Download Period 8 Volcanism

Lecture 8:
Volcanism
EAS 2200
Introduction to the Earth System
Today’s Plan
Introduction
Melting in the Earth
mid-ocean ridges
subduction zones
mantle plumes
Crystallization of igneous rocks
Volcanic eruptions
Introduction
 Volcanic eruptions are among the most
spectacular natural phenomena.
 Where does the magma come from?
 Why does most volcanism occur only in
certain areas?
 What causes eruptions to sometimes be
catastrophic and sometimes quiescent?
 Why is there such a variety of igneous
rocks?
Where does magma come
from?
 Early ideas:
 Hot vapors produce melting
 Burning coals layers provide heat for melting
 Global layer of molten rock at depth
 Modern ideas:
 Decompression melting
 Flux melting
 Intrusions of magma into the crust (but this
begs the question of the origin of the original
magma).
 Deep burial of low melting point material
(rare).
Melting of Rock
 Complex (“multi-phase”) substances
progressively melt over a range of
temperatures.
 The lowest temperature at which melt exists
(temperature at which melting begins) is known as
the solidus.
 The highest temperature at which solid persists
(temperature at which melting is complete) is known
as the liquidus.
 The melting range for most rocks (difference in
solidus and liquidus) is several hundred
degrees C.
 In essentially all cases, melting in the Earth is
believed to be partial (i.e., liquidus temperature
Volcanoes are like Clouds
Decompression Melting
 Solidus temperature of
rock decreases with
decreasing pressure.
 Temperature of rising
mantle rock also
decreases with pressure
(adiabatic
decompression).
 Adiabat is steeper than
solidus, so that rising
mantle rock eventually
reaches solidus and
Melting and Mantle
Convection
 We can expect
melting to occur
within hot, rising
mantle convection
cells.
 As we have seen,
plate tectonics is
part of mantle
convection, with
mantle rising
beneath divergent
plate boundaries.
 Thus melting
beneath mid-ocean
ridges (and
Volcanism at Mid-Ocean
 More than 90% of
terrestrial volcanism
occurs at mid-ocean
ridges - but we never see
it!
 The lava erupted there is
particularly uniform in
composition and given
the acronym MORB (midocean ridge basalt).
 The lava is quickly
quenched by seawater to
form pillows.
Magma Chambers and
Structure of the Oceanic
 Magma rising into the
oceanic crust “ponds” to
form magma chamber.
 Most of the magma does not
erupt, but instead
crystallizes at depth forming
gabbro.
 Consequently, the oceanic
crust is layered:
 Lava flows on top
 Sheeted dikes - pathways of
magma to surface.
 Gabbro layer - magma that
crystallized within the
magma chamber.
Ridges and Rises
East Pacific Rise
Mid-Atlantic Ridge
Why are mid-ocean
ridges ridges?
 Ridges stand above the surrounding seafloor by ~ 2 km.
 The are not elevated because of a build-up of lava flows.
The oceanic crust is typically 6 km thick everywhere (if
anything, crust is thinner right at the axis).
 Ridges and rises are elevated is because they are hot and
thermally expanded.
 A thought experiment:
 Coefficient of thermal expansion, α, is only 10-5.
 If the outer 100 km (lithospheric thickness) is 200°C hotter,
then:
 200˚C × 10-5 × 100 km = 2 km
 After formation, the lithosphere slowly cools and
thermally contracts. Consequently, the seafloor gets
progressively deeper.
 The cooling depends only on time (decreases with the
East Pacific Rise (EPR)
S-wave image of the East
Pacific Rise
Mid-Atlantic Ridge
 Mid-Atlantic Ridge
(MAR) has rift valley,
EPR does not.
 MAR has steep
flanks, EPR does not.
 EPR has permanent
(“steady state”)
magma chamber,
MAR does not.
 Why the difference?
Ridges and Rises: the
difference is spreading
 Graben forms on MAR
because of stretching of
strong lithosphere
 On EPR, volcanism is too
frequent & lithosphere is
too weak for a graben to
develop (faulting still
happens)
 Difference in flank
steepness is due to
difference in spreading
rate.
 On the MAR, magma flux
(and therefore heat flux)
is not high enough to
keep the magma chamber
from freezing.
Hydrothermal Processes
Basalt
fractures as it
cools, allowing
water to
penetrate the
young oceanic
crust.
Water is
heated and
reacts with the
oceanic crust.
Principle Hydrothermal
 Precipitation of Anhydrite
(CaSO4).
