GLACIAL GEOLOGY

Glacial Geology

• What was it like to be an early geologist?
• No perception of Earth history
• What was it like to find the first evidence of dinosaurs? Massive Pleistocene ice sheet?

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.

Glaciers

• 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
• Types of moraine (Fig. 17-13)
• ground
• lateral
• medial
• terminal - carried in front of glacier; marks furthest advance
• recessional - formed as glacier recedes in steps

Alpine Glaciers

• Anatomy of an alpine glacier
• zone of accumulation - higher elevation zone of snow accumulation
• zone of ablation - lower elevation zone; more snow melts in summer than accumulates in winter
• snowline - boundary between zone of accumulation and zone of ablation
• Glacial movement
• rates of a few cm to meters/day
• two mechanisms
• basal slip - wet base moves faster than dry base
• plastic flow
• Erosional Features (Fig. 17-13b)
• 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

Continental Glaciers

• 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

Earth's Climate Modes
"Ice house" vs. "Greenhouse"

• Ice house periods - those with continental glaciers
• 2 b.y. - Precambrian
• Late Precambrian to Cambrian
• Late Ordovician to Early Silurian
• Carboniferous to Permian
• Oligocene - present
(Figure at right modified from Frakes et al., 1992)

Quaternary Glaciation

• Glacial Cycles in the Quaternary
• cyclical: glacial and interglacial cycles
• recorded in ¹⁸O/¹⁶O ratios of marine CaCO3 (foraminifera from marine sediment cores)

Controls on Glaciation

• Controls of Glaciation
• Location of continents
• Ocean circulation - circum-polar current
• Concentration of CO2 in atmosphere
• Ex: formation and burial of coal in Carboniferous

Cause of Glacial Cycles

• Variations in Earth's orbit - 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