Section 1 Weathering

Objectives

• Explain how mechanical weathering and chemical weathering shape and reshape Earth’s surface.

• Describe how ice, water, wind, gravity, plants, and animals cause mechanical weathering.

• Describe how plants and animals can shape Earth’s landscape.

• Describe how water, acids, plants, and air cause chemical weathering of rocks.

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Section 1 Weathering

Mechanical Weathering

• Small rocks often are pieces of larger rocks. The process by which rock materials are broken down by the action of physical and chemical processes is known as weathering.

• Mechanical weathering is the breakdown of rock into smaller pieces by physical means, such as ice, wind, water, gravity, plants, or animals.

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Section 1 Weathering

Mechanical Weathering, continued

• Ice The alternate freezing and thawing of soil and rock, called frost action, is a form of mechanical weathering. One type of frost action is ice wedging.

• Ice wedging starts when water seeps into cracks during warm weather. When temperatures drop, the water freezes and expands. The ice then pushes against the sides of the crack, causing the crack to widen.

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Section 1 Weathering

Ice Wedging

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Section 1 Weathering

Mechanical Weathering, continued

• Abrasion is the grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles.

• Wind, Water, and Gravity are three forms of abrasion. Wind blows sand and silt against rock. Rocks scrape against each other as they roll along the bottom of swiftly flowing rivers. Abrasion also occurs when rocks fall on one another.

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Section 1 Weathering

Mechanical Weathering, continued

• Plants often send roots into cracks in rocks. As the plant grows, the expanding root widens the crack and can eventually split the rock apart.

• Animals cause weathering by burrowing in the soil and loosening rocks to be exposed to weathering.

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Section 1 Weathering

Chemical Weathering

• The process by which rocks break down through chemical reactions is called chemical weathering.

• Common agents of chemical weathering are water, weak acids, and air.

• Water can break down even hard rock, such as granite. But it may take thousands of years.

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Section 1 WeatheringChapter 7

Section 1 Weathering

Chemical Weathering, continued

• Acid precipitation is rain, sleet, or snow with a high concentration of acids. High levels of acidity can cause rapid weathering of rock.

• Sulfuric and nitric acids from natural sources, such as volcanoes, can make precipitation acidic. Also, acid precipitation can be caused by air pollution from the burning of fossil fuels.

• If the acidity is too high, acid precipitation can be harmful to plants and animals.

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Section 1 Weathering

Chemical Weathering, continued

• Acids in Groundwater In certain places ground-water contains weak acids, such as carbonic or sulfuric acid. These acids react with the rocks in the ground.

• Acids in Living Things Living organisms such as lichens, which consist of fungi and algae, produce acids that can slowly break down rock.

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Section 1 Weathering

Chemical Weathering, continued

• Air The oxygen in air reacts with metals causing rust in a process called oxidation.

• Oxidation is a chemical reaction that occurs when an element, such as iron, combines with oxygen to form an oxide. Water speeds up the process.

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Section 2 Rates of Weathering

Differential Weathering

• Differential weathering is the process by which softer, less weather-resistant rocks wear away and leave harder, more weather-resistant rocks behind.

• Weathering is a process that takes a long time, however, some rock will weather faster than other rock.

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Section 2 Rates of Weathering

Differential Weathering, continued

• Devils Tower in Wyoming was once a mass of molten rock deep inside an active volcano. When the molten rock cooled and hardened, it was protected from weathering by the outer rock of the volcano.

• The soft outer parts of the volcano have worn away after thousands of years of weathering, leaving just the harder, more resistant rock.

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Section 2 Rates of Weathering

Differential Weathering, continued

• This illustration is an artist’s idea of how the original volcano may have looked. Inside is Devils Tower as it appears today.

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Section 2 Rates of Weathering

The Shape of Rocks

• Weathering takes place on the outer surface of rocks. The more surface area that is exposed to weathering, the faster the rock will be worn down.

• A large rock has a large surface area, but it also has a large volume. Because of its large volume, the large rock will take a long time to wear down.

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Section 2 Rates of Weathering

The Shape of Rocks, continued

• If a larger rock is broken into smaller rocks, then weathering happens more quickly because the smaller rocks have more surface area to volume.

• The next slide shows this concept in detail.

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Section 2 Rates of Weathering

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Section 2 Rates of Weathering

Weathering and Climate

• Temperature and Water The rate of chemical weathering happens faster in warm, humid climates.

• Water also increases the rate of mechanical weathering, through processes like ice wedging.

• Temperature is another major factor. Climatic regions that experience frequent freezes and thaws have a greater rate of mechanical weathering.

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Section 2 Rates of Weathering

Weathering and Elevation

• Rocks at high elevations weather rapidly because they are exposed to large amounts of wind, rain, and ice. Rocks at sea level are affected by wave action also causing rapid weathering.

• Gravity also affects weathering as the steepness of mountain slopes increases the effects of mechanical and chemical weathering.

Chapter 7