Unit 1 Lesson 2 Weathering Copyright © Houghton Mifflin Harcourt Publishing Company.

Unit 1 Lesson 2 Weathering

Copyright © Houghton Mifflin Harcourt Publishing Company

Break It Down


What is weathering?

• Weathering is the breakdown of rock material by physical and chemical processes.

• Two kinds of weathering are physical weathering and chemical weathering.


What causes physical weathering?

• Physical weathering is the process by which rock is broken down into smaller pieces by physical changes.

• The composition of the material does not change during physical weathering.

• Agents of physical weathering include temperature changes, pressure changes, plant and animal actions, water, wind, and gravity.




• Changes in temperature can cause a rock to break apart by weakening the structure of the rock.

• Heat causes the rock to expand; cold causes it to contract.

• Ice wedging, or frost wedging, causes cracks in rocks to widen with repeated cycles of freezing and thawing.




• How can a small crack in a rock eventually split the rock into two or more pieces?



Ice Wedging

What causes physical weathering?• Rocks that formed under

pressure deep within Earth can be exposed to the surface.

• As material is removed above the rock, the pressure decreases and the rock expands.

• Exfoliation is the process by which the outer layers of rock slowly peel away due to pressure changes.



What causes physical weathering?• Animals such as prairie dogs,

can cause physical weathering by digging burrows.

• New rocks, soils, and other materials become exposed at the surface as a result of animal actions.

• Materials exposed at the surface are more likely to undergo weathering than those below.




• Abrasion is the breaking down and wearing away of rock material by the mechanical action of other rocks.

• Three agents of physical weathering that can cause abrasion are moving water, wind, and gravity.

• Rocks suspended in a glacier can also cause abrasion of other rocks on Earth’s surface.




• Roots of plants start out as tiny strands that may grow in small cracks in rocks.

• As the roots grow, they put more pressure on the rock, causing the rock to expand and eventually break apart.



Reaction


What causes chemical weathering?

• Chemical weathering is the breakdown of rocks by chemical reactions.

• Chemical weathering changes both the composition and appearance of rocks.

• Agents of chemical weathering include oxygen in the air and acids.



• Oxidation is the process by which chemicals in rock combine with oxygen in the air or in water.

• Rock surfaces, especially those containing iron, sometimes change color, indicating that a chemical reaction may have occurred due to rust production.




• Acids can cause chemical weathering by breaking down minerals faster than water alone.

• Acids in the atmosphere are created when chemicals (due to the burning of fossil fuels) combine with water in the air.

• Acid precipitation occurs when strong acids fall to Earth as rain, sleet, or snow.




• Acids in groundwater can cause rock to dissolve.

• A small crack in the rock can result in the formation of extensive cave systems carved out over time.

• Rock material dissolved in groundwater can be carried and deposited in new locations over time.




• Acids produced by living things such as lichens and mosses, can cause chemical weathering.

• Chemical reactions occur as these acids move through tiny spaces in rock material.

• As the acids seep deeper, cracks form. Eventually, the rock can break apart.



Unit 1 Lesson 2 Weathering Copyright © Houghton Mifflin Harcourt Publishing Company.

Documents

agents of physical weathering

chemical weathering

kinds of weathering

agents of chemical weathering

physical changes

breakdown of rock material

cold causes

pressure changes