PTYS 554 Evolution of Planetary Surfaces Volcanism I

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PTYS 554 Evolution of Planetary Surfaces Volcanism I

PTYS 554 Evolution of Planetary Surfaces Volcanism I

PYTS 554 – Volcanism I l Volcanism I n n l Volcanism II n

PYTS 554 – Volcanism I l Volcanism I n n l Volcanism II n n l 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 n n Interaction with volatiles (Maars, Tuyas etc…) Ash columns and falls, Surges and flows Igminbrites, tuffs, welding Pyroclastic deposits 2

PYTS 554 – Volcanism I l 3 Volcanoes on all the terrestrial bodies (and

PYTS 554 – Volcanism I l 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 l Volcanism on Earth n Mostly related to plate

PYTS 554 – Volcanism I l Volcanism on Earth n Mostly related to plate tectonics Mostly unseen. ~30 km 3 per year (~90%) never reaches the surface n Rift-zone and subduction-zone volcanism has very different causes n 4

PYTSForming 554 – Volcanism I Heating Crusts and l A melt has a bulk

PYTSForming 554 – Volcanism I Heating Crusts and l A melt has a bulk chemical composition, but no crystals l Minerals are mechanically separable crystals with a distinct composition n l Terrestrial planets are dominated by silicon-oxygen based minerals – silicates Silicate rocks are built from Si. O 4 tetrahedra 5

PYTSForming 554 – Volcanism I Heating Crusts and 6 Connected with metals l Depending

PYTSForming 554 – Volcanism I Heating Crusts and 6 Connected with metals l Depending on how Oxygen is shared n Olivine Share O atom w Isolated tetrahedra joined by cations (Mg, Fe) w (Mg, Fe)2 Si. O 4 (forsterite, fayalite) n Pyroxene w Chains of tetrahedra sharing 2 Oxygen atoms w (Mg, Fe) Si. O 3 (orthopyroxenes) w (Ca, Mg, Fe) Si. O 3 (clinopyroxenes) n Feldspars w Framework where all 4 oxygen atoms are shared 3 D-share O atoms

PYTS 554 – Volcanism I l Partial melting n n Rocks (incl. mantle rocks)

PYTS 554 – Volcanism I l Partial melting n n 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 l Magma is characterized by silica and alkali metal

PYTS 554 – Volcanism I l Magma is characterized by silica and alkali metal content n l 8 Partial melting of fertile mantle produces basalts Higher temperatures mean more Olivine is melted (lowers Si/O ratio) n n 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 l The geotherm rolls over when radiogenic isotopes are

PYTS 554 – Volcanism I l The geotherm rolls over when radiogenic isotopes are in the crust n n Steady-state solution: T = T 0 + (Q/k) z – (H/2 k) z 2 When d. T/dz=0 then z = Q/H ~ 100 km w H~0. 75 μW m-3 (for continental crust on Earth) w Q~0. 08 W m-2 l Ordinarily mantle material would never melt l Three ways to get around this (ranked by importance) n n Lower the pressure by moving mantle material upwards Change the solidus location (adding water) w Important only on Earth n Raise the temperature (plumes melting the base of the crust) 9

PYTS 554 – Volcanism I Decompression melting 10 Lithosphere δ<<h z h l ΔT

PYTS 554 – Volcanism I Decompression melting 10 Lithosphere δ<<h z h l ΔT Convection creates near-isothermal mantle n n Convection rates of cm/year Temperature changes accommodated across boundary layers by conduction T l Mantle temperatures follow an adiabat n n α : Thermal expansion coefficient Cp : Heat capacity n 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 n Material can cross the melting curve n n Ignore the lithosphere/asthenosphere boundary in this figure

PYTS 554 – Volcanism I l Most important mechanism for rift zones n n

PYTS 554 – Volcanism I l Most important mechanism for rift zones n n l Mantle plumes can also create hot-spot volcanism with this mechanism n l Requires a thin lithosphere Melting starts at ~50 km Ocean island basalts Accounts for ~75% of terrestrial volcanism n …and probably 100% of planetary volcanism on other terrestrial planets 11

