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Chapter 5.1

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What is Weathering? Weathering can be defined as any chemical or mechanical process by which rocks exposed to weather undergo changes in character or are broken down into smaller pieces.

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Mechanical Weathering Mechanical Weathering is a type of weathering that breaks down rocks into smaller pieces without changing the chemical composition of the rock. For example, Granite can be broken down into smaller pieces, but the smaller pieces are still Granite.

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Agents of Mechanical Weathering Frost Wedging When water freezes in the cracks of rocks causing them to expand and break the rock. The products of frost wedging are large piles of rocks called talus.

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Frost Wedging Talus produced by Frost Wedging

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Agents of Mechanical Weathering Unloading and Exfoliation When large sections of rock buried underneath the ground are exposed at the surface, enormous amounts of pressure are released. This release of pressure is referred to as unloading. After unloading, the rocks begin to flake off in sections like an onion. This is referred to as exfoliation.

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Unloading and Exfoliation Giant Slabs of rock flake off As a result of Unloading and Exfoliation

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Agents of Mechanical Weathering Biologic Activity Weathering that occurs as a result of the actions of living organisms like plants and animals. Plants often cause mechanical weathering when their roots penetrate down into the cracks of rocks. When the roots grow they expand. Animals burrowing into the ground can also create cracks in the rocks.

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Biologic Activity Roots penetrating the cracks in rocks Gophers and Earthworms Burrowing Also contribute to Mechanical Weathering

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Chemical Weathering When rocks that weather change their composition into a new type of rock, this is referred to as chemical weathering. For example, if limestone (calcite) dissolves as a result of acid rain, then it becomes carbon dioxide gas and various ions.

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Chemical Weathering The three main types of chemical weathering. Carbonation occurs when carbonic acid (H 2 CO 3 ) dissolves minerals in rocks. Hydrolysis occurs when hydrogen in water dissolves minerals in rocks. Oxidation occurs when oxygen reacts with minerals in rocks.

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Chemical Weathering Examples of each type include. Carbonation occurs when acid rain (carbonic acid) dissolves limestone bedrock to produce caves. Hydrolysis transforms feldspar (in granite), to produce clay sediments. Oxidation transforms rocks with iron (like basalt and gabbro) into hematite (rust).

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Chemical Weathering Oxidation of Mafic/Basaltic Rocks To produce the mineral Hematite Weathering of Granite to Produce clay sediments Carbonic Acid dissolves limestone To produce caverns

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Rate of Weathering The most important factors that affect the rate (time) of weathering are Surface Area - More cracks means more surface area, more surface area means more weathering. Composition Certain minerals are more resistant to weathering than others. Quartz for example is more resistant to weathering than calcite.

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Rate of Weathering How Weathering Affects the Surface Area and the Rate of Weathering

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Rate of Weathering Climate Hot, humid climates have a fast rate of weathering. Cold, dry climates have a slow rate of weathering. Vegetation Places with more vegetation tend to have a higher rate of weathering because certain plants help produce acids that help speed up the rate of weathering.

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Rate of Weathering Varies depending upon the Climate