Geology - Glaciers and Glaciation - Quick Guides | Geology

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Study GuideGeology–Glaciers and GlaciaƟon1. How Glaciers Develop1.1 From Snow to IceGlaciers begin with snow. Over many years, new layers of snow pile up on top of older snow. Theweight of these layers presses down, squeezing the snowbelow. As this happens, the light, delicatesnowflakes lose their shape, and the air trapped between them is pushed out.Gradually, the snow turns into small, rounded grains calledfirn. Firn is partly compacted snow that isloosely held together by newly formed ice, which acts like a natural cement. As more snow builds upabove, the pressure increases. This causes further compaction and recrystallization, eventuallyforming thick, solidlayers of glacial ice.1.2 Movement, MelƟng, and CalvingAs a glacier grows heavier, it begins to move downslope under the pull of gravity. This movement iscalledglacial flow. While flowing, the glacier scrapes up loose rocks and sediment from the groundbelow and may even break pieces off the rocky surface beneath it.When a glacier reaches its farthest point of advance, ice is lost through a process calledwasting, orablation. Most of this loss happens when ice melts. Some ice also changes directly from solid tovapor and escapes into the atmosphere,especially from the glacier’s warmed surface.In places where glaciers end in lakes or oceans, large chunks of ice may break off from the glacier’sfront. This process is calledcalving, and the broken pieces become icebergs. In extremely coldregions, calving can be the main way glaciers lose ice.

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Study Guide1.3 Glacier Budget: Gaining and Losing IceA glacier’sbudgetdescribes the balance between how much ice it gains and how much it loses.•Apositive budgetmeans the glacier gains more ice from snowfall than it loses from melting.Even if the front of the glacier is melting, the added weight at the top pushes the glacierforward, causing it to advance.•Anegative budgetmeans the glacier loses more ice than it gains. In this case, the glacier isreceding. It may still move downhill, but melting happens faster than the ice can replace itself.•Abalanced budgetmeans the glacier gains and loses ice at the same rate, so it neitheradvances nor retreats.1.4 Zones of a GlacierGlaciers are divided into two main areas based on where ice is gained and lost.•Thezone of accumulationis the upper part of the glacier. This area stays covered in snowall year, and ice builds up here.•Thezone of wastageis the lower part of the glacier, where melting, evaporation, and calvingremove ice.The boundary between these two zones is called thesnow line. The snow line is not fixed—it movesdepending on climate conditions and the glacier’s budget. If the snow line moves down the glacier, itmeans the glacier has a positive budget. If it moves upward, the glacier has a negative budget.1.5 The Terminus and Climate InfluenceTheterminusis the front end of a glacier. When a glacier has a positive budget, its terminus movesforward down the valley. When the budget is negative, the terminus retreats upslope.Temperature is not the only factor that controls glacier movement. A glacier may advance becausewinters are wetter and produce more snowfall, or because summers are cooler and cloudier, whichreduces melting. On the other hand, a glacier may retreat if winters are dry (even if they are cold) or ifsummers are sunnier and warmer, increasing ice loss.Scientists estimate that a worldwide drop in average annual temperature of just4 to 5°Ccould beenough to start another glacial period.

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Study Guide2. Glacier MovementGlaciers may look slow and frozen, but they are constantly moving. The way a glacier moves dependson its size, shape, slope, and temperature. Let’s explore howdifferent types of glaciers move andwhat controls their motion.2.1 Movement of Ice SheetsAnice sheetis a massive body of ice that spreads out in many directions from a high central area.Unlike valley glaciers, ice sheets are not confined to valleys or channels. Instead, they flow outwardlike thick syrup spreading across a flat surface.Ice sheets must expand because snow and ice are constantly accumulating at the top. However, theymove more slowly than alpine (valley) glaciers. This slower movement happens because ice sheetsusually sit on gentler slopes and contain enormous amounts ofice, which makes them harder tomove.Most of the movement in ice sheets occurs throughplastic flow, where ice slowly deforms under itsown weight. At the South Pole, ice sheets are so thick—more than3,000 meters—that theycompletely bury entire mountain ranges beneath them.2.2 Movement of Valley GlaciersValley glaciers flow downhill through mountain valleys, and they can move surprisingly fast—sometimesmore than 15 meters per day. Ice moves faster where the glacier is thicker and the slopeis steeper, usually higher up the valley. Lower down, where slopes are gentler, movement slows.This faster movement upslope is important because it helps replace the ice that melts in thezone ofwastage. Without this steady flow, glaciers would shrink much more quickly.Glaciers intemperate regionstend to move the fastest. That’s because ice at the base can partiallymelt, creating water that acts like a lubricant. Other factors that affect how fast a glacier movesinclude:•The roughness of the rock beneath the glacier (friction)•The amount of meltwater present•The total weight and thickness of the glacier

