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All About Glaciers What types of glaciers are there? Mountain glaciers Variegated Glacier is a valley glacier that winds through the Saint Elias Mountains in Alaska, terminating near Yakutat Bay. —Credit: Photograph by Austin Post. 1965. Variegated Glacier: From the Glacier Photograph Collection. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media. These glaciers develop in high mountainous regions, often flowing out of icefields that span several peaks or even a mountain range. The largest mountain glaciers are found in Arctic Canada, Alaska, the Andes in South America, and the Himalaya in Asia. Valley glaciers Commonly originating from mountain glaciers or icefields , these glaciers spill down valleys, looking much like giant tongues. Valley glaciers may be very long, often flowing down beyond the snow line, sometimes reaching sea level. Tidewater glaciers As the name implies, these are valley glaciers that flow far enough to reach out into the sea. Tidewater glaciers are responsible for calving numerous small icebergs , which although not as imposing as Antarctic icebergs, can still pose problems for shipping lanes.

Piedmont glaciers A classic tidewater glacier, Lamplugh Glacier terminates in a small embayment in Alaska's Glacier Bay National Park and Preserve. —Credit: Photograph by William Osgood Field. 1941. Lamplugh Glacier: From the Glacier Photograph Collection. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media. Piedmont glaciers occur when steep valley glaciers spill into relatively flat plains, where they spread out into bulb-like lobes. Malaspina Glacier in Alaska is one of the most famous examples of this type of glacier, and is the largest piedmont glacier in the world. Spilling out of the Seward Icefield, Malaspina Glacier covers about 3,900 square kilometers (1,500 square miles) as it spreads across the coastal plain. Hanging glaciers When a major valley glacier system retreats and thins, sometimes the tributary glaciers are left in smaller valleys high above the shrunken central glacier surface. These are called hanging glaciers . If the entire system has melted and disappeared, the empty high valleys are called hanging valleys . Cirque glaciers Cirque glaciers are named for the bowl-like hollows they occupy, which are called cirques .

Typically, they are found high on mountainsides and tend to be wide rather than long. Ice Aprons These small, steep glaciers cling to high mountainsides. Like cirque glaciers, they are often wider than they are long. Ice aprons are common in the Alps and in New Zealand, where they often cause avalanches due to the steep inclines they occupy. Rock Glaciers This satellite image shows the Larsen Ice Shelf, Antarctica, in 1998, four years before the shelf disintegrated. Taken at the beginning of the Antarctic winter, the ice shelf is clearly visible. Notice that sea ice is forming over the ocean to the right the ice shelf. —Credit: NSIDC, data from the Moderate Resolution Imaging Spectroradiometer (MODIS) Rock glaciers sometimes form when slow-moving glacial ice is covered by debris. They are often found in steep-sided valleys, where rocks and soil fall from the valley walls onto the ice. Rock glaciers may also form when frozen ground creeps downslope. Ice shelves Ice shelves occur when ice sheets extend over the sea and float on the water. They range from a few hundred meters to over 1 kilometer (0.62 mile) in thickness.

Ice shelves surround most of the Antarctic continent. Icefields Icefields are similar to ice caps , except that their flow is influenced by the underlying topography, and they are typically smaller than ice caps. Ice caps Ice caps are miniature ice sheets, covering less than 50,000 square kilometers (19,305 square miles). They form primarily in polar and sub-polar regions that are relatively flat and high in elevation. Ice streams The Southern Patagonian Ice Field spans the border between Argentina and Chile and covers 12,363 square kilometers (4,773 square miles). —Credit: NASA Ice streams are large ribbon-like glaciers set within an ice sheet—they are bordered by ice that is flowing more slowly, rather than by rock outcrop or mountain ranges. These huge masses of flowing ice are often very sensitive to changes such as the loss of ice shelves at their terminus or changing amounts of water flowing beneath them. The Antarctic ice sheet has many ice streams. Ice sheets Found now only in Antarctica and Greenland, ice sheets are enormous continental masses of glacial ice and snow expanding over 50,000 square kilometers (19,305 square miles).

