Lecture 17 - Glaciers and Ice Ages

Outline

I. What are Glaciers?

II. How Glaciers Move

III. Glacial Landscapes

IV. Ice Ages

I. What are Glaciers?

In places where snow falls in the winter months and does not completely melt during the summer months it will accumulate and eventually the growing weight of the overlying snow will cause the snow to recrystallize forming a mass of ice. This process is analogous to metamorphism of rock and thus glacier ice is a metamorphic rock consisting of the mineral "ice". As soon as the ice mass is large enough to start to move under the force of gravity, its called a glacier.

Glacier- a permanent body of ice that shows evidence of down slope movement due to the pull of gravity.

Based on the shape, size and location of glaciers we can distinguish several kinds of glaciers:

  1. cirque glacier- occupies a cirque, or bowl shaped depression on a mountainside
  2. valley glacier- flows from a cirque onto and along the floor of a valley
  3. piedmont glacier- broad lobe of ice that terminates on open slopes, usually fed by more than one valley glacier
  4. ice sheet- continent sized masses of ice like that which presently covers Greenland

Glaciers accumulate new ice from snowfall in winter months and lose ice during melting that occurs in summer months. Accumulation of ice occurs in the higher altitudes in a region called the accumulation zone, and loss occurs at the lower latitudes, in a region called the ablation zone. The point where these two regions meet is called the firn line and marks the highest level of retreat of winter snow. The firn line varies from place to place and from year to year depending on the climate. If accumulation is greater than ablation, the glacier will grow and if ablation is greater than accumulation the glacier will shrink or retreat.

II. How Glaciers Move

Glaciers move in two ways: by internal flow and by sliding of the basal ice over underlying rock or sediment.

Internal Flow - As ice thickens, the mass will begin to deform ductiley or flow. This flow takes place by the movement of individual ice crystals similar to the way that cards slide by each other if you shear the deck. Rocks undergo ductile deformation in the same way. The surface of the glacier, which is not under large forces due to the weight of ice above it, deforms brittley by cracking. These cracks are called crevasses

Basal Sliding - some melted water exists at the base of the glacier, which lubricates the bottom of the glacier so it can slide over the bedrock

Glaciers move at centimeters to meters per day or about as fast as groundwater percolates through rock.

III. Glacial Landscapes

A. Erosional

Ice is a far more efficient agent of erosion than either water or wind. Glaciers tear up and crush bedrock from their sides and base creating many different erosional features:

  1. U-shaped valley- as a glacier moves down a narrow river valley, it carves out the valley leaving a flat bottom and steep sides. Example- Yosemite Valley
  2. hanging valley- tributary glaciers can join main glaciers at higher topographic levels such that when the glaciers melt, tributary carved valleys are left hanging above the main glacially carved valley. Example are valleys from which Bridal Veil and numerous other water falls originate
  3. striations- grooves in the bedrock created by rock fragments dragged along the base of glaciers. The orientation of the striations provides information about the direction of the glacier
  4. roches moutonees (sheep rock)- small hill smoothed by the ice on the up current side and plucked to a rough face on the down current side as the ice pulls fragments from joints and cracks. These also provide information on the direction of ice movement . Example- Lembert Dome
  5. cirque- bowl shaped depression carved by a glacier. Cirques at the heads of adjacent valley glaciers may meet in sharp jagged cliffs called arêtes.

B. Depositional - Sediment deposited by glaciers is termed glacial drift. Unlike sediment carried by a stream, glacial drift is not sorted.

  1. Glacial till and erratics-non-sorted drift deposited directly by ice, if the particle size is boulder or larger we call the drift a glacial erratic.
  2. outwash- glacial drift reworked by melt water and sorted
  3. moraine- an accumulation of glacial till usually along the sides of a glacier (lateral moraine) or at its end (terminal moraine)

IV. Ice Ages

Glacial features that we see today in alpine regions that have glaciers are apparent over much of the presently temperate regions of the northeast and Midwest of the United States and in northern Europe. This lead Louis Agassiz, a geologist to speculate that in the not too distant past, large ice sheets covered much of this region and have since melted away. Careful work has since revealed that several different layers of glacial till exist with intermediate soil layers containing fossil evidence of intervening warm periods. Four distinct periods of glaciations or "ice ages" have been recognized in the last 1 million years (Pleistocene) through radioactive Carbonating of logs found in the glacial till. The last ice age occurred about 12,000 years ago. The geologic record tells us that ice ages have existed numerous times throughout geologic time. A very important consequence of ice ages is that the water that makes up the huge continental glaciers comes out of the ocean; so that during ice ages sea level is lower and during interglacial periods (with little ice tied up in continental glaciers) more water is in the oceans and sea level rises. These rises and falls of sea level are responsible for the changing sedimentary environments from marine sedimentation to continental sedimentation that we have frequently seen in the geology of many of the National Parks.

What causes ice ages? - Ice ages are produced when temperatures over large areas decrease. This seems to occur due to changes in the orbital parameters of the Earth that place us farther away from the Sun providing us with less solar radiation. Reduction in the concentration of certain gases in the atmosphere can also reduce temperatures because gases like carbon dioxide prevent heat from the Sun that is absorbed by the Earth from escaping back out to space and thus heal up the Earth. Therefore low concentrations of these gases allow more solar radiation to escape and temperatures to decline. High concentrations of these gases heating up the Earth is referred to as the "greenhouse effect".

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