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Study Guide2.3 Basal Sliding and PlasƟc FlowValley glaciers move in two main ways:basal slidingandplastic flow.2.3.1 Basal SlidingBasal sliding happens when the entire glacier moves as one unit over the rock below it. Theenormous weight of the glacier creates pressure that melts some of the ice at the base. This thin layerof water reduces friction, allowing the glacier to slide downhill more easily.2.3.2 PlasƟcFlowPlastic flow occurs deeper within the glacier. Ice buried more than about 50 meters below the surfacebecomes soft enough to bend and flow slowly, much like thick plastic or putty. In this zone, ice flowscontinuously downhill.Just like water in a river, thecenter and upper partsof a glacier move faster than the ice near thebottom and sides. Friction with the valley walls and the ground slows the ice along the edges. Overall,plastic flow usually moves icefaster than basal sliding.2.4 Crevasses, the Zone of Fracture, and IcefallsAbove a depth of about 50 meters, the ice is not under enough pressure to flow plastically. This upperlayer is rigid and brittle and is called thezone of fracture. Instead of flowing smoothly, this zone iscarried along on top of the softer ice below—almost like riding piggyback.Sometimes, the zone of fracture moves faster than the ice beneath it. When this happens—especiallyon steep slopes—the surface cracks open, forming deep fractures calledcrevasses.Crevasses can also form when a glacier curves. Ice along the outside of the curve moves faster thanice on the inside, causing the surface to stretch and crack.On very steep slopes, many crevasses can form quickly. This creates anicefall, which looks like achaotic pile of broken ice blocks tumbling downhill.

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Study Guide3. Glacial ErosionGlaciers don’t just move slowly across the land—they are powerful agents of erosion. As they flow,they reshape the landscape beneath and around them in several important ways. Let’sbreak it downstep by step.3.1 How Glaciers Wear Down RockOne major way glaciers erode rock is throughbasal sliding. This happens when a glacier slides overthe rock surface beneath it. A thin layer of meltwater at the base helps the ice move, but it also playsa role in breaking rocks apart.Water seeps into cracks in the bedrock. When temperatures drop, the water freezes and expands.This expansion forces the cracks wider and eventually breaks pieces of rock loose. This process isvery similar tofrost wedging.3.2 Rocks Trapped in the IceOnce pieces of bedrock break off, they become frozen into the bottom of the glacier. These rocksdon’t stay smooth for long. As the glacier moves, the rocks scrape and grind against each other andagainst the bedrock below.The heavier and thicker the glacier, the more pressure it puts on these rocks. More pressure meansstronger grinding and more erosion.Some rocks at the base becomefaceted rocks, meaning they are worn down and flattened byconstant rubbing.3.3 Polishing and Scratching the BedrockThe grinding action of rocks inside the glacier works like a giant rock tumbler. Over time, both thebedrock and the rock fragments become smooth and polished.At the same time, rocks stuck in the ice scratch long, deep lines into the bedrock. These scratchesare calledstriations. Striations are important because they show the direction the glacier wasmoving.

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Study Guide3.4 Rock Powder and Milky MeltwaterAll this grinding produces a fine powder made from crushed rock. When the glacier melts—a processcalledablation—this powder mixes with the meltwater. That’s why glacial meltwater often lookscloudy or milky white.3.5 Other Erosion Caused by GlaciersGlaciers also increase erosion in surrounding areas:•Mass wastingincreases as glaciers cut deeper into valleys, making valley sides steeper andless stable.•Frost wedgingcontinues on exposed rock surfaces, leading to rockfalls.•Avalanchescarry rock and soil onto the glacier, adding even more material for erosion.4. Glacial LandformsGlaciers don’t just erode rock—they also create some of the most dramatic landforms on Earth. Manyof the most eye-catching features are formed byalpine glaciers, which flowthrough mountainousregions. Let’s explore how these landforms develop and what makes each one unique.4.1 From V-Shaped Valleys to U-Shaped ValleysBefore glaciers arrive, rivers usually carveV-shaped valleys. When an alpine glacier moves into oneof these valleys, everything changes.Glacial ice is extremely thick and heavy. Instead of following the narrow river channel, the glacierpushes straight through the valley. As it moves, it deepens and widens the valley, carving it into abroadU-shaped valley, also called aglacial trough.As the glacier flows, it also cuts off the ends of ridges that stick into the valley from the sides. Theseshortened ridges are known astruncated spurs.

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Study Guide4.2 Hanging Valleys and WaterfallsSide valleys, calledtributaries, often contain smaller glaciers. These smaller glaciers do not erode asdeeply as the main glacier. When the ice melts, the tributary valleys are left higher than the mainvalley.These elevated valleys are calledhanging valleys, and they are often marked by waterfalls that dropinto the main glacial trough.4.3 Cirques, Bergschrunds, and Mountain PeaksAt the very top of a glacial valley, erosion creates a bowl-shaped depression called acirque. Cirqueshave steep sides and form as ice, frost wedging, and mass wasting break down the rock.Near the back wall of a cirque, the glacier can pull away from the rock, creating a deep crack called abergschrund. This crevasse often fills with fallen rock debris.When several cirques erode a mountain from different sides, they can carve out sharp, pointed peakscalledhorns. Extending downward from a horn is a narrow, steep ridge known as anarête, whichseparates neighboring glacial valleys.4.4 Tarns: Lakes Carved by IceAs a glacier advances, it can gouge out shallow depressions in the bedrock. After the glacier melts,these depressions fill with water, formingrock-basin lakes, also calledtarns.Tarns are most common where the bedrock is soft or heavily cracked. In contrast, smooth and hardbedrock is more difficult for the glacier to break apart, so tarns are less likely to form there.4.5 Landforms Created by Ice SheetsUnlike alpine glaciers,ice sheetscover huge areas and move more slowly. Because of their massiveweight, they tend to grind down the land rather than carve sharp features.Ice sheets smooth and flatten the landscape, polishing exposed bedrock and leaving behindstriationsthat show the direction of ice movement. The rounded landforms and broad, flat bedrocksurfaces seen in places like Ontario, Canada, are excellent examples of how ice sheets shape theEarth.

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