
• Average Earth: ~30% albedo
• Tropical forest: 5 - 10%
• Snow fields and glaciers: 80 - 90%
• Ice sheet area increases albedo
• Causes cooling;  promotes more glaciation - positive feedback

• Paleogeography
• Location of continents near poles
• Cambrian versus Carboniferous
• Effect on ocean circulation - Circum-polar current
• Paleogene versus Neogene

• Quaternary glacial cycles
• ¹⁸O/¹⁶O ratios of foraminifera (shells) from marine sediment cores
• 100-ky cycles over last million years; 42 ky cycles from 1 - 2 m.y.

Causes of Glaciation
Variations in Earth’s Orbit (cont.)

• Variations in Earth’s orbit (Fig. 12.36) - Milankovitch Hypothesis
• Eccentricity - shape of earth’s orbit; 100,000 year cycle
• Obliquity - change in angle of earth’s axis to plane of earth’s orbit; 41,000 year cycle
• Precession - cycle of wobble in earth’s axis = 23,000 years
• Glaciation enhanced by mild winters, cool summers
• Decreased tilt will reduce seasonality in high latitudes