The Solid Earth - Crust rocks and soil The rocks and...

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The Solid Earth Time-scales for changes in the crust Short-term: earthquakes, volcanic eruptions, landslides Medium term (10 3 years): melting of glaciers, uplift Long-term (10 7 years): appreciable horizontal movement of the crust. Sea floor spreading. Very long term (10 8 -10 9 years): Production of crustal motions, mountains and continents formed. Below the crust the density is seen to vary with depth, but also with longitude. We see the crust subject to external forces like levelling down/production of sediments, and to internal forces which produce, for example, uplift (the evidence being sea-shells on the tops of mountains). There has been a long history of debate between those who believe in "catastrophic Processes" and those who believe that geological change has happened gradually. The latter seem most likely to be correct the majority of times. This philosphy can be summarised by the the "Principle of Uniformity of Process" as described by James Hutton (1726-1797).
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Unformatted text preview: Crust rocks and soil The rocks and material of the crust are of four types: sedimentary igneous metamorphic magma Metamorphic rocks (e.g. slMetamorphic rocks (e.g. slate) are usually found deep inside mountains, and not too often at the surface. Most common rocks are igneous, and much of the surface is sedimentary. Sediments are: Clastic (consisting of broken up pieces) in order of decreasing particle size: gravel, sand (sandstone), silt and clay (silt and clay constituing shales ). Non-Clastic - mainly chemical deposits - salt, gypsum and coal for example. By biological deposition, e.g. limestone CaCO 3 and dolostone CaMg(CO 3 ) 2 These sediments are sometimes modified by pressure and temperature to become metamorphic rocks with different characteristics - e.g marble is limestone or dolostone after high T,p....
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