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Understanding Weathering: Mechanical and Chemical Processes Affecting Rocks

Weathering is the process that breaks down rocks through exposure to surface conditions on Earth. It can be classified into mechanical (physical) weathering, which involves the disintegration of rocks into smaller fragments without altering their composition, and chemical weathering, which transforms minerals into different substances. Factors such as freeze-thaw cycles, abrasion, plant growth, and chemical reactions involving water and acids contribute to these processes. Understanding weathering is essential for studying landscapes, soil formation, and geological features.

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Understanding Weathering: Mechanical and Chemical Processes Affecting Rocks

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  1. Ch.12, Sec.1 - Weathering • Weathering - weathering is the breakup of rock due to exposure to processes that occur at Earth’s surface - mechanical weathering, or disintegration, takes place when rock is split or broken into smaller pieces of the same material without changing its identity - mechanical weathering is also called “physical weathering”

  2. Ch.12, Sec.1 - Weathering - chemical weathering, or decomposition, takes place when rock’s minerals are changed into different substances - mechanical & chemical weathering almost always act together

  3. Ch.12, Sec.1 - Weathering • Mechanical Weathering - water occupies 10% more space when it freezes, so more pressure is applied to surrounding walls - when water freezes in the cracks of rocks, it wedges the rock apart - frost wedging only occurs in areas where the temperature fluctuates above and below freezing point

  4. Ch.12, Sec.1 - Weathering - frost wedging occurs on mountaintops, in porous rocks, and also on our highways (potholes)

  5. Ch.12, Sec.1 - Weathering

  6. Ch.12, Sec.1 - Weathering

  7. Ch.12, Sec.1 - Weathering - mechanical weathering can also create abrasion, which is the grinding of rocks, pebbles, and boulders against one another - sand is a product of abrasion as pebbles get ground down into particles of sand as they are carried by rivers, streams, and ocean waves

  8. Ch.12, Sec.1 - Weathering - the growth of plants and the activities of animals also contribute to the mechanical weathering of rock - the roots of trees, shrubs, & mosses help to split rocks as they grow - ants, earthworms, rabbits, woodchucks, and other animals dig holes in soil allowing air & water to weather bedrock

  9. Ch.12, Sec.1 - Weathering - the upward expansion of rocks that are formed deep underground may result in a mechanical weathering process called exfoliation - reduced pressure on the surface of granite can create curved joints that are parallel to the surface – eventually they get exposed and break down over time Half Dome (exfoliation dome) at Yosemite National Park, California Yosemite National Park

  10. Ch.12, Sec.1 - Weathering

  11. Ch.12, Sec.1 - Weathering Stonewall Jackson, Jefferson Davis, & Robert E. Lee (Civil War Heroes for the Confederacy) Stone Mountain (exfoliation dome) in Georgia

  12. Ch.12, Sec.1 - Weathering Sugarloaf Mountain (exfoliation dome) in Brazil Sugarloaf Mountain in Rio de Janeiro, Brazil

  13. Ch.12, Sec.1 - Weathering • Chemical Weathering - chemical weathering occurs almost everywhere because water or water vapor is found almost everywhere - all chemical weathering involves at least one of the following: water, water vapor, oxygen, and acids - the chemical weathering by reaction of water with other substances is called hydrolysis

  14. Ch.12, Sec.1 - Weathering - water’s chemical effect on minerals is increased by the presence of acids that are dissolved in the water: carbonic acid & sulfuric acid - carbonic acid is created from excess carbon dioxide (vehicles) & sulfuric acid is created from sulfur burn-off in manufacturing plants - these acids mix with rain in the atmosphere and cause acid rain, which further increases chemical weathering

  15. Ch.12, Sec.1 - Weathering - carbonic acid has a dissolving effect on limestone and can create caverns once it mixes with water and seeps into the ground Perry’s Cave at Put-in-Bay, Ohio Mammoth Cave in Kentucky

  16. Ch.12, Sec.1 - Weathering - the brown or red color of some exposed rocks may be the result of a process called oxidation - oxidation is a chemical reaction of oxygen with other substances, especially iron-rich rocks Cathedral Rocks, Sedona, Arizona Zion National Park, Springdale, Utah

  17. Ch.12, Sec.1 - Weathering • Rates of Weathering - under average conditions, weathering is a slow process - the three factors that increase weathering are: surface area, composition of the rock, and climate - more surface area exposed to the elements means faster weathering of the rock more surface area = faster mechanical/chemical weathering

  18. Ch.12, Sec.1 - Weathering - granite, quartz, and marble are not easily weathered, whereas gypsum, feldspar, and limestone weather at faster rates - warm, wet climates are conducive to faster mechanical/chemical weathering because of the excessive moisture, changes in pressure, and changes in temperature - cold, dry climates are conducive to faster mechanical weathering because of the excessive winds McMurdo Dry Valley Desert, Antarctica

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