Lecture 10 Up from the Inferno Magma and Igneous Rocks Igneous Rocks are rocks that form from a melt Solidified molten rock or melt which freezes at high temp Earth is mostly Igneous rock Magma subsurface Lava 0melt at the surface Magma erupts via volcanoes Melted rocks can cool above or below ground There are many varieties of igneous rocks Why does magma form Earth is hot inside Why Planetesimal and meteorite accretion Gravitational compression Differentiation Radioactive decay Magma Formation Partial melting in crust upper mantle Melting is from pressure release Volatile addition heat transfer Geothermal gradient Earth is hot inside Geothermal gradient varies form place to place Pressure release Due to high pressure the rock does not melt A drop in pressure initiates decompressional melting Heat transfer Rising magma carries mantle heat This raises T in crustal rock Crustal rock melts at lower T Addition of Volatiles Volatiles cause rocks to melt at much lower Temp Water Carbon dioxide Adding volatiles to hot dry rocks initiates melting Important subduction process What is Magma made of Magmas have 3 components solid liquid and gas Solid solidified are carried by the liquid Liquid the melt itself is comprised of mobile ions Different mixes of elements yields different magmas Magma Compositions Composition controls density Temp And viscosity These characteristics govern eruptive style Why are there different magma compositions Magmas vary chemically due to Initial source rock compositions Partial melting Assimilation Fractional crystallization Magma variation Source rock dictates initial magma composition Mantle source ultramaifc and mafic magmas Crustal source Mafic intermediate and felsic magmas Partial melting Upon heating silica rich minerals melt first Partial meting then yields a silica rich magma Removing a partial melt from its source creates Felsic magma Mafic residue Assimilation Magma melts the country rock it passes through Assimilated materials change magma composition Fractional Crystallization As magma cools early crystals settle by gravity Melt composition changes as a result Magma migration Magma is less dense than rock it moves upward o Magma moves by o Injection into cracks o Melting overlying rocks o Squeezed by overburden thereby increasing viscosity Viscosity controls migration ease Viscosity depends on temp volatiles and silica Pressure decrease with upward migration releases volatiles bubbles Temp Hotter lower viscosity Cooler higher viscosity Volatile content More volatiles lower viscosity Less volatiles higher viscosity Silica content Less Silica lower viscosity More silica higher viscosity Igneous Environments Two major categories based on cooling site o Extrusive settings cool at or near the surface Cool rapidly Chill too fast to grow big crystals Intrusive settings cool at depth o Lose heat slowly o Crystals grow large Most mafic magmas extrude Most felsic magmas do not Extrusive characteristic Lava flows sheets of cooled lava Lava flows exit volcanic vents and flow outward Lava cools as it flows eventually solidifying Low viscosity lava basalt can flow long distances Intrusive Characteristics Intrusive rocks cool at depth they don t surface Magma invading colder country rock initiates o Thermal heat metamorphism and melting o Inflation of fractures pushing the rock aside o Incorporation of country rock fragments xenoliths o Hydrothermal hot water alteration Intrusive contacts preserve evidence of high heat o Baked zone rim of heat altered country rock o Chill margin magma at contact that cooled rapidly Xenolith country rock fragment in magma o Thermally altered o Magma cooled before xenolith could be assimilated Intrusive activity Tabular intrusions o Magma intrudes into other rocks in 2 ways o As planar tabular bodies dikes and sills and o As balloon shaped blobs plutons o Tend to have a uniform thickness o Can be traced laterally o Two major subdivisions sill and dike o Dikes and sills o Cause the rock to expand and inflate o Thermally alter the country rock o Dikes o Sills o Spread rocks sideways o Dominate in extensional settings e g MORs o Lift entire landscapes skyward o Usually intruded near the surface o Both dikes and sills exhibit wide variability o Size o Thickness or width o Lateral continuity Plutonic activity Most magma is emplaced at depth in the Earth o A large deep igneous body is called a pluton Plutonic intrusions modify the crust Plutons may coalesce to form a larger batholith o Plutons are created at subduction zones o Magma generations may occur of over 10s of Ma o Long subduction history linked to large batholiths Intrusive and Extrusive o Intrusive and extrusive rocks commonly co occur o Magma chambers feed overlying volcanoes o Magma chambers may cool to become plutons o Many igneous geometries are possible Cooling Rates o Haw fast is heat lost o Depth deep is hot shallow is cool Deep plutons cool slowly Shallow flows cool rapidly o Shape surface to volume ratio Spherical bodies cool slowly Tabular bodies cool faster o Ground water Ground water removes heat What you should know o Definition of an igneous rock o Igneous rock classification o Basic igneous minerals o Bowen s reaction series o Basic textural properties of igneous rocks Igneous classification o Igneous rock classification is based on composition and texture o Composition reveals magma source o Texture reveals cooling history o Fast cooling smaller crystals o Slow cooling larger crystals Composition Felsic 66 to 76 Silica SiO2 dominated by quartz and feldspar Intermediate 52 to 66 silica between felisc and mafic composition Mafic 45 to52 silica contains Fe and Mg minerals Ultramafic 38 to 45 silica dominated by Olivine Minerals composition from a cooling melt o Bowen s reaction series the sequence of minerals that crystallize from silicate melt o Mafic minerals e g olivine form at higher temperatures than felsic minerals o Mineral composition of rock indicates temperature at which it crystallized AS silicate minerals crystallize at lower temperatures they form more complex structures Igneous Rocks Vesicular Texture o Igneous rocks are names on the basis of heir texture and composition o Composition of a rock is the assemblage of minerals it contains o Texture of a rock is the size and arrangement of the minerals it contains o Vesicular texture many pits from gas escape Basalt o Extrusive rock fast cooling Glassy Texture o Obsidian o Glassy texture without
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