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Explore the fascinating world of igneous rocks, from the formation of magma beneath the Earth's surface to the crystallization processes creating diverse rock compositions. Learn about the Bowen's Reaction Series, igneous textures, and the significance of plate tectonics in the origins of various igneous rock types.
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Igneous Rocks • Magma – molten rock below the Earth’s surface • Granite • Diorite • Gabbro • Peridotite • Lava – molten rock extruded on/at the Earth’s surface • Rhyolite • Andesite • Basalt • Komatite
Igneous Rocks • Compose or underlie all of the Ocean floors • Mid-Ocean ridges • Origin of continents • First continents must have appeared as some extrusive/intrusive complex • Make up some of the continents oldest rocks • Zoroaster Granite in the Grand Canyon’s Inner Gorge
Creating Magma • Partial melting • Ice – single mineral melts all at once • Rocks – several minerals that melt at different temps • Heat • Temperature increases with depth • 50-250km are the depths at which rocks begin to melt • Geothermal Gradient • Radioactive decay • Residual heat from formation of planet • Friction at plate boundaries • Pressure • > depth > pressure • Increases melting temperature • Fluids • Mostly water • Decreases melting temperature • Important at plate boundaries
Melting Rocks and Crystallizing Magma • Melting preexisting rocks • Various minerals have different melting temperatures • Composition of melt/liquid changes as new elements are introduced through melting process • Crystallizing magmas • Reverse the melting process • High temp minerals begin to crystallize • Composition of melt changes as elements are extracted through crystallization • Mineral crystals grow as cooling continues and eventually form interlocking crystal structure • Size of crystals is directly related to pace of cooling • Fast cooling small crystals • Slow cooling large crystals
Viscosity of Magmas • Viscosity – resistance to flow • Increases with decreasing temperature • Increases with increasing silica content • Magma rises because • Less dense than surrounding rock • Hotter than surrounding rock • Gas content expands as it rises • Surrounding pressure squeezes it upwards
Bowens Reaction Series • The sequence at which silicate minerals crystallizes as a magma cools • Continuous and discontinuous sides • High temp – Olivine and Ca-plagioclase • Low temp – Quartz and K-feldspar
Igneous Textures • Intrusive – Plutonic • Phaneritic – large crystals, easily seen and identified • Cooled slowly underground • Pegmatities – very large crystals, low temp, high water content, often associated with economic deposits • Extrusive – Volcanic • Aphanitic – small crystals, hard to identify without magnification • Cooled quickly, usually above ground • Volcanic glass – obsidian, pumice
Igneous Compositions(Table 4.7) • Felsic – light in color • >65% silica • High viscosity • Al, K, Na • Granites/Rhyolites • Intermediate – usually light in color • 55-65% silica • Medium viscosity • Al, Ca, Na, Fe, Mg • Diorite/Andesite
Igneous Compositions • Mafic – dark in color • 45-55% silica • Low viscosity • Al, Ca, Fe, Mg • Gabbro/Basalt • Ultramafic – dark in color • <40% silica • Very low viscosity • Peridotite/Komatiite
Gabbro/Basalt • Most abundant rock in Earth’s crust • 45-55% silica content • Low viscosity • Pyroxene, Ca-feldspar, olivine (no quartz) • Mafic, dense • Ocean plates, Hawaiian Islands, Northwest U.S. (Columbia Plateau), San Francisco Peaks and surrounding volcanic field (cinder cones – Sunset Crater)
Diorite/Andesite • 55-65% silica content • Sometimes difficult to distinguish between basalt • Amphibole, pyroxene, Ca-to-Na-plagioclase • Second most abundant rock at Earth’s surface • Flows and explosive events • Andes Mtns., Cascades • San Francisco Peaks, Mt. Fuji, Kilimanjaro, Rainier, Vesuvius, Mt St. Helens
Granite/Rhyolite • 65% or more silica content • Intrusive much more common than extrusive • K-feldspars, quartz, Na-plagioclases, sometimes micas • Explosive eruptions • Mt. Elden (intrusive dacite, endogenous/exogenous dome) • Sierra Nevada batholiths • Prescott – Granite Mtn. and Dells • Inner Gorge Grand Canyon – Zoroaster Granite
Plate TectonicsOrigin of Gabbros/Basalts • Mid-ocean ridges • Divergent plate boundaries • Creation of new ocean crust • Gabbros overlain by basalts overlain by sediments (ophiolitic suite) • Ocean island hot-spots • Some continental hot-spots • Varied composition due to assimilation of surrounding country rock
Plate Tectonics Origin of Diorites/Andesites • Subduction zones • Plate collision boundary where one plate overrides another (subduction zone) • Oceanic to cont or oceanic to oceanic • Pacific Ring of Fire • Borders Pacific Ocean • High rate of volcanism and seismic activity • Produced by partial melting of the upper mantle • Inclusion of water and felsic material from the subducting plate • Assimilation of felsic material from surrounding country rock and magma rises
Plate TectonicsOrigin of Granites/Rhyolites • Nearly all occur on continents • Originate from partial melting of crust • Most appear at or near subduction zones • Very viscous • Rise slowly • Generally cool before eruption • Hence larger population of intrusive than extrusive • Produce many economic deposits • Hydrothermal deposits near plutons
Plutons • Tabular - book shaped usually located within zones of weakness • Concordant • Sills – usually in bedding planes etc. • Discordant • Dikes – cut across pre-existing rocks • Massive • Concordant • Laccoliths (mushrooms) • Discordant • batholiths
Andesite Mt. St. HelensIntermediate/mod silica/mod viscosity
