CEE 1030 1nd Edition Lecture 3 Igneous Rocks Outline of Last Lecture I Minerals Outline of Current Lecture II Igneous rocks magma and lava a Nature of magma lava III Classifying igneous rocks IV Igneous textures V Minerals in igneous rocks a Examples VI Viscosity and the origins of magma VII Review rocks in Iowa and ore Current Lecture I II Igneous Rocks a Form by the cooling and solidification of magma i Magma molten rocks in the mantle ii Magma at the Earth s surface is called lava b Crystallization of magma lava i Water changes from liquid to solid at 0 deg C ii Magma changes from liquid to solid over a temperature interval of 200300 deg C iii As magma cools different silicate materials begin to crystallize and grow c Igneous rocks are formed from lava at the Earth s surface these are called extrusive or volcanic rocks d Igneous rocks formed from magma at a greater depth are called intrusive or plutonic rocks e The appearance of rocks changes based on where they are formed f The nature of magma lava i Liquid portion of magma is called melt ii Solids are silicate material iii When magma is deep in the ground it can dissolve CO2 in water more easily higher up there is less pressure so the CO2 gas does not dissolve and can erupt Classifying Igneous Rocks These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute III IV a Texture what does the rock look like What are the sizes of the crystals i Crystal size is determined by the cooling rate of the rocks 1 Faster cooling means a smoother texture b Types of minerals in rocks c Chemical composition of the rock Igneous Textures a Aphranitic i Fine grained with crystals too small to identify with the naked eye ii Rapid rate of cooling volcanic iii May contain vesicles holes from gas bubbles b Phaneritic i Coarse grained large visible crystals ii Slow rate of cooling plutonic c Porphyritic i Large crystals phenocrysts embedded in a matrix of smaller crystals groundmass ii Minerals form at different temperatures d Pegmatitic i Extremely coarse grained rock ii Forms in late stages of crystallization of magma e Pyroclastic i Fragmental texture ii Produced by violent volcanic eruptions iii Often appear more similar to sedimentary rocks f Glassy i Very rapid cooling of lava resulting rock is called obsidian ii Used by cultures to make sharp blades and arrow heads obsidian can be broken into sharp edges iii Pumice frothy glass is linked to depressurization forms bubbles in magma as gasses 1 Practical uses for pumice it is abrasive which makes it good for skin exfoliating and soap 2 It is also lightweight which makes it a good cement additive for building underwater Minerals In Igneous Rocks a Silicon and oxygen are the fundamental building blocks b Dark mafic silicates c Light felsic silicates d Igneous rock compositions i Granitic composition felsic makes up the continental crust ii Basaltic composition mafic makes up the oceanic crust e Examples of rocks i Felsic V VI 1 Granite plutonic 2 Rhyolite volcanic ii Intermediate 1 Diorite plutonic 2 Andesite volcanic iii Mafic 1 Gabbro plutonic 2 Basalt volcanic iv Ultramafic 1 Earth s mantle is ultramafic Viscosity and the Origins of Magma a Chemical composition of magma determines its physical properties b The more silicate in magma the higher the viscosity i A higher viscosity means the magma is more sticky as opposed to runny ii Basaltic magma has less silicate therefore it is more runny consistency of ketchup iii Rhyolitic has more silicate therefore it is more sticky consistency of peanut butter c Viscosity determines effusive lava flow vs explosive eruption i Runny magma means that bubbles of CO2 can escape therefore there is less likelihood of the magma exploding d Origin of Magma i Basaltic magma is the most common 1 Partial melting of the Earth s mantle 2 Linked to plate tectonics ii Magma only forms under certain conditions 1 Increase temperature thermal 2 Decrease pressure decompression 3 Add water fluid induced iii A single volcano may erupt lavas with different compositions Review Rocks in Iowa and Ore a Review i Silicate materials in igneous rocks silicon and oxygen ii Different types of magma ranked from lowest to highest viscosity basalt andesite rhyolite b Igneous rocks in Iowa i Mid Continent Rift System 1 Part of the Midwest tried to break apart so there are now volcanic rocks deeply buried along that rift c Ore deposits i Ore economically viable metals