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PTYS 554 Evolution of Planetary Surfaces Volcanism I PYTS 554 – Volcanism I Volcanism I Volcanism II Mantle convection and partial melting Magma migration and chambers Dikes, sills, laccoliths etc… Powering a volcanic eruption Magma rheology and volatile content Surface volcanic constructs Behavior of volcanic flows Columnar jointing Volcanism III Interaction with volatiles (Maars, Tuyas etc…) Ash columns and falls, Surges and flows Igminbrites, tuffs, welding Pyroclastic deposits 2 PYTS 554 – Volcanism I 3 Volcanoes on all the terrestrial bodies (and then some…) Mercury – Smooth plains Earth – Mount Augustine Moon – Maria Mars – Olympus Mons Venus – Maat Mons Io – just about everywhere PYTS 554 – Volcanism I Volcanism on Earth Mostly related to plate tectonics Mostly unseen. ~30 km3 per year (~90%) never reaches the surface Rift-zone and subduction-zone volcanism has very different causes 4 PYTSForming 554 – Volcanism I Heating Crusts and A melt has a bulk chemical composition, but no crystals Minerals are mechanically separable crystals with a distinct composition Terrestrial planets are dominated by silicon-oxygen based minerals – silicates Silicate rocks are built from SiO4 tetrahedra 5 PYTSForming 554 – Volcanism I Heating Crusts and Depending on how Oxygen is shared Olivine Isolated tetrahedra joined by cations (Mg, Fe) (Mg,Fe)2SiO4 (forsterite, fayalite) Connect with metals Pyroxene Can have Chains of tetrahedra sharing 2 Oxygen atoms impuriti (Mg, Fe) SiO3 (orthopyroxenes) es (Ca, Mg, Fe) SiO3 (clinopyroxenes) 6 Share O atoms Feldspars Framework where all 4 oxygen atoms are shared K rich Na rich Ca rich 3D-share O atoms PYTS 554 – Volcanism I Partial melting Rocks (incl. mantle rocks) are messy mixtures of many minerals In a pyroxene-olivine mixture the pyroxene melts more readily than the olivine More silica-rich minerals melt even more readily Melting mantle at the Eutectic has a specific composition – generally basaltic 7 PYTS 554 – Volcanism I Magma is characterized by silica and alkali metal content 8 Partial melting of fertile mantle produces basalts Higher temperatures mean more Olivine is melted (lowers Si/O ratio) Proportionally lower Silica in melt Proportionally more Iron etc… Io volcanism probably ultramafic High-temp melting of Earth’s mantle in early history produced Komatiite – primitive basalt Ultrabasic Primative Acidic Evolved Basic Fe rich Dark Dense Fe poor Light Less-dense PYTS 554 – Volcanism I The geotherm rolls over when radiogenic isotopes are in the crust dT d 2T H =k 2 + dt dz rc Steady-state solution: T = T0 + (Q/k) z – (H/2k) z2 When dT/dz=0 then z = Q/H ~ 100 km H~0.75 μW m-3 Q~0.08 W m-2 Ordinarily mantle material would never melt Three ways to get around this (ranked by importance) Lower the pressure by moving mantle material upwards Change the solidus location (adding water) Important only on Earth Raise the temperature (plumes melting the base of the crust) 9 PYTS 554 – Volcanism I Decompression melting Lithosphere δ<<h z h Convection creates near isothermal mantle ΔT Temperature changes accommodated across boundary layers Heat transport across boundary layer is conductive Rates of cm/year T Mantle temperatures follow an adiabat α : Thermal expansion coefficient Cp : Heat capacity dT Ta = dP adiabatic rCP Works out to only ~ 0.25-0.5 K/km Material rises and cools at this rate (i.e. not much) …but pressure drop is large Material can cross the melting curve 10 Ignore the lithosphere/asthenosphere boundary in this figure PYTS 554 – Volcanism I Most important mechanism for rift zones Mantle plumes can also create hot-spot volcanism with this mechanism Requires a thin lithosphere Melting starts at ~50km Ocean island basalts Accounts for ~75% of terrestrial volcanism …and probably 100% of planetary volcanism on other terrestrial planets 11 PYTS 554 – Volcanism I Adding water changes the