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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2 EXTRUSIVE IGNEOUS ROCKS There are 4 main things that come out a volcano or other opening in the Earth’s surface: 1. Lava : magma that has cooled and solidified on the Earth’s surface. - may be ropey (pahoehoe) or blocky (aa) in appearance - vesicular: a volcanic rock that has holes ( vesicles ) in it where gases have bubbled off after eruption - amygdaloidal: vesicles have been infilled with later minerals 2. Pyroclastic material : lacks a crystalline texture - consists of fragments ( pyroclasts ) of solidified lava, broken glass pieces, ash, crystals,… - result of violent explosive volcanic eruption - solidifies into rock called tuff - or volcanic breccia (coarse fragmental rock) 3. Volcanic glass : a) Pumice : a solidified glassy rock so full of trapped gas that it will float b) Obsidian : very dense volcanic glass lacking any crystals or grains 4. Porphyry : an igneous rock texture in which there are 2 distinct crystal sizes – larger crystals ( phenocrysts ) in a finer grained matrix. - result of 2 distinct cooling stages (larger crystals formed at depth and then the magma was brought to the surface where the rest of the liquid cooled quickly)
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3 B. CHEMICAL AND MINERAL COMPOSITION : Chemical classification of igneous rocks based on relative amounts of silicate minerals present in the rock. Main silicate minerals: Quartz Potassium Feldspar Plagioclase Feldspar Biotite Amphibole Pyroxene Olivine Classify both intrusive and extrusive rocks based on how much of each mineral is in the rock. Felsic : igneous rocks rich in silica and feldspar minerals - tend to be light in colour Intermediate : Mafic : igneous rocks rich in magnesium and iron silicates - tend to be dark in colour Ultramafic: igneous rocks very high in Mg and Fe and low in silica - coarse-grained rocks ( no equivalent extrusive rock) - very dark coloured ** You can have intrusive or extrusive igneous rocks with the same chemical / mineralogical composition. **
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4 HOW DO MAGMAS FORM? A rock melts and becomes magma when the temperature and
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This note was uploaded on 05/14/2010 for the course EARTH SCIE 100 taught by Professor John during the Spring '10 term at Simons Rock.

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

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