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Transcript
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