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Doc Brown's Chemistry - Earth Science & Geology Revision Notes

for KS4 Science, GCSE, IGCSE & O Level Courses

6. Metamorphic Rocks - Slate & Marble etc.

Metamorphic rocks are formed from pre-existing sedimentary rocks or igneous rocks. Under the effect of heat and pressure these pre-existing recrystallise without melting into another type of rock with different physical properties. The formation of the metamorphic rocks marble, slate, gneiss, quartzite and schist are described.


GCSE/IGCSE/O Level KS4 Earth Science-Geology ANSWER-REVISION-NOTES 1. Evolution of the Earth's atmosphere, Gases in Air, Carbon Cycle, Origin of Life ... 2. Rock Cycle, Types of rock ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Layered Structure of the Earth ... 8. Tectonic plate theory, Wegener's theory, evidence for continental drift ... 9. More on Plate Tectonics, effects of plate movement, volcanoes, earthquakes, faults etc. ... 10. A few geology and atmosphere notes on the Moon and Planets


6. Metamorphic Rocks - formed through the action of heat and pressure!

doc b's Earth Science Notes  (c) doc bFig 6.1 Volcanoes, intrusions and metamorphic rock formation

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.

top6(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 without melting. 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 8.1 Find the metamorphic zone!

doc b's Earth Science Notes

 

Fig 6.2 Contact Metamorphism

doc b's Earth Science Notesdoc b's Earth Science Notes

top6(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!

  • Gneiss is a metamorphic rock, with pronounced crystalline layering, formed by the action of heat and pressure on igneous rocks.
  • Quartzite is formed by the action of heat and pressure on the sedimentary rock sandstone.
  • Schists are formed in regional metamorphism, they tend to be course grained and easily split into layers.
  • NOTE: The terms ....
  • Regional Metamorphism refers to large scale metamorphic rock regions associated with mountain building from tectonic activity (See Fig 8.1 or above)
  • Contact Metamorphism refers to localised metamorphic rock formation around an igneous intrusion. (see Figs 4.1, 6.1, 6.2)

Fig 4.1 Igneous and intrusion and metamorphic rock formation

doc b's Earth Science Notes

 

Fig 6.1 Another example of metamorphic rock formation

doc b's Earth Science Notes

 

Fig 6.2 A dome formed by an igneous rock intrusion

doc b's Earth Science Notes

 

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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GCSE/IGCSE/O Level KS4 Earth Science-Geology ANSWER-REVISION-NOTES 1. Evolution of the Earth's atmosphere, Gases in Air, Carbon Cycle, Origin of Life ... 2. Rock Cycle, Types of rock ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Layered Structure of the Earth ... 8. Tectonic plate theory, Wegener's theory, evidence for continental drift ... 9. More on Plate Tectonics, effects of plate movement, volcanoes, earthquakes, faults etc. ... 10. A few geology and atmosphere notes on the Moon and Planets


Studying Revision for KS4 Earth Science GCSE/IGCSE/O level Chemistry Information Study Notes for revising for AQA GCSE Earth Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  WJEC gcse science chemistry CCEA/CEA gcse science chemistry (revise courses equal to US grade 8, grade 9 grade 10)

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