1. External Forces of the Earth

2. Two Types:
Weathering
and
Erosion

3. Two Types – mechanical and chemical
Weathering
Two Types –
mechanical and chemical

4. Mechanical weathering
Rock is actually broken or weakened physically.

5. Mechanical weathering:
Ex. -Frost Wedging
Water freezes to ice in a crack.
Ice widens the crack and splits the rock.

6. Mechanical weathering
Ex. -Seeds
Seeds will take root and grow in the cracks of a rock. As the plant grows, the rock will split.

7. Chemical weathering
Alters the rock’s chemical make-up by changing the minerals that form the rock.

8. Chemical weathering: Example #1
Water mixed with carbon dioxide from the air seeps into the cracks in certain rocks & dissolves the rocks away.
Examples: caves

9. Chemical weathering Example #2
Acid Rain - Chemicals in the polluted water combine with water vapor and then fall back to the earth.
Destroys many things

10. Erosion
3 types

11. Wind

12. Wind Erosion Moves dust, sand, and soil from one place to another.
Wears away the layer of soil that covers the earth’s crust
Plants help protect the land from wind erosion.

13. Wind Erosion
Wind grinds away rock and hills by blasting them with sand and dirt

14. Glaciers

15. Glacial Erosion As glaciers move they pick up rocks and soil
Change landscapes – destroy forests, carve out valleys, alter paths of rivers.
As glaciers melt, rocks and soil are deposited.

16. Water

17. Water Erosion
Rivers carve away at the surrounding land to create deep V-shaped valleys
Fast rivers can wash away soil and carry it down stream
River flooding can create floodplains – good farming!

18. Water Erosion
Ocean waves wear away shorelines As water flows, it cuts into the land.