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Chapter 4

by: Taylor Notetaker

Chapter 4 EAR 110 - M010

Taylor Notetaker
Dynamic Earth
S. Samson

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Lecture and Textbook Notes for chapter 4
Dynamic Earth
S. Samson
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This 9 page Bundle was uploaded by Taylor Notetaker on Monday September 28, 2015. The Bundle belongs to EAR 110 - M010 at Syracuse University taught by S. Samson in Fall 2015. Since its upload, it has received 64 views. For similar materials see Dynamic Earth in Earth Sciences at Syracuse University.

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Date Created: 09/28/15
4 Magma Igneous Rocks and Intrusive Activity 41 Magma the Parent Material of Igneous Rock Igneous rock forms as molten rock cools and solidi es The parent of igneous rock is called magma formed by the partial melting that occurs at various levels within Earth s crust and upper mantle to depths of 250km When molten rock reaches earth s surface it s called lava o Emitted as fountains that are produced with escaping gasses propel it from a magma chamber 0 Explosively ejected producing a dramatic steam of ash and eruptions The Nature of Magma Most magmas consist of three materials liquid component solid component and a gaseous component 0 Melt is the liquid component of magma composed mainly of mobile ions of the eight most common elements found in Earth s crust Solid components of magma are crystals of silicate minerals Gaseous components of magma are volatiles materials that vaporize at surface pressures 0 Water vapor carbon dioxide and sulfur dioxide are common ones 0 They re usually con ned by immense pressure exerted by the overlying rocks but they separate from the melt and move to the surface and can eventually propel magma from the vent 0 When magma bodies buried deep in the Earth crystallize the remaining volatiles collect as hot waterrich uids that migrate through the surrounding rocks key in metamorphism From Magma to Crystalline Rock 0 Solid rock ions vibrate slightly but as they are heated up they vibrate more rapidly and collid with neighbors Heating causes ions to occupy more space and expands the solid 0 During melting ions vibrate so vigorously to the point where they break their chemical bonds and ions lose their crystalline ordered structure Crystallization is when temperature drops and ions pack closely together as movement slows and the forces co chemical bonds recon ne the ions into an orderly structure 0 Silicon and oxygen atoms link together rst to form tetrahedra o Earliest formed crystal minerals tend to have better developed crystal faces than those that formed later Igneous Processes Igneous rocks can from at depth forming an intrusive igneous rock or plutonic rock or on Earth s surface forming an extrusive igneous rock or volcanic rocks 0 Intrusive rocks are typically observed at the surface where uplifting and erosion have stripped away overlying rocks Extrusive rocks form when lava solidi es or when volcanic debris falls onto Earth s surface 42 Igneous Compositions Granitic Felsic versus Basaltic Ma c Compositions Rocks composed mostly of lightcolored silicates have granitic composition or are called felsic o 10 dark silicate minerals o Rocs that have at least 45 dark silicate minerals have basaltic composition and called ma c o Denser than granitic 0 Make ip ocean oor and volcanic is Other Compositional Groups Rocks with composition between granitic and basaltic are said to have andesitic or intermediate compositions 0 At least 25 dark silicate minerals 0 Associated with volcanic activity on seaward continental margins and volcanic island arcs Peridotite is composed of almost entirely ferromagnesian minerals so it s referred to as ultrama c Silica Content as an Indicator of Composition Felsic rocks typically have high amounts of silica and rocks and ma c or ultrama c rocks have low amounts of silica Chemical makeup of an igneous rock can be assumed directly from its silica content 0 Granitic magma high in silica is viscous thick and may erupt in temperatures as low as 6509C while basaltic magmas are more uid and erupt at higher temperatures 10509C 43 Igneous Textures What Can They Tell Us 0 Texture describes overall appearance of a rock based on size shape and arrangement of its mineral grains not how it feels to touch 0 Reveals a lot on originof the rock 0 Three factors that in uence the textures of igneous rocks 0 The rate at which molten rock cools dominant factor 0 The amount of silica present 0 The amount of dissolved gases in the magma Slowcooling magma causes fewer but larger crystals because ions have time to migrate then join an already forming crystal 0 Rapid cooling magma cools too quickly so ions have to rush to form crystals causing numerous intergrown crystals Types of lgneous Textures o Aphanitic ne grained texture o Rapidly cooling environment 0 Crystals so small that minerals can only be distinguished under a microscope o Characterized as light intermediate or dark in color Phaneritic course grained texture 0 When large masses of magma slowly crystallize at great depth 0 Mass of intergrown crystals roughly equal in size with distinguishable minerals to the bare eye or magnifying glass 0 Porphyritic texture 0 When magma crystallizes under different environmental conditions there are typically large crystals embedded in small crystals 0 Large crystals are referred to as