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Home , Magma, Volcanism

Lecture 8:

EAS 2200 Introduction to the System Today’s Plan

Introduction Melting in the Earth mid-ocean ridges  zones  plumes  of igneous rocks Volcanic eruptions Introduction

Volcanic eruptions are among the most spectacular natural phenomena. Where does the 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 at depth Modern ideas: Decompression melting  Intrusions of magma into the (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  We can expect melting to occur within hot, rising mantle convection cells.  As we have seen, plate 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 erupted there is particularly uniform in composition and given the acronym MORB (mid- ocean ridge ).  The lava is quickly quenched by seawater to form pillows. Magma Chambers and Structure of the Oceanic  Magma rising into the “ponds” to form .  Most of the magma does not erupt, but instead crystallizes at depth forming .  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 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 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 surfaces to catalyze reactions.  Insulation from the hostile surface environment  Protection from UV radiation  Some protection meteorite, bombardment  Highly variable  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 .  The water released by this dehydration rises into the overlying, hotter causing . Water, Volcanism & 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, , the world’s most active . What causes melting in this situation? Mantle Plumes  Most 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”, “oceanic ” - 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 of the (source of most magma) consists of (>50%), clinopyroxene, orthopyroxene, and either , , 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,  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 , such as the Sierra

Glaciers carved Yosemite Valley out of part of the immense Sierra Nevada granitic Origin of Granites, etc. Non-basaltic can be produced in a variety of ways: Under certain circumstances, melting in the mantle can produce . 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 . Assimilation of 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 :

*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 assimilation. Composition of Igneous Rocks Volcanic Eruptions Sometimes volcanoes erupt catastrophically, at other times, they erupt with a quiet efusion of lava. Why?  TwoFactors main factors A governfecting Volcanic how a volcano will erupt:

content (mainly H2O, but also CO2)  Magma  Explosive eruptions occur when gas exsolves from magma and is unable to escape. The resulting expanding bubbles cause the magma to fragment.  These are known as Plinian Eruptions.  (Quiescent eruptions Eruptions and Magma Composition  Both gas content and magma viscosity increase as fractional crystallization proceed.  Gas concentrations increase because the gas remains in the magma as minerals crystallize out.

 Viscosity increases with SiO2 content as the tetrahedra becoming increasingly linked or polymerized in the magma. High viscosity makes it difcult for gas bubbles to leave the magma.  Consequently, explosive eruptions are more common in dacitic and rhyolitic magmas. Basaltic magmas essentially never erupt in Plinian eruptions.  Water contents are higher in subduction zones magmas, hence subduction zone volcanoes are more explosive.  The water is derived from dehydration of subducting oceanic Melts  Primary structural element is still the tetrahedron  As in solids, these are linked to various degrees by bridging .  Main diference between melts and solids is the presence of long range structure in the latter.  (Note: this is also true of water - its structure is much like ice, with many individual molecules bound together by hydrogen bonds.) Silicate Melt  Physical and chemical properties depend on the degree of this linkage, or Polymerizatio polymerization.  Network forming : n  Si4+, Al3+, Fe3+,Ti4+  Promote polymerization  Network modifiers:  Mg2+, Ca2+, Na+, K+, H+,  Since their charge can only be balances by non- bridging O, these promote depolymerization.  On a weight percent basis, H has very large efect because of its low mass.  (Addition of ions also breaks up structure of water) Shield Volcanoes

Because basaltic are quite fluid (low viscosity), they build shield volcanoes with gentle slopes, such as . of Mt. St. Helens, 1980 Plinian Eruptions  Gas solubility decreases with pressure. So as magma rises, gas exsolves, forming bubbles (very much like uncorking champagne).  If sufcient gas is present and it cannot escape, it will eventually disrupt the magma in an . Dome-forming eruptions

2/15/06 2006-2008 eruption on Mt. St. Helens produced a new without significant . The dacitic lava degassed before erupting, but was too viscous to flow. Hazards of Explosive  The principle hazard of explosive eruptions are not the per se, or the ash falls, but pyroclastic flows.  Pyroclastic flows are hot debris avalanches that can travel at 200 km/hr.  They can be generated in several The Tragedy of St. Pierre

 Perhaps the most famous case of destructive pyroclastic flows was that of Vesuvius destroying Pompei in 79 AD.  More recently, pyroclastic flows destroyed the town of St. Pierre on and killed 29,000 Formation Large Plinian eruptions sometimes result in the roof of the magma chamber collapsing, producing (not to be confused with craters).