Geology - The Rock Cycle Key Definitions

Key terms from the AS Geology OCR specification. 

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  • Created by: Phil
  • Created on: 02-01-13 11:51

Extrusion & Intrusion

  • An extrusion is the emission of magma onto the Earth's surface where it forms a lava flow. 
  • An intrusion is igneous rock formed below the Earth's surface. The magma can be forced into pre-existing rocks, for example, either along bedding planes and  joints, or by cutting across the existing rocks. 
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Weathering & Erosion

  • Weathering is the breakdown of rocks in situ
  • Erosion is the removal of weathered material, usually by the physical action of transported fragments. 
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The Rock Cycle - Sedimentary

  • Transport is the means by which weathered material is moved from one place to another by water, wind, ice or gravity. 
  • Deposition is the laying down of a sediment that occurs when a transporting agent loses energy. 
  • Burial occurs when sediment is covered by younger layers of sediment accumulating on top of it. 
  • Diagenesis defines all processes that take place in sediments at low temperature and pressure at or near the Earth's surface. 
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The Rock Cycle - Metamorphic

  • Recrystallisation is the solid state process that changes minerals into new crystalline metamorphic minerals. 
  • Metamorphism is the changing of rocks in the Earth's crust by heat and/or pressure and/or volatile content. It is isochemical and occurs in the solid state. 
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The Rock Cycle - Igneous

  • Partial melting is the incomplete melting of rock in the lower crust or upper mantle. 
  • Magma accumulation is magma collecting within a magma chamber. 
  • Crystallisation occurs during the cooling of magma or lava so that solid mineral crystals form. 
  • Intrusions are igneous rock formed below the Earth's surface. The magma can be forced into pre-existing rocks, for example, either along bedding planes and  joints, or by cutting across the existing rocks. 
  • Uplift is the return of buried rocks to the Earth's surface by tectonic forces. 
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Thin Section

  • A thing (0.03mm) slice of rock that is translucent, mounted on a glass slide and viewed through a microscope. 
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Rock

  • An aggregate or mixture of one or more minerals. 
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Minerals

  • Naturally occurring inorganic crystalline compounds with a definite chemical composition. 
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Clast

  • A fragment of broken rock.
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Sedimentary, Metamorphic and Igneous Rock

  • A sedimentary rock is a rock composed of fragments that have been deposited compacted and cemented. 
  • A metamorphic rock is formed by the recrystallisation of other rocks in the solid state due to pressure, temperature or both. 
  • An igneous rock is a rock that has crystallised from a magma. 
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Sillicates

  • The most important and abundant group of rock-forming minerals, with an atomic structure containing SiO4 arranged as tetrahedra
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Foliation

  • A texture in metamorphic rocks formed by the preferred alignment of flat/tabular minerals. 
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The Geological Column

  • The geological column is a table that contains all the eras and systems in the correct time sequence. 
  • An era is a major unit of time that contains several periods/systems. 
  • A System refers to rocks laid down in a named time period and is shorter than an era. 
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