 Removal of Mg from
seawater, acidification:
 Mg2+ + Mg2Si2O6 + 3H2O
→ Mg3Si2O5(OH)4 + 2H+
 Reduction of sulfate:
 SO42- + 8FeO → S2– +
4Fe2O3
 Dissolution of Fe, Mn, Zn,
Cu, etc.
 Fe(solid) + 3H+ → Fe
(diss) + 3H+(solid)
 Precipitation of sulfides
and hydroxides
Consequences of Ridge Crest
Hydrothermal Activity
 “Buffers” composition
of seawater (e.g.,
important ‘sink’ for
Mg)
 Responsible for many
“base metal” (e.g., Cu,
Zn, Pb) ores
 Metamorphoses and
“hydrates” oceanic
crust
 Sustains unique
chemosynthetic
communities.
 Major reaction site in
global water cycle
Did life originate at
hydrothermal vents?
 Energy source
 Chemosynthesis is simpler
than photosynthesis
 Chemical raw materials
 Variety of chemical raw
materials
 Also, variety of mineral
surfaces to catalyze
reactions.
 Insulation from the hostile
surface environment
 Protection from UV radiation
 Some protection meteorite,
asteroid bombardment
 Highly variable climate
 Vent bacteria are among the
simplest, most primitive
Subduction Zone
Volcanism
 Why is there
melting above
subduction zones,
at convergent plate
boundaries, where
cold lithosphere is
sinking and
compressing?
 Here, flux melting
seems to be
important - the
addition of water to
mantle rock lowers
the solidus.
Melting in Subduction
Zones
 As oceanic crust and
sediment sink into
the mantle, they are
subjected to
increasing heat and
pressure causing
breakdown of
hydrous minerals.
 The water released by
this dehydration rises
into the overlying,
hotter mantle wedge
causing partial
melting.
Water, Volcanism & Plate
Tectonics
H2O added by
hydrothermal systems
at mid-ocean ridge
Released H2O causes
melting and explosive
volcanism
H2O released by
dehydration during
subduction
“Intraplate”
Volcanism
 Some volcanoes
occur within
lithospheric plates
rather than at
plate boundaries.
 An example is
Kilauea, Hawaii,
the world’s most
active volcano.
 What causes
melting in this
situation?
Mantle Plumes
 Most intraplate
volcanism is thought to
be caused by
decompression melting
in mantle plumes.
 Mantle plumes are rising
convection currents not
directly related to plate
tectonics.
 They are the cause of
Wilson’s hot spots.
 These mantle plumes
are thought to begin at
the core mantle
Current Mantle Plumes
Tomographic Images of Plumes
Mantle Plumes, Large Igneous
Provinces, and Climate
 Theory says that new
plumes need large heads
to initiate buoyant rise.
 When these “heads” reach
the surface, they produce
large pulses of volcanism,
know as “flood basalts”,
“plateau basalts”, “oceanic
plateaus” - collectively
called “large igneous
provinces”.
 May be important in
continent formation.
 CO2 released by these
events may change
climate & lead to mass
Basalts and
Partial
Melting
Piles of Basaltic Lava Flows in the Columbia
River Gorge
 When rock undergoes
partial melting, the
composition of melt is
not the same as that of
the original rock.
 The peridotite of the
upper mantle (source of
most magma) consists
of olivine (>50%),
clinopyroxene,
orthopyroxene, and
either plagioclase,
spinel, or garnet.
 When peridotite melts,
it produces basalt,
which crystallizes to
mostly of clinopyroxene
and plagioclase (+ ~10%
olivine).
 The melt is less dense
than the solid and hence
rises.
 At first by percolation,
Granites
 If melting of the
mantle produces
basalt, how do we
explain the great
variety of igneous
rocks at the surface
of the Earth?
 In particular, how do
we explain the vast
expanses of granites
and granodiorites,
such as the Sierra
Glaciers carved Yosemite Valley out of part of
the immense Sierra Nevada granitic batholith
Origin of Granites, etc.
 Non-basaltic magmas can be produced in
a variety of ways:
 Under certain circumstances, melting in the
mantle can produce andesite.
Melting of subducted basalt.
Melting peridotite with high water concentrations at
shallow depth.
 Melting of sediments and other rocks within
the crust.
A rock that is metamorphosed to the extent that it
starts to melt is called a migmatite.
 Assimilation of country rock in the crust by
Fractional Crystallization
 As in melting,
crystallization takes
place over a range of
temperature.
 The composition of
minerals crystallizing
from the magma are
different in composition
from the magma.
