* GCSE Earth Science: 6. Metamorphic Rocks at Doc Brown's
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6. Metamorphic Rocks - Slate & marble etc. Revision KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science (revise courses equal to US grades 9-10) based on a GCSE Geological & Earth Science TASK SHEET * Earth Science Homepage * 5 multi-word fill GCSE worksheets + answers * GCSE Earth Science Quiz: Foundation-easier m/c Quiz and Higher-harder level m/c Quiz ANSWERS-NOTES 1. The Evolution of the Earth's atmosphere and Carbon Cycle ... 2. The Rock Cycle and types of rock (details 'evolve' through sections 3. to 9.) ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Structure of the Earth ... 8. Plates and their movement ... 9. Plate Tectonics ... 10. The Moon and Planets 6. Metamorphic Rocks formed through the action of heat and pressure!
Large scale movements of the Earth's crust can cause mountain ranges to form very slowly over millions of years. These replace older mountain ranges worn down by weathering and erosion. Metamorphic rocks are associated with the Earth movements (tectonic activity) which created present-day and ancient mountain belts. They are evidence of the high temperatures and pressure created by these mountain-building processes. 6(a) A metamorphic rock is one that is formed directly from a pre-existing rock. Heat and pressure are the 'driving forces' for metamorphic rock formation in which the grains of pre-existing rocks are re-crystallised. The pre-existing rocks involved are usually deep in the crust where they are subjected to great pressure. The high temperatures often needed, are due to rocks being near the hot mantle, or when an igneous intrusion rises, or volcanic rock heats other surrounding rock and when continental plate meets oceanic plate (see (3) in Fig 8.1). Fig 6.2 Contact Metamorphism
6(b) The link between metamorphic rocks and igneous intrusions is shown in the two diagrams above. The rising magma heats up the surrounding sedimentary (or igneous) rocks producing metamorphic rocks such as marble, slate, gneiss or schist. Note: There are high-low grades of metamorphism depending on the high-low temperatures and pressures particular pre-existing rocks are subjected to. For example, the rocks become 'less metamorphic' the further you go from the igneous intrusion as you go to a lower temperature. 6(c) Slate is formed from sedimentary rocks such as shale, mudstone or clay deposits and the re-crystallised layers are easily split - hence its use in roofing. Sometimes, but rarely, fossil traces are preserved in the layers through the crystallisation process. 6(d) Marble is a hard rock formed from the action of heat and pressure on the sedimentary rock limestone. It will still give carbon dioxide with acid but is much harder physically than limestone or chalk. 6(e) Gneiss, quartzite and schist are metamorphic rocks formed by the action of heat and pressure on pre-existing igneous or sedimentary rocks. They can form from igneous rocks* like granite or basalt, from metamorphic rocks* like slate or from sedimentary rocks like shale, mudstone or sandstone, and chemically they are mainly 'silica' SiO2. * Note, the original pre-existing rock does not have to be sedimentary!
6(f) Metamorphic rock has the same chemical composition as the original rock it was formed from (in terms of % elements). This is because no minerals are added or lost in the recrystallisation process. For example, the Ca:C:O ratio is the same in the sedimentary limestone rock as it is in the resulting metamorphic rock marble, because chemically they are both mainly calcium carbonate CaCO3.
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Doc Brown's Chemistry - Earth Science Revision Notes
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