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Lecture 8: Volcanism

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Lecture 8: Volcanism Lecture 8: Volcanism EAS 2200 Introduction to the Earth System Today’s Plan Introduction Melting in the Earth mid-ocean ridges subduction zones mantle plumes Crystallization of igneous rocks Volcanic eruptions Introduction Volcanic eruptions are among the most spectacular natural phenomena. Where does the magma come from? Why does most volcanism occur only in certain areas? What causes eruptions to sometimes be catastrophic and sometimes quiescent? Why is there such a variety of igneous rocks? Where does magma come from? Early ideas: Hot vapors produce melting Burning coals layers provide heat for melting Global layer of molten rock at depth Modern ideas: Decompression melting Flux melting Intrusions of magma into the crust (but this begs the question of the origin of the original magma). Deep burial of low melting point material (rare). Melting of Rock Complex (“multi-phase”) substances progressively melt over a range of temperatures. The lowest temperature at which melt exists (temperature at which melting begins) is known as the solidus. The highest temperature at which solid persists (temperature at which melting is complete) is known as the liquidus. The melting range for most rocks (diference in solidus and liquidus) is several hundred degrees C. In essentially all cases, melting in the Earth is believed to be partial (i.e., liquidus temperature Volcanoes are like Clouds Decompression Melting Solidus temperature of rock decreases with decreasing pressure. Temperature of rising mantle rock also decreases with pressure (adiabatic decompression). Adiabat is steeper than solidus, so that rising mantle rock eventually reaches solidus and Melting and Mantle Convection We can expect melting to occur within hot, rising mantle convection cells. As we have seen, plate tectonics is part of mantle convection, with mantle rising beneath divergent plate boundaries. Thus melting beneath mid-ocean ridges (and Volcanism at Mid-Ocean More than 90% of terrestrial volcanism occurs at mid-ocean ridges - but we never see it! The lava erupted there is particularly uniform in composition and given the acronym MORB (mid- ocean ridge basalt). The lava is quickly quenched by seawater to form pillows. Magma Chambers and Structure of the Oceanic Magma rising into the oceanic crust “ponds” to form magma chamber. Most of the magma does not erupt, but instead crystallizes at depth forming gabbro. Consequently, the oceanic crust is layered: Lava flows on top Sheeted dikes - pathways of magma to surface. Gabbro layer - magma that crystallized within the magma chamber. Ridges and Rises East Pacific Rise Mid-Atlantic Ridge Why are mid-ocean ridges ridges? Ridges stand above the surrounding seafloor by ~ 2 km. The are not elevated because of a build-up of lava flows. The oceanic crust is typically 6 km thick everywhere (if anything, crust is thinner right at the axis). Ridges and rises are elevated is because they are hot and thermally expanded. A thought experiment: Coefcient of thermal expansion, α, is only 10-5. If the outer 100 km (lithospheric thickness) is 200°C hotter, then: 200˚C × 10-5 × 100 km = 2 km After formation, the lithosphere slowly cools and thermally contracts. Consequently, the seafloor gets progressively deeper. The cooling depends only on time (decreases with the East Pacific Rise (EPR) S-wave image of the East Pacific Rise Mid-Atlantic Ridge Mid-Atlantic Ridge (MAR) has rift valley, EPR does not. MAR has steep flanks, EPR does not. EPR has permanent (“steady state”) magma chamber, MAR does not. Why the diference? Ridges and Rises: the diference is spreading Graben forms on MAR because of stretching of strong lithosphere On EPR, volcanism is too frequent & lithosphere is too weak for a graben to develop (faulting still happens) Diference in flank steepness is due to diference in spreading rate. On the MAR, magma flux (and therefore heat flux) is not high enough to keep the magma chamber from freezing. Hydrothermal Processes Basalt fractures as it cools, allowing water to penetrate the young oceanic crust. Water is heated and reacts with the oceanic crust. Principle Hydrothermal Precipitation of Anhydrite (CaSO4). Removal of Mg from seawater, acidification: 2+ Mg + Mg2Si2O6 + 3H2O + → Mg3Si2O5(OH)4 + 2H Reduction of sulfate: 2- 2– SO4 + 8FeO → S + 4Fe2O3 Dissolution of Fe, Mn, Zn, Cu, etc. Fe(solid) + 3H+ → Fe (diss) + 3H+(solid) Precipitation of sulfides and hydroxides Consequences of Ridge Crest Hydrothermal Activity “Bufers” composition of seawater (e.g., important ‘sink’ for Mg) Responsible for many “base metal” (e.g., Cu, Zn, Pb) ores Metamorphoses and “hydrates” oceanic crust Sustains unique chemosynthetic communities. Major reaction site