IGNEOUS ROCKS EXTRUSIVE (Volcanic)- Fine-grained INTRUSIVE (Plutonic)- Coarse-grained MAGMA O Molten Rock – Usually with dissolved gasses O Generated at depth O If doesn’t reach surface, Solidifies underground – Intrudes country rock – Intrusive contact – Chill zone – Zenolith Igneous Rocks O Names based on mineral composition (which reflects chemical composition of the magma) and... Grain size – Course-grained: > 1 mm. – Fine-grained: < 1 mm. – Porphyritic – Pegmatite-very coarse-grained: >5 cm Igneous Rocks-Classification O Course-grained- Plutonic – GRANITE – Diorite – Gabbro – Ultramafic O Fine-Grained – Rhyolite – ANDESITE* – BASALT* Igneous Rock Identification O Granite (& Rhyolite) – High in Si + O – Low in Fe + Mg – Mostly feldspar & quartz – Light-colored O Basalt (& Gabbro) – “Low” in Si + O – High in Fe + Mg – no quartz, abundant ferromagnesian minerals – Dark colored O Andesite (& Diorite- intermediate) O Ultramafic rock (Peridotite) INTRUSIVE BODIES (STRUCTURES) O Bodies that solidified underground O Volcanic neck- shallow intrusion O Fills cracks- tabular bodies – DIKE- • If no layering in country rock • If country rock is layered- Discordant – SILL- less common • Concordant- parallel to layering in country rock O Pluton O BATHOLITH- – Large intrusive body – Exposed in an area greater than 100 square Km. – Coalesced smaller plutons O smaller bodies are called STOCKS O Batholiths a gathering of smaller blobs O Magma moves upward from depth as diapirs DISTRIBUTION OF PLUTONIC ROCK – Granite most abundant • Common in mountain ranges • In ancient rock that were mountain ranges that are now plains – Ultramafic rock in the mantle How magma forms O Partial melting of rock at depth O Source of heat – Geothermal gradient – Mantle plumes O Factors that control melting temperatures – Pressure – Water under pressure – Mixing of minerals How magmas evolve O Differentiation – Bowen’s Reaction Series • Discontinuous Branch • Continuous Branch – Crystal Settling • Ore deposits due to crystal settling O Partial melting O Assimilation O Mixing of magmas PLATE TECTONICS & IGNEOUS ACTIVITY O 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 O INTRAPLATE IGNEOUS ACTIVITY – Attributed to mantle plumes O CONVERGENT BOUNDARY – Origin of Andesite • Partial melting of asthenosphere above subducted crust • Water lowers melting temperature producing mafic magma • Ascending magma modified into intermediate magma – Origin of Granite • Partial melting of lower continental crust • Heat from mafic magma underplating the crust O Alternate hypotheses for explaining andesitic and granitic magmas – Partial melting of basalt in subduction zone – Assimilation of crustal rocks – Mixing of magmas – Melting of sedimentary rocks The End.