Glaciers – College Essay Example
Firn and ne’ve’ are repeated melting and refreezing forms of granules Background info Unconstrained these glaciers cover vast areas. Topography does not play a major role in the extent of these glaciers. Ice sheets and ice caps fall into the same category. The difference between them is one of scale. Ice sheets are larger. Typically the dividing line is around 50,000 km2. The glaciers that cover Antarctica and Greenland are ice sheets, and the glacier that covers Iceland is an ice cap.
Two main components of ice sheets and ice caps are ice domes and outlet glacier. Ice shelves An ice shelf is a very thick sheet of ice that has been shoved out over the sea floor from a land-based glacier. It is still attached to land on one side but most of it is afloat. Massive icebergs (glossary) may cal...
ve off of ice shelves. Constrained These are the glaciers that are found in rugged topography and are typically bound within a valley or depression.
An icefield is an extensive area of land ice covering a mountain region; its surface is approximately level and can be distinguished from an ice cap because it does not achieve the characteristic domelike shape, and because flow is strongly influenced by the underlying topography. Valley glaciers A valley glacier flows between the walls of a mountain valley in all or part of its length. It may originate in an icefield or a cirque.
Cirque glaciers A cirque glacier is a small ice mass generally wide in relation to its length and characteristically occupying an armchair-shaped bedrock hollow. It is the most common type of glacier in the mountains of the
western United States. Other small glaciers This category includes a wide variety of glaciers whose forms are closely controlled by the underlying topography. The permutations are almost limitless. Typically small glaciers are found in hollows or slight depressions in mountainous terrain or bordering coastlines.
Cirque glacier-glacier confined by valley ,forms cirque(semicircular basin at head of valley formed by plucking of bedrock by glacier moving down hill Alpine glacier-form on mountain side and move downward through valleys. Create and deepen valleys by pushing dirt soil out of their way, Found in high mountains of every continent except Australia When two glaciers meet and merge at base of mountain, new glacier is called piedmont glacier- if it lows to the sea, called tidewater glacier Valley glacier-form in a valley Ice caps-cover mountain tops Icesheets- not limited to mountain areas, Form broad dome, spread out from center in all direction, Cover everything around them with ice Continental glaciers-largest ice sheets (antartica and island of Greenland) Dome shaped and flow away from central region and are largely unaffected by underlying topography Hanging glaciers-don’t flow down the entire length of mountain Avalanches and icefalls transfer glacial ice from hanging glaciers to a larger glacier beneath them, or directly to the valley below If a valley glacier extends down a valley and then covers a gentle slope beyond the mountain range, it is called a piedmont glacier.
A View from Above Suppose stones are placed in a straight line across the surface of a valley glacier. In a year or two, the line would no longer be straight; it would be displaced and bent down-valley. This elementary study demonstrates that the center of
a glacier, where the ice is the thickest, moves more rapidly than at the edges.
A Sliced View
Suppose a bendable pipe is inserted into a deep, narrow hole that was drilled vertically through a glacier from top to bottom. In a year or two, the inclination and position of the pipe would change. The pipe would be bent into a curve by greater movement at the top of the glacier. In addition to the pipe bending, it would also have moved down-valley, revealing that the glacier slipped over its bed. This experiment demonstrates the two dominant types of glacial movement: Mysterious Movement Suppose a child broke his or her mother’s favorite vase while playing baseball indoors and foolishly wished to dispose of the pieces in a glacier.
He or she would eventually realize his/her error of not taking into consideration the vertical component of glacial movement. If the vase is buried in the accumulation zone, it will ultimately emerge in the ablation zone. Eventually their mother would find the evidence. The basic pattern of flow in the accumulation zone is downward, but changes to upward in the ablation zone. Such variations in ice movement are associated with two different types of flow regimes within a glacier: Extending Flow A glacier is extended and thinned where flow is accelerating. Compressive Flow A glacier is compressed and thickened where flow is decreasing in velocity.