melting point As solid stability increases Olivine – isolated tetrahedra Pyroxenes – chains Amphiboles – double chains Feldspar – sheets Quartz – 3D frameworks Water breaks the Si-O bonds SiO2 + H2O -> 2 Si OH Acts in the same way that raising temperature does Descending slabs loose volatiles From hydrated minerals e.g. mica at 100km From decomposition of marine limestones Causes mantle melting – leads to island arc basalts Melosh, 2011 12 PYTS 554 – Volcanism I Magma transport Mantle melt forms at crystal junctions High surface energy Wetting angle determines whether melt can form an interconnected network <60° required for permeability Less dense liquid flows upwards through the permeable mantle. At mid-ocean ridges the asthenosphere comes all the way up to the base of the crust Melt collects in magma chamber 13 PYTS 554 – Volcanism I 14 Things are harder when there’s a lithosphere No partial melting (otherwise it wouldn’t be rigid) so no permeable flow Pressures at the base of the lithosphere are too high to have open conduits Magma ascends through the lithosphere (and oceanic crust) in dikes Fine as long as ρ(magma) < ρ(country rock) Magma ascends to the level of neutral bouyancy Lithosphere Magma Tilling and Dvorak, 1993 PYTS 554 – Volcanism I What about under continents? Rising basaltic melt encounters continental crust Thin crust: basaltic volcanism still possible SW United states during Farallon subduction Thick crust: Basalts don’t reach the surface Andes today Basalt underplates the crust and heats the continental rock Melting produces felsic magma Intermediate states are common so we have a wide variety of magma compositions in continental volcanism Likewise for continental hotspot volcanism… Under continental crust transport is harder Density change at the Moho Now ρ(magma) > ρ(country rock) Magma chamber at the base of the crust Felsic melts are still buoyant and can rise to form shallower magma chambers 15 PYTS 554 – Volcanism I Differentiation occurs within magma chambers Minerals condense and fall to the floor Cumulates Follows Bowens reaction series Melts become more felsic Volatiles no longer kept in solution H2O and CO2 Starts to build pressure in the chamber Pressure can force out magma – Eruptions! 16 Intrusive eruptions cool slowly below the surface Extrusive eruptions cool quickly on the surface Discontinuous Continuous PYTS 554 – Volcanism I Felsic magmas tend to have more water Water is a necessary component to form felsic melts and granites 17 PYTS 554 – Volcanism I Intrusive structures Sills Dikes 18 PYTS 554 – Volcanism I Intrusive structures Laccolith – bows up preexisting layers, so shallow Lopolith – subsidence from overlying layers - deep 19 PYTS 554 – Volcanism I Batholith Many frozen magma chambers 20 PYTS 554 – Volcanism I Formation of bubbles Reduces magma density – helps magma rise to the surface Also increases viscosity Less water in the melt - Allows silica to polymerize Expanding bubbles cool magma Emptying the magma chamber causes decompression More volatiles degassed – faster ascent etc… Leads to a ‘detonation front’ that propagates downwards Runaway effect until the magma chamber empties Magma shredded by exploding bubbles If volatile content is very high If viscosity is very high and bubbles can’t escape Generates volcanic pumice and ash 21 PYTS 554 – Volcanism I Volcanism I Volcanism II Mantle convection and partial melting Magma migration and chambers Dikes, sills, laccoliths etc… Powering a volcanic eruption Magma rheology and volatile content Surface volcanic constructs Behavior of volcanic flows Columnar jointing Volcanism III Interaction with volatiles (Maars, Tuyas etc…) Ash columns and falls, Surges and flows Igminbrites, tuffs, welding Pyroclastic deposits 22 PYTS 554 – Volcanism I Released volatiles power the eruption Injection of new magma Fractional crystallization Collapse of overburden Interaction with ground water Etc… 23