PYTS 554 – Volcanism I l 12 Adding water changes the melting point n

PYTS 554 – Volcanism I l 12 Adding water changes the melting point n As solid stability increases w w w n Olivine – isolated tetrahedra Pyroxenes – chains Amphiboles – double chains Feldspar – sheets Quartz – 3 D frameworks Water breaks the Si-O bonds w Si. O 2 + H 2 O -> 2 Si OH w Acts in the same way that raising temperature does l Descending slabs loose volatiles n n n From hydrated minerals e. g. mica at 100 km From decomposition of marine limestones Causes mantle melting – leads to island arc basalts Melosh, 2011

PYTS 554 – Volcanism I l Magma transport n Mantle melt forms at crystal

PYTS 554 – Volcanism I l Magma transport n Mantle melt forms at crystal junctions w High surface energy n Wetting angle determines whether melt can form an interconnected network w <60° required for permeability l n Less dense liquid flows upwards through the permeable mantle. n 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 l 14 Things are harder when there’s a lithosphere

PYTS 554 – Volcanism I l 14 Things are harder when there’s a lithosphere n 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 n Magma ascends through the lithosphere (and oceanic crust) in dikes n w Fine as long as ρ(magma) < ρ(country rock) n Magma ascends to the level of neutral buoyancy Lithosphere Magma Tilling and Dvorak, 1993

PYTS 554 – Volcanism I l What about under continents? n Rising basaltic melt

PYTS 554 – Volcanism I l What about under continents? n Rising basaltic melt encounters continental crust n Thin crust: basaltic volcanism still possible w SW United states during Farallon subduction n Thick crust: Basalts don’t reach the surface w Andes today w Basalt underplates the crust and heats the continental rock w Melting produces felsic magma n Intermediate states are common so we have a wide variety of magma compositions in continental volcanism w Likewise for continental hotspot volcanism… l Under continental crust transport is harder n Density change at the Moho Now ρ(magma) > ρ(country rock) n Magma chamber at the base of the crust n Felsic melts are still buoyant and can rise to form shallower magma chambers n 15

PYTS 554 – Volcanism I l Differentiation occurs within magma chambers l Minerals condense

PYTS 554 – Volcanism I l Differentiation occurs within magma chambers l Minerals condense and fall to the floor n Cumulates l Follows Bowens reaction series l Melts become more felsic l Volatiles no longer kept in solution n H 2 O and CO 2 Starts to build pressure in the chamber n Pressure can force out magma – Eruptions! n 16 w Intrusive eruptions cool slowly below the surface w Extrusive eruptions cool quickly on the surface Discontinuous Continuous

PYTS 554 – Volcanism I l Intrusive structures n n Sills Dikes 17

PYTS 554 – Volcanism I l Intrusive structures n n Sills Dikes 17

PYTS 554 – Volcanism I l Intrusive structures n n Laccolith – bows up

PYTS 554 – Volcanism I l Intrusive structures n n Laccolith – bows up preexisting layers, so shallow Lopolith – subsidence from overlying layers - deep 18

PYTS 554 – Volcanism I l Batholith n Many frozen magma chambers 19

PYTS 554 – Volcanism I l Batholith n Many frozen magma chambers 19

PYTS 554 – Volcanism I l Felsic magmas tend to have more water n

PYTS 554 – Volcanism I l Felsic magmas tend to have more water n Water is a necessary component to form felsic melts and granites 20

PYTS 554 – Volcanism I l Formation of bubbles n n Reduces magma density

PYTS 554 – Volcanism I l Formation of bubbles n n Reduces magma density – helps magma rise to the surface Also increases viscosity w Less water in the melt - Allows silica to polymerize w Expanding bubbles cool magma l Emptying the magma chamber causes decompression n l 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 n n n 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 l Volcanism I n n l Volcanism II n

PYTS 554 – Volcanism I l Volcanism I n n l Volcanism II n n l 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 n n Interaction with volatiles (Maars, Tuyas etc…) Ash columns and falls, Surges and flows Igminbrites, tuffs, welding Pyroclastic deposits 22

PYTS 554 – Volcanism I 23 Long-lived radioisotopes l Mantle concentrations of K, Th

PYTS 554 – Volcanism I 23 Long-lived radioisotopes l Mantle concentrations of K, Th and U are 2 orders of magnitude lower than crust n Large mantle volume compensates for that ~34 TW for Earth ~13 extra TW for primordial heat and core phase changes

PYTS 554 – Volcanism I l Released volatiles power the eruption n n Injection

PYTS 554 – Volcanism I l Released volatiles power the eruption n n Injection of new magma Fractional crystallization Collapse of overburden Interaction with ground water Etc… 24