A bergschrund is a deep and often wide gap or crevasse, or series of closely spaced crevasses, in ice or firn at or near the head of a valley glacier. A bergschrund separates the moving ice and snow from the relatively immobile ice and snow adhering
to the headwall of a valley (or cirque). Crevasses A crevasse is a deep (some as much as 100 meters deep), nearly vertical fissure, crack, or rift in a glacier. Crevasses are caused by stresses resulting from differential movement over the uneven surface underlying a glacier. Crevasses may be concealed by snowbridges, and are, therefore, hazards when traversing a glacier. Crevasse-similar to reading tree ring width Foliation the banding in ice that occurs in the ablation zone.
Foliation consists of alternating layers of white bubbly ice and bluish ice. Layers vary from several millimeters to several meters in width. It is related to glacier movement, and that it is best displayed on those glaciers that have undergone considerable deformation, as for example at the foot of an icefall. Ice Falls Icefalls occur on the parts of a glacier that flow over convex bedrock surfaces, and are thereby very steep. Icefalls are highly crevassed. Extending flow predominates over icefalls. Ice Streams Ice streams are currents of ice in an ice sheet or ice cap that flow more rapidly than the surrounding ice. Ice streams are usually flowing to an ocean or an ice shelf and not constrained by exposed rock.
Ogives are broad banded surface patterns that generally curve down-glacier as a result of faster ice movement toward the center of a glacier. Ogives are common below icefalls. Behaviors Stable A stable glacier is still a moving body of ice; it just isn’t increasing in size of changing location. A stable glacier occupies about the same position year-after-year, except for minor seasonal fluctuations. In a stable state, the rate of a glacier’s forward movement is balanced by recession
of the ice edge through melting. It is neither advancing nor retreating. In short, ablation equals accumulation. Advancing In order for a glacier to be considered a glacier, it must be moving. Moving” and “advancing” are not the same when used to describe glaciers. By definition, a glacier is always moving, but it may not be advancing. Advancement in this situation has more to do with a glacier’s mass balance than a glacier’s motion. In short, accumulation exceeds ablation. Retreating / Receding During extended warm periods of a year or more, glaciers will retreat/recede if the amount of snow and ice that is lost through melting—and to a lesser extent through evaporation, sublimation, wind erosion, and calving—is greater than the amount of snow that is gained through precipitation or wind deposition. In short, ablation exceeds accumulation.
Calving During the last century and a half, many coastal Alaskan glaciers have receded at rates far in excess of rates of retreat on land. Their dramatic recession is due to frontal calving, in which icebergs progressively break off from the front of glaciers that terminate in deep water. Although the base of such a glacier may lie far below sea level along much of its length, its terminus can remain stable as long as it is grounded, say against a shallow submarine ridge, such as a moraine shoal. Once the glacier retreats off the shoal, however, water will replace the space that had been occupied by ice. With the glacier now terminating in water, conditions are ripe for calving. Surging When a glacier surges, you can actually see it move or build up at its terminus.
A surge of ice
may locally increase velocities by as much as 10 to 100 times the normal velocity. Rates of five meters per hour have been recorded. Surging is initiated when a threshold of instability is reached and ice in the upper ablation zone begins to move rapidly down-glacier. Surging is still one of the least understood aspects of glacier movement; it is necessary to explain both the mechanism that triggers a surge and the high flow velocity subsequently attained.