The ice sheet on Antarctica is over 4.7 kilometers (3 miles) thick in some areas, covering nearly all of the land features except the Transantarctic Mountains, which protrude above the ice. Another example is the Greenland Ice Sheet. In the past ice ages, huge ice sheets also covered most of Canada (the Laurentide Ice Sheet) and Scandinavia (the Scandinavian Ice Sheet), but these have now disappeared, leaving only a few ice caps and mountain glaciers behind. See Also NSIDC's Glacier Glossary - Search and browse terms related to glaciers in NSIDC's comprehensive cryospheric glossary. NSIDC Glacier Photograph Collection - NSIDC archives a Glacier Photograph Collection of historical photos, which includes both aerial and terrestrial photos for the 1880s to 1975. The photos are primarily of Alaskan glaciers, but coverage also includes the Pacific Northwest and Europe. Related Resources Glossary - Terms Glacier Types ·Glossary Home· Types of Glaciers: July 1986 LANDSAT TM image of the terminus of Hubbard Glacier at the head of Yakutat Bay, Wrangell - St Elias National Park, Alaska.

At the time that this digital image was made, the glacier's terminus was blocking the entrance to Russell Fiord. Image from USGS EROS Data Center. ·Calving Glacier A glacier with a terminus that ends in a body of water (river, lake, ocean) into which it calves icebergs. Photograph of the calving terminus of Tyndall Glacier, located at the head of Taan Fiord, Icy Bay, Wrangell - St. Elias National Park, Alaska. The large mountain to the right is Mt. St. Elias. ·Cirque glacier A small glacier that forms within a cirque basin, generally high on the side of a mountain. Northeast-looking oblique aerial photograph showing a small, unnamed cirque glacier located on the south flanks of Mount Fairweather, Fairweather Range, Glacier Bay National Park, Alaska. North-looking oblique aerial photograph showing a small, unnamed hanging glacier located in the Chugach Mountains, near Cordova Peak, Chugach National Forest, Alaska. ·Hanging glacier A glacier that originates high on the wall of a glacier valley and descends only part of the way to the surface of the main glacier.

Avalanching and icefalls are the mechanisms for ice and snow transfer to the valley floor below. Northeast-looking photograph showing three small, unnamed hanging glaciers descending to the surface of the Vaughn Lewis Glacier, Juneau Icefield, Tongass National Forest, Coast Mountains, Alaska. Northwest-looking oblique aerial photograph showing several small, unnamed hanging glaciers located in the Chugach Mountains, near Cordova Peak, Chugach National Forest, Alaska. Only the hanging glacier in the center of the photograph reaches the debris-covered glacier on the valley floor. North-looking NOAA AVHRR satellite image of the western side of the Greenland Ice Cap. The body of water west of Greenland is Baffin Bay. ·Ice Cap A dome-shaped accumulation of glacier ice and perennial snow that completely covers a mountainous area or island, so that no peaks or Nunataks poke through. North-looking oblique aerial photograph of an unnamed, snow-covered ice field, located on the south side of Blackstone Bay, Kenai Mountains, western Prince William Sound, Chugach National Forest, Alaska. ·Ice Field A continuous accumulation of snow and glacier ice that completely fills a mountain basin or covers a low-relief mountain plateau to a substantial depth.

When the thickness become great enough, tongues of ice overflow the basins or plateaus as Valley Glaciers. South-looking oblique aerial photograph of the Harding Ice Field, Kenai Fjords National Park, Kenai Mountains, Alaska. The cloud-covered Gulf of Alaska is at the top of the photograph. ·Ice Sheet A thick, subcontinental to continental-scale accumulation of glacier ice and perennial snow that spreads from a center of accumulation, typically in all directions. Also called a Continental Glacier. North-looking photograph across the Antarctic Ice Sheet. In the foreground are some of the research facilities located at the Amundsen-Scott South Pole Station, South Pole Antarctica. July 1985 LANDSAT TM image of the part of the piedmont lobe terminus of Malaspina Glacier, located on the west side of Yakutat Bay, Wrangell - St Elias National Park, AK. At the time that this digital image was made, only a small quantity of snow was present on the glacier's terminus, hence many folded moraines are visible. Image from USGS EROS Data Center. ·Piedmont glacier A fan or lobe-shaped glacier, located at the front of a mountain range.

