Chapter 7 Section 3 Glaciers


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Chapter 7 Section 3 Glaciers

  1. 1. Section 3: Glaciers<br />Chapter 7: Erosion Forces<br />
  2. 2. I. How Glaciers Form and Move<br />When snow does not melt it piles up. <br />As it accumulates slowly, the increasing weight of the snow becomes great enough to compress the lower layers into ice. <br />Eventually there can be enough pressure on the ice so that it becomes plastic like. <br />The mass slowly begins to flow in a thick, plastic like lower layer, and ice slowly moves away from its source. <br />A large mass of ice and snow moving on land under its own weight is a GLACIER. <br />
  3. 3. II. Ice Eroding Rock<br />Glaciers are agents of erosion. <br />As glaciers pass over land, they erode it, changing features on the surface. <br />Glaciers then carry eroded material along and deposit it somewhere else. <br />Glacial erosion and deposition change large areas of Earth’s Surface.<br />
  4. 4. II. Ice Eroding Rock<br />Plucking<br />When glacial ice melts, water flows into cracks in rocks. <br />Water refreezes in the cracks, expands, and fractures the rock. <br />Pieces of rock are lifted out by the ice as shown. <br />Results in boulders, gravel, and sand being added to the sides and bottom of the glacier. <br />
  5. 5. II. Ice Eroding Rock<br />Transporting and Scouring<br />As a glacier moves forward over land huge volumes of sediment and rock can be transported. <br />Plucked rock fragments and sand at its base scour and scrape the soil and bedrock like sandpaper against wood, eroding the ground below even more. <br />When bedrock is gouged deeply by rock fragments being dragged along, marks such as those in figure 10 (next slide) are left behind. <br />These marks, called grooves, are deep, long, parallel scars on rocks. <br />Shallower marks are called striations. <br />Grooves and striations indicate the direction in which the glacier moved. <br />
  6. 6. II. Ice Eroding Rock<br />
  7. 7. III. Ice Depositing Sediment<br />When glaciers begin to melt they are unable to carry much sediment. <br />Sediment drops, or is deposited, on the land. <br />When glaciers melt and shrink back, it is said to retreat. <br />As it retreats, a jumble of boulders, sand, clay, and silt is left behind. <br />This mixture of different sized sediments is called TILL. <br />Till deposits can cover huge areas of land. <br />Thousands of years ago, huge ice sheets in the Northern U.S. left enough behind to fill valley completely and make these areas appear flat. <br />Some examples are: wheat farms running NW from Iowa to Montana, some farmland in parts of Ohio, Indiana, and Illinois, and the rocky pastures of New England. <br />
  8. 8. III. Ice Depositing Sediment<br />Moraine Deposits<br />Till also is deposited at the end of a glacier when it is not moving forward. <br />This type of deposit does not cover a wide area. <br />Rocks and soil are moved to the end of the glacier, much like items on the belts at Way-Mart. <br />Because of this, a big ridge of material piles up that looks like it has been pushed along by a bulldozer. <br />Such a ridge is called a MORAINE. <br />Moraines are also deposited along the sides of a glacier. <br />
  9. 9. III. Ice Depositing Sediment<br />Outwash Deposits<br />When glacial ice starts to melt, the melt-water can deposit sediment that is different from till. <br />Material deposited by the melt-water from a glacier is called outwash. <br />Melt-water carries sediments and deposits them in layers. <br />Heavier sediments drop first, so bigger pieces of rock are deposited closer to the glacier. <br />Outwash from a glacier can also form into a fan shaped deposit when the stream of melt-water deposits sand and gravel in front of the glacier. <br />
  10. 10. III. Ice Depositing Sediment<br />Eskers<br />When a glacier melts, a winding ridge of sand and gravel, called and esker, is left behind. <br />The deposit forms in a melting glacier when melt-water forms a river within the ice. <br />
  11. 11. IV. Continental Glaciers<br />Continental Glaciers cover ten percent of Earth, mostly near the poles in Antarctica and Greenland. <br />Continental glaciers are huge masses of ice and snow. <br />Thicker than some mountain ranges.<br />Continental Glaciers have pieces that break off as icebergs. <br />
  12. 12. IV. Continental Glaciers<br />Climate Changes<br />In the past Continental Glaciers covered as much as 28 percent of Earth. <br />Periods of widespread glaciation are known as Ice Ages. <br />During the recent Ice Age the average temperature was 5 degrees less than it is today. (Celsius)<br />The last major advance of ice reached its maximum 18,000 years ago. <br />Currently ice sheets began to recede, or move back, by melting. <br />
  13. 13. V. Valley Glaciers<br />Valley Glaciers<br />Occur even in today’s warmer global climate. <br />In the high mountains where the average temperature is low enough to prevent snow from melting during the summer, valley glaciers grow and creep along. <br />
  14. 14. V. Valley Glaciers<br />Evidence of Valley Glaciers<br />If you visit the mountains, you can tell whether valley glaciers ever existed there. <br />You might look for striations, then search for evidence of plucking. <br />Glacial plucking often occurs near the top of a mountain where a glacier is mainly in contact with the solid rock. <br />Valley glaciers erode bowl-shaped basins called CIRQUES, into the sides of mountains. <br />
  15. 15. V. Valley Glaciers<br />Evidence of Valley Glaciers<br />Arete (ah RAYT)<br />If two valley glaciers form side by side a long ridge called an arete forms between them. <br />If Valley Glaciers erode a mountain from several directions, a sharpened peak called a HORN might form. <br />
  16. 16. V. Valley Glaciers<br />Evidence of Valley Glaciers<br />Valleys that have been eroded by glaciers have a different shape from those eroded by streams. <br />Stream-eroded valleys are normally V-Shaped. <br />Glacially eroded valleys are U-Shaped because a glacier plucks and scrapes soil and rock from the sides as well as the bottom. <br />
  17. 17. VI. Importance of Glaciers<br />Glaciers have had a profound effect on Earth’s surface. <br />They have eroded mountaintops and transformed valleys. <br />Vast areas of the continents have sediments that were deposited by great ice sheets. <br />Glaciers leave behind sediments that are economically important. <br />The sand and gravel deposits from glacial outwash and eskers are important resources for construction of roads and buildings. <br />