Precipitation, Temperature solar radiation has the most significant influence on ablation, the snow that falls needs to stick around, and chances are improved if summers are cool. , Latitude (High latitudes receive less annual solar radiation and experience prolonged winters at sub-zero temperatures. glaciers prefer polar regions), Altitude (Glaciers are found at sea level at the South Pole, but in the mid and low latitudes, glaciers are related to highlands. Alpine regions are prime candidates for glacier formation), Relief (topography has powerful influence where enough land surface, deepness of surface, or surface is jagged or hollows exist), Aspect(The orientation of the ground surface with respect to incoming solar radiation is important The proper slope aspect lowers snowline. Steep north-facing slopes receive the least direct radiation in the northern hemisphere, but lopes facing east of north are the coolest. ) Snow remains in the same area year-round, where enough snow accumulates to transform into ice new layers of snow bury and compress the previous layers. This compression forces the snow to re-crystallize slowly compact and increase in density. After about two winters, the snow turns into firn—an intermediate state between snow and glacier ice. At this point, it is about
half as dense as water. Over time, larger ice crystals become so compressed that any air pockets between them are very tiny. In very old glacier ice, crystals can reach several inches in length. For most glaciers, this process takes over a hundred years
Glacier formation Snow compresses, become denser(tighter) Changes crystals to hard ice pellets Becomes dense grainy ice called firn Process called firnification Firn builds up, fuses into solid ice Compression melting- glacier put so much pressure that firn and snow melt without temp increase Water, unlike others can melt under pressure This causes the glacier to move and slide around GLACIER SPEED AND MOVEMENT.
Most glaciers move very slowly—only a few centimeters a day. Some, though, can move 50 meters (160 feet) a day. These fast-moving rivers of ice are called galloping glaciers Alpine glaciers form on mountainsides and move downward through valleys.
Sometimes, alpine glaciers create or deepen valleys by pushing dirt, soil, and other materials out of their way. Alpine glaciers are found in high mountains of every continent except Australia (although there are many in New Zealand). The Gorner Glacier in Switzerland and the Furtwangler Glacier in Tanzania are both typical alpine glaciers. Alpine glaciers are also called valley glaciers or mountain glaciers. As years go by, layers of firn build on top of each other. When the ice grows thick enough—about 50 meters (160 feet)—the firn grains fuse into a huge mass of solid ice. The glacier begins to move under its own weight. It does this through a process called compression melting.
The glacier is so heavy and exerts so much pressure that the firn and snow melt without any increase
in temperature. (Most substances do not melt under pressure. Water is somewhat unusual. ) The meltwater makes the bottom of the heavy glacier slicker and more able to spread across the landscape. Pulled by gravity, an alpine glacier moves slowly down a valley. Some glaciers, called hanging glaciers, don't flow the entire length of a mountain. Avalanches and icefalls transfer glacial ice from hanging glaciers to a larger glacier beneath them, or directly to the valley below. An ice sheet spreads out from its center. The great mass of ice in a glacier behaves plastically, or like a liquid.
It flows, oozes, and slides over uneven surfaces until it covers everything in its path. Different parts of a glacier move at different speeds. The flowing ice in the middle of the glacier moves faster than the base, which grinds slowly along its rocky bed. The different speeds at which the glacier moves causes tension to build within the brittle, upper part of the ice. The top of the glacier fractures, forming cracks called crevasses. Crevasses are in the top 50 meters (160 feet) of the glacier. Crevasses can be very dangerous for mountaineers. They can open quickly and be very deep. Although glaciers move slowly, they are extremely powerful.
Like huge bulldozers, they plow ahead year after year, crushing, grinding, and toppling almost everything in their paths. Forests, hills, and mountainsides are no match for glaciers. Sometimes, glaciers form on volcanoes. When these volcanoes erupt, they are especially dangerous. They send floods of water, ice, and rocks over the land and into the atmosphere. Alpine glaciers begin to flow downhill from bowl-shaped mountain hollows called cirques. As
the glaciers overflow the cirque, they move downward. They dig deep into the terrain, forming rugged, dramatic landscapes. As they move, glaciers erode or wear away the land beneath and around them.
Glaciers carry great amounts of soil, rock, and clay. Some of the boulders they carry are as big as houses. Rocks carried hundreds and even thousands of kilometers by glaciers are called glacial erratics. Glacial erratics differ significantly from the landscape in which they were deposited. The Big Rock, for instance, is a 15,000-ton quartzite boulder near Okotoks, Alberta, Canada. The Big Rock was deposited from what is now northern Alberta, about 1,640 kilometers (500 miles) away, during the last ice age. Embedded, or stuck, in a glacier’s base, these large rocks grind against the ground like the prongs of a rake. They dig long grooves, called striations, in the surface of the Earth.