phenocrysts and smaller crystals are groundmass o A rock with porphyritic texture is called a porphyry Vesicular Texture 0 Many extrusive rocks have voids called vesicles left by gas bubbles that escape as lava solidi es giving rocks a vesicular texture o Often occur in the quotupper zonequot of lava ow Glassy Texture o Unordered ions are frozen in place 0 Obsidian is a natural glass prized for its sharp hard edge because of its conchoidal fracture o Granitic magma when extruded as an extremely viscous mass can solidify to form obsidian o Pryoclastic fragmental texture o Consolidation of individual rock fragments ejected during explosive volcanic eruptions 0 Some are fragments that solidi ed before impact and became cemented together at some later time o Pegmatitic Texture o Exceptionally course grained rocks crystals are larger than 1cm in diameter 0 Form later in crystallization when water carbon dioxide chlorine and uorine make up a high percentage of the melt Fluid rich environment enhances crystallization 44 Naming Igneous Rocks Granitic Felsic Igneous Rocks 0 Granite 0 1020 quartz 50 Kfeldspar about 10 dark silicates biotite amphibole and muscovite o quartz grains appear rounded in shape glassy and clear to gray o feldspar are generally white grey or salmon and blockrectangular o porphyritic texture o Rhyolite o Finegrained equivalent of granite and is composed essentially of lightcolored silicates o Buff pink to light gray in color 0 Rhyolite deposits are less common and smaller than granite 0 Obsidian 0 Dark colored glassy rock usually forming when highly silica rich lava cools quickly at Earth s surface 0 Unordered arrangement 0 Same chemical composition as granite o Pumice o Volcanic rock with glassy and vesicular texture with voids varying from large to small 0 Sometimes ow lines are visible 0 pumice and obsidian can often be found at the same rock mass Andesitic intermediate Igneous Rocks Andesite o Mediumgray fine grained rock typically of volcanic origin 0 Porphyritic texture o Phenocrysts are often light rectangular crystals of plagioclase feldspar or black elongated amphibole crystals 0 Diorite o Intrusive equivalent to andesite 0 Course grained rock looks like gray granite 0 Little to no visible quartz crystals and higher percentage ma c than granite 0 Salt and pepper appearance bc of light colored feldspar and dark colored amphibole crystals Basaltic Ma c Igneous Rocks 0 Basalt 0 Very dark green to black 0 Fine grained o Pyroxene and carich plagioclase feldspar lesser amounts of olivine and amphibole 0 When porphyritic it contains small lightcolored feldspar phenocrysts or green glassyappearing olivine grains embedded in dark groundmass 0 Most common extrusive igneous rock 0 Gabbro o Intrusive equivalent to basalt 0 Dark green to black in color 0 Pyroxene and carich plagioclase feldspar Pyroclastic Rocks 0 Tuff o Composed of tiny ashsize fragments that were later cemented together 0 When ash particles remained hot enough to fuse the rock is welded tuff 45 Origin of Magma Majority of crust and mantle are composed of primarily solid not molten rock Magma originates in Earth s uppermost mantle mostly produced at divergent plate boundaries and when crustal rocks are heated to sufficiently melt Generating Magma from Solid Rock 0 Temperature change going deeper into the Earth averages at about 259C per km in the upper crust known as the geothermal gradient 0 The rate at which Peridotite common in mantle melts is higher than gradient so in normal conditions the mantle is solid 0 Tectonic processes trigger melting through processes that decrease the melting point of the mantle rock Decrease in Pressure Decompression Melting Pressure greatly in uences melting because melting occurs at progressively higher pressures with increased depth 0 The less pressure that s on a rock LOWERS its melting points Decompression melting is triggered when a hot solid mantle rock ascents to zones of convective upwelling o Produces magma rock on divergent boundariesocean dges Addition of Water 0 Water and volatiles causes rock to melt at lower temperatures o It lowers the temperature of hot mantle rock enough to generate magma When ocean plates sink into the mantle at convergent plate boundaries water from the ocean crust seeps into the mantle and melts rock at 100km in depth Temperature Increase Melting Crustal Rocks 0 When mantlederived basaltic rocks gather below the less dense silicarich crustal rock the crustal rock begins to melt and create silica magma When silica magma reaches the surface it produces explosive eruption associated with convergent plate boundaries Crustal rocks also melt during continental collisions that result in the formation fo large mountain belts o The crust is greatly thickened and crustal rocks end up being buried into depths and start melting 46 How Magmas Evolve Bowen s Reaction Series and the Composition of lgneous Rocks 0 As magma cools minerals tend to crystallize in a systematic fashion some minerals have higher melting points than others which causes partial melting 0 During crystallization the composition of the liquid magma changes because whatever mineral was crystallized will be taken out of the liquid mixture Bowen39s Reaction series is when the solid components of magma come in contact and react with the remaining melt chemically changing the mineralogy Imam 7 Tampaalum nal j E16113 lMll lquot illifil l 0 Order of melting rst to last felsic rocks intermediate rocks ma c rocks ultrama