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EASC%20101%20Ch%203_Igneous%20Rocks

EASC%20101%20Ch%203_Igneous%20Rocks - EASC 101 Chapter 3...

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1 EASC 101 Chapter 3: IGNEOUS ROCKS Igneous rocks are formed by crystallisation from a liquid magma Magma : - a mixture of liquid rock, crystals and gas. - characterised by a wide range of chemical compositions, with high temp. and properties of a liquid. - less dense than surrounding rocks and will therefore move upward. HOW DO IGNEOUS ROCKS DIFFER FROM ONE ANOTHER? Igneous rocks divided into 2 categories according to their texture and chemical/mineral composition : A. TEXTURE : the interrelationship of crystals making up a rock Æ related to the different cooling rates of magmas Æ reflection of emplacement history of rock INTRUSIVE (Plutonic): magma cools very slowly when it remains below the Earth’s surface - coarse-grained rocks Æ phaneritic - eg. granite EXTRUSIVE (Volcanic): magma cools very quickly on or near the surface of the Earth - fine-grained rocks Æ aphanitic - eg. basalt, rhyolite - or glassy (eg. obsidian)
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