2 Glacial Erosion and Deposition

Transcription

1 CHAPTER 17 SECTION Glaciers 2 Glacial Erosion and Deposition KEY IDEAS As you read this section, keep these questions in mind: What landscape features does glacial erosion produce? What landscape features does glacial deposition produce? How do glacial lakes form? What Landscape Features Does Glacial Erosion Produce? Glaciers formed many of the landforms that exist in the northern United States and Canada. Large lakes, solitary boulders on flat plains, and jagged ridges are just a few examples. Glaciers produced these landforms through the processes of erosion and deposition. Like rivers, glaciers can erode the landscape into new shapes. Also, rivers and glaciers can both pick up and carry rocks and sediment. However, landforms produced by glaciers are very different from those made by rivers. A moving glacier may loosen and pull out rock from the bedrock. The glacier drags that rock across the bedrock, leaving long, parallel grooves in the bedrock. The grooves show the direction of the glacier s movement. READING TOOLBOX Categorize This section has a lot of information about different landscape features. Organize this information by creating a table. As you read, list all of the landscape features in your notebook. Then, classify each feature by writing down its size, shape, and location, as well as how it formed. Grooves Direction of glacier movement 1. Infer The grooves in the bedrock are parallel to the direction of the glacier s movement. However, they don t tell the whole story. What evidence indicates that the glacier in the figure was moving right to left instead of left to right? The grooves in the bedrock point in the direction that the glacier is moving. Holt McDougal Earth Science 251 Glaciers

2 2. Describe How did the V-shaped valley on the left become the U-shaped valley on the right? LANDFORMS Glaciers have shaped many mountain ranges and other landforms through erosion. The process of erosion begins in the upper end of a valley where an alpine glacier forms. As the glacier moves through a river valley, rock from the valley walls breaks off. Losing rock makes the valley walls steeper. Rounded peaks Glacier V-shaped valley The advance of a glacier changes the shape of a river valley. 3. Compare What is the difference between a cirque and an arête? The moving glacier also pulls blocks of rock from the valley floor. These actions produce a bowl-shaped depression called a cirque. Sharp, jagged ridges called arêtes form between cirques. When several arêtes join, they form a sharp, pyramid-like peak called a horn. As the glacier flows through a valley, it also picks up large amounts of rock. These rock fragments become embedded in ice. Remember that large rocks can carve grooves in the bedrock. The rocks in a glacier may also smooth the bedrock, like sandpaper makes wood smooth. Glaciers can also slide over rock projections, making one side of the rock rounded. The rounded side of one of these landforms, called roches moutonnées, faces the direction the glacier came from. The ice pulls rock away from the other side, making that side steep and jagged. Cirques Horns Arêtes Cirques, arêtes, horns, and roches moutonnées are some of the landscape features formed by glacial erosion. Roches moutonneés Holt McDougal Earth Science 252 Glaciers

3 VALLEYS In addition to creating individual landforms, glaciers also change the overall shape of valleys through erosion. Many valleys have a V shape because rivers carved them out of the rock. As a glacier scrapes away a valley s walls and floor, this V shape changes to a U shape. Glacial erosion is the only process that forms U-shaped valleys. Therefore, scientists can look for this shape to determine whether or not a glacier has eroded a valley. Smaller glaciers in nearby valleys may flow into a main alpine glacier. These smaller glaciers cannot cut as deeply into the rock as the main glacier can. As a result, the valleys created by the smaller glaciers are not as deep as the main valley. When the ice melts, the smaller valley is suspended high above the main valley floor. These valleys are called hanging valleys. When a stream flows from a hanging valley, a waterfall forms. 4. Explain Why is the presence of a U-shaped valley evidence that glaciers once existed in an area? Hanging valley 5. Infer When a stream flows out of a hanging valley, it forms a waterfall. Why do you think this happens? U-shaped valley The main glacier changes the valley from a V shape to a U shape. Smaller glaciers can create hanging valleys. CONTINENTAL GLACIAL EROSION Continental glaciers erode the landscape differently than alpine glaciers do. As they move, continental glaciers smooth and round exposed rock surfaces. This process produces a smooth, level landscape. Rocks carried at the base of the continental glacier also scratch and groove rock surfaces. These scratches and grooves lie parallel to the direction of the glacier s movement. Holt McDougal Earth Science 253 Glaciers

4 6. Identify Which term refers to glacial drift that is not sorted? What Landscape Features Does Glacial Deposition Produce? Glaciers also change landscapes through the process of deposition. Deposition occurs when a glacier melts. As it melts, the glacier deposits much of the material that it has collected. This material may range in size from fine sediment to large rocks. Large rocks that a glacier carries from a distant source are called erratics. A glacier can carry an erratic over very long distances. As a result, the composition of an erratic is typically different from that of the bedrock. Other landforms can also develop as glaciers melt and deposit sediment. The general term for all sediments that a glacier deposits is glacial drift. Remember that a sample of sorted sediments is made up of particles that are about the same size. Unsorted sediments are made up of particles of a variety of sizes. Glacial drift can be either sorted or unsorted. These two types of glacial drift form different kinds of landforms. Till is unsorted glacial drift that is made up of sediments that come from the base of the glacier. The sediments are left behind when the glacier melts. Glacial drift can also be sorted into layers by streams flowing from the melted ice, also known as meltwater. This type of glacial drift is called stratified drift. Sorted glacial drift and unsorted glacial drift create different kinds of landforms. Types of Materials Deposited by a Melting Glacier Type of Material Description the general term for all sediment that has been carried by a glacier large rock or boulder carried over a long distance by a glacier glacial drift that has been deposited directly by the melting glacier; unsorted sediment 7. Summarize Complete the table to summarize the information from the text. glacial drift that has been deposited by streams flowing from the melted ice of the glacier; sediment that has been sorted into layers by the streams Holt McDougal Earth Science 254 Glaciers