It forms when one or more valley glaciers flow from a confined valley onto a plain where it expands. The 30-mile wide Malaspina is the largest in Alaska. East-looking oblique aerial photograph of the piedmont lobe terminus of the advancing Taku Glacier, located in Taku Inlet, Coast Mountains, Tongass National Forest, Alaska. The width of the glacier's terminus is ~ 5 miles. December 1986 photograph of the author standing at the South Pole, on the snow-covered surface of the polar glacier that covers Antarctica. ·Polar Glacier A glacier with a thermal or temperature regime in which ice temperatures always remain below the freezing point. NOAA AVHRR satellite image of the Antarctica, showing the polar glacier ice that covers more than 97% of the continent. Southwest-looking oblique aerial photograph of the reconstituted Ogive Glacier, located on the shore of Northwestern Fjord, Kenai Fjords National Park, Kenai Mountains, Alaska.

·Reconstituted Glacier A glacier formed below the terminus of a hanging glacier by the accumulation, and reconstitution by pressure melting (regelation), of ice blocks that have fallen and/or avalanched from the terminus of the hanging glacier. Also called Glacier Remanié. North-looking photograph of several small unnamed reconstituted glaciers, located at the head of Icy Bay, Wrangell - St Elias National Park, St Elias Mountains, Alaska. Northwest-looking oblique aerial photograph of an unnamed rock glacier heading in a cirque, located on the southeast side of the Talkeetna Mountains, Alaska. ·Rock Glacier A glacier-like landform that often heads in a cirque and consists of a valley-filling accumulation of angular rock blocks. Rock glaciers have little or no visible ice at the surface. Ice may fill the spaces between rock blocks. Some rock glaciers move, although very slowly. South-looking oblique aerial photograph of an unnamed rock glacier with multiple flow lobes, located in the Metal Creek drainage on the north side of the Chugach Mountains, Alaska.

·Temperate Glacier A glacier with a or temperature-regime in which liquid water coexists with frozen water (glacier ice) during part or even all of the year. Northeast-looking photograph showing the stream and pond covered-surface of the Taku Glacier, Juneau Icefield, Tongas National Forest, Coast Mountains, Alaska. ·Tidewater Glacier A glacier with a terminus that ends in a body of water influenced by tides, such as the ocean or a large lake. Typically, tidewater glaciers calve ice to produce icebergs. Northwest-looking oblique aerial photograph of the tidewater terminus of the calving Chenega Glacier, located at sea level in western Prince William Sound, Kenai Mountains, Alaska. ·Valley Glacier A glacier that flows for all or most of its length within the walls of a mountain valley. Also called an Alpine Glacier or a Mountain Glacier. Northeast-looking oblique aerial photograph of the upper part of the Bucher Glacier, an outlet glacier of the Juneau Icefield, Coast Mountains, Tongass National Forest, Alaska. The length of the glacier shown is ~ 5 miles.

Alpine glaciers | Glaciers | Earth processes | OneGeology Kids | eXtra | OneGeology Get involved Alpine glaciers Alpine glaciers begin high up in the mountains in bowl-shaped hollows called cirques. As the glacier grows, the ice slowly flows out of the cirque and into a valley. Several cirque glaciers can join together to form a single valley glacier. When valley glaciers flow out of the mountains, they spread out and join to form a piedmont glacier. The ice in contact with the valley walls contains a lot of rock and sediment which makes the ice dark grey or brown. When several cirque glaciers merge into a single valley or piedmont glacier the ice has a striped appearance. Zones of an alpine glacier: Click to view larger and see the legend. Truncated spur 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 - Relatively small glaciers which occur at higher elevations in mountainous regions.

