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IGNEOUS ROCKS • EXTRUSIVE (Volcanic) – Fine-grained • INTRUSIVE (Plutonic) – CoarseCoarse-grained Rock Cycle • Equilibrium • Interrelationships between: – – – – – igneous rocks sediment sedimentary rocks metamorphic rocks weathering and erosion Intrusive structures • PLUTON • BATHOLITH – Large intrusive body – Exposed in an area greater than 100 square km – Coalesced smaller plutons • Smaller bodies are called STOCKS • Magma moves upward from depth as diapirs Building a batholith out of plutons Classification of Igneous Rocks • Mineralogy • Chemistry • Mafic to Silicic • Ultramafic rock – Mantle peridotite • Varieties of granite – Proportions of qtz/plag/K-spar Sierra Nevada Batholith California Granodiorite (plagioclase-rich granite) Torres del Paine, Patagonian Batholith, southern Chile Granite pluton Intrusive contact Chalten or Cerro Fitzroy Patagonian Andes, Argentina Granite quartz + plagioclase + K-spar + biotite) Gabbro Adamello Complex, Italian Alps Diorite pluton, Plag + hbl + xenoliths of country rock Distribution of Plutonic Rocks • Granite most abundant in continents – Common in mountain ranges – Ancient mountain ranges that are now plains • e.g, Wolf River Batholith in Wisconsin • Ultramafic rock comprises the upper mantle Cordilleran Batholiths North and South America How does magma form? • Partial melting of rock at depth • Source of heat – Geothermal gradient • Rate of Temperature increase with depth • Average 30 oC/km depth • Not same everywhere • Tells us that heat is being conducted to the surface of the earth from the interior (Core > 5000 deg C) How does magma form? • Factors that control melting temperatures – Pressure • melting points of minerals increase with pressure – This is why increasing temperature along the geotherm alone fails to melt crustal rocks • reduction in pressure can therefore induce melting – Water added under pressure • lowers melting point of minerals – Mixing of minerals • lowers melting point of minerals Partial melting at top of mantle plume • Mantle plumes – Narrow upwelling of hot mantle rock from great depth – Brings deep rocks rapidly to pressures at which they can melt 500 km Partial melting beneath the Mid-Ocean Ridge • Diverging plates – Lithospheric plates spread apart – Creates narrow upwelling zone where hot mantle rock of the asthenosphere rises to fill in the gap – Decompression results in partial melting – Spreading animation – Whole mantle upwelling zones Partial melting in subduction zones • Converging plates • Addition of water from subducted crust into mantle lowers melting point of mantle rock • Mantle rock partially melts to form basalt • Basalt rises into crust – Evolves to andesite and rhyolite – Partially melts crust to create granite – Forms Granite batholiths Sea level Oceanic crust Sedimentary rocks Basalt Mantle (“rigid”) Oceanic lithosphere Kilometers 0 100 Asthenosphere (mantle) Continental crust Continental lithosphere Asthenosphere Sea level Oceanic crust Sedimentary rocks Oceanic lithosphere Kilometers 0 100 Basalt Mantle (“rigid”) Asthenosphere (mantle) Continental crust Continental lithosphere “Dewatering” of oceanic crust; water triggers melting in asthenosphere Asthenosphere Water from subducting crust Sea level Oceanic crust Sedimentary rocks Continental crust Basalt Mantle (“rigid”) Oceanic lithosphere Kilometers 0 100 Asthenosphere (mantle) Continental lithosphere “Dewatering” of oceanic crust; water triggers melting in asthenosphere Asthenosphere 800°C 1000°C 1200°C 1 1 2 80 00 00º 0º 0ºC C C Water from subducting crust Sea level Oceanic crust Sedimentary rocks Oceanic lithosphere Kilometers 0 100 Continental crust Basalt Mantle (“rigid”) Asthenosphere (mantle) Continental lithosphere “Dewatering” of oceanic crust; water triggers melting in asthenosphere Asthenosphere 800°C 1000°C 1200°C 1 1 2 80 00 00º 0º 0ºC C C Zone where wet mantle partially melts Water from subducting crust Sea level Oceanic crust Sedimentary rocks Oceanic lithosphere Kilometers 0 100 Continental crust Basalt Mantle (“rigid”) Continental lithosphere Asthenosphere (mantle) Mafic magma “Dewatering” of oceanic crust; water triggers melting in asthenosphere Asthenosphere 800°C 1000°C 1200°C 1 1 2 80 00 00º 0º 0ºC C C Zone where wet mantle partially melts Water from subducting crust Sea level Mixing of magma Intermediate magma Granitic plutons emplaced Silicic magma Partial melting of silicic crust Mafic magma Kilometers 0 100 How do magmas evolve? • Partial melting of mantle or crust is followed by: • Differentiation – Crystal Settling • Assimilation of country rock • Mixing of magmas How do magmas evolve? • Differentiation – Crystal Settling How do magmas evolve? • Assimilation of country rock How do magmas evolve? • Mixing of initially separate magmas Review: Plate Tectonics & Igneous activity • DIVERGENT BOUNDARY – Notably at mid-oceanic ridges – Sea floor Spreading – Magma from asthenosphere • Partial melting – Due to reduced pressure • Produces mafic magma • Solidifies into basalt and gabbro • Becomes oceanic crust • Unmelted residue remains as ultramafic rock Review: Plate Tectonics & Igneous activity • INTRAPLATE IGNEOUS ACTIVITY – Attributed to mantle plumes • Partial melting due to reduced pressure on upwelling mantle rock • Creates mafic magma (basalt) – Ocean Island chains – “Hot spot” magmatism Review: Plate Tectonics & Igneous activity • CONVERGENT BOUNDARY – Origin of Andesite • Partial melting of asthenosphere above subducted crust • Water lowers melting temperature producing mafic magma • Ascending magma evolves into intermediate magma – Origin of Granite • Partial melting of lower continental crust • Heat from mafic magma underplating the crust