Prof. Stephen A. Nelson
Glaciers and Glaciation
|Glaciers constitute much of the Earth that makes up the cryosphere, the part of the
Earth that remains below the freezing point of water. Most glacial ice today is found in
the polar regions, above the Arctic and Antarctic Circles. While glaciers are of
relatively minor importance today, covering only about 10% of the surface, evidence exists that the Earth's climate has undergone
fluctuations in the past, and that the amount of the Earth's surface covered by glaciers
has been as much as 30% in the past. In fact, much of the topography in
the northern part of North America, as well as in the high mountain regions of the west,
owe their form to erosional and depositional processes of glaciers. The latest glaciation
ended only 10,000 years ago.
The Earth has experienced numerous glaciations, the most recent during the Pleistocene Epoch between 1.8 million years ago and 11,000 years ago. Other episodes occurred in the Permian, Ordovician, and Late Precambrian.
Definition of a glacier
A glacier is a permanent (on a human time scale, because nothing on the Earth is really
permanent) body of ice, consisting largely of recrystallized snow, that shows evidence of
downslope or outward movement due to the pull of gravity.
Types of Glaciers
(note: images of these features are shown in your textbook and will be shown in class.)
Mountain Glaciers - Relatively small glaciers which occur at higher elevations in mountainous regions.
Ice Sheets (Continental glaciers) - are the largest types of glaciers on Earth.
They cover large areas of the land surface, including mountain areas. Modern ice sheets
cover Greenland and Antarctica. These two ice sheets comprise about 95% of all glacial ice
currently on Earth. They have an estimated volume of about 24 million km3.
If melted, they contain enough water to raise sea level about 66m (216 ft.). This would
cause serious problems for coastal cities (L.A., NY, Washington DC, New Orleans, Miami, SF
etc). The Greenland ice sheet is in some places over 3000 m (9800 ft) thick and the weight
of ice has depressed much of the crust of Greenland below sea level. Antarctica is covered
by two large ice sheets that meet in the central part along the Transantarctic Mountains.
These are the only truly polar ice sheet on earth (North Pole lies in an ocean covered by
thin layer of ice).
Ice Shelves - Ice shelves are sheets of ice floating on water and attached to land. They usually occupy coastal embayments, may extend hundreds of km from land and reach thicknesses of 1000 m.
Glaciers can also be classified by their internal temperature.
The Formation of Glacial Ice
Three conditions are necessary to form a glacier: (1) Cold local climate (polar latitudes or high elevation). (2) snow must be abundant; more snow must fall than melts, and (3) snow must not be removed by avalanches or wind.
Changes in Glacier Size
A glacier can change its size by Accumulation, which occurs by addition of snowfall, compaction and recrystallization, and Ablation, the loss of mass resulting from melting, usually at lower altitude, where temperatures may rise above freezing point in summer. Thus, depending on the balance between accumulation and ablation during a full season, the glacier can advance or retreat (see figure 22.9 in your text book).
Movement of Glaciers
Glaciers move to lower elevations under the force of gravity by two different processes:
The upper portions of glaciers are brittle, when the lower portion deforms by internal flow, the upper portions may fracture to form large cracks called crevasses. Crevasses occur where the lower portion of a glacier flows over sudden change in topography (see figure 22.6 in your text).
The velocity of glacial ice changes throughout the glacier. The velocity is low next to the base of the glacier and where it is contact with valley walls. The velocity increases toward the center and upper parts of the glacier (see figure 22.8 in your text).
