igneousrocks

igneousrocks - Up from the Inferno Magma and Igneous Rocks...

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Unformatted text preview: Up from the Inferno: Magma and Igneous Rocks J. D. Griggs/U.S. Geological Survey 1 Geology Rocks! Geology A naturally occurring consolidated naturally mixture of minerals or mineralmixture like substances Blah, Blah, Blah… Rocks are composed of Minerals! Remember, Minerals are: 1. Naturally occurring 2. Crystalline substance 3. Solid 4. Inorganically formed 5. Definite chemical 5. composition composition 6. Homogeneous 7. Orderly arrangement of atoms in 7. crystal lattice crystal Classification of Rocks IGNEOUS SEDIMENTARY METAMORPHIC QuickTime ™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Kilauea, active lava lake until 1929 A really big headache… IGNEOUS ROCKS IGNEOUS -Most abundant rock type on Earth -Why? Why? -All of the Mantle, Oceanic crust, and a bulk All of continental crust is Igneous in origin of -Think Rock Cycle -All rocks “born’ from igneous/volcanic source -Weathering, re-deposition, and Weathering, metamorphism account for the variability we observe observe Two “Breeds” EXTRUSIVE (Volcanic) — cooled on the above ground surface (lava) Vs. INTRUSIVE (Plutonic) below ground — cooled beneath the surface (Granite) 7 MAGMA PROPERTIES * Completely or partially molten material, which cools to form an igneous rock * Liquid portion called the melt (usually ions of Si + O, with lesser amounts of Mg, Al, K, Ca, Na + Fe). *Hot 600-1200 C * Loses heat as it crystallizes * Chemical composition - each magma different * “Chemical Fingerprint” * Silica content controls viscosity and therefore flow * Volatile content controls melting temperature – high volatiles lowers melting temperature * Melt less dense than rock so rises * Heat, differentiation causes change in buoyancy 8 Why is Magma a Liquid? Why -Remember, the Mantle is ductile and Remember, “flows” at VERY slow rates (2-15 cm/yr) VERY -But it’s solid! -How do you convert solid, superheated How rock into liquid magma? rock LIQUID SOLID ? PRESSURE Heat- Geothermal Gradient Normally, 1400 degrees would melt A LOT of Stuff! But there is too much PRESSURE for Magma to exist in Liquid phase… The Role of Pressure Pressure increases with depth in the Earth. Why? Melting occurs at higher temperatures with depth because of confining pressure the greater the pressure, the higher the melting point When confining pressure drops, decompression melting occurs Occurs when rock ascends as a result of convective upwelling (recall that the mantle is solid, but flows at very slow rates. peridotite Decompression melting mixture of solid + melt Second method of magma genesis is HEAT TRANSFER…. Hotter, ascending magma melts rocks with lower melting points Note: common in lithosphere (crust and lithospheric mantle) Effect of volatiles (gas at P+T conditions of earth’s surface) Key “Players”- (H2O, CO2,SO2 H2S) VOLATILE ADDITION! 13 Crystallization of a magma Amount of TIME Amount TIME that the magma cools controls the ending grain size of the igneous rock the Also different types: Why? Different minerals Why? (Mafic vs. Felsic) (Mafic have different have “melting/freezing” points 14 Faster! Fast Slow Cooling rate often depends on depth… Exposure to surroundings… Extrusive (Fine-grained) Intrusive (Coarse-grained) Coarsely Crystalline Granite SLOW FAST Finely Crystalline Basalt 17 Crystallization Ideally, crystallization is the opposite of melting. In fact, the process is more complicated than that because rocks are complex aggregates of many minerals with different melting (crystallization) points. * 18 Simple crystallization Example: Quartz When melt reaches the crystallization temperature of a mineral, the mineral forms and undergoes no further changes with subsequent cooling. Quartz beach sand19 Continuous crystallization Example: Plagioclase feldspar 1. Mineral begins to crystallize of a given composition 2. But composition of the crystal’s exterior (and therefore the entire crystal) changes due to changes in the composition of the magma. 