Smallest of these occupy hollows or bowl-shaped depressions on sides of mountains (cirque glaciers). As cirque glaciers grow larger they may spread into valleys and flow down the valleys as valley glaciers. Paths these valley glaciers take are controlled by existing topography. If a valley glacier extends down to sea level, it may carve a narrow valley into the coastline. These are called fjord glaciers, and the narrow valleys they carve and later become filled with seawater after the ice has melted are fjords. If a valley glacier extends down a valley and then covers a gentle slope beyond the mountain range, it is called a piedmont glacier. If all of the valleys in a mountain range become filled with glaciers, and the glaciers cover then entire mountain range, they are called ice caps. 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. Temperate glaciers - Ice in a temperate glacier is at a temperature near its melting point. Polar glaciers - Ice in a polar glacier always maintains a temperature well below its melting point. 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. Glaciers can only form at latitudes or elevations above the snowline, which is the elevation above which snow can form and remain present year round. The snowline, at present, lies at sea level in polar latitudes and rises up to 6000 m in tropical areas. Glaciers form in these areas if the snow becomes compacted, forcing out the air between the snowflakes. As compaction occurs, the weight of the overlying snow causes the snow to recrystallize and increase its grain-size, until it increases its density and becomes a solid block of ice. A glacier is actually a metamorphic rock. 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: Internal Flow - called creep, results from deformation of the ice crystal structure - the crystals slide over each other like deck of cards. This type of movement is the only type that occurs in polar glaciers, but it also occurs in temperate glaciers. Basal sliding - meltwater at base of glacier reduces friction by lubricating the surface and allowing the glacier to slide across its bed. Polar glaciers are usually frozen to their bed and are thus too cold for this mechanism to occur. 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 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 deposition. Glacial Erosion - Glaciers erode in several ways.: Abrasion – Rock fragments carried by the glacier scrape against rock causing abrasion, like sandpaper. Plucking – Ice breaks off and removes bedrock fragments Ice melts by pressure against the up-ice side of an obstruction. Entering cracks in bedrock, this water re-freezes to the ice. Glacial movement plucks away bedrock chunks (see figure 22.13 and 22.15 in your text).

Small scale erosional features (note: most of this material will be presented as slides in class) Glacial striations - long parallel scratches and grooves that are produced at the bottom of temperate glaciers by rocks embedded in the ice scraping against the rock underlying the glacier (see figure 22.12 in your text). Glacial polish - rock that has a smooth surface produced as a result of fined grained material embedded in the glacier acting like sandpaper on the underlying surface (see figure 22.12 in your text). Landforms produced by mountain glaciers (see figure 22.14 in your text) Cirques - bowl shaped depressions that occur at the heads of mountain glaciers that result form a combination of frost wedging, glacial plucking, and abrasion. Sometimes small lakes, called tarns occur in the bottom of cirque. Glacial Valleys - Valleys that once contained glacial ice become eroded into a "U" shape in cross section. Stream erosion, on the other hand, produces valleys that are "V" shaped in cross section.

Arï¿½tes - If two adjacent valleys are filled with glacial ice, the ridges between the valleys can be carved into a sharp knife-edge ridge, called an arï¿½te. Horns - Where three or more cirques are carved out of a mountain, they can produce a sharp peak called a horn. Hanging Valleys - When a glacier occupying a smaller tributary valley meets the larger valley, the tributary glacier usually does not have the ability to erode its base to the floor of the main valley. Thus, when the glacial ice melts the floor of the tributary valley hangs above the floor of the main valley and is called a hanging valley. Waterfalls generally occur where the hanging valley meets the main valley. Fjords - Fjords are narrow inlets along the seacoast that were once occupied by a valley glacier, called a fjord glacier. Landforms produced by Ice Caps and Ice Sheets Abrasional features - The same small-scale abrasional features such as striations and glacial polish can occur beneath ice caps and ice sheets, particularly in temperate environments.