Geologists can tell in what direction an ancient glacier moved by studying striations left in rock Mass balance is the difference between the amount of snow and ice accumulation on the glacier and the amount of snow and ice ablation (melting and sublimation) lost from the glacier Mass balance is measured by determining the amount of snow accumulated during winter, and that is remaining at the end of the melt season, and measuring the amount of snow and ice removed by melting in the summer. The difference between these two parameters If the amount of snow accumulated during the winter is larger than the amount of melted snow and ice during the summer, the mass balance is positive and the glacier has increased in volume. f the melting of snow and
ice during the summer is larger than the supply of snow in the winter, the mass balance is negative and the glacier volume decreases. Mass balance is reported in meters of water equivalent. This represents the average thickness gained (positive balance) or lost (negative balance) from the glacier during that particular year. A typical glacier that is not calving must have 60-70% of its area snowcovered at the end of the summer to be in balance mass balance general indicator of the health of the glacier. Long-term mass balance records can be indicators of local changes in climate -different between accumulation and ablation
Ablation is measured by emplacing stakes in the glacier at the end of the previous melt season or the beginning of the melt season Accumulation is measured by either probing or crevasse stratigraphy to determine the annual snowpack thickness at many locations Glacial Erosion Main three types- plucking, abrasion, freezethaw Plucking-melt water from glacier freezes around lumps of cracked and broken rock Abrasion is when rock frozen to the base scraps the bed rock Freeze thaw is when melt water or rain gets into cracks in bed rock(usually back wall), Crack gets bigger and rock break away erosional landforms bays -when Less resistant rock erodes more rapidly than hard rock. Headlands-when area of hard rock sticks out into sea-because rock has alternating hard and soft rock Braided streams-ice melt, water moves away from glacial snout and into fast streams, while transport sediment.
Erratcis- large boulders transported by glacier-if found original position, can determine direction of ice flow and plot past ice movements Eskers-produced as result of running water (under ice) Kames -sediment which are
deposited along the front of a slowly melting or stationary glacier, A depositional feature of glaciation; a small hill of poorly sorted sand and gravel that accumulates in crevasses or in ice caused indentations in the surface Kettle lakes form when a piece of glacier ice breaks off and becomes buried by glacial till or moraine deposits. Over time the ice melts, leaving a small depression in the land, filled with water. Kettle lakes are usually very small, and are more like ponds than lakes.
Kettle Forms when an isolated block of ice persists in a ground moraine, an outwash plain or valley floor after a glacier retreats; as the block finally melts, it leaves behind a steep sided hole that frequently fills with water Moraine-(ground-till deposited over valley floor, found where ice meets rock under ice) (lateral-forms along glacier edges, material form valley wall falls onto ice and ridge forms) (medial-formed from 2 laterals) (push-forms by glaciers that have retreated and then advance again, evidence of climate becoming poorer after warm period, identification individual rocks that have been pushed upwards from their original horizontal positions) (recessional- form at end of glacier, found across valley, not along valley, form where retreating glacier remained stationary for some time to make material.
Process is same as terminal, but occur when retreating ice paused) (terminal- forms at snout of glacier, marks the furthest extent of ice, forms across valley floor, large mounts of debris, marks the end of unsorted deposits and start of sorted material) (Supraglacial-material on the surface of the glacier, including lateral and medial moraine, loose rock debris and dust settling out from the atmosphere) (englacial- any material
trapped within the ice. It includes material that has fallen down crevasses and the rocks being scraped along the valley floor) Outwash Plain-largest areas of glacial sediment deposited by meltwater streams, furthest away from snout, formed from gravel, sand, clay farthest away from snout b/c small material carried far, material already deposited are sorted by streams forming outwash plain, thickness of outwash plain can be 50m thick Rock Flour- finely grinded remains of rock.
