Glaciers and Glaciation

Glaciers and the Hydrologic Cycle

        Glaciers are masses of ice that flow under the influence of gravity. The term glacier does not include icebergs, sea ice, or immobile snow fields in mountainous areas. Glaciers cover about 10% of the Earth's land surface, with the Greenland and Antarctica ice sheets accounting for about 96% of the land covered. Glaciers are the largest reservoir of fresh water and contain about 2.15% of the world's water. Mountain glaciers are found all over the world, even near the equator. Glacial ice eventually melts or vaporizes and returns to the hydrologic cycle.

        Glacial ice forms by the recrystallization of snow. Ice is a mineral, and glacial ice is a rock. The conversion of snow to ice involves several steps:

• Accumulation of snow - snowfields grow in areas above the snow line where more snow accumulates in the winter than melts during the summer. Freshly fallen snow has about 80% void space.

• Formation of ice granules - as snow accumulates and gets thicker, sublimation (transformation of solid to gas) and pressure change it into firn (ice granules).

• Formation of glacial ice - with further accumulation, compaction and pressure melting (released water that refreezes to cement ice granules together) cause firn to be transformed into glacial ice (mass of interlocking crystals). Ice has only about 10% void space. When ice reaches a thickness of about 40 meters, it begins to flow and becomes a glacier.

    There are three basic categories of glaciers:

• Valley Glaciers - flow down valleys in mountainous areas, and are fed by the snow fields of high mountain ranges. They usually consist of a main ice mass with smaller tributary glaciers feeding into it.
• Continental glaciers (ice sheets) represent great ice sheets that obscure most of the topography over large sections (at least 50,000 km2) of a continent. The glacial ice spreads laterally under its own weight from points of maximum thickness. During the Ice Age (>10,000 years ago), continental glaciers covered large portions of the continents in the Northern Hemisphere.
• Ice caps - similar to continental glaciers, but smaller in size (<50,000 km2), these ice masses may form by valley glaciers merging together or on fairly flat terrain at high latitudes.

        Glaciers expand in response to accumulation and contract from wastage (loss of ice). Glaciers can be divided into two zones:

• Zone of accumulation - zone above the snow line where snow accumulates faster than it is removed by melting and evaporation.
• Zone of wastage (ablation) - zone below the snow line (or firn limit) where wastage exceeds accumulation. The firn limit may change position from year to year. Wastage processes include:

1. Melting - caused by friction at sides and bottom of ice mass and by warming during the summer months.
2. Sublimation - conversion of ice directly to water vapor without an intermediate liquid phase.
3. Calving - breaking off of blocks of ice at ends of glaciers that reach the ocean, where icebergs are produced.

        Several factors determine whether and how fast ice masses move:

• Advance versus retreat of glacial systems

1. If accumulation > wastage, the glacial front advances. Firn limit moves down the glacier and ice mass increases.
2. If accumulation equals wastage, the glacial front is stationary. Glacier is said to have a balanced budget. Firn line remains constant.
3. If wastage > accumulation, the glacial front retreats. Even though the edge of the glacier retreats, ice is still flowing toward the edge. If a glacier thins enough, it will cease to flow and become a stagnant glacier.

• The way ice moves through a combination of:

1. Plastic flow - Under pressure, ice can flow plastically. Glacial movement inside the ice mass takes place by this mechanism (zone of flow). Uppermost part of ice sheet (zone of fracture) is not under pressure and cracks as the ice below it moves, locally producing deep crevasses (cracks). Ice falls result when a glacier passes over a steep slope and crevasses break the ice sheet into large blocks and spires.
2. Basal slip - The base of the glacier moves slowest because of friction. Friction produces melt water which lubricates the ice mass, allowing it to slip when under enough pressure.

        In general, valley glaciers move faster than continental glaciers:

• Valley Glaciers - rates vary from centimeters per day to tens of meters per day. The steeper the slope, the faster the rate of movement. Larger ice masses move faster than smaller ice masses. In a valley glacier, ice moves fastest at the upper center part of zone of plastic flow. Basal slip is most rapid in warmer months and can produce brief periods of rapid movement called surges. Surges can be produced by unusuallyheavy precipitation and by avalanches loading the upper part of a valley glacier.
• Continental Glaciers - average rate of movement is a few cm/day or a few m/day. Flow rates are fastest in the zone of accumulation and decrease below the firn line toward the margins. Thicker ice sheets have higher flow rates than thinner ones. These glaciers show little basal slip and may be frozen to the underlying surface.

        Glaciers can erode and transport huge quantities of materials and once covered much larger areas than they do presently. Glaciation helped form the topography of the northern areas of the US and Canada. Glaciers erode by:

• Plucking (Quarrying) - similar to frost wedging. Meltwater penetrates into bedrock cracks and refreezes, prying angular blocks of rock loose. These blocks may be incorporated into the ice, produceing boulders known as glacial erratics which may be transported long distances.
• Abrasion - rock fragments carried by ice function as "sandpaper" that scours the surface over which the ice moves. This process produces rock flour (very fine particles of pulverized rock), striations (long grooves and scratches cut into bedrock), and glacial polish (a very smooth surface produced by fine abrasion of bedrock by rock flour).
• Bulldozing - glacier pushes loose material in its path.

