GEOL 104 Lect2

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Geological Sciences and Geological Engineering
GEOL 104
Michael T.Mc Bride

Igneous Rocks With the exception of Earths core the remainder of the planet is basically a huge mass of igneous rock that is partially covered by a thin layer of sedimentary and metamorphic rocksIgneous rock forms as molten rock coolssolidifiesParent material for igneous rockmagmamagma is formed by a process called partial melting Magma rises to the Earths surface because of buoyancy It is less dense than the surrounding rocksMagma that reaches the Earths surface is called lavaExtrusiveVolcanicIgneous rocks that form when magma solidifies at the surfaceIntrusivePlutonicIgneous rocks that form when magma crystallises at depth before reaching the surfaceThe greatest amount of igneous activity occurs at divergent plate boundariesSubstantial amounts of magma are also produced at subduction zonesEarths crust and mantle are composed primarily of solid not molten rockGeothermal gradientthe rate at which temperature increases as the depth increasesEnough heat can be generated from friction especially at subduction zones Crustal rocks are heated as they are pushed into the mantle during subductionHot mantle rocks can rise and intrude crustal rocksPressure increases with depthMelting occurs at higher temperatures atlower depths due to greater confining pressureHence an increase in confining pressure results in an increase in melting point of the rockdecompression melting is triggered when confining pressure drops enough This can occur as a result of convective upwelling hereby forcing the rock into areas of lower pressureThis process generates magma along ocean ridges where plates are rifting apartWater content and other volatiles lower the melting point of rocksThe effect of volatiles is magnified by pressure MagmaMost magma consists of three different partsThe melt The liquid portionmostly made up of oxygen and siliconThe solid components if any
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