Produced by grinding rocks together, when glacier moves across valley floor, material include material from freeze thaw and debris fallen from valley wall, it is removed from glacier by meltwater streams and often colors it, presence of it is milk blue white color of water Varves-found at deposits of glacial lakes, two layers of sediment, lower of light color sandy material, upper of dark silt, number of varves can determine age of lake, thick valves is warm temp and increased melt, thin varves is little deposition because of less melting and outwash, analysis of original debris from varves have info about plant types at time of deposition Cirques are created when glaciers erode backwards, into the mountainside, creating rounded hollows shaped like a shallow bowls. 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 Aretes are jagged, narrow ridges created where the back walls of two cirque glaciers meet, eroding the ridge on both sides. Aretes - 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 arete. Horns, such as the famous Matterhorn in Switzerland, are created when several cirque glaciers erode a mountain until all that is left is a steep, pointed peak with sharp, ridge-like aretes leading up to the top. Horns - Where three or more cirques are carved out of a mountain, they can produce a sharp peak called a horn Braided Streams When glacial ice melts, the water moves away from the glacial snout in fast flowing streams and rivers.
The water transports vast quantities of sediment and larger debris. If the sediment load is very large in relation to the velocity of the stream, the more coarse material may start to block the stream, choking it and forcing it to constantly change it's course. The stream starts to diverge, splitting into numerous segments which split and join repeatedly. The small islands formed within the stream are called eyots. Braided streams are typically shallow and wide, surrounded by poorly sorted rock debris. Drumlins are long, tear-drop-shaped sedimentary formations. What caused drumlins to form they were created subglacially as the ice sheets moved across the landscape during the various ice ages.
Theories suggest that drumlins might have been formed as glaciers scraped up sediment from the underlying ground surface, or from erosion or deposition of sediment by glacial meltwater, or some combination of these processes. Because the till, sand and gravel that form drumlins, are deposited and shaped by glacier movement, all drumlins created by a particular glacier face the same direction, running parallel to the glacier's flow. Often, hundreds to thousand of drumlins are found in one place, composed of till (unstratified, unsorted)
and is streamline in the direction of continental ice movement-blunt end upstream and tapered end downstream with a rounded summit. 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. Valleys carved by tributary glaciers 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 (see figure 16. 20 in your text). Fjords - Fjords are narrow inlets along the seacoast that were once occupied by a valley glacier, called a fjord glacier. A drowned glaciated valley or glacial trough along a seacoast 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. 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. Outwash Terraces - If the outwash streams cut down into their outwash deposits, the banks from river terraces called outwash terraces. 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.
Paternoster Lake: One of a series of small,circular stair-stepped lake formed in individual rock basins aligned down thecourse of a glaciated valley. Col the lowest point of a ridge or saddle between two peaks, typically affording a pass from one side of a mountain range to another A saddle-like narrow depression formed by two headward eroding cirques that reduce an arete Horn A pyramidal, sharp-pointed peak that results when several cirques glaciers gorge an individual mountain summit from all sides Bergschrund These form when a crevasse or wide crack opens along the headwall of a glacier; most visible in the summer when covering snow is gone Tarn A small mountain lake especially one that collects in a cirque basin behind risers of rock material or in an ice gouged depression.
Paternoster Lake One of a series of small circular stair-stepped lake formed in individual rock basins aligned down the course of a glaciated valley U shaped valley post glaciation conditions where the continual freeze and thaw has weathered away the rock wall Erratics An unique rock carried by a glacial formation that deviates in size and or type relative to the native area Truncated Spurs Occur where a glacier carves its way though rock, cutting off the edges of interlocking spurs
- Deposition land by alpine glaciers Glacial Drift: A general term for all glacial deposits both sorted and unsorted. Stratified Drift Sediments deposited by glacial meltwater that are sorted by size.
- Tills: Unstratified and unsorted debris from ice deposits
- Valley Train Deposits: Material deposited downvalley of a glacier via melt-water
water entering a crack and freezing. Frost Action The 9% expansion of water as it freezes creates a strong mechanical force.
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.
- are deposits of till that have a form different from the underlying bedrock.
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