Текст 5A

External Forces That Change the Earth's Surface

Section Review

Key Ideas

· Mechanical and chemical weathering are forces that change landforms.

· Erosion is another external force that alters the surface of the earth.

Key Terms

weathering, mechanical weathering, frost wedging, chemical weathering, acid rain, erosion, sediment, floodplain, delta, mouth, loess, glacier, moraine

In the soil of the Hawaiian Islands, there is crumbly gray clay that is older than the volcanic rock of the islands themselves. For years, scientists wondered how this had happened. Now they think that the clay comes from a desert in faroff China. Blown across the ocean by the wind, it has been deposited on the islands by centuries of rainstorms. Byproviding a mineral needed for plant growth, the clay enriches Hawaii’s land. This process is still going on. As one conservationist explained:

• So much soil from the Asian mainl and blows over the Pacific Ocean that scientists taking air samples at the Mauna Loa observatory in Hawaii can now tell when spring plowing starts in North China.

Wind is the only one of several external forces that change the earth’s surface, build and destroy landforms, and affect the soil in which plants grow. The actions of all these forces are usually grouped into two categories: weathering and erosion.

Weathering

Changes in the earth’s surface usually take place very slowly, over thousands or millions of years. The process of weathering, for example, breaks down rock at or near the earth’s surface into smaller and smaller pieces. While it may take millions of years, weathering can reduce a mountain to gravel. Depending on the forces involved, weathering is either mechanical or chemical.

Mechanical Weathering

When rock is actually broken or weakened physically, the process is termed mechanical weathering. The most common type of mechanical weathering takes place when water freezes to ice in a crack in the rock. Because water expands by nearly 10 percent when it freezes, the ice widens the crack and eventually splits the rock. This process is known as frost wedging. Frost wedging over time can even cause huge parts of a mountainside to break and fall away. Another kind of mechanical weathering occurs when seeds take root in cracks in rocks. In the same way as sidewalks crack and rise over tree roots, a rock will split as plants or trees grow within a fracture.