Streamlined forms - The land surface beneath a moving continental ice sheet can be molded into smooth elongated forms called drumlins (see figure 22.20 in your text). Other elongated hills carved into bedrock by plucking and abrasion are called roche mountomées (see figure 22.15 in your text). 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 Till - nonsorted glacial drift deposited directly from ice. Till consists of a random mixture of different sized fragments of angular rocks in a matrix of fine grained, sand- to clay-sized fragments that were produced by abrasion within the glacier.

This fine-grained material is often called rock flour because it is really ground up rock. A till that has undergone diagenesis and has turned into a rock is called a tillite. Erratics - a glacially deposited rock or fragment that now rests on a surface made of different rock. Erratics are often found many kilometers from their source, and by mapping the distribution pattern of erratics geologists can often determine the flow directions of the ice that carried them to their present locations. Moraines - are deposits of till that have a form different from the underlying bedrock. Depending on where it formed in relation to the glacier moraines can be: Ground Moraines - these are deposited beneath the glacier and result in a hummocky topography with lots of enclosed small basins. End Moraines and Terminal Moraines are deposited at the low elevation end of a glacier as the ice retreats due to ablation (melting) (see illustration on page 820-21 in your text). It is notable that Long Island, NY and part of Cape Cod, MA are terminal moraines from the last glaciation (see figure 22.19 in your text)..

Lateral Moraines are deposits of till that were deposited along the sides of mountain glaciers. Medial Moraines - When two valley glaciers meet to form a larger glacier, the rock debris along the sides of both glaciers merge to form a medial moraine (see figures 22.17). These black streaks in an active glacier, as well as the deposits left behind after the ice melts are called medial moraines. Glacial Marine drift - Glaciers that reach the oceans or even lakes, may calve off into large icebergs which then float on the water surface until they melt. Upon melting, the rock debris that they contain becomes immediately deposited on the sea floor or lake bed as an unsorted chaotic deposit. Sometimes single large rock fragments fall out on the floor of the water body, and these are called dropstones. Stratified Drift - Glacial drift can be picked up and moved by meltwater streams which can then deposit that material as stratified drift. Outwash Plains - Streams running off the end of a melting glacier are usually choked with sediment and form braided streams, which deposit poorly sorted stratified sediment in an outwash plain. These deposits are often referred to as outwash.

Kettle Lakes (called kettle holes in your text) - If depressions form underneath a glacier and remain after the glacier is melted then water filling these depressions become small lakes where fine-grained sediment is deposited. The state of Minnesota is called the land of ten thousand lakes, most of which are kettle lakes. Kames and Kame Terraces - Streams and lakes forming on top of or on the sides of stagnant ice may deposit stratified sediment on top of the glacier. When the glacier melts these deposits are set down on the ground surface and become kames. Eskers - Eskers are long sinuous ridges of sediment deposited by streams than ran under or within a glacier. The sediment deposited by these streams becomes an esker after the ice has melted (see figure 22.21 in your text). Meltwater Lakes - Depressions created by glacial erosion and deposition collect water released by melting glaciers. Sediment that collects in the bottom on the lakes is often finely layered with coarser grained layers forming during times of warmer temperatures and fine grained layers formed during cold times when no new sediment is entering the lake. Such finely layered strata are termed varves.

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 During glacial periods much sea water was tied up in glaciers so sea level was lower. during interglacial periods sea level was higher due to melting of the ice. 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. Glacial Ages 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 ages.

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. Oxygen has two major isotopes, 18O, which is considered heavy, and 16O, which is considered light. Both of these isotopes are stable and non-radiogenic, so their ratio is constant through time. Because 16O is lighter, it is preferentially evaporated with sea water from the oceans, and thus gets concentrated in the water that eventually falls on the continents as rain or snow. Because of this, 18O gets concentrated in ocean water.

During constant climatic conditions the 16O lost to evaporation returns to the oceans by rain and streams, so that the ratio of 18O to 16O (18O / 16O) is constant. But, during a glaciation, some of the 16O gets tied up in glacial ice and does not return to the oceans. Thus during glaciations the 18O / 16O ratio of sea water increases. During an interglaciation, on the other hand, the 16O that was tied up in glacial ice returns to the oceans causing a decrease in the 18O / 16O ratio of seawater. 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. Long term variations in climate (tens of millions of years) on a single continent are likely caused by drifting continents.

