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)

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

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