GEOL 101 Glaciers and Glaciation
GEOL 101 Glaciers and Glaciation GEOL 101
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This 7 page Class Notes was uploaded by Victoria Williams on Tuesday April 19, 2016. The Class Notes belongs to GEOL 101 at George Mason University taught by Mark Uhen in Fall 2016. Since its upload, it has received 14 views. For similar materials see Introductory Geology in Geology at George Mason University.
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Date Created: 04/19/16
Glaciers and Glaciation How much would sea level rise if the Ross Ice Shelf collapsed? (1m = 3.3ft) If the shelf collapses, it will displace a ton of water. Answer: 15 meters (3.316.4 meters) Ice shelfs are pieces of ice attached to a larger body of ice on a continent. An example of this is the Ross Ice Shelf in Antarctica. The shelf is HUGE! Others are the Filchner Ice Shelf and the Larson B Ice shelf. Glaciers o A thick mass of ice that forms over hundreds of years from accumulation of snow. o Glaciers form where more snows fall in the winter than melts in the summer. o Snow turns to ice as it is compacted and recrystallized. o Makes up 2.8% of the ocean and is most of the fresh water on earth. o If all glaciers melted, sea level would rise 60 to 70 meters. o For most of earth’s history, there were no glaciers. We are still in an ice age but we are rapidly coming out of it because of human activity. Two main types of glaciers o A valley (or alpine) glacier are glaciers that form in a mountain valley, often one that was previously occupied by a stream. Ice in an alpine glacier flows downhill. Alpine glaciers are called that because they occur like glaciers do in the alps o An ice sheet is a glacier that forms over a broad region. Today there are only two ice sheets on the planet. One near the North Pole on Greenland and the other at the South Pole on Antarctica. Greenland – 1.7mil. square km, averages 1500 m thick, over 3000m thick in places Antarctica – 13.9 million square km. Over 4300 m thick in places. Cover 10% of the earth’s surface Ice flows downhill like water. Ice covers part of the sea, forming ice shelves. This ice actually floats on the sea. Other types of glaciers, minor glacial types o Ice cap – an expanse of ice covering an upland area. Smaller than an ice sheet o Outlet glacier – a tongue of ice extending out of an ice cap or ice sheet. Basically a mini alpine glacier (but attached to ice sheet) forming in a valley. o Piedmont glacier – a glacier covering the broad lowlands at the bases of steep mountains. These are basically alluvial fan glaciers that merge together into a glacial blob like a bajada. Formation of Glacial Ice o Extremities of snowflake crystals evaporate, and water vapor condenses near the center, thus snowflakes become smaller and thicker. o As the snowflakes become more compact, they pack more tightly together forcing air out. o This compacted, granular snow is called firn. As more snow is added, the weight compacts the firn into solid glacial ice. However, some bubbles of gas remain in the ice. These bubbles are samples of air from hundreds of thousands of years ago. Glacial movement o Plastic flow takes place within the ice, a brittle solid on time and down to 50m below, that ice moves like silly putty. The top part that is brittle is called the zone of fracture. o Basal Slip is movement between the ice and the surface underneath, meltwater beneath the glacier can act as a lubricant. Alpine glaciers move about 0.5 to 2 meters a day. But in the winter, the movement slows down because the meltwater is mostly frozen. Rates of glacial movement Glacial Surface – includes the zone of fracture where the ice cracks and breaks at or near the surface rather than flowing plastically. A large crack in the surface is called a crevasse. Glacial Budget o Glaciers are constantly gaining and loosing ice, this set of processes is called the glacial budget. Even it doesn’t look like its moving, the ice is always changing. o The portion of the glacier where new ice is being added to the glacier is called the zone of accumulation. o The boundary of the zone of accumulation is marked by the snowline. Above the line, you get snow, below the line, you don’t get snow. o Downstream of the snowline is the zone of wastage. Here, more ice is being lost that accumulated from new snowfall. This is below the snowline. o At the front of the glacier, large pieces can break off in a process called calving. If these pieces drop into the ocean, they become icebergs. Ice bergs are not stable. o A negative glacial budget leads to a net loss of ice. A positive glacial budget leads to a net gain of ice. o The loss of ice in the zone of wastage is called ablation. Glacial Erosion o Flowing ice has both an enormous capacity and competence. It can literally