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Forces in the Earth’s CrustEarth Comm: Understanding Your
Environment, pages 43-47Earth and Space Science
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I. Forces in the Earth’s Crust
• Natural forces fold, tilt, and move rock layers These actions are called deformation Can cause layers of rock to be folded, break, and move
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There are three types of forces that can deform rocks.
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1. Tensional forces
– Forces which stretch / pull parts of the rock in opposite directions
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2. Compressional forces
– Forces that squeeze together parts of the rock
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3. Shear forces
– Forces that try to cause materials to slide past each other
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• They can act alone or as a combination * If any of these forces are greater than the material’s strength, the material will deform(bend, break, stretch, compress, twist, etc.)
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A. Folding and Faulting
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Temperature plays a key role as to what type of deformation occurs
• Low temperature rocks are brittle and break easily - deform by fracturing / faulting - generally the rocks in the upper crust are cooler in temperature
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Temperature plays a key role as to what type of deformation occurs
• High temperature rocks tend to deform by changing in shape continuously instead of breaking
• Generally the rocks in the lower crust are warmer in temperature
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• Folding is an example of how rocks can change their shape continuously without breaking
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Speed of deformation plays a key role
if force builds up slowly = folding is more likely if force builds up quickly = faulting is more likely
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Fault
• A crack in the lithosphere which can allow rock bodies to move past each other
can be caused by shear forces, compressional forces, and/or tensional forces or a combination
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Folding
• A bend in a layer of rock, usually due to deformation
- usually caused by compressional forces- very common in layered rocks such as sedimentary
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* These processes occur over long periods of time and cannot be directly observed in nature