20 Conditions in middle Conditions Are different from conditions Around the edges Bowen’s reaction series Series of chemical reactions that take place in silicate magmas as they cool First investigated in the 1920s and 1930s by N. L. Bowen Important experiments that help us understand the evolution of magmas 22 Bowen’s Reaction Series Fe-Mg minerals Ca-Na plagioclase Isolated tetrahedra ultramafic Single chain silicates Framework silicates Double chain silicates Sheet silicates Useful? 23 Magmatic Differentiation Process of developing more than one rock type from a common magma 24 Crystal settling 25 The Palisades Sill Fig. 4.10 26 How do we characterize igneous rocks? Igneous Textures GRAIN SIZE> fine grained - aphanitic - can’t see by naked eye > coarse grained - phaneritic - can see by naked eye > mixed grain size - porphyritic - porphyry > Phenocrysts – the large grains > ground mass or matrix - the fine material Tells us information on cooling history and possible parental magma 27 Mixed grain sizes 28 Amount of TIME Amount TIME that the magma cools controls the ending grain size of the igneous rock rock 29 Fine grained – cooled fast Same chemistry Coarse grained – cooled slow 30 Glassy Igneous Rocks Glassy Glassy Igneous RocksGlassy Forms very rapidly very Obsidian- Black to Brown Black volcanic glass volcanic Pumice- contains contains abundant pores and vesicles (trapped gas bubbles). Can be less dense than water, floats dense Scoria- glassy mafic rock glassy that has 30% vesicles, larger bubbles than pumice pumice OBSIDIAN Glassy 32 Crystalline Igneous Rocks- Distinguished by Crystalline silica content AND grain size What can color tell us? What Black, Brown, Dark Grey, “Dark” Black, colors = Mafic Mafic Pink, Tan, Light Grey = Felsic Pink, Felsic SILICA is KING! SILICA FELSIC MAGMA- 66-76% silica- abundant in Quartz and Feldspar- Low density! Think continental crust! FeldsparINTERMEDIATE MAGMA- 52-66% silica MAFIC MAGMA- 45-52% silica- abundant in mafic minerals, high MgO, FeO, Fe2O3. Dense- Think oceanic crust high ULTRAMAFIC MAGMA- 38-45% silica- characteristic of Peridotite and MANTLE rocks Peridotite Silica % influences viscosity: “resistance to flow,” “stickySilica viscosity ness” -High silica (felsic) are more viscous (Mt. St. Helens) -Low silica (mafic) flow easily (Hawaii) Note the % Silica, types of minerals, and the ROCK NAME Ocean crust Continental crust Mantle Arcs 35 Plutonic Volcanic Basalt Gabbro MAFIC Granite Rhyolite FELSIC 36 “Middle” silica Intermediate Silicic Mafic High silica Low silica 37 Pegmatite Pegmatite Very coarse grained, HUGE (10 cm +! long) crystals, slow cooling Very in a water rich melt. Fragmental Igneous Rocks Fragmental Describe size of fragments and Describe how they are bound together how Volcanic debris erupted and Volcanic thrown together are pyroclastic rocks rocks Tuff- fine grained with ash and Tufffragmented lava/pumice fragmented Volcanic Breccia: larger Volcanic fragments of lava fragments More to come in VOLCANOES Lecture!!! DIKE W. W. Norton SILL 40 Sill 41 Dike – cuts Upwards, across the original layering 42 Dike 43 Difference between dikes + sills Back to the Palisades Back Paul Hoffmann A single igneous body can crystallize in many different intrusive forms 46 Pluton - A large igneous rock body that has cooled beneath the surface of the Earth 47 Stephen Marshak Changing landscape through time. Pluton W. W. Norton Why the difference? Igneous rocks are Why Usually “harder” than sedimentary rocks, Usually And are more “resistant” to erosion! This is what we see now 48 ...
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This note was uploaded on 04/03/2012 for the course GEOL 100 taught by Professor Neitzke during the Fall '10 term at Rutgers.

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