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Title: 7.1 Weathering (Mechanical Weathering.) Page #:48 Date: 1/2/13 Students will be able to identify the factors that affect mechanical weathering. Weathering: The process in which materials on or near Earth’s surface break down and change. Mechanical Weathering (Physical Weathering): When rocks and minerals break into smaller pieces. Only changes the size and shape of the rock. Effect of Temperature: • Water expands when it freezes. • Frost Wedging: When water in the cracks of a rock freezes and expands causing the rock to break. Example: Potholes Result from freeze-thaw cycles of moderate climates. Frost Wedging Frost Wedging: Potholes Note: They all collect water! Effect of Pressure: 1. Added Pressure: Causes Wedging and Cracking. Example: Tree roots growing between cracks of rocks, expand and break the rock. Added pressure Of tree roots Breaks rocks Apart. Effect of Pressure: 2. Removed Pressure: Large rocks form under Earth’s surface. When overlying rock is removed by erosion (or mining) pressure on bedrock is reduced. Removal of pressure allows bedrock surface to expand. Expansion of bedrock causes cracks to form parallel to the surface of the bedrock. Cracks are called joints. Batholith: Underground magma Chamber. When a batholith cools and is Exposed to the Earth’s surface It forms a large round rock like Half Dome at Yosemite National Park. 2. Removed Pressure: Rock layers peel away from surface of rock like the skin of an onion. Exfoliation: The process by which outer rock layers are stripped away.