Glaciers and Glaciation

Ice

Ice is a rock made up of individual crystals

Glaciers

Large masses of ice, flowing downhill. Flows as a viscous liquid, sowhat like a viscous lava flow

o Continental glaciers (ice sheet) covers large region eg. Greenland (4.5 million sq km), Antarctica

o Valley glacier - found on high mountains or on edges of continental glaciers

How glaciers accumulate

o Low temperatures - high mountains in (lower mountains in Antipodes)

o Adequate snow

o Accumulation of snow feathery snow crystals are compacted, plus air is expelled

Ablation - removal of ice from glaciers

Usually by melting at end of glacier when it encounters warmer zone

When reach the ocean, then masses break off - calving - to for icebergs [Titanic and all that]

Glaciers grow (or shrink) depending if accumulation greater than ablation (or vice versa)

How glaciers move

Dry glaciers: plastic flow of ice crystals or slip between crystals.

Wet glaciers: basal slip. In less cold areas where tunnels of water at base of glacier

Crevasses - huge cracks that occur when glacier flows over break in slope. [In pleistocene, in Asia, mammoths have fallen int these and have been frozen whole, only to appear in the zone of ablation 1000's of years later]

Speed of flow: meters to 10's of meters per year sometimes undergo surges - periods of much more rapid movement

Glacial landscapes

Although affect in smaller areas than river systems erosion by glaciers and to lesser degree deposition, produces dramatic landscapes. Nearly all rugged "mountain landscapes" show evience of glaciation

Erosional features

Striations

Roches moutonees

Cirques

aretes

Horns - pyramidal peaks formed at intersection of three or more cirques Matterhorn, Everest etc

Ushaped valleys and hanging valleys

Depositional features

Glacial drift

Erratics

Moraines (particularly terminal moraines)

[ Drumlins and eskers optional ]

Ice ages: the Pleistocene Glaciation

Large areas of moderate to high latituded have evidence of glacial erosion or deposition in landscape where glaciers no longer exist (eg Kansas)

Late 19th C Lous Aggasiz proposed that must have been glaciation in the last few thousand years. Beginning of importan notion of global climate change

Now know that there were at least 4

Glacial Interglacial

Present

8000a Wisconsionian 75000a

Sangamonian (Miami Fm age)

125000 Illinoan 300000

Yarmouthian

.6 Ma Kansan .9Ma

Aftonian

1.3 Ma Nebraskan 1.6 Ma

Paleotemperatures from O isotopes

Use oxygen isotopes in shells to estimate T O18/O16 greater the coller is the T. From this can get an idea of fluctuations in temp

and climatic cycles

Glaciations and Sea level changes

In ice age (or cooler periods) more of Earths oceans as solid H2O in icecaps, therefore sea level lower - reverse when climate warmer in interglacials

This oscillating sea level effects coastal environments, downcutting rates of rivers etc.. Miami 125000 years ago very different shallow sea (like Bahamas today) perfect environment for oolite formation.

Climate fluctuations and Global warming

Recent research (involving tree rings, ice core studies as well as rock studies) has shown that climate fluctuates over many different time periods. Climate is not a constant.

Also clear that Sea level is rising, glaciers are ablating indicating global warming

How fast? - contraversial UN uses figure that sea level will be 25cm higher in 2050

Cause? Antropogenic CO2? possibly, but could be part of some natural cycle

(anthro CO2 does not help, of course)