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Download Igneous Rocks • igneous rocks are formed from cooling lava
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Igneous Rocks • igneous rocks are formed from cooling lava / magma • differences in rocks are caused by rate of cooling and by magma composition Composition • the mineral content of the molten material determines the composition of the rock • colour is used as an indicator of composition • there are four classifications > felsic / granitic: contains mostly light coloured minerals > intermediate/ andesitic: contains a mixture of light and dark coloured minerals > mafic / basaltic: contains mostly dark coloured minerals > ultramafic: contains only dark coloured minerals Light Coloured Elements aluminum calcium copper sodium silicon Minerals quartz (silicate) orthoclase feldspar muscovite mica Dark Coloured Elements Minerals iron magnesium olivine hornblende pyroxene 1 Questions 1) Granite is primarily made up of silicate minerals with small amounts of aluminum, iron and mica present. What is the magma composition of granite? 2) Peridotite is entirely made up of pyroxene and olivine. What is the magma composition of peridotite? 3) Diorite is roughly 50% silicate materials. There is also a large amount of hornblende and some pyroxene. What is the magma composition? 2 Bowen's Reaction Series • different minerals melt and solidify at different temperatures • the two branches in Bowen's reaction series occur at the same time 1) Discontinuous Series • minerals crystallize in order with some overlap • the minerals in this series are distinctly different from each other • all of these minerals are dark in colour 2) Continuous Series • plagioclase feldspar (pfeldspar) crystallizes at a high temperature will a high calcium concentration • calcium rich pfeldspar is gray and is found in many mafic rocks • as the magma cools, the pfeldspar that crystallizes will have a higher sodium • sodium rich pfeldspar is white and is found in many intermediate to felsic rocks 3) Final Stage • the final cooling stage will result in the crystallizing light minerals, such as > muscovite mica > quartz > orthoclase feldspar (also known as potassium feldspar or k feldspar) 3 4 Ex) Which rocks could contain 60% pyroxene and 10% olivine? Ex 2) An igneous rock contains 20% quartz. What other minerals would it likely contain? Ex 3) What is the range of calcium rich plagioclase feldspar contained in basalt? 5 Igneous Texture • remember that differences in igneous rocks are caused by rate of cooling and by magma composition • the rate of cooling affects crystal growth • the faster the cooling, the smaller the individual crystals will be Which cooled faster? Describe the rate of cooling for this rock. no crystals Describe the rate of cooling for this rock. "big" crystals surrounded by "small" crystals 6 Textures 1) Phaneritic (coarse grained) • crystals large enough to see unaided • magma cools slowly under the surface (intrusive), usually in large masses 2) Aphanitic (fine grained) • crystals too small to see unaided • forms when lava cools at the surface (extrusive) or when small masses of magma cools When a large mass of magma cools, the edges may cool slower than the inner portion. This can lead to a phaneritic core with an aphanitic "crust" 3) Porphyritic (coarse grains in fine grain) • two rates of cooling > slow cooling first to develop large crystals (phenocrysts) > fast cooling second to develop small crystals (groundmass) • this could be due to a magma flow cooling slightly and then being extruded to the surface 7 4) Glassy (no crystals) • cools instantly so no crystals can develop • this can be due to molten material being ejected into air, water or ice • there are two types of glass texture > compact, such as obsidian > frothy, such as pumice 5) Vesicular (full of holes or hollow spaces) • actually a subtype of texture • holes in the rock formed by water or air passing through the molten material as it cools • these cavities make the rocks much lighter than normal ex) pumice and scoria 8 Felsic Intermediate Mafic Ultramafic Phaneritic Aphanitic Glassy Vesicular 9 1) Which rock has the same composition as basalt? 2) Which rock contains the highest amount of dark minerals? 3) Which rocks have the largest crystals? 4) Which rocks would feel the lightest? 5) Which rock that formed deep underground also has a high silca content? 10
