Characteristics of Magma Igneous rocks form as molten rock cools and solidifies 1 2 General characteristics of magma a Parent material of igneous rocks b Forms from partial melting rocks inside Earth c Magma that reaches the surface is called lava 3 The nature of magma a Consists of three components i Liquid portion called melt that is composed of mobile ions ii Solids if any are silicate minerals that have already crystalized from the iii Volatiles which are gases dissolved in the melt including water vapor melt carbon dioxide and sulfur dioxide 1 Steam explosion 2 Not all magmas contain water iv Confining pressure weight of the rock layers on the magma chamber that raises the boiling point of any liquid in the magma chamber 4 Rocks formed from lava at the surface are classified as extrusive or volcanic rocks 5 Rocks formed from magma that crystalizes at depth are termed intrusive or plutonic a Texture in igneous rocks is determined by the size and arrangement of mineral rocks 6 Crystallization of magma grains Igneous rocks are typically classified by b i Mineral composition ii Texture 7 Igneous rocks are composed primarily of silicate minerals a Dark or ferromagnesian silicates i Olivine ii Pyroxene iii Amphibole iv Biotite mica 8 Igneous rocks are composed primarily of silicate minerals a Light or nonferromagnesian silicates i Quartz ii Muscovite mica iii Feldspars 9 Granit versus basaltic compositions a Granit composition i Composed of light colored silicates ii Designated as being felsic feldspar and silica in composition iii Contains high amounts of silica iv Major constituents of continental crust 10 Basaltic composition a Composed of dark silicates and calcium rich feldspar b Designated as being mafic magnesium and ferrum for iron in composition c More dense than granitic rocks d Comprise the ocean floor as well as many volcanic islands 11 Other compositional groups a Intermediate andesitic composition i Contain at least 25 dark silicate minerals ii Associated with explosive volcanic activity b Ultramafic composition i Rare composition that is high in magnesium and iron ii Composed entirely of ferromagnesian silicates 12 Silica is an indicator of composition a Silica content in crustal rocks exhibits a considerable range i Low of 45 ultramafic rocks ii More than 70 in felsic rocks 13 Silica content influences a magma s behavior a Granitic magma i High silica content ii Extremely viscous iii Liquid exists at tempts As low as 700 degrees Celsius b Basaltic magma i Much lower silica content ii Fluid like behavior iii Crystalizes at higher temps arrangement of interlocking minerals 15 Factors affecting crystal size a Rate of cooling 16 Factors affecting crystal size a Rate of cooling i Fast rate forms many small crystals ii Very fast rate forms glass b Amount of silica present c Amount of dissolved gases 17 Types of igneous textures a Aphanitic fine grained texture i Rapid rate of cooling of lava or magma ii Microscopic crystals iii May contain vesicles holes from gas bubbles b Phaneritic coarse grained texture i Slow cooling i Slow rate promotes the growth of fewer but larger crystals 14 Texture is used to describe the overall appearance of a rock based on the size shape and ii Crystals can be identified without a microscope Igneous Textures 1 Types of igneous textures a Porphyritic texture i Minerals form at different temps as well as differing rates ii Large crystals called phenocrysts are embedded in a matrix of smaller crystals called groundmass b Glassy texture i Very rapid cooling of molten rock ii Resulting in a rock called obsidian 1 Volcanic glass 2 Types of igneous textures a Pyroclastic texture i Various fragments ejected during a violent volcanic eruption ii Textures often appear more similar to sedimentary rocks b Pegmatite texture i Exceptionally coarse grained ii Form in late stages of crystallization of granitic magmas Naming Igneous Rocks 1 Granite felsic rocks a Granite i Phaneritic ii More than 25 quartz about 65 or more feldspar iii May exhibit a porphyritic texture iv Very abundant because it is often associated with mountain building v The term granite covers a wide range of mineral compositions b Rhyolite i Extrusive equivalent of granite ii May contain glass fragments and vesicles iii Aphanitic texture iv Less common and less voluminous that granite c Obsidian d Pumice i Dark colored ii Glassy texture i Volcanic ii Glassy texture iii Frothy appearance with numerous voids 2 Andesitic intermediate rocks a Andesite i Volcanic origin ii Aphanitic texture iii Often resembles rhyolite b Diorite i Plutonic equivalent of andesite ii Coarse grained iii iv Composed mainly of intermediate feldspar and amphibole Intrusive 3 Basaltic mafic rocks a Basalt i Volcanic origin ii Aphanitic texture iii Composed mainly of pyroxene and calcium rich plagioclase feldspar iv Most common extrusive igneous rock Intrusive equivalent of basalt i ii Phaneritic texture consisting of pyroxene and calcium rich plagioclase iii Makes up a significant percentage of the oceanic crust b Gabbro 4 Pyroclastic rocks a Composed of fragments ejected during a volcanic eruption b Varieties i Tuff ash sized fragments ii Volcanic breccia particles larger than ash Origin of Magma 1 Highly debated topic 2 Generate magma from solid rock a Produced from partial melting of rocks in the crust and upper mantle b Role of pressure i An increase in confining pressure causes an increase in a rock s melting temp or conversely reducing the pressure lowers the melting temp ii When confining pressures drop decompression melting occurs c Role of heat i Temp increases within Earth s upper crust called the geothermal gradient average between 20 30 degrees Celsius per km ii Rocks in the lower crust and upper mantle are near their melting points iii Anny additional heat from rocks descending into the melting or rising heat from the mantle may induce melting d Role of volatiles i Volatiles primarily water cause rocks to melt at lower temps ii This is particularly important where oceanic lithosphere descends into the mantle Evolution of Magmas 1 A single volcano may extrude lavas exhibiting very different compositions 2 Bowen s reaction series and the composition of igneous rocks a N L Bowen demonstrated that as a magma cools minerals crystallize in a systematic fashion based on their melting points 3 Bowen s reaction series continually changes a During crystallization the composition of the liquid portion of the