Glaciers. Chapter 17

Transcription

1 Glaciers Chapter 17

2 Vocabulary 1. Glacier 2. Snowfield 3. Firn 4. Alpine glacier 5. Continental glacier 6. Basal slip 7. Internal plastic flow 8. Crevasses 9. Glacial grooves 10. Ice shelves 11. Icebergs 12. Cirque 13. Arête 14. Horn 15. Erratic 16. Glacial drift 17. Till 18. Moraine 19. Lateral moraine 20. Medial moraine 21. Ground moraine 22. Terminal moraine 23. Drumlin 24. Outwash plain 25. Kettle 26. Esker 27. Finger lakes 28. Ice age 29. Glacial period 30. Interglacial period 31. Milankovitch theory

3 I.Glaciers: Moving Ice A.Formation of Glaciers 1. Glacier: large mass of moving ice 2. Cycles of partial melting & refreezing change the snow into a grainy ice called firn 3. Pressure on deep layers of snow & firn compresses the ice grains 4. As this material continues to build up, a glacier is formed 5. Glaciers move downslope or outward under their own weight

4 6. Size of a glacier depends on the amount of snowfall received & the amount of ice lost a. Several factors can affect these amounts: i. Differences in average annual temperatures ii. Varying amounts of snowfall b. Glaciers will get bigger when snowfall > ice melt, and smaller when snowfall < ice melt

5 B. Types of Glaciers 1. Alpine glacier: narrow, wedge-shaped mass of ice that forms in a mountainous region a. Confined to a small area by surrounding topography b. Exist in Alaska, Himalayas, Andes, Alps, New Zealand 2. Continental glacier: massive sheet of ice a. May cover millions of square kilometers & be thousands of meters thick b. Not confined by surrounding topography -- free-moving c. Currently only exist in Greenland & Argentina

6 C. Movement of Glaciers 1. Glaciers, like rivers, flow according to gravity -- downward! a. Cannot move as quickly or move around obstacles like water can b. Rates of glacial movement vary; in a year, some glaciers travel a few centimeters, while others can move a kilometer or more 2. There are two processes of glacial movement

7 a. Basal slip: process causing the ice at the base of a glacier to melt and the glacier to slide i. Weight of the ice in a glacier applies pressure that lowers the melting point of ice ii. Causes ice to melt where the glacier touches the ground iii. Water mixes with sediment at the base of the glacier; acts as a lubricant between the ice & ground iv. Allows glacier to slide along the water-sediment mixture layer

8 b. Internal Plastic Flow: process by which glaciers flow slowly as grains of ice deform under pressure and slide over each other i. Rate varies for different parts of a glacier ii. Rate is determined by the slope of the ground & the thickness and temperature of the ice iii. Edges of a glacier move more slowly than the center because of friction with underlying rock

9 D. Features of Glaciers 1. Glaciers flow unevenly beneath the surface, causing regions of tension & compression under brittle surface a. This results in large cracks, crevasses, that form on the surface 2. Some parts of the ice sheets may move out over the ocean & form ice shelves. a. When tides rise and fall, large blocks of ice, icebergs, may break from the ice shelves & drift into the ocean

10 II.Glacial Erosion & Deposition **See Directed Reading Packet**

11 III. Ice Ages A.Glacial & Interglacial Periods 1. Ice age: long period of climatic cooling during which continents are glaciated repeatedly a. There have been several ice ages in Earth s history; most recent began about 4 mya 2. An ice age consists of several glacial & interglacial periods a. In a glacial period, climate is cooler & glaciers advance i. In the most recent glacial period, ⅓ of Earth s surface was glaciated b. In an interglacial period, climate warms & glaciers retreat i. We are currently in an interglacial period of the most recent ice age

12 3. Glaciation in North America a. Canada & mountainous parts of Alaska were covered b. Mountains in western US had numerous small alpine glaciers which combined to form larger glaciers c. Continental ice sheet covered Hudson Bay region of Canada down to the Missouri & Ohio rivers

13 4. Glaciation in Eurasia & the Southern Hemisphere a. Continental ice sheet over Germany, Belgium, the Netherlands, Great Britain, Ireland, Poland, & Russia b. Alpine glaciers in Alps & Himalayas c. Continental ice sheet in Siberia d. Andes mountains & New Zealand covered in mountainous ice fields & alpine glaciers

14 B. Causes of Glaciation 1. Milankovitch theory: over thousands of years, cyclical changes in Earth s orbit & the tilt of Earth s axis occur; this causes changes in solar energy reaching Earth s surface, causing climatic changes 2. Periodic changes include: a. Eccentricity: shape of Earth s orbit; changes from nearly circular to elliptical and back to circular every 100,000 years b. Tilt of Earth s axis varies between 22.2 and 24.5 every 41,000 years c. Precession: circular motion of Earth s axis; causes axis to change its position in a wobble, completes a circle every 25,700 years

15 3. Biological Evidence of Glaciation a. Shells of marine animal fossils i. Amount of dissolved oxygen in shells ii. Temp > 8 C shells coil to right iii.temp < 8 C shells coil to left 4. Other Explanations for Glaciation a. Changes in solar energy caused by varying amounts of energy produced by the sun b. Changes in solar energy caused by volcanic dust blocking sun s rays