Geology 101 Spring 2017 Lab 9 page 1 Background information on Glaciers. Glaciers begin to form when snow remains in the same area year-round, where enough snow accumulates to transform into ice. Each year, new layers of snow bury and compress the previous layers. This compression forces the snow to recrystallize, forming grains similar in size and shape to grains of sugar. Gradually the grains grow larger and the air pockets between the grains get smaller, causing the snow to 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 two-thirds 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 more than a hundred years. Crevasses rumple the surface of Crane Glacier in Antarctic – Credit: Ted Scambos, NSIDC Glaciers are dynamic, and several elements contribute to glacier formation and growth. Snow falls in the accumulation area, usually the part of the glacier with the highest elevation, adding to the glacier's mass. As the snow slowly accumulates and turns to ice, and the glacier increases in weight, the weight begins to deform the ice, forcing the glacier to flow downhill. Further down the glacier, usually at a lower altitude, is the ablation area, where most of the melting and evaporation occur. Between these two areas a balance is reached, where snowfall equals snowmelt, and the glacier is in equilibrium. Whenever this equilibrium is disturbed, either by increased snowfall or by excessive melting, the glacier either advances or retreats at more than its normal pace. The Aletsch Glacier in Switzerland has a prominent (medial) moraine on its surface. Credit: Photograph by Harry Fielding Reid, 1902. Several visible features are common to most glaciers. At locations where a glacier flows rapidly, friction creates giant cracks called crevasse, which may make travel across a glacier treacherous. Other common glacial features are moraine, created when the glacier pushes or carries rocky debris as it moves. These long, dark bands of debris are visible on top and along the edges of glaciers. Medial moraines run down the middle of a glacier, lateral moraines along the sides, and Geology 101 Spring 2017 Lab 9 page 2 terminal moraines are found at the terminus, or snout, of a glacier. Sometimes one glacier flows into another, creating combined wider moraines. Often these linear deposits of rocks are left behind, almost intact, after the ice in a glacier has melted away. Studying these rocky debris remnants, and the sediments that were once beneath the glacier, is the subject of glacial geology and geomorphology. (http://nsidc.org/cryosphere/glaciers/questions/components.html) Pre-Lab Question 1: Use the terms below to fill in the blanks for the reading on glaciers. Aardvark Ablation Accumulation Arête Cirque Equilibrium line freeze-thaw cycles Global climate change Hanging valleys Horn Icebergs Mass Moraine Paternoster lakes Retreat Snout Tarns U-shaped valley V-shaped valley Zombies Alpine glaciers are very interesting features on the landscape. They originate high on a mountainside as snow that falls in the winter fails to melt completely in the summer. Over time the snow builds up. The weight of the overlying snow and repeated ___________________________ transform the underlying snow into ice. The ice carves a depression into the mountainside. This semicircular depression is called a(n) ______________. With continued accumulation and mass the ice begins to flow down the mountainside. The down-slope end of the glacier is called the ________________. Out in front of the glacier there may be an accumulation of rocks and dirt built up into a(n) __________________________________. If the glacier ends in a body of water, ________________ may calf off the glacier and go crashing into the water with a huge splash. An established glacier is said to have a zone of __________________ where mass is being added to the glacier and a zone of ___________________ where mass is being lost. The line where there is a balance between gain and loss is called the ______________________________. Nearly every alpine glacier in the world today is in ____________________. That is, it does not come as far down the mountain as it did before and it is losing ____________________________. Alpine glaciers carve their mountains into unique landforms that survive long after the glaciers have melted away. The semicircular _______________ where the snow first accumulated and turned into ice make natural amphitheaters on the mountainsides. Where glaciers carved way at three sides of the same peak a ________________ will form. The famous Matterhorn in the Alps is evidence of just such activity as is Triple Divide Peak in Glacier National Park. _______________________form where two alpine glaciers carve their _______________________ very close together leaving a narrow ridge between the deep cuts. The Garden Wall in Glacier National Park is one such feature. _____________________ are lakes that fill in small areas gouged out by a glacier as it makes its way down slope. Several of these lakes in a row are said to form ________________________________________ since they reminded people of the beads on a Catholic Rosary. (A Rosary is also called an “Our Father” since they are the first words of the Rosary prayer when recited in Latin.) Spectacular ____________________________________, some with dramatic water falls, form where a smaller glacier joins a larger one with a deeper valley. While we may lose our beautiful glaciers to _______________________________________, at least these magnificent landforms will remain.
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