Glaciation: is the modification of the land surface by the action of
glaciers. Glaciations have occurred so recently in N. America and Europe, that weathering,
mass wasting, and stream erosion have not had time to alter the landscape. Thus, evidence
of glacial erosion and deposition are still present. Since glaciers move, they can pick up
and transport rocks and thus erode. Since they transport material and can melt, they can
also deposit material. Glaciated landscapes are the result of both glacial erosion and
Glacial Erosion - Glaciers erode in several ways.:
Small scale erosional features (note: most of this material will be presented as slides in
Landforms produced by mountain glaciers (see figure 22.14 in your text)
Landforms produced by Ice Caps and Ice Sheets
Glacial Deposition and Deposits
Since glaciers are solid they can transport all sizes of sediment, from huge house-sized boulders to fine-grained clay sized material. The glacier can carry this material on its surface or embedded within it. Thus, sediment transportation in a glacier is very much different than that in a stream. Thus, sediments deposited directly from melting of a glacial can range from very poorly sorted to better sorted, depending on how much water transport takes place after the ice melts. All sediment deposited as a result of glacial erosion is called Glacial Drift.
Ice Laid Deposits
Stratified Drift - Glacial drift can be picked up and moved by meltwater streams which can then deposit that material as stratified drift.
Other Consequences of Glaciation
Ice Loading and Glacial Rebound
The weight of glacial ice sheets depress the lithosphere into the mantle causing the crust to subside. After the ice melts, the depressed lithosphere rebounds. The rebound process is still taking place today (see figures 22.23 in your text).
Sea Level Changes
Ice Dams, Drainage Reversals, and Lakes
When glacial ice forms, it can block existing drainages causing the formation of new lakes and forcing streams to find new pathways that develop into new drainage networks. Once the ice melts, the new drainage network become well established and the old drainage networks are often abandoned.
Such a change in drainage networks took place as a result of the last ice age in North America (see figure 22.24 in your text book). Prior to glaciation, streams in the northern U.S. and Canada drained to the northeast into what is now Hudson Bay and only the southern part of the U.S. drained into the Mississippi River system. Because the glacial ice retreated toward the north, the Mississippi drainage system became the major drainage system for much of the U.S.
During the Pleistocene Epoch, large lakes formed both as result of ice dams and melting of glaciers. Examples include the Great Lakes of the northern U.S., and a now much reduced lake, Lake Agassiz the formed from northern Minnesota, into the Canadian provinces of Manitoba, Saskatchewan and Ontario. As ice melted, lakes were also formed in the western U.S. at large distances from the glacial source. For example in the Basin and Range Province, basins were filled with large lakes formed by internal drainage. One of these lakes. Lake Bonneville, covered much of western Utah, eventually draining and evaporating leaving the remnant called the Great Salt Lake.
The last glaciation ended about 11,000 years ago. But the period between 11,000
years ago and 2 million years ago (the Pleistocene epoch) was a time of many glacial and interglacial
Based on evidence from glacial deposits and glacial erosion features geologists have been able to document at least 4 glaciations during the Pleistocene, two of which are poorly documented. But recent studies of deep-sea sediments and dating of these deposits suggest that there were at least 30 glaciations that occurred during the Pleistocene. This evidence comes from studies of fossils found in deep-sea sediment cores, and what they tell us about ocean surface temperatures in the past. The results come from studies of the isotopes of oxygen.
Thus, we expect that during glaciations the 18O / 16O ratio in seawater will be high, and during interglaciations the 18O / 16O ratio in seawater will be low.
Since organisms that live in the oceans extract Oxygen from seawater to form their carbonate (CO3-2) shells, measuring the 18O / 16O ratio in the shells of dead organisms gives a record of past ocean temperatures. The record for the past two million years is shown here and in figure 22.30 in your text. The data suggests about 30 glaciations separated by interglaciations during the past 2 million years.
During the last 1 million years it appears that each glacial - interglacial cycle has lasted about 100,000 years, but earlier cycles were about 40,000 years long.
Other periods of glaciation are known from the geologic record, mainly from preserved glacial striations and tillites (consolidated till). The earliest recognized glaciation occurred about 2.3 billion years ago, but at least 50 other glaciations are recognized to have occurred during the Paleozoic era.
Causes of Glacial Ages
In order to understand what causes these cycles of glacial - interglacial episodes we need a much better understanding of what causes global climate changes. Because human history is so short compared to the time scales on which global climate change occurs, we do not completely understand the causes. However, we can suggest a few reasons why climates fluctuate.
Examples of questions on this material that could be asked on an exam.