c rocks 0 Rocks with higher silica ions melt faster than those with higher metallic ions 0 Minerals that form in the same general temperature are typically found together in the same igneous rocks Magmatic Differentiation and Crystal Setting 0 Crystal setting is when the earlierformed minerals are denser and tend to sink towards the bottom of the magma chamber allowing for separation of various components to occur 0 When the remaining rock solidi es it moves into fractures of the surrounding rocks causing for a rock to have different minerology from the parent magma The formation of one or more secondary magmas from a single parent magma is called magmatic differentiation Assimilation and Magma Mixing A magma can change composition through assimilation incorporating some of the surrounding rock into the magma body 0 During the ascent of two chemically distinct magma bodies the more buoyant mass takes over the slower rising body and they join stirring the two magma into one single mass called magma mixing 47 Partial Melting and Magma Composition 0 Partial melting is the incomplete melting of rocks that produces most magma 0 Partial melting of ultrama c rocks yield ma c magmas partial melting of ma c rocks yield intermediate magmas and partial melting of intermediate rocks yield felsic magmas Formation of Basaltic Magma Most of the magma that erupts on Earth s surface is basalticma c so that means that the magma being partially melted is ultrama c rock Peridotite leading us to assume that majority of the mantle is made up of that rock source 0 Basaltic magmas that originate from partial melting are called primary or primitive magmas because they have not yet evolved Formation of Andesitic and Granitic Magmas o Andesitic magma can form when a rising mantlederived basaltic magma undergoes magmatic differentiation forming secondary magmas from a single parent magma 0 Iron rich components would crystallize rst leaving the remaining melt with an andesitic composition Evolvedchanged magmas are called secondary magmas Andesitic magma also forms when basaltic magma assimilates with silica rich magma Granitic magma can form through 0 Magmatic differentiation of andesitic magma 0 When hot basaltic magma ponds and melts granitic crust creating large quantities of granitic magmas 48 Intrusive Igneous Activity 0 Most magma crystallizes at depth not from volcanic eruptions Nature of Intrusive Bodies o Intrusions or plutons are structure that arise from forcibly displacing preexisting rocks Intrusions are classi ed by their shape 0 Tabular meaning tabletablet shaped 0 Massive meaning blob shaped They re also classi ed by their orientation with respect to the hostpreexisting body 0 Discordant cuts across the existing rock 0 Concordant injects parallel to features such as sedimentary strata Tabular Intrusive Bodies Again tabular intrusive bodies are produced when magma is forcible injected to a fracture or zone of weakness Dikes are discordant bodies that are formed when magma vertically cut through bedding surfaces 0 Tends to form in roughly parallel groups called dike swarms 0 They can also radiate from an eroded volcano similar to the shape of spokes on a wheel 0 They weather slower than the surrounding body and eventually start to take the shape of a walllike structure Sills are concordant bodies that are nearly horizontal and are caused when magma seeps into weaknesses between sedimentary beds 0 Bikes and sills are relatively consistent in thickness and are made of uid mobile magma so they can go on for kilometers Columnar jointing is when igneous rocks cool and develop shrinkage fractures that produce elongated pillarlike columns that most often have six sides Massive Intrusive Bodies Batholiths Stocks and LaccoIiths o Batholiths are the largest igneous bodies mammoth linear structures that can be several hundred km long and up to 100km wide 0 Most are less than 10km thick 0 Typically composed of felsic or intermediate rocks 0 Forming plutons crowd and penetrate one another creating the massive structure that comes to be over millions of years 0 MUST have a surface area greater than 100 square km Smaller plutons are called stocks but most are a portions of larger bodies that would later be considered batholiths Emplacement of Batholiths Magma rises because it is less dense than surrounding rock and magma bodies form quotdiapirsquot invertedteardropshaped masses Dikelike structures may provide conduits for the ascent of magma At great depths where rock is able to ow magma may push aside the overlying rock a process called shouldering 0 Some of the host rock that was displaced will ll in the space left behind by the magma body When the magma body encounters brittle rock that isn t easily pushed aside a process called stoping may occur in which blocks of the roof of overlying hit rising magma may become dislodged and sink through the magma 0 Some plutons contain suspended blocks of country rock called xenoliths that support the theory of stoping Laccoliths ACTIVE quotHE LEWISH lgneous intrusions can lift the sedimentary strata they penetrate creating laccoliths which arch the bed above while leaving those below relatively at Mountains are not laccoliths but stocks but stocks are the source of material for laccoliths which branch off from the central magma bodies Mill El39illT VQLEMWIE FEATUREE h mum dndlr 39E l iE39 stock with radiating 5 rent llama ws 1M Iii lli SEl


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