Volcanism I

1. Volcanism I

2. Volcanism I • Mantle convection and partial melting • Magma migration and chambers • Dikes, sills, laccoliths etc… • Powering a volcanic eruption • Volcanism II • 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

3. Volcanoes on all the terrestrial bodies (and then some…) Moon – Maria Venus – Maat Mons Mercury – Smooth plains Io – just about everywhere Earth – Mount Augustine Mars – Olympus Mons

4. 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

5. Volcanic material derived from the mantle • Silicate composition built from SiO4 tetrahedra • Mantle rocks built from Olivine and Pyroxene • Olivine • Isolated tetrahedra joined by cations (Mg, Fe) • (Mg,Fe)2SiO4 (forsterite, fayalite) • Pyroxene • Chains of tetrahedra sharing O atoms • (Mg,Fe)SiO3 (orthopyroxenes) • (Ca, Mg, Fe) SiO3 (clinopyroxenes)

6. 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. Magma is characterized by silica and alkali metal content • 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

8. Recall that for the geotherm rolls over when radiogenic isotopes are in the crust • 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. Decompression melting • Convection creates near isothermal mantle • Temperature changes accommodated across boundary layers • Heat transport across boundary layer is conductive • Rates of cm/year Lithosphere δ<<h • Mantle temperatures follow an adiabat • α : Thermal expansion coefficient • Cp : Heat capacity • 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 z h ΔT T Ignore the lithosphere/asthenosphere boundary in this figure

10. Most important mechanism for rift zones • Requires a thin lithosphere • Melting starts at ~50km • Mantle plumes can also create hot-spot volcanism with this mechanism • Ocean island basalts • Accounts for ~75% of terrestrial volcanism • …and probably 100% of planetary volcanism on other terrestrial planets

11. 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. 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. 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

14. 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. 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! • Intrusive eruptions cool slowly below the surface • Extrusive eruptions cool quickly on the surface Continuous Discontinuous

16. Felsic magmas tend to have more water • Water is a necessary component to form felsic melts and granites

17. Intrusive structures • Sills • Dikes

18. Intrusive structures • Laccolith – bows up preexisting layers, so shallow • Lopolith – subsidence from overlying layers - deep

19. Batholith • Many frozen magma chambers

20. 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. Volcanism I • Mantle convection and partial melting • Magma migration and chambers • Dikes, sills, laccoliths etc… • Powering a volcanic eruption • Volcanism II • 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. Released volatiles power the eruption • Injection of new magma • Fractional crystallization • Collapse of overburden • Interaction with ground water • Etc…