oCauses of MeltingDecrease in pressure (P)—decompressionThe base of the crust is hot enough to melt mantle rock. But, due to high P, the rock doesn’t melt. Melting will occur if P is decreased.oP drops when hot rock is carried to shallower depths.Mantle plumesBeneath riftsBeneath mid-ocean ridgesoP drops when hot rock is carried to shallower depths.Mantle plumesBeneath riftsUnder mid-ocean ridgesAddition of volatiles (flux melting)Volatiles lower the melting T of a hot rock.Common volatiles include H2O and CO2.Subduction carries water into the mantle, melting rock.oMagmas have three components (solid, liquid, and gas).Solid—solidified mineral crystals are carried in the melt.Liquid—the melt itself is composed of mobile ions.Dominantly Si and O;lesser Al, Ca, Fe, Mg, Na,and KOther ions to a lesserextent.Different mixes of elements yielddifferent magmas.oGas—variable amounts of dissolved gasoccur in magma.Dry magma—scarce volatilesWet magma—up to 15% volatilesWater vapor (H2O)Carbon dioxide (CO2)Sulfur dioxide (SO2)Nitrogen (N2)Hydrogen (H2)oMajor Types of MagmaThere are four major magma types based on % silica (SiO2).
Felsic (feldsparand silica)66–76% SiO2olight tan to pink or maroon Intermediate52–66% SiO2olighter gray or greenish grayMafic (Mg- and Fe-rich)45–52% SiO2oBlack or dark grayUltramafic 38–45% SiO2Why are there different magma compositions?Magmas vary chemically due toinitial source rock compositions.partial melting.assimilation.magma mixing.Different magmas may blend ina magma chamber. The result combines thecharacteristics of the two. Classifying Igneous Rocks according to textureoCrystalline Igneous RocksGabbro, Diorite, GraniteCooled slowly inplutonsIntrusiveLong time to growBasalt, andesite and rhyolite Cooled quickly in lava flows or near surface dikes and sillsExtrusive, Fine Grained, don’t have time to growDistinguish among thedifferent compositionalclasses on the basis of silicacontent (ultramafic, mafic,intermediate, felsic) and wedistinguish among thedifferent rock types containdifferent groups of mineralsOne of the major types ofigneous rocks in peridotiteoGlassy Igneous RocksGlassy texture in felsic igneous rocks because high composition of silicaIn some cases, a rapidly cooling lava freezes while it still contains a high concentration of gas bubbles-these bubbles remain as open holesknown as vesiclesTypesObsidianoSolid, felsic glass
oBlack or brownoIt is very hard to crack itTachyliteoVesicle-free maws consisting of more than 80% mafic glass oRarer than obsidianPumiceoFelsic volcanic rock oConsists of tiny vesicles, each of which is surrounded bya thin screen of glassoIt can look like spongeoForms from quickly cooling, volatile-rich lava Scoria
You've reached the end of your free preview.
Want to read all 23 pages?
- Fall '09
- Plate Tectonics, Gypsum