If a continent drifts toward the equator, the climate will become warmer. If the continent drifts toward the poles, glaciations can occur on that continent. Short-term variations in climate are likely controlled by the amount of solar radiation reaching the Earth. Among these are astronomical factors and atmospheric factors. Astronomical Factors - Variation in the eccentricity of the Earth's orbit around the sun has periods of about 400,000 years and 100,000 years. Variation in the tilt of the Earth's axis has a period of about 41,000 years. Variation in the way the Earth wobbles on its axis, called precession, has a period of about 23,000 years. The combined effects of these astronomical variations results in periodicities (called Milankovitch Cycles) similar to those observed for glacial - interglacial cycles (see figure 22.46 in your test book) Atmospheric Factors- the composition of the Earth's atmosphere can be gleaned from air bubbles trapped in ice in the polar ice sheets.

Studying drill core samples of such glacial ice and their contained air bubbles reveals the following: During past glaciations, the amount of CO2 and methane, both greenhouse gasses that tend to cause global warming, were lower than during interglacial episodes. During past glaciations, the amount of dust in the atmosphere was higher than during interglacial periods, thus more heat was likely reflected from the Earth's atmosphere back into space. The problem in unraveling what this means comes from not being able to understand if low greenhouse gas concentration and high dust content in the atmosphere caused the ice ages or if these conditions were caused by the ice ages. Changes in Oceanic Circulation - small changes in ocean circulation can amplify small changes in temperature variation produced by astronomical factors. Other factors The energy output from the sun may fluctuate. Large explosive volcanic eruptions can add significant quantities of dust to the atmosphere reflecting solar radiation and resulting in global cooling. Impacts of large asteroids with the Earth can cause extensive amounts of dust and soot to be placed in the atmosphere. Dust and soot would block incoming solar radiation and cause a cooling effect for as long as the dust remains in the atmosphere. Examples of questions on this material that could be asked on an exam.

Describe the different types of mountain glaciers. How does glacial ice cause erosion? Describe the erosional features produced by glaciers. What are the characteristics of sediment deposited directly from glaciers? Describe the various types of ice laid deposits. How does glaciation modify the landscape in terms of drainage networks and lakes? During the Pleistocene Epoch, how many glacial/interglacial episodes are suspected and what is the evidence for this? What factors can cause long term and short term variations in climate that may be responsible for the glacial/interglacial cycles. Why is Minnesota called the Land of Ten Thousand Lakes? Glaciers Concepts Importance of the Cryosphere Snow surface at Dome C Station, Antarctica. Image taken by Stephen Hudson (GNU). The cryosphere is part of the hydrosphere and includes glaciers and frozen sea ice. It is located mostly at the north and south poles. The cryosphere is an effective agent of erosion and deposition. It can significantly modify landscapes. The cryosphere impacts Earthï¿½s climate by cooling water and air, and setting up oceanic and atmospheric circulation patterns.

The cryosphere impacts the biosphere because water cooled by polar ice has the largest concentration of dissolved oxygen and supports a diverse fauna (oxygen gas is most soluble in cold water). Types of Glaciers Glacier- A large, long-lasting mass of ice that forms on land and moves downhill under its own weight. The two general categories of glaciers: 1. Alpine/valley glaciers ï¿½ glaciers formed in mountainous regions of high elevation (Example: Glacier National Park in Montana). They are confined to a valley and flow downhill due to gravity. St. Mary Lake, Glacier National Park. Public Domain Image taken by Ken Thomas. 2. Continental/ice sheet glaciersï¿½ glaciers that exist over a large part of a continent (> 50,000 km2) in regions near the earthï¿½s north or south poles (Examples: Antarctica, Greenland). Antarctica has 85% of glacier ice currently on earth, and Greenland has 10%. If Antarcticaï¿½s ice were to melt, the sea level would rise 70 m or 230 ft.

