Chapter 4 magma molten rock Igneous rocks are ones that form by cooling of magma either quickly volcanic eruptions or intrusion of thin bodies of magma along fractures within the earth or slowly intrusive or plutonic igneous rocks that are completely crystallized lava magma that erupts at the earth s surface Lava will erupt quietly to form lava flows with vari ous features if the dissolved gas content of the magma is low If a magma with a relatively high dis solved gas content rises to the surface the gas can come out of solution to form bubbles that can drive more violent volcanic eruptions that result in formation of pyroclastic layers including volcanic ash Melting rocks pressure decrease rock is under pressure from overlaying rock so despite the heat it stays solid A de crease in pressure permits melting so magma forms when hot mantle rock rises to shallow depths which explains how basalt magma forms by melting of rising mantle rock in the asthenosphere beneath divergent plate boundaries and above hot spots volatile addition substances that evaporate easily mix with hot dry rock and break chemical bonds so rock begins to melt the example we discussed was the release of OH from heating of subducted water rich basalt of oceanic lithosphere at subduction zones to form andesite magma heat transfer from rising magma melting of rocks in the crust to form rhyolite magma that could happen at continental rifts hot spots beneath a continent rhyolite at Yellowstone hot spots beneath oceanic crust rhyolite at Iceland sub duction zones beneath continental plates Andes etc Table 4 1 organizes magmas according to the magma s silica weight SiO2 content Magmas that are mafic basalt have fairly low SiO2 45 52 whereas felsic rhyolite magmas have the high est SiO2 66 76 Intermediate magmas andesite have SiO2 in between mafic and felsic Because ultramafic magmas are vanishingly rare among modern magma types we won t focus on them in this course You should be familiar with Basalt Andesite Rhyolite as examples of Mafic Intermediate Felsic magmas The main factors processes that affect magma chemistry include Source rock composition magma chemistry is influenced by the chemistry of the rock that melts to form the magma crustal melting vs mantle melting Partial melting A magma that forms by a small melting of a rock will be highly enriched in those atoms that are most weakly held to crystal structures As the of melting increases the magma s overall chemical composition becomes more and more like that of the rock that is un dergoing melting Assimilation rising magma can interact with blocks of rock that drop into the magma from the walls or roof or it can just interact chemically with whatever rock happens to be adjacent to the magma chamber Atoms that are more abundant in the wallrocks will tend to be incorporated in the magma diffusion along concentration gradients just normal chemistry and vice versa Magma mixing if basalt magma and rhyolite magma both occur in the same location perhaps rhyolite was formed by heat released from the basalt and not all the basalt has crystallized yet it is possible to blend the two compositions to form intermediate chemistry mixtures Fractional crystallization Bowen s Reaction Series provides a way to change the chemistry of a cooling and crystallizing magma by dropping out crystals whose chemistry is different from the average chemistry of the magma they form from Loss of such crystals will increase the rela tive in the magma of any atoms that are more abundant in the magma than in the crystals and will cause a decrease in the relative in the magma of any atoms whose abundance in the crys tals is greater than that atom s abundance in the magma Generally mafic magmas become more felsic by this process Magma rises because buoyancy drives magma upward because magma is less dense than surrounding rock and because the weight of overlying rock creates pressure at depth that squeezes magma upwards viscosity resistance to flow magmas with low viscosity flow more easily than those with high Magma with more molecular chains flow easier felsic magmas are more viscous than mafic magmas so mafic magma spreads out more quickly once it erupts to cover a wide area with a thin sheet but not rhyolite Cooling magma depth of intrusion magma deep in crust cools slowly shape and size of magma body the greater the surface area the faster it cools and presence of circulating groundwater water carries away heat The chemistry of magma always changes during crystallization because the chemistry of the minerals that form from the magma olivine pyroxene amphibole biotite muscovite quartz K feldspar Ca Na feldspar are not identical to the magma s chemistry Bowen s Reaction Series it is useful to memorize the sequence of crystal appearance shown in Box 4 1 Figure 1 b shows the typical order in which crystals appear during cooling of a magma Mafic magmas have enough Ca Mg Fe mantle source rock chemistry influences magma chemistry to start at the top and crystallize Mg rich minerals like olivine and Ca rich feldspar As you crystallize and remove olivine the SiO2 of the remaining magma increases because olivine s SiO2 is lower than that of the magma until you have enough SiO2 to stabilize single chain structures like pyroxene And so on If you start with a felsic magma rich in Si Na K and fairly low Ca Mg Fe contents you won t start at the top of Bowen s reaction series but instead you see early formation of K feldspar maybe biotite or muscovite mica etc When you freeze a pan of water the chemistry of the remaining water doesn t change because you re crystallizing a mineral ice whose chemistry is identical to that of the liquid With magmas and silicate minerals that is never true so all silicate magma compositions evolve due to fractional crystallization during cooling In volcanic systems you generally erupt the magma a few crystals into the air and it cools too quickly for Bowen s Reaction Series to go very far Magma that intrudes deep within the earth will cool slowly enough to undergo fairly extensive chemical differentiation over the course of its cooling down cycle Volcanic products erupt at the surface of the earth and are pulled down by gravity onto pre exist ing earth materials soil rocks mud on the seafloor etc This permits geologists to use super position as a philosophical tool in deciding relative ages of events in the volcanic realm If a lava flow sits on top of a pyroclastic ash layer