• Valley Glaciers are associated with:

1. U-shaped valleys - characteristic shape of glaciated valleys, as opposed to characteristic V-shape of stream valleys. Glaciers follow pre-existing stream valleys, making them broader and deeper.
2. Truncated spurs - Triangular cliffs that are formed by glacial erosion of ridges that once extended into the valley at stream meanders.
3. Paternoster lakes - produced when water fills rock basins (bedrock depressions produced by glacial plucking) in the valley floor.
4. Fiords - deep sea inlets formed by the flooding of glacial valleys. Restricted to high latitudes, they can be up to 1,300 meters deep.
5. Hanging valleys - tributary glacier valleys, where main glacier cuts its valley deeper than the tributary glaciers. After the ice melts, smaller valleys are left hanging above the main glacier valley. Streams in hanging valleys form waterfalls.
6. Cirques - bowl-shaped depressions at the head of a glacial valley formed by glacial plucking and enlarged by abrasion, plucking, and mass wasting. Cirques may be occupied by small lakes called tarns.
7. Horns - steep, pyramid-like peaks formed where at least three cirques approach a summit crest.
8. Aretes - Narrow, sharp-edged ridges between glacial valleys produced by plucking, abrasion, and mass movement. Aretes form from headward erosion of two cirques on opposite sides of a ridge or from erosion in two parallel glacial troughs.
9. Cols - a glaciated mountain pass formed when two adjacent glaciers erode away the wall between their cirques.
10. Roche moutonnees - asymmetric bedrock knob, formed by glacial abrasion and plucking, has a gentle slope that faces side of glacial advance.

• Continental Glaciers are associated with:

1. Land, smoothed and rounded from glacial abrasion, produces a flattened topography with rounded hills.
2. Erosion strips soil and sediment away to expose bedrock, producing ice-scoured plains.
3. Stream drainage patterns are disrupted, producing deranged drainage patterns with numerous lakes and swamps.

        All sediment of glacial erosion is known as drift and is subdivided into:

• Till - unsorted, unlayered material deposited directly by a glacier. Landforms composed of till include:

• Moraines - landforms composed of till deposited at or near the margins of glaciers by moving ice.

• end moraine - a ridge of till that forms at the terminus of a stationary glacier.
• recessional moraine - a series of end moraines formed by a receding glacier that periodically stabilized.
• terminal moraine - the last recessional moraine representing the point of farthest glacier advance.
• ground moraine - a gently-rolling layer of till deposited by a receding glacier.
• lateral moraine - only produced by valley glaciers, these are ridges of till paralleling the valley walls, deposited at the margins of the glacier. Sediment is abraded and plucked from valley walls and mass wasted onto the glacier surface.
• medial moraine - central moraine formed when two valley glaciers merge and combine their lateral moraines.

 
• Drumlins - Only produced by continental glaciers, these are smooth, elongate, parallel hills of reworked glacial drift that are thought to form when glaciers advance over previously deposited drift. The steep slope faces the direction of glacier advance. Clusters are called drumlin fields.

Glacial Erratics - Erratics are pieces of rock carried by a glacier and left stranded on bedrock of different composition. Boulder trains are linear or fan-shaped deposits composed of large numbers of erratics that came from the same source.

• Stratified drift - sorted, stratified sediment laid down by glacial meltwater (often by braided streams). Landforms composed of stratified drift and deposited by glacial meltwater:

• Outwash plains - area beyond the margins of a continental glacier where meltwater (as braided streams) deposited sand, gravel, and mud washed out from the melting ice. When confined to a mountain valley, outwash is called a valley train.
• Kettles - depressions in deposits of glacial drift formed where a block of ice was partially buried, and then melted. The depression can fill with water to form a lake. Kettles form in outwash plains and in end moraines.
• Kames - steep-sided conical hill of stratified drift that collected in openings or lakes in the ice sheet.
• Eskers - steep-walled, sinuous ridge of coarse-grained stratified material deposited by streams of meltwater which flow in tunnels within or beneath the ice. Eskers can be up to 100 m high and over 100 km long.
• Varves - Pairs of coarse- and fine-grained (light and dark colored) sediment beds deposited in a single year in glacial lakes. Dropstones, gravel to boulder-size rocks deposited with the varves, represent material carried into lake by icebergs and released by melting.

        A great Ice Age occurred between 1.6 million and 10,000 years ago, causing widespread glaciation on northern continents. Venetz, a Swiss engineer, in 1821 proposed that Swiss glaciers had once expanded on a great scale. Agassiz, a zoologist and skeptic, did extensive field work in Switzerland that led him to propose the Glacial Theory in 1837. Certain featues produced by glacial ice are produced by no other known process. Agassiz related the activity of modern glaciers to the ancient deposits, using the concept of Uniformitarianism. Modern analytical techniques allow analysis of ice and sediment cores to accurately trace historical climate changes.

