5.1 Weathering

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5.1 WEATHERING
2 TYPES OF WEATHERING
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Mechanical Weathering
• Occurs when physical forces break rock into smaller & smaller pieces without
changing the rock’s mineral composition
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Chemical Weathering
• The transformation of rock into 1 or more new compounds
MECHANICAL WEATHERING
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Breaking a rock into smaller pieces increases the total surface area
• more SA is exposed to chemical weathering
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Unloading
• Exfoliation (layers of onion)
• Causes the outer layers of the rock to expand more than the rock below
• Igneous rock may be exposed by uplift & erosion of overlying rocks. Pressure
exerted on the igneous rock is reduced.
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Frost Wedging
• Water freezes and expands
• Frozen water in rocks expand cracks
• Freeze – thaw cycles = rock breaks into pieces
CHEMICAL WEATHERING
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Water
• Most important factor in CW
• Absorbs gases in the atmosphere & ground
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Chemical Weathering of Granite
• Can change the properties of rocks
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Chemical Weathering of Silicate Minerals
• Make up most of Earth’s crust
• Na, Ca, K, Mg they contain dissolve & are carried away by groundwater
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Spheroidal Weathering
• Form of CW
• Corners & edges become more rounded by CW
RATE OF WEATHERING
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Differential Weathering
• Different parts of rock mass often weather at different rates
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Factors that affect the rate of chemical weathering:
• Rock Characteristics
• Climate
• Temperature, Moisture
• High Temps. & high moisture are best for chemical weathering
• Mechanical Weathering
• By breaking rock into small pieces MW accelerates CW by increasing the SA

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