GLACIERS AND THEIR EFFECTS GLACIER A body of ice Formed on land Recrystallization of snow Evidence of movement Theory of glacial ages – at present ~10% of land surface glaciated – in past, ~32% of land surface glaciated GLACIOLOGY Distribution of present glaciers – Alpine vs. Continental glaciers – Develop where all of annual snow doesn’t melt away in warm seasons » Various climates – Most glacial ice in Antarctica – Glaciers in US Types of Glaciers – Valley glaciers- “ice streams” » Move down valley – Ice Sheets & Ice Caps » Flow from high part outward Antarctic slide show Growth of glaciers – Snow => Firn => Glacier Ice Wastage (ablation) of glaciers (“shrinkage”) – Melting » more melting at lower elevations – Evaporation – Calving into Icebergs » where a glacier flows onto a sea GLACIER BUDGET Positive--Advancing Glacier – Terminus moves forward Negative--Receding Glacier – Terminus retreats Zone of accumulation – Where some snow remains after the melt season Zone of Wastage – Where all snow & some glacier melt Snow line – Boundary between two zones Movement of Valley Glaciers Gravity driving force Faster above & in middle of glacier Basal sliding Plastic flow – “flowage” Internal flowage (plastic flow) Rigid zone – Crevasses may form here EROSION BY GLACIERS Under glacier – Polishing and rounding » “Sheep Rocks” – Striations- scratches & grooves on rock – Rock flour produced & washed out of glacier Above glacier – Frost wedging takes place – Erosion by glaciers steepen slopes – Angular & rugged landscapes created Alpine landscapes due to erosion – U-shaped valleys – Hanging valleys – Truncated spurs – Cirques- at heads of valley glaciers – Rock steps » Rock basin lakes – Horns (saw tooth mountains) – Aretes – Landscapes due to continental glaciation DEPOSITION BY GLACIERS Till – Unsorted debris Erratic Moraine- body of till – Lateral Moraine – Medial Moraine- where tributary joins glacier – End moraine- » Terminal » Recessional – Ground moraine » Drumlins Outwash – Stream-deposited sediment » sorted – Braided streams typical – Kettles Glacial Lakes – (Varves) Effects of Glacial Ages 4 major advances during past 2 million years (Pleistocene) – Latest peaked 20,000 years ago – Ended 10,000 years ago Direct effects – Scoured much of Canada & New England – Cut Great Lakes – Flattened Midwest – Extent of ice in N. America – Sierra & other mountains had larger valley glaciers Indirect effects – Lowered sea level 400 feet – Wetter climate – Pluvial Lakes » Some were large lakes in west Lake Bonneville Death Valley a lake – Crustal rebound Older Glacial Ages Late Paleozoic Late Precambrian .