Glaciers

1. Glaciers Chapter 15

2. Topic 15.1What is a Glacier? • Much of Earth’s fresh water is frozen in valley glaciers or continental glaciers. Glaciers form above the snowline from compressed snow called firn. The snow line elevation is high in areas close to the equator and gets lower in areas farther from the equator. Formation of glaciers requires adequate snowfall and low temperatures. The two main types of glaciers are valley glaciers and continental ice sheets.

3. Topic 15.2Glacial Movement and Erosion • Gravity causes glaciers to move. The ice front of a glacier may advance or retreat depending on conditions. If a glacier’s ice front is near the sea, icebergs may break off. As glaciers move, they erode the underlying surface, leaving behind characteristic erosional features. Glaciers move by basal slip and internal plastic flow. Glaciers erode the valleys through which they flow, producing characteristic landforms.

4. Topic 15.3Glacial Deposits • When glaciers retreat, the leave behind deposits. Till is unsorted, unstratified material deposited directly by ice. Outwash is sorted, stratified material deposited by meltwater from the ice. When glaciers melt, they deposit their sediments, creating features that can be readily identifiable. Glaciers may form lake basins by eroding the land or by depositing sediments.

5. Topic 15.4Ice Ages • Earth has experienced a number of periods of widespread glaciation. Evidence for such ice ages comes from erosional and depositional features. Scientists have identified a number of possible causes for glaciation. During an ice age, glacial advances alternate with glacial retreats. The Milankovitch theory is the most commonly accepted theory of the cause of the ice ages.

6. Key Terms calving cirque continental glacier crevasse drumlin erratic valley glacier firn glacial valley glacier ice age kame internal plastic flow kettle moraine outwash striation snowline Milankovitch theory till outwash plain esker

7. Topic 15.1What is a Glacier? • About 75% of Earth’s freshwater is frozen as glaciers. Natural forces compact snow to make enormous masses of moving ice.

8. glacier A large mass of ice and snow that exists year-round and moves under the influence of gravity. • They have carved some of the most spectacular features on the surface of the earth. They form in areas that are always covered by snow. Layer after layer

9. Snow line The lowest elevation that permanent snow reaches in summer. • The elevation of the snowline varies from place to place. At the equator, it is 5,500 m above sea level. Near the poles, it is at sea level. Snowfields are formed by the accumulation of snow and ice above the snowline. They cover most of the land in the polar regions and the tops of some mountains at lower latitudes.

11. firn Partially compacted and refrozen snow which has yet to become a glacier.

12. In the deepest layers of accumulating snow, the pressure of the overlying ice becomes so great that the ice grains flatten and the air between the firn is squeezed out. It loses its white color and becomes a bight steel-blue color. When the mass becomes large enough, it begins to move downslope due to gravity. Where are temperatures are low and snowfall is high, glacial formation is greatest.

13. There are two main types of glaciers: • 1. Valley • 2. Continental

14. Valley glacier A long, slow moving, wedge shaped glacier that moves within valley walls. • They develop in high mountain regions and flow down slope due to the force of gravity. They are also known as alpine glaciers, after the Alps in south-central Europe. They vary in size.

16. continental glacier A large sheet of ice that covers a large part of a continent. • Today, they are found only in Greenland and Antarctica. The Antarctic ice sheet is 1 ½ times the size of the United States. In some places it is more than 4,000 m thick. 90% of Greenland is covered by its ice sheet. It is about the size of Mexico and is 3,000 m thick. Scientists estimate that if melted, the ice would release enough water to raise sea level by 60 m if melted.

23. Homework • Read Textbook pages 318 - 320. • Complete Regents Questions

24. Topic 15.2Glacial Movement and Erosion Glaciers are sometimes called rivers of ice. Continental glaciers may move lass than a centimeter a day, while valley glaciers can move several meters in a day. As they move, they erode and deposit earth materials. However, they move very differently than water does.

25. Examine the seasonal migration of snow cover. • http://www.classzone.com/books/earth_science/terc/content/visualizations/es1501/es1501page01.cfm?chapter_no=visualization

26. They move in two ways: • 1. Basal slip • 2. Internal plastic flow

27. Basal slip Movement at the base of the glacier. • The water from the melted ice acts like a lubricant between the ice and the ground. This allows the ice to slip on a thin layer of water. The ice can push against a rock causing some of the ice to melt, letting it flow around the barrier. In extremely cold places, basal slip does not occur.

29. internal plastic flow Grains of ice deform, or permanently change shape and slip past each other to create forward movement. This movement is usually out from the glacier’s center. • The speed of internal plastic flow is not the same at all parts of the glacier. Speed is determined by the slope and thickness of the ice. It is faster near the surface than at the base. Friction caused by contact with the rock below causes the speed to slow. The center also moves faster than the sides.

30. Because of the pressure from above, the inside of the glacier moves by internal plastic flow. The base moves due to slip, while the surface remains brittle. This tension and compression causes cracks and buckles.

32. crevasses A great fissure or crack in a glacier. • They can be more than 30 m deep

34. Ice sheets move outward from the center in all directions. It can even move out over the ocean forming an ice shelf. The rise and fall of tides causes large blocks to break off. These chunks are called icebergs. Remember that only the top third of an iceberg is above the surface.

36. Most glaciers extend below the snowline. As a glacier moves down-slope, ice and snow melt away due to warmer temperatures causing it to thin and melt. It tends to be the thinnest where it melts and moves the fastest.

38. Ice front The end of a glacier. • As long as the rate of movement and melt are equal, the ice front is stationary. Where the snowline is close to sea level, glaciers reach the sea. As they extend out over the ocean, great blocks of ice break off and fall into the sea. This process is called calving. In Antarctica, the snowline is at sea level. Blocks of ice the size of Connecticut are not uncommon.

40. Glaciers are powerful agents of erosion. As they move, they carve out the terrain over which the move. The glacial processes begin at the upper end of the valley where a valley glacier forms. As it wedges its way through a valley, it breaks off rock making the valley steeper. Particles can be as small as a fine powder or as large as a house. The particles can ride on top of the glacier or can be dragged along the bottom of the glacier.

41. Observe how glaciers erode bedrock surfaces. • http://www.classzone.com/books/earth_science/terc/content/visualizations/es1502/es1502page01.cfm?chapter_no=visualization

42. till Unsorted and unstratified rock material deposited dirtectly by glacial ice. • Landforms made from glacial till are called moraines.

44. moraine A deposit of till left behind when a glacier retreats. • The soil of a ground moraine is very rocky. Long Island is a terminal moraine. They are ridges of unsorted rock material on the ground or on the glacier.

46. Streams flowing from melted ice are called meltwater. Melting occurs in a glacier during the summer months. Streams of meltwater flow from the edges, the surface, and beneath the glacier. Because of the presence of small rock particles, the meltwater often looks like milk.

47. Glaciers weather and erode bedrock when they drag rocks with them, cutting and scraping. Smaller particles smooth and polish.

48. striations Long, parallel scratches left on rocks and bedrock by glacial movement. • They show the direction of movement of the ice, often creating long parallel grooves in the landscape.

50. cirque A semicircular basin found at the head of a glacial valley formed by a valley glacier. • Sharp jagged ridges form between the cirques.