Planet Earth Section 4

Section 4: Weathering and Erosion

Preview• Key Ideas • Bellringer • Physical Weathering • Chemical Weathering• Erosion

Planet Earth Section 4

Key Ideas

〉How does physical weathering affect rocks?

〉How are rocks affected by chemical weathering?

〉What is erosion, and what causes it to happen?

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Bellringer

1. The Colorado River flows through the Grand Canyon. What do you think made the Grand Canyon?

2. What are some other environmental factors that change the land?

3. How do you think potholes form in a road?

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Physical Weathering

〉How does physical weathering affect rocks?

〉Physical, or mechanical, weathering breaks rocks into smaller pieces but does not alter their chemical composition.

– Abrasion caused by particles suspended in wind or water is an example of physical weathering.

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Visual Concept: Mechanical Weathering

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Physical Weathering, continued

• Ice can break rocks.– A common kind of mechanical weathering is called

frost wedging.– Water seeps into cracks or joints in rock and then

freezes.– When water freezes it expands, pushing rock apart.– Every time the ice thaws and refreezes, it wedges

farther into the rock.

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Physical Weathering, continued

• Plants can also break rocks.– The roots of plants can also act as wedges as the

roots grow into cracks in the rocks.– As the plant grows, the roots exert constant pressure

on the rock, eventually causing pieces to break off.

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Chemical Weathering

〉How are rocks affected by chemical weathering?

〉Chemical weathering can cause rocks to weaken, decompose, or dissolve, and it can affect the chemical composition of rock.

– Chemical weathering occurs when a rock is broken down due to chemical reactions with the environment.

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Visual Concept: Chemical Weathering

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Chemical Weathering, continued

• Carbon dioxide can cause chemical weathering.– Carbon dioxide can react with water in the air to form

carbonic acid. – Minerals dissolved by carbonic acid may be washed

away, leaving underground pockets, or caves.

• Water plays a key role in chemical weathering.– Some minerals react with water, which changes their

physical properties.– Some minerals dissolve in water and are carried to

new locations.

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Chemical Weathering, continued

• Acid precipitation can slowly dissolve minerals.– Sulfur dioxide and nitrogen oxides enter the air as a

result of burning fossil fuels.– These chemicals can react with water in the air,

forming sulfuric acid, nitric acid.– When this happens, the precipitation that results is

acidic.

• acid precipitation: precipitation, such as rain, sleet, or snow, that contains a high concentration of acids, often because of pollution in the atmosphere

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Acid Precipitation

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Erosion

〉What is erosion, and what causes it to happen?

〉Erosion is the removal and transportation of weathered and nonweathered materials by gravity, running water, wind, waves, ice, and underground water.

• erosion: a process in which the materials of the Earth’s surface are loosened, dissolved, or worn away and transported from one place to another by a natural agent, such as wind, water, ice, or gravity

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Erosion, continued

• Water erosion shapes Earth’s surface.– Water is the most effective physical weathering agent.

• Deposition occurs if a stream has too much sediment.– Rivers carry sediment to the ocean, and create

canyons and riverbeds.– The faster the water flows, the larger the sediment it

can carry.

• deposition: the process in which material such as sediment is laid down, or deposited as a result of erosion

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Erosion, continued

• Glaciers erode mountains.– Large masses of ice, known as glaciers, can exert

tremendous forces on rocks.– Glaciers can carve U-shaped valleys in mountains.– Moving glaciers grind rocks below them into fine

powder.– Glacial meltwater streams carry the sediment away

from the glacier.

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Erosion, continued

• Wind can also shape the landscape.– Fast moving wind can carry fine sediment.– Sediment carried by wind can smooth Earth’s surface

and weather the landscape by abrasion.– Wind blows weathered particles away from their

source.

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Erosion, continued

• Gravity moves weathered materials downhill.– The movement of rock fragments down a slope in

large quantities is known as mass movement.– Landslides and mudflows are common types of mass

movement.

• Humans can also cause erosion.– Humans often create conditions that speed up

erosion.– Deforestation is an example of a human created

condition that speeds up the erosion process.

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Visual Concept: Erosion