 Therefore, as
crystallization of the
magma proceeds, the
The Chemistry of
Fractional Crystallization
Wt % Olivin Parent
e
Magma
of:
39.0
50.0
Magma after 20%
olivine
crystallization
52.8
MgO 42.0
10.0
2.0
FeO
9.0
6.5
15.0
18.8
SiO2
19.0
Al2O3 0.0
Bowen’s Reaction Series
 Sequence of minerals
precipitating from a melt is
sometimes called “Bowen’s*
reaction series”.
 First minerals to crystallize
are rich in Mg and Fe and
poor in SiO2, subsequent ones
are progressively richer in
SiO2.
 Consequently, the remaining
melt becomes progressively
richer in SiO2, alkalis, etc.,
and poorer in Mg and Fe.
 Olivine (Mg,Fe)2SiO4
crystallizes first. It will
subsequently react with the
magma to form pyroxene:
+ experimental
SiO2 = (Mg,Fe)
 (Mg,Fe)
*Norman
L. Bowen
was4 an
petrologist working at the Geophysical Laboratory of Carnegie
2SiO
Institution
of Washington in the first half of the twentieth century. He first championed the idea that the
2Si2O6
variety of igneous rocks are produced by fractional crystallization.
Assimilation
 Composition of
magma will also
change if the
surrounding
country rock melts
and this melt
mixes with the
original magma.
 This process is
called
assimilation.
Composition of Igneous Rocks
Volcanic
Eruptions
 Sometimes
volcanoes erupt
catastrophically,
at other times,
they erupt with
a quiet effusion
of lava.
 Why?
Factors Affecting Volcanic
 Two main factors govern
how a volcano will erupt:
 Gas content (mainly H2O, but
also CO2)
 Magma viscosity
 Explosive eruptions occur
when gas exsolves from
magma and is unable to
escape. The resulting
expanding bubbles cause
the magma to fragment.
 These are known as Plinian
Eruptions.
 (Quiescent eruptions
Eruptions and Magma
Composition
 Both gas content and magma viscosity increase as
fractional crystallization proceed.
 Gas concentrations increase because the gas remains in the
magma as minerals crystallize out.
 Viscosity increases with SiO2 content as the silicon tetrahedra
becoming increasingly linked or polymerized in the magma.
High viscosity makes it difficult for gas bubbles to leave the
magma.
 Consequently, explosive eruptions are more common
in dacitic and rhyolitic magmas. Basaltic magmas
essentially never erupt in Plinian eruptions.
 Water contents are higher in subduction zones
magmas, hence subduction zone volcanoes are more
explosive.
 The water is derived from dehydration of subducting oceanic
Silicate
Melts
 Primary structural element
is still the tetrahedron
 As in solids, these are
linked to various degrees by
bridging oxygens.
 Main difference between
melts and solids is the
presence of long range
structure in the latter.
 (Note: this is also true of
water - its structure is
much like ice, with many
individual molecules bound
together by hydrogen
bonds.)
Silicate Melt
Polymerizatio
n
 Physical and chemical
properties depend on the
degree of this linkage, or
polymerization.
 Network forming ions:
 Si4+, Al3+, Fe3+,Ti4+
 Promote polymerization
 Network modifiers:
 Mg2+, Ca2+, Na+, K+, H+,
 Since their charge can only
be balances by nonbridging O, these promote
depolymerization.
 On a weight percent basis,
H has very large effect
because of its low mass.
 (Addition of ions also
breaks up structure of
water)
Shield Volcanoes
Because basaltic lavas are quite fluid (low viscosity), they
build shield volcanoes with gentle slopes, such as Mauna
Loa.
Plinian Eruption of Mt. St.
Helens, 1980
Plinian
Eruptions
 Gas solubility
decreases with
pressure. So as
magma rises, gas
exsolves, forming
bubbles (very much
like uncorking
champagne).
 If sufficient gas is
present and it cannot
escape, it will
eventually disrupt the
magma in an
explosive eruption.
Dome-forming eruptions
2/15/06
2006-2008 eruption on Mt. St. Helens produced a new lava dome without
significant explosions. The dacitic lava degassed before erupting, but was
too viscous to flow.
Hazards of Explosive
 The principle hazard
of explosive
eruptions are not the
explosion per se, or
the ash falls, but
pyroclastic flows.
 Pyroclastic flows are
hot debris avalanches
that can travel at 200
km/hr.
 They can be
generated in several
The Tragedy of St. Pierre
 Perhaps the most
famous case of
destructive
pyroclastic flows
was that of Vesuvius
destroying Pompei
in 79 AD.
 More recently,
pyroclastic flows
destroyed the town
of St. Pierre on
Martinique and
killed 29,000
Caldera Formation
 Large Plinian eruptions
sometimes result in the roof
of the magma chamber
collapsing, producing
calderas (not to be confused
with craters).