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Geology - Glaciers and Glaciation

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    Study Guide Geology – Glaciers and Glacia Ɵ on 1. How Glaciers Develop 1.1 From Snow to Ice Glaciers begin with snow. Over many years, new layers of snow pile up on top of older snow. The weight of these layers presses down, squeezing the snow below. As this happens, the light, delicate snowflakes lose their shape, and the air trapped between them is pushed out. Gradually, the snow turns into small, rounded grains called firn . Firn is partly compacted snow that is loosely held together by newly formed ice, which acts like a natural cement. As more snow builds up above, the pressure increases. This causes further compaction and recrystallization, eventually forming thick, solid layers of glacial ice. 1.2 Movement, Mel Ɵ ng, and Calving As a glacier grows heavier, it begins to move downslope under the pull of gravity. This movement is called glacial flow . While flowing, the glacier scrapes up loose rocks and sediment from the ground below 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 called wasting , or ablation . Most of this loss happens when ice melts. Some ice also changes directly from solid to vapor 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’s front. This process is called calving , and the broken pieces become icebergs. In extremely cold regions, calving can be the main way glaciers lose ice.

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    Study Guide 1.3 Glacier Budget: Gaining and Losing Ice A glacier’s budget describes the balance between how much ice it gains and how much it loses. • A positive budget means 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 glacier forward, causing it to advance. • A negative budget means the glacier loses more ice than it gains. In this case, the glacier is receding. It may still move downhill, but melting happens faster than the ice can replace itself. • A balanced budget means the glacier gains and loses ice at the same rate, so it neither advances nor retreats. 1.4 Zones of a Glacier Glaciers are divided into two main areas based on where ice is gained and lost. • The zone of accumulation is the upper part of the glacier. This area stays covered in snow all year, and ice builds up here. • The zone of wastage is the lower part of the glacier, where melting, evaporation, and calving remove ice. The boundary between these two zones is called the snow line . The snow line is not fixed — it moves depending on climate conditions and the glacier’s budget. If the snow line moves down the glacier, it means the glacier has a positive budget. If it moves upward, the glacier has a negative budget. 1.5 The Terminus and Climate In fl uence The terminus is the front end of a glacier. When a glacier has a positive budget, its terminus moves forward 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 because winters are wetter and produce more snowfall, or because summers are cooler and cloudier, which reduces melting. On the other hand, a glacier may retreat if wi nters are dry (even if they are cold) or if summers are sunnier and warmer, increasing ice loss. Scientists estimate that a worldwide drop in average annual temperature of just 4 to 5°C could be enough to start another glacial period.

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    Study Guide 2. Glacier Movement Glaciers may look slow and frozen, but they are constantly moving. The way a glacier moves depends on its size, shape, slope, and temperature. Let’s explore how different types of glaciers move and what controls their motion. 2.1 Movement of Ice Sheets An ice sheet is 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 outward like thick syrup spreading across a flat surface. Ice sheets must expand because snow and ice are constantly accumulating at the top. However, they move more slowly than alpine (valley) glaciers. This slower movement happens because ice sheets usually sit on gentler slopes and contain enormous amounts of ice, which makes them harder to move. Most of the movement in ice sheets occurs through plastic flow , where ice slowly deforms under its own weight. At the South Pole, ice sheets are so thick — more than 3,000 meters — that they completely bury entire mountain ranges beneath them. 2.2 Movement of Valley Glaciers Valley glaciers flow downhill through mountain valleys, and they can move surprisingly fast — sometimes more than 15 meters per day . Ice moves faster where the glacier is thicker and the slope is 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 the zone of wastage . Without this steady flow, glaciers would shrink much more quickly. Glaciers in temperate regions tend to move the fastest. That’s because ice at the base can partially melt, creating water that acts like a lubricant. Other factors that affect how fast a glacier moves include: • 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 Guide 2.3 Basal Sliding and Plas Ɵ c Flow Valley glaciers move in two main ways: basal sliding and plastic flow . 2.3.1 Basal Sliding Basal sliding happens when the entire glacier moves as one unit over the rock below it. The enormous weight of the glacier creates pressure that melts some of the ice at the base. This thin layer of water reduces friction, allowing the glacier to slide dow nhill more easily. 2.3.2 Plas Ɵ c Flow Plastic flow occurs deeper within the glacier. Ice buried more than about 50 meters below the surface becomes soft enough to bend and flow slowly, much like thick plastic or putty. In this zone, ice flows continuously downhill. Just like water in a river, the center and upper parts of a glacier move faster than the ice near the bottom and sides. Friction with the valley walls and the ground slows the ice along the edges. Overall, plastic flow usually moves ice faster than basal sliding . 2.4 Crevasses, the Zone of Fracture, and Icefalls Above a depth of about 50 meters, the ice is not under enough pressure to flow plastically. This upper layer is rigid and brittle and is called the zone of fracture . Instead of flowing smoothly, this zone is carried 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 — especially on steep slopes — the surface cracks open, forming deep fractures called crevasses . Crevasses can also form when a glacier curves. Ice along the outside of the curve moves faster than ice on the inside, causing the surface to stretch and crack. On very steep slopes, many crevasses can form quickly. This creates an icefall , which looks like a chaotic pile of broken ice blocks tumbling downhill.