In Antarctica, two ice sheets (west and east) flow downward and outward from a central point: the Transantarctic Mountains. The East Antarctic Ice Sheet = 2700 m thick and the West Antarctic Ice Sheet = 4800 m thick. Public Domain Image (generated by NASA's Blue Marble data set). Formation and Growth of Glaciers Transition from snow to glacier ice: Powdery snow → Granules → Firn (somewhat compacted)→ Glacier ice. Public Domain Image by Luis Marï¿½a Benï¿½tez. B. Glacier ice moves downhill and is eventually lost (ablated) by melting, evaporation, or calving of icebergs. Icebergs calving off the coast of Greenland. Photo by Mila Zinkova (GNU). Zone of accumulation- The upper part of a glacier where snowfall is collected. Zone of ablation- The lower part of a glacier where ice is lost. Equilibrium line- The boundary between these zones. It marks the highest point at which the glaciers snow cover is lost during the warm season. The equilibrium line may migrate from year to year depending upon where there has been more accumulation or more ablation.

If there is more accumulation than ablation→ the equilibrium line moves down the glacier (positive budget). If there is more ablation than accumulation→ the equilibrium line moves up the glacier (negative budget). Terminus- The lower edge of a glacier (the position depends on the glacierï¿½s budget). The velocity of ice movement varies across the glacier with velocities being highest in the thick center and lowest at the thin edges (due to drag). Movement of Alpine Glaciers Glacier speed varies from millimeters to tens of meters per day (depends on slope). The base of a glacier slides over underlying rock on a thin layer of meltwater (basal sliding). The thicker parts of glaciers (the centers) move faster than thinner parts of glaciers (the edges) which experience more frictional drag. The lower part of glaciers flows plastically as individual ice grains move relative to each other (the zone of plastic flow). Here the ice grains deform to accommodate movement. The upper part of the glacier moves rigidly. Here fractures called crevasses may develop here. Erosion by Glaciers Meltwater at the base of a glacier may work its way into cracks and freeze.

Pieces of bedrock may be worked loose and picked up by the moving glacier in a process is called plucking. Image by Luis Marï¿½a Benï¿½tez (GNU). Rocks carried along at the base of the glacier may grind and polish the bedrock (abrasion). Large rocks may produce glacial striations in the bedrock as they move over it. Erosional Features of Alpine Glaciation (Angular Topography): Alpine Glaciation is characterized by the following types of erosional features: Public Domain Image by Luis Marï¿½a Benï¿½tez. 1. U-shaped valley ï¿½ The characteristic cross section profile of a valley carved by glacial erosion. New Zealand's Franz Joseph glacier. Copyright, Matthew Cox 2. Hanging valley ï¿½ A small valley that terminates abruptly high above a main valley. Bridal Veil Falls in Yosemite National Park flowing from a hanging valley. (GNU Image) 3. Truncated spur ï¿½ The triangular facet produced where the lower end of a ridge has been eroded by glacial ice. 4. Rock-basin lake ï¿½ A lake occupying a depression caused by glacial erosion of bedrock.

5. Cirque ï¿½ A steep-sided, amphitheater-like hollow carved into a mountain at the head of a glacial valley. 6. Horn ï¿½ A sharp peak formed where cirques cut back into a mountain on several sides. 7. Arï¿½tes ï¿½ A sharp ridge that separates adjacent glacial valleys. Horn and arï¿½te in the Fairweather Range, Alaska. ï¿½ Michael Collier Erosional Features of Continental Glaciation (Rounded Topography): The weight of the moving ice sheet produces rounded knobs and striated bedrock. The orientation of the striations can be used to interpret the direction of movement of the ice sheet. Image by Luis Marï¿½a Benï¿½tez (GNU). Thick ice sheets may bury mountain ranges, rounding and streamlining them in the direction of ice movement.

Question: What have types called – continental, mountain and piedmont?
Answer: Glaciers