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Geology - crystallized at some depth below the...

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(A) Igneous Rocks Igneous rocks are those formed at very high temperatures, crystallized from a molten silicate material known as magma. The crystal size (grain size) of an igneous is determined fundamentally by cooling rate: All other factors being equal, slower cooling means larger crystals, and the rocks are coarsely crystalline. Very rapid cooling produces finer-grained or glassy (i.e. no obvious crystals) rocks. Typically, plutonic rocks, which crystallize at depth, are more coarsely crystalline than volcanic rocks. Phorphry: formed if a magma began to crystalline slowly at depth and then was suddenly erupted from a volcano; coarse crystals (phenocrysts) embedded in the finer groundmass Igneous rocks are classified on the basis of depth of crystallization, crystal size and chemical composition. Plutonic (hypabyssal) igneous rocks
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Unformatted text preview: crystallized at some depth below the surface (intrusive) their exposure at the surface implies the erosion of the rocks that overlay them at the time of crystallization coarser grain sizes e.g. syenite, granite, granodiorite, diorite, gabbro, peridotite, dunite Volcanic igneous rocks crystallized at or near the surface (extrusive) finely grained or glassy e.g. rhyolite, andesite, basalt • Intrusive rock structures (pluton) Both the properties of the magma and the properties of the surrounding rocks (country rocks) play a role in determining the shape of intrusion formed. Concordant: approximately parallel to any structure (e.g. layering or folds) in the country rock e.g. sills, laccoliths, lopoliths, batholiths Discordant: cut across the structure of the country rock e.g. dykes...
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