move mountain size rocks that liquid water could never even budge. o Glaciers can erode by plucking. Plucking is the process by which meltwater seeps into cracks in the groundwater and freezes, breaking chunks of rock that are incorporated into the base of the glacier. This can break monumental size rocks, medium rocks, small rocks, or really tiny rocks all the same. o Glaciers can also erode by abrasion. The ice and sediment in the base of the glacier act like sandpaper on underlying rock. o Large rocks can be randomly deposited in a landscape where there was once a glacier that melted. These rocks are called glacial erratic. o Bedrock that was once covered by a glacier usually has lines carved into them. These are called glacial grooves, and they are caused by the rocks that the glacier has plucked. o But when a glacier has very fine grain sediment, it smooths out the rock it’s moving over. This is called glacial polish. Glacial land forms – Alpine Glaciers o A horn is a peak pyramidal peak projecting high above the landscape. These tend to be at the highest peaks. An example is the Matterhorn in the Alps. These are formed by glaciers moving and plucking away rock to leave the horn behind. o An arête is a sharp ridge between adjacent valleys. These tend to be next to the horns. o These both form by plucking and frost action of adjacent glaciers. o These glaciers form like rivers, they can move together in tributaries. o A cirque is a bowlshape area that contains the head of the glacier. They are open on one side where the ice can flow out. Like a culdesac o A truncated spur is the eroded end of a ridge that formerly extended into the glacial valley. o PostGlacial Landforms o Pater noster lakes are strings of lakes formed in depressions where bedrock was plucked out. o A tarn is a lake formed in a cirque o A glacial trough is a UShaped valley formed occupied by a glacier. o Hanging valleys are formed where a glacial tributary met the glacier. o A fjord is a glacial trough flooded by the sea. o PostGlacial Landforms o Roches moutonnées are formed where a glacier flows over a protruding knob of bedrock. Glacial Deposits o Glacial drift is any sedimentary deposit derived from glacial processes (any glacier) o Till is the term for deposited directly by a glacier, in contact with the ice. Glacial erratics are large boulders found in other till or lying free on the surface. They are called erratics because they differ from the underlying bedrock. They also tend to be smooth because they’ve been polished by the glacier. With freeze/thaw cycles, these rocks are brought to the surface. Places that have till tend to be very rich farm land. Moraines are large piles of till. Different kinds of moraines are distinguished by how they were deposited. Moraines that are on either side of the glacier are called lateral moraines. But if two glaciers come together, both with lateral moraines, a medial moraine will form in the middle between the glaciers. Ice Sheet Moraines At the end of a glacier where the ice melts, an end moraine forms. The ice keeps moving and melting and keeps dumping sediment like a conveyer belt. The edge of the glacier is stable. A ground moraine forms when an ice sheet melts rather quickly. As the end of the glacier melts, it dumps the sediment at moving locations. Here, the edge of the glacier was not stable. A place where this happened is around NorthernMid U.S. like in Wisconsin, Illinois, Michigan, etc. On the East Coast, this happened on Cape Cod and Long Island Other Glacial Landforms Esker – An esker is a long, meandering hill. It represents a stream in or on the ice. When it was a stream, it was laden with sediment which got deposited. Drumlin – elongate, asymmetrical hills made of till. Usually found in groups called drumlin fields. They are formed when glaciers reshape previously deposited till. Kame – Reverse of a Kettle, it is a mound or steep sided hill. They are formed by sediment deposited in or on the ice or as deltas in meltwater streams. https://youtu.be/2QhlVuF9xQg Kettle Lake – A kettle is a depression formed by an isolated block of melting ice, often contains a lake called a kettle lake. A leftover chunk of ice can have sediment surround it, and when the ice melts a hole is left over and that can be filled with water. These tend to be almost circular. https://www.youtube.com/watch? v=aIAd7erc2YU GlacialFluvial – moving water of the glacier Basal till – Base underneath the glacier. o Stratified drift is the term for sediments laid down by glacial meltwater. This happens at the ‘toe’ of the glacier. Very poorly sorted rocks. How much of earth’s land was covered with ice during the last ice age? About 30%. We are still in the ice age but we don’t have as much ice as back then.
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