Iron formations- generally in old rocks, when less oxygen was present in the atmosphere Chapter 6 Wednesday, February 28, 2007 12:42 AM Coal made up of diagenetically altered swamp vegetation. Oil and gas (not rocks) formed from diagenetic alteration of organic material in pores of sedimentary rocks. Found mainly in sandstones and limestones. Only sedimentary rocks contain fossils. Chapter 6 - Metamorphic Rocks Produced when igneous or sedimentary rocks are subjected to high pressure and temperature for long periods of time, inside the Earth. Can also (re)metamorphose a metamorphic rock (subtle changes only). A good place is near convergent plate boundaries, and especially deep inside mountain belts (e.g. continent - continent collision). Going on today, deep inside Alps, Himalayas. Will only see rocks perhaps hundreds of m.y., later when brought to (and exposed) at the surface. The new rock develops new minerals and texture, with all "changes" taking place in the solid state (no melting). However, fluids often play a major role. Regional Metamorphism. Large volumes of rocks, subject to high P and high T for long periods of time. Deformation (bending or breaking) of rocks is not uncommon. Contact Metamorphism. Smaller volumes of rock, subject to high T for shorter periods of time (e.g. around cooling plutons). Generally, minerals in rocks formed at low temps change (more) readily during metamorphism; e.g. clay ---> mica. Regional metamorphism takes place close to convergent plate boundaries. Plates pushing against each other, cause directed pressure; force (pressure) in one of 3 dimensions more than in other
2. This directed pressure gives rise to new texture called foliation/lineation in many metamorphic rocks. Temperature (T) increases steadily with depth inside the Earth, as does the pressure (P); but not the same everywhere Rate at which temp increases with depth inside the Earth is called the geothermal gradient. Average value is 30 degrees C/km for metamorphic conditions. Pressure more difficult concept. At depth of 15 km, rocks squeezed together ~4000 times more than at the surface of the Earth. In P-T space can define areas where we get low, intermediate, high grade, metamorphic rocks. (Most) regional metamorphic conditions are represented through central (diagonal) part of Fig. 6.2 Regional metamorphism - cf. cooking large turkey in oven. Can have high/low (grade) temp of cooking for long periods of time - cannot simulate increase in pressure. Contact metamorphism - cf. frying a steak on skillet. Relatively high temp for short period of time. (Little/no increase in pressure).
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- Spring '07
- Geology, Sedimentary rocks, Fig, sedimentary rocks Fig