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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
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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
