Geology - Igneous Rocks Key Definitions

Key terms from the AS Geology OCR specification. 

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  • Created by: Phil
  • Created on: 02-01-13 19:12

Igneous Rocks

  • Rocks that have been cooled from magma. 
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Silica Percentage

  • Silicic igneous rocks have a composition rich in silica, of more than 66% and are light coloured. (leucocratic) 
  • Intermediate igneous rocks have a silica content of 66 to 52% and are grey (mesocratic).
  • Mafic igneous rocks have a silica content of 52 to 45% and are dark (melanocratic). 
  • Ultramafic igneous rocks have a silica content of less than 45%. 
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Felsic and Mafic Minerals

  • Felsic minerals are light coloured and silica rich. 
  • Mafic minerals are dark coloured, silica poor and rich in magnesium and iron. 
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Extrusive & Intrusive

  • Extrusive is the general term for all igneous rocks that cool at the surface - both lava and pyroclasts
  • Intrusive is the term for igneous rocks that cool below the surface. 
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Hypabyssal & Plutonic

  • Hypabyssal is when igneous rocks form at relatively shallow depths below the surface. 
  • Plutonic is when igneous rocks form deep below the surface. 
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Plate Margins

  • Divergent plate margins are where two plates are moving apart and magma is rising up between them. 
  • Convergent plate margins are where two plates are colliding and magma is formed above a subduction zone or deep in the crust. These can be ocean-oceanocean-continental orcontinental-continental
  • Hot spots are formed by a fixed mantle plume bringing magma to the surface. 
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Partial Melting

  • Occurs where some of the minerals in a rock melt to form a magma. 
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Country Rock

  • Is any rock, whether sedimentary, metamorphic or igneous into which an igneous rock is intruded. 
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Minor & Major Intrusions

  • Minor intrusions cool at hypabyssal depth below the surface and include sills and dykes. 
  • Major intrusions are plutonic and cool deep the surface and include batholiths. 
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  • Is where the igneous rock meets the country rock. 
  • chilled margin is where the igneous rock has cooled rapidly so it has fine crystals. 
  • baked margin is in the country rock where it was heated by the intrusion and altered. 
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Metamorphic Aureole

Is a large area around a batholith where the rocks have been metamorphosed. 

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  • Is a vent at the surface of the Earth through which magma and other volcanic materials are ejected. 
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  • Pyroclast - An individual fragment ejected during an eruption. 
  • Pyroclastic - Describes all the fragmental materials formed by explosive eruptions, including bombs, blocks lapili and ash. 
  • Pyroclastic rocks - Formed when the pyroclastic material is compacted into rocks. 
  • Pyroclastic flow - A hot mixture of pyroclastic material and gas. A nuee ardente is a type of pyroclastic flow. 
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  • A line joining points of equal thickness of a deposit such as ash. The maps may be called isopach maps. 
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  • Mudflows of wet ash and volcanic debris that can flow rapidly down a mountainside. 
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  • A measure of a fluid's resistance to flow and controls the stickiness of a magma, which in turn depends on the silica content. 
  • Low viscosity is where magma is fluid and flows freely.
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Shield Volcanoes

  • Have gentle slopes of less than 10 degrees and a roughly circular shape around a central vent. 
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Active, Extinct and Dormant Volcanoes

  • An active volcano has shown eruptive activity within recorded history. Currently there are about 600 active volcanoes and each year 50 to 60 actually erupt. 
  • An extinct volcano has not shown any historic activity and each year 50 to 60 actually erupt. 
  • dormant volcano is one that has not shown eruptive activity with recorded history, but shows geological evidence of activity within the geologically recent past. 
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  • The term used to describe the fluid, non-explosive, basalt lavas. 
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