        With further study, scientists became convinced that ice had advanced and retreated over the continents several times in the recent geological past. The Ice Age (Pleistocene Epoch) can be divided into four major stages of glaciation in North America, named for the states where deposits of a particular period were first studied or where they are well exposed:

1. Nebraskan - sometimes lumped with Kansan period and called Pre-Illinoian. Nebraskan and Kansan periods represent several advances and retreats, rather than two.
2. Kansan
3. Illinoian
4. Wisconsinan - actually multiple glaciation events.

           Glaciers advanced about 2-3 million years ago and retreated for the final time about 10,000 years ago. About 27% of the land surface was covered by ice during the Wisconsin age, and the glaciers were up to 3,000 meters thick. In North America, ice reached as far south as New Jersey in the east, St. Louis in the mid-west, and to southcentral New Mexico in the western mountains. Greenland, Scandinavia, Great Britain, Ireland, and part of northern Russia were also covered with ice.

• Caused important climatic changes. Pluvial lakes formed in desert areas, showing cooler and wetter conditions. Proglacial lakes formed from glacial meltwater. Deposits of loess (wind-deposited dust) were laid down in temperate areas. Some areas were much drier due to cooler temperatures causing less evaporation from the oceans.

• Changed sea levels. Glaciers stored more than 70 million km3 of water, lowering sea level 130 meters. Continental shelves were partially exposed and a land bridge connected Alaska and Siberia. If all remaining glacial ice were to melt, sea ievel would rise by about 70 meters, flooding densely populated coastal areas.

• Forced plants and animals to migrate. Temperate, subtropical, and tropical climate zones were shifted toward the equator, causeing extinctions.

• Diverted stream drainage patterns and caused downcutting by streams. Rivers in the northern part of North America once drained northward, but were blocked by glacial ice. Lowered sea level caused streams to erode downward.

• Weight of ice depressed continental crust. Plastic asthenosphere acts like a "mattress", as in some places, crust was depressed by 300 meters. After ice melted, the crust has been gradually rebounding (isostasy).

        Earlier periods of extensive glaciation have been recently identified. Evidence is less abundant because of subsequent erosion and deposition, but several older periods of glaciation occurred besides the Ice Age:

• End of Paleozoic (250 million years ago) - widespread glaciation in southern Africa, South America, India, Australia and Antarctica.
• Middle Paleozoic (350 million years ago) - glaciation in South America
• Early Paleozoic (500 million years ago) - glaciation in Africa
• End of Precambrian (600 million years ago) - glaciation in various continents of the Northern Hemisphere.
• Precambrian (850 million, 1.2 billion, and 2.2 billion years ago) - various parts of Gondwanaland (a great southern landmass composed of India, Australia, Africa, and South America) were covered with ice.

        There are basically two apparent kinds of glaciation events:

1. glaciation involving rapid climatic change with closely-spaced advances and retreats of ice (averages every 100,000 years)
2. longer, more widely spaced glaciation events caused by gradual climate changes (averages every 100 million years).

• Long-term changes in climate may be due to:

1. Increasing continentality - average elevation of continents has doubled since the mid-Cenozoic. This results in a general drop in temperature (about 3 degrees) from the increase in elevation, and interferes with heat transfer from the equator to the poles via wind and ocean currents.
2. Continental Drift - cooling can be initiated when continents move over polar regions, but can't explain rapid advances and retreats of ice. It may explain widely spaced periods of glaciation.

• Climatic changes of shorter duration may be best explained by rapid climatic fluctuations (10,000 to 100,000 year cycles) due to variation in the Earth's orbit (orbital or Milankovitch Theory). Such variations can result from:

1. Eccentricity - variation in the shape of the Earth's orbit. Earth moves farther away from the sun every 100,000 years, which decreases solar energy reaching the Earth.
2. Obliquity - changes in the angle the Earth's axis makes with its orbit. Obliquity changes by about 1.5 degrees every 44,000 years, and can make the contrast between seasons less.
3. Precession - wobbling of the Earth's axis because of the gravitational pull of the sun, moon, and planets that occurs on a cycle of 22,000 years. Backward calculations of these cyclic changes has shown that solar heat maxima and minima occurred during the Pleistocene. Climatic evidence from deep sea cores correlates well with calculations and shows 20 periods of warming and cooling during the 2 million year Ice Age.

         It seems likely that periods of glaciation will occur again in the future. A short-term cooling of the Earth occurred from about 1500 to the mid-to-late 1800's Little Ice Age and which cannot be explained by Milankovitch Theory. We are presently in an interglacial period of warmer temperatures where most of the Earth's glaciers are in retreat. Past cycles in temperature suggest that we might expect another glacial stage in about 23,000 years with progressive cooling from now on. However, the build-up of carbon dioxide in the atmosphere from fossil fuel combustion ("greenhouse" effect) could delay the temperature decrease for some 2,000 years.