Explain how glaciers cut into rocks and identify landforms caused by glacial erosion.

Erosion by glaciers is less geographically widespread than erosion by streams. Where they occur, however, glaciers exert a powerful erosive force and are capable of grinding down and wearing away even the hardest of bedrock. Even in the tropics, as mountain ranges are uplifted through geologic time, they enter colder reaches of the troposphere where glaciers can form. As the glaciers begin flowing, they grind away at the mountains—the higher mountains are lifted, the more they are eroded by glaciers.

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Many of Earth’s most spectacular landforms were carved during the last glacial maximum (see Section 6.2). With natural climate warming, those glaciers have melted away, but they have left behind their mark of erosion.

Flowing glaciers cut into bedrock through plucking and abrasion. Plucking is the process by which a glacier pulls up and breaks off pieces of bedrock as it moves downslope. Abrasion, as we saw in Section 16.2, is the process by which movement of one material wears away another material.

When meltwater accumulates at the base of a glacier, then refreezes around protrusions in the bedrock, it allows the glacier to pull loose, or pluck, fragments of bedrock. The ice grabs the bedrock and pulls it up, much as you would pull a staple out of paper with a staple remover. Fragments of bedrock ranging from siltsized particles to massive boulders become embedded in the base of the glacier as a result of glacial plucking.

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As a glacier flows downslope, the rock fragments that have been plucked and embedded in its base grind against the bedrock much like sandpaper sanding wood. Grooves gouged into the surface of bedrock by glacial abrasion are called glacial striations. Often, a smoothed surface called glacial polish forms on the bedrock over which the glacier flows (Figure 17.17A). Glacial abrasion pulverizes rock into a fine powder called glacial flour. When this powder is suspended in water, it can color the water bright turquoise or cyan (Figure 17.17B).

Most glaciers carry rock debris that has fallen on them through mass movement of overlying slopes as well as rock fragments they have picked up through plucking. As Figure 17.18 shows, rocks on glaciers and rocks embedded within them are transported downslope by the moving ice. Flowing glaciers are like conveyor belts that move ice and rock debris downslope. Rock material can be transported all the way to the toe of the glacier, where it is deposited in piles of unsorted sediments called moraines (see Section 17.4).

In summer, a glacier produces streams of meltwater that flow over the surface of the glacier (called supraglacial streams), inside the glacier in tunnels, and beneath the glacier between its base and bedrock (called subglacial streams) (Figure 17.19). Subglacial streams transport glacial sediments downslope. Much like streams that form deltas, subglacial streams exiting a glacier (called outlet streams) form a flat outwash plain as the sediments they carry accumulate (see Figure 17.20). Because of their heavy sediment loads, streams on outwash plains are typically braided (see Section 16.3). Outwash plains are examples of glaciofluvial landforms, meaning that both flowing ice and flowing water combine to create them.

Proglacial lakes (meaning lakes “in front of” the glacier) form at the toe of a glacier where a depression has been excavated by the ice or where sediments dam an outlet stream (Figure 17.20). Proglacial lakes are becoming more common as valley glaciers and outlet glaciers retreat upslope in response to the atmospheric warming of the last century.

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Subglacial streams excavate tunnels through the ice. The ceilings of these tunnels sometimes become thin enough for light to pass through. As Picture This shows, when this happens, the interior of the glacier becomes illuminated, resembling stained glass.

Glacial geomorphology is the study of landforms created by glaciers. Understanding how glaciers influence a landscape is important for understanding and describing the development of a region’s topography and its climate history. In the remainder of this section, we explore landforms created by glacial erosion, starting with those that form in high-elevation settings and concluding with those found at sea level.

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Cirques and Tarns

Cirques and tarns are often found at the highest elevations in mountain ranges. A cirque is a bowl-shaped depression with steep walls, called headwalls, formed by a cirque glacier. A tarn is a mountain lake that forms within or just below a cirque. As a cirque glacier flows, it transports rockfall material downslope, rather than letting it accumulate in a talus cone and fill the cirque (Figure 17.21).

Arêtes, Cols, and Horns

Where glaciers erode opposite sides of a mountain, a sharp, steep-sided ridge, called an arête, can form. A low area or pass over a ridge formed by two cirque glaciers positioned on opposite sides of the ridge is called a col. Erosion by cirque glaciers can also form a pyramid-shaped mountain peak called a horn. The cirque glaciers do not reach the top of the horn; instead, the glaciers cut into and steepen the headwalls. These erosional features are shown in Figure 17.22.

Glacial Valleys and Paternoster Lakes

As we learned in Section 16.2, a stream’s energy is focused on its narrow stream channel. As a result, streams cut V-shaped valleys. Glaciers, on the other hand, carve broad U-shaped glacial valleys (or glacial troughs), which often have steep or vertical valley walls. Because the base of a glacier is mostly flat, plucking and abrasion carve a flat valley. Where the sides of the valley are undercut by the glacier, rockfall often causes steep valley walls to develop.

Like trunk streams and tributary streams (see Section 16.1), trunk glaciers have a larger ice volume than tributary glaciers. As a result, they have more erosive power and cut deeper into the bedrock. The surface heights of trunk and tributary glaciers, however, are equal. After the glaciers have melted, a hanging valley is left where a tributary glacial valley feeds into a larger glacial valley with a deeper valley floor, as shown in Figure 17.23.

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Relatively resistant portions of bedrock and ridges of debris called recessional moraines (discussed further in Section 17.4) sometimes form obstructions to stream flow, called glacial steps, along the floor of a glacial valley. A small lake may form behind each of these steps. Lakes that form in this way are called paternoster lakes (from Latin, meaning “our father,” in reference to their similarity to religious rosary beads) (Figure 17.24).

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Also found on the floors of glacial valleys are protruding outcrops of bedrock called roches moutonnées (derived from the French mouton, for their resemblance to fleecy sheep). A roche moutonnée is an elongated and asymmetrical ridge of glacially carved bedrock (Figure 17.25).

Drowned Glacial Valleys: Fjords

Many high-latitude coastlines, such as those of southern Chile, southern Alaska, Norway, Iceland, and Greenland, look remarkably similar because they are dominated by fjords. Fjords are long U-shaped glacial valleys that have been flooded by the sea. Global sea level was drawn down by some 85 m (280 ft) during the last glacial maximum, when ice sheets covered much of North America and Eurasia (see Section 6.3). Outlet glaciers flowing from these ice sheets, as well as valley glaciers, eroded deep glacial valleys that reached below the present-day sea level. By roughly 10,000 years ago, most of this ice had melted. Sea level rose 85 m, and the glacial valleys were flooded, creating fjords (Figure 17.26).

Climate and Glacial Landforms

Latitude and elevation are important factors that determine whether glaciers will develop in a region and give rise to landforms like those we have explored in this section. Ultimately, climate controls the growth of glaciers in any given region. Like fluvial landforms (described in Chapter 16), glacial landforms develop through time as climate changes (Figure 17.27).

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