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    Study Guide 3. Glacial Erosion Glaciers 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’s break it down step by step. 3.1 How Glaciers Wear Down Rock One major way glaciers erode rock is through basal sliding . This happens when a glacier slides over the rock surface beneath it. A thin layer of meltwater at the base helps the ice move, but it also plays a 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 is very similar to frost wedging . 3.2 Rocks Trapped in the Ice Once pieces of bedrock break off, they become frozen into the bottom of the glacier. These rocks don’t stay smooth for long. As the glacier moves, the rocks scrape and grind against each other and against the bedrock below. The heavier and thicker the glacier, the more pressure it puts on these rocks. More pressure means stronger grinding and more erosion. Some rocks at the base become faceted rocks , meaning they are worn down and flattened by constant rubbing. 3.3 Polishing and Scratching the Bedrock The grinding action of rocks inside the glacier works like a giant rock tumbler. Over time, both the bedrock 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 scratches are called striations . Striations are important because they show the direction the glacier was moving.

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    Study Guide 3.4 Rock Powder and Milky Meltwater All this grinding produces a fine powder made from crushed rock. When the glacier melts — a process called ablation — this powder mixes with the meltwater. That’s why glacial meltwater often looks cloudy or milky white. 3.5 Other Erosion Caused by Glaciers Glaciers also increase erosion in surrounding areas: • Mass wasting increases as glaciers cut deeper into valleys, making valley sides steeper and less stable. • Frost wedging continues on exposed rock surfaces, leading to rockfalls. • Avalanches carry rock and soil onto the glacier, adding even more material for erosion. 4. Glacial Landforms Glaciers don’t just erode rock — they also create some of the most dramatic landforms on Earth. Many of the most eye - catching features are formed by alpine glaciers , which flow through mountainous regions. Let’s explore how these landforms develop and what makes each one unique. 4.1 From V - Shaped Valleys to U - Shaped Valleys Before glaciers arrive, rivers usually carve V - shaped valleys . When an alpine glacier moves into one of these valleys, everything changes. Glacial ice is extremely thick and heavy. Instead of following the narrow river channel, the glacier pushes straight through the valley. As it moves, it deepens and widens the valley, carving it into a broad U - shaped valley , also called a glacial trough . As the glacier flows, it also cuts off the ends of ridges that stick into the valley from the sides. These shortened ridges are known as truncated spurs .

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    Study Guide 4.2 Hanging Valleys and Waterfalls Side valleys, called tributaries , often contain smaller glaciers. These smaller glaciers do not erode as deeply as the main glacier. When the ice melts, the tributary valleys are left higher than the main valley. These elevated valleys are called hanging valleys , and they are often marked by waterfalls that drop into the main glacial trough. 4.3 Cirques, Bergschrunds, and Mountain Peaks At the very top of a glacial valley, erosion creates a bowl - shaped depression called a cirque . Cirques have 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 a bergschrund . This crevasse often fills with fallen rock debris. When several cirques erode a mountain from different sides, they can carve out sharp, pointed peaks called horns . Extending downward from a horn is a narrow, steep ridge known as an arête , which separates neighboring glacial valleys. 4.4 Tarns: Lakes Carved by Ice As a glacier advances, it can gouge out shallow depressions in the bedrock. After the glacier melts, these depressions fill with water, forming rock - basin lakes , also called tarns . Tarns are most common where the bedrock is soft or heavily cracked. In contrast, smooth and hard bedrock is more difficult for the glacier to break apart, so tarns are less likely to form there. 4.5 Landforms Created by Ice Sheets Unlike alpine glaciers, ice sheets cover huge areas and move more slowly. Because of their massive weight, they tend to grind down the land rather than carve sharp features. Ice sheets smooth and flatten the landscape, polishing exposed bedrock and leaving behind striations that show the direction of ice movement. The rounded landforms and broad, flat bedrock surfaces seen in places like Ontario, Canada, are excellent examples of how ice sheets shape the Earth.

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