in global water cycle Did life originate at hydrothermal vents? Energy source Chemosynthesis is simpler than photosynthesis Chemical raw materials Variety of chemical raw materials Also, variety of mineral surfaces to catalyze reactions. Insulation from the hostile surface environment Protection from UV radiation Some protection meteorite, asteroid bombardment Highly variable climate Vent bacteria are among the simplest, most primitive Subduction Zone Volcanism Why is there melting above subduction zones, at convergent plate boundaries, where cold lithosphere is sinking and compressing? Here, flux melting seems to be important - the addition of water to mantle rock lowers the solidus. Melting in Subduction Zones As oceanic crust and sediment sink into the mantle, they are subjected to increasing heat and pressure causing breakdown of hydrous minerals. The water released by this dehydration rises into the overlying, hotter mantle wedge causing partial melting. Water, Volcanism & Plate Tectonics Released H O causes H2O added by 2 hydrothermal systems melting and explosive at mid-ocean ridge volcanism H2O released by dehydration during subduction “Intraplate” Volcanism Some volcanoes occur within lithospheric plates rather than at plate boundaries. An example is Kilauea, Hawaii, the world’s most active volcano. What causes melting in this situation? Mantle Plumes Most intraplate volcanism is thought to be caused by decompression melting in mantle plumes. Mantle plumes are rising convection currents not directly related to plate tectonics. They are the cause of Wilson’s hot spots. These mantle plumes are thought to begin at the core mantle Current Mantle Plumes Tomographic Images of Plumes Mantle Plumes, Large Igneous Provinces, and Climate Theory says that new plumes need large heads to initiate buoyant rise. When these “heads” reach the surface, they produce large pulses of volcanism, know as “flood basalts”, “plateau basalts”, “oceanic plateaus” - collectively called “large igneous provinces”. May be important in continent formation. CO2 released by these events may change climate & lead to mass Basalts and When rock undergoes partial melting, the Partial composition of melt is not the same as that of Melting the original rock. The peridotite of the upper mantle (source of most magma) consists of olivine (>50%), clinopyroxene, orthopyroxene, and either plagioclase, spinel, or garnet. When peridotite melts, it produces basalt, which crystallizes to mostly of clinopyroxene and plagioclase (+ ~10% olivine). The melt is less dense Piles of Basaltic Lava Flows in the Columbia than the solid and hence River Gorge rises. At first by percolation, Granites If melting of the mantle produces basalt, how do we explain the great variety of igneous rocks at the surface of the Earth? In particular, how do we explain the vast expanses of granites and granodiorites, such as the Sierra Glaciers carved Yosemite Valley out of part of the immense Sierra Nevada granitic batholith Origin of Granites, etc. Non-basaltic magmas can be produced in a variety of ways: Under certain circumstances, melting in the mantle can produce andesite. Melting of subducted basalt. Melting peridotite with high water concentrations at shallow depth. Melting of sediments and other rocks within the crust. A rock that is metamorphosed to the extent that it starts to melt is called a migmatite. Assimilation of country rock in the crust by Fractional Crystallization As in melting, crystallization takes place over a range of temperature. The composition of minerals crystallizing from the magma are diferent in composition from the magma. Therefore, as crystallization of the magma proceeds, the The Chemistry of Fractional Crystallization Wt % Olivin Parent Magma after 20% of: e Magma olivine crystallization SiO2 39.0 50.0 52.8 MgO 42.0 10.0 2.0 FeO 19.0 9.0 6.5 Al2O3 0.0 15.0 18.8 Bowen’s Reaction Series Sequence of minerals precipitating from a melt is sometimes called “Bowen’s* reaction series”. First minerals to crystallize are rich in Mg and Fe and poor in SiO2, subsequent ones are progressively richer in SiO2. Consequently, the remaining melt becomes progressively richer in SiO2, alkalis, etc., and poorer in Mg and Fe. Olivine (Mg,Fe)2SiO4 crystallizes first. It will subsequently react with the magma to form pyroxene: *Norman (Mg,Fe) L. Bowen2SiO was4 an+ experimentalSiO2 = (Mg,Fe) petrologist working at the Geophysical Laboratory of Carnegie InstitutionSi ofO Washington in the first half of the twentieth century. He first championed the idea that the variety2 of igneous2 6 rocks are produced by fractional crystallization. Assimilation Composition of magma will also change if the surrounding country rock melts and this melt mixes with the original magma. This process is called
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