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EARTHSC 1G03 (139)

8 - Weathering

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Earth Sciences
Maureen Padden

CHAPTER 8: WEATHERING AND SOIL Processes that alter rocks: weathering, erosion and transportation  Weathering: destructive processes that change the physical and chemical character of the rock at or near the surface  Erosion: physical removal of rock particles by an agent such as running water or glaciers  Transportation: movement of eroded particles by agents such as rivers, waves, glaciers, or wind WEATHERING Alteration of Rocks:  Mechanical weathering: physical disintegration; break rock into smaller pieces o Increases surface area o Increases chemical weathering rate  Chemical weathering: decomposition of rock from exposure to atmospheric agents o Proportional to the surface area Effects:  Spheroidal weathering: rock has been rounded from initial blocky shape b/c weathering acts quicker and more intense on corners than surface; example of “small scale exfoliation”  Differential weathering: varying rates of weathering in an area b/c differential resistance among rocks to weathering o Ex. Of resistant type: sandstone o Ex. Of less resistant: shale  When weathered, shale forms gentle slopes of eroded rock debris called talus MECHANICAL WEATHERING Pressure release: reduction of pressure on top of a body of rock; causes rocks at depth to crack as they expand upwards  Sheet joints: fracturing parallel to rock surface or in concentric layers develop parallel as outer part of rock expands more o Exfoliation: Process of spalling off of rock layers from the top o Exfoliation domes: large, rounded landforms developed in massive rock by exfoliation Frost Action:  Mechanical effect of freezing water on rocks o Frost Wedging: water expands as it freezes in joints; this wedges the rock apart  Northern climates or mountainous regions o Frost heaving: lifts rock and soil vertically  Evident in winter with bulging ground surface Salt Crystallization:  Occurs when saline solutions enter cracks/joints and evaporate, leaving salt crystals; when heated, salt crystals exert pressure on rock and decompose it o Arid climates  B/c stronger heating results in stro
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