Glaciers and the Great Ice Ages
Pleistocene Epoch: the Great Ice Ages 2.0 Ma to 10,000 years ago Four (or more) distinct episodes expansion and melting of ice sheets (continental glaciers) Each ice advance lasted for 40,000 to 200,000 years Warm spells lasted 10,000 to 30,000 years Are we living in a warm period now between ice ages?
Why did the climate change so drastically and repeadedly? Climate change and variations in Earth’s orbit Eccentricity: elliptical to nearly circular cycles Obliquity: cyclic changes in tilt of axis Precession: cyclic change in”wobble” of the axis Variations affect amount and distribution of solar energy received by Earth Milankovitch cycles are in the right time range to possibly explain cyclic changes in climate
How does a glacier form? Cold climate is necessary Annual snow accumulation is greater than annual rate of melting Snow builds up over time Snow recrystallizes to ice
Glacial Flow Ice begins to flow (plastic deformation) under the influence of gravity Glacier = mass of flowing ice
Glaciers are dynamic Flow is constant Some parts of glacier are melting Rate of accumulation vs. rate of melting determines if glacier will advance or if end of glacier will melt back (glaciers never flow backwards)
Glaciers and Landscapes Flowing ice picks up sediment, soil, etc. Breaks loose pieces of bedrock Modifies landscape by erosion and deposition
Erosion by Glaciers Striations: “scratch”marks made as ice drags rocks across bedrock
Erosion by Glaciers Modifies “V” shaped river valleys to a “U” shape
Deposition by Glaciers Erratics – pieces of “exotic” rock left by glacier Till – unsorted, unstratified sediments dumped by ice Outwash – sorted and stratified sand and gravel deposited by meltwater
Deposition by glaciers Lacustrine sediments Lakes form between melting edge of ice and obstructions (often deposits of till) Fine silt and clay settle to the bottom in calm water Clay may be used in pottery and brick making If lakes drain or fill with sediment: bogs and swamps – ecologically important wetlands Water-logged lacustrine sediments may cause tilting and sinking of buildings constructed on them
Glaciers and Landforms Characteristic landforms