PTYS 554
Evolution of Planetary Surfaces
Volcanism I
PYTS 554 – Volcanism I
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Volcanism I
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Volcanism II
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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
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Interaction with volatiles (Maars, Tuyas etc…)
Ash columns and falls, Surges and flows
Igminbrites, tuffs, welding
Pyroclastic deposits
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PYTS 554 – Volcanism I
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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
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Volcanism on Earth
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Mostly related to plate tectonics
Mostly unseen. ~30 km3 per year (~90%) never reaches the surface
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Rift-zone and subduction-zone volcanism has very different causes
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PYTS 554 – Volcanism I
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Volcanic material derived from the mantle
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Silicate composition built from SiO4 tetrahedra
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Mantle rocks built from Olivine and Pyroxene
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Olivine
 Isolated tetrahedra joined by cations (Mg, Fe)
 (Mg,Fe)2SiO4 (forsterite, fayalite)
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Pyroxene
 Chains of tetrahedra sharing O atoms
 (Mg,Fe) SiO3 (orthopyroxenes)
 (Ca, Mg, Fe) SiO3 (clinopyroxenes)
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PYTS 554 – Volcanism I
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Partial melting
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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
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PYTS 554 – Volcanism I
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Magma is characterized by silica and alkali
metal content
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Partial melting of fertile mantle produces basalts
Higher temperatures mean more Olivine is
melted (lowers Si/O ratio)
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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
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Recall that for the geotherm rolls over when
radiogenic isotopes are in the crust
dT
d 2T H
=k 2 +
dt
dz
rc
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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
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Ordinarily mantle material would never melt
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Three ways to get around this (ranked by importance)
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Lower the pressure by moving mantle material upwards
Change the solidus location (adding water)
 Important only on Earth
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Raise the temperature (plumes melting the base of the
crust)
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PYTS 554 – Volcanism I
Decompression melting
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Lithosphere
δ<<h
z
h
Convection creates near isothermal mantle
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ΔT
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Temperature changes accommodated across
boundary layers
Heat transport across boundary layer is
conductive
Rates of cm/year
T
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Mantle temperatures follow an adiabat
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α : Thermal expansion coefficient
Cp : Heat capacity
dT
Ta
=
dP adiabatic rCP
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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
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Material can cross the melting curve
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Ignore the
lithosphere/asthenosphere
boundary in this figure
PYTS 554 – Volcanism I
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Most important mechanism for rift zones
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Mantle plumes can also create hot-spot volcanism with this mechanism
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Requires a thin lithosphere
Melting starts at ~50km
Ocean island basalts
Accounts for ~75% of terrestrial volcanism
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…and probably 100% of planetary volcanism on other terrestrial planets
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PYTS 554 – Volcanism I
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Adding water changes the melting point
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As solid stability increases
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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
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Descending slabs loose volatiles
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From hydrated minerals e.g. mica at 100km
From decomposition of marine limestones
Causes mantle melting – leads to island arc basalts
Melosh, 2011
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PYTS 554 – Volcanism I
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Magma transport
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Mantle melt forms at crystal junctions
 High surface energy
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Wetting angle determines whether melt
can form an interconnected network
 <60° required for permeability
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Less dense liquid flows upwards through
the permeable mantle.
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At mid-ocean ridges the asthenosphere
comes all the way up to the base of the
crust
Melt collects in magma chamber
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PYTS 554 – Volcanism I
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Things are harder when there’s a lithosphere
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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
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Magma ascends through the lithosphere (and oceanic crust) in dikes
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 Fine as long as ρ(magma) < ρ(country rock)
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Magma ascends to the level of neutral bouyancy
Lithosphere
Magma
Tilling and Dvorak, 1993
PYTS 554 – Volcanism I
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What about under continents?
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Rising basaltic melt encounters continental
crust
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Thin crust: basaltic volcanism still possible
 SW United states during Farallon subduction
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Thick crust: Basalts don’t reach the surface
 Andes today
 Basalt underplates the crust and heats the continental
rock
 Melting produces felsic magma
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Intermediate states are common so we have a
wide variety of magma compositions in
continental volcanism
 Likewise for continental hotspot volcanism…
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Under continental crust transport is harder
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Density change at the Moho
Now ρ(magma) > ρ(country rock)
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Magma chamber at the base of the crust
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Felsic melts are still buoyant and can rise to form shallower magma chambers
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PYTS 554 – Volcanism I
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Differentiation occurs within magma chambers
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Minerals condense and fall to the floor
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Cumulates
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Follows Bowens reaction series
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Melts become more felsic
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Volatiles no longer kept in solution
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H2O and CO2
Starts to build pressure in the chamber
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Pressure can force out magma – Eruptions!
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 Intrusive eruptions cool slowly below the surface
 Extrusive eruptions cool quickly on the surface
Discontinuous
Continuous
PYTS 554 – Volcanism I
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Felsic magmas tend to have more water
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Water is a necessary component to form felsic melts and granites
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PYTS 554 – Volcanism I
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Intrusive structures
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Sills
Dikes
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PYTS 554 – Volcanism I
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Intrusive structures
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Laccolith – bows up preexisting layers, so shallow
Lopolith – subsidence from overlying layers - deep
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PYTS 554 – Volcanism I
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Batholith
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Many frozen magma chambers
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PYTS 554 – Volcanism I
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Formation of bubbles
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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
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Emptying the magma chamber causes decompression
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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
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If volatile content is very high
If viscosity is very high and bubbles can’t escape
Generates volcanic pumice and ash
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PYTS 554 – Volcanism I
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Volcanism I
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Volcanism II
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
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
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PYTS 554 – Volcanism I
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Released volatiles power the eruption
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Injection of new magma
Fractional crystallization
Collapse of overburden
Interaction with ground water
Etc…
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