5 TILL DEPOSITS When a glacier deposits till, the resulting landforms are called moraines. A moraine is a ridge of unsorted sediment found on the ground or on the glacier. There are several different types of moraines. A lateral moraine lies along the sides of an alpine glacier. It typically looks like a long ridge bordering the glacier. When two or more alpine glaciers join, their lateral moraines combine and form a medial moraine. Any unsorted material that is left beneath the glacier when the ice melts is called ground moraine. The soil of a ground moraine is typically very rocky. A continental glacier may sometimes push ground moraine into clusters called drumlins. Drumlins are long, low mounds of till. They are generally tear shaped. The longer axes of these drumlins are parallel to the direction the glacier moved. Terminal moraines are small ridges of till that lie at the leading edge of a melting glacier. Terminal moraines have many depressions. These depressions often contain lakes or ponds. 8. Describe How do lateral moraines form a medial moraine? Critical Thinking 9. Apply Concepts How could you determine how the glacier that formed a drumlin was moving? Alpine glacier Medial moraine Terminal moraine Moraines are one group of landforms formed by glacial deposition. 10. Identify On the figure, label the unidentified landform. Holt McDougal Earth Science 255 Glaciers

6 OTHER FEATURES FORMED BY GLACIAL DEPOSITION In addition to moraines, glacial deposition can also form other landforms. When a glacier melts, streams of meltwater flow from its edges, surface, and underside. The melted ice carries glacial drift and deposits it in front of the glacier. This deposit forms a large outwash plain. An outwash plain lies in front of the glacier s terminal moraine. Many of the streams formed by the glacier s melted ice run across the plain. Most outwash plains contain many small depressions called kettles. A kettle forms when a chunk of glacial ice becomes buried in the glacial drift. As the ice melts, a hole forms in the drift. Eventually, the drift collapses into the hole and forms a depression. Kettles often fill with water, forming kettle lakes. When continental glaciers recede, they may leave behind eskers. Eskers are long, winding ridges of gravel and sand. These ridges consist of stratified drift that were deposited by streams of meltwater. Some extend for many kilometers. They may resemble raised, winding roadways. 11. Define What is an esker? Outwash plain Kettle 12. Synthesize Text and Graphics On the figure, locate and label a drumlin and an erratic. Meltwater streams Continental glacier Land that has been glaciated can contain many landforms, such as erratics, drumlins, and kettles. Holt McDougal Earth Science 256 Glaciers

7 How Do Glacial Lakes Form? Glacial lakes can form in several different ways. Lake basins may form where glaciers erode surfaces and leave depressions in the bedrock. Other lakes may form as a result of deposition rather than erosion. Many lakes form in the uneven surfaces of ground moraines. In addition, long, narrow lakes called finger lakes can form where terminal and lateral moraines block existing streams. HISTORY OF THE GREAT LAKES The Great Lakes of North America formed as a result of erosion and deposition. Glacial erosion widened and deepened existing valleys. Moraines then blocked off the ends of these valleys. As the glacier melted, the meltwater was trapped, forming the Great Lakes. 13. Describe How do finger lakes form? SALT LAKES Salt lakes are a special kind of glacial lake. Like other glacial lakes, these lakes formed during the last glacial advance. However, topographic and climate changes resulted in changes to the structure of these lakes. Unlike other glacial lakes, salt lakes no longer have outlet streams that allow water to leave the lake. Water can leave the lake only by evaporation. When the water from the lakes evaporates, salt that was dissolved in the water is left behind. Thus, the water becomes increasingly salty. Salt lakes typically form in dry climates, where evaporation is rapid and there is little precipitation. 14. Explain How do minerals such as salt become concentrated in the Great Salt Lake? Streams and rivers carry dissolved minerals to the Great Salt Lake in Utah, but there are no streams to carry water and minerals out of the lake. Water evaporates from the lake, but minerals are left behind. Holt McDougal Earth Science 257 Glaciers

8 Section 2 Review SECTION VOCABULARY arête a sharp, jagged ridge that forms between cirques cirque a deep and steep bowl-like depression produced by glacial erosion erratic a large rock transported from a distant source by a glacier esker a long, winding ridge of gravel and coarse sand deposited by glacial meltwater streams glacial drift rock material carried and deposited by glaciers horn a sharp, pyramid-like peak that forms because of the erosion of cirques kettle a bowl-shaped depression in a glacial drift deposit moraine a landform that is made from unsorted sediments deposited by a glacier till unsorted rock material that is deposited directly by a melting glacier 1. Describe In your own words, describe how a kettle forms. 2. Compare Rivers and glaciers can both erode the surrounding landscape. Name one similarity between how rivers and glaciers erode the landscape. Then, name one difference. 3. List What are some features you would expect to find in a valley that was eroded by an alpine glacier? 4. Compare How are erratics different from glacial drift? 5. Describe What is one way in which a glacial lake can form? 6. Explain Why are salt lakes more common in dry climates than in wet ones? Holt McDougal Earth Science 258 Glaciers