geohazzards and the publicees2.geo.rpi.edu/pe1geo1_08/ss/04_igneous-rocks.pdf · igneous rocks are...
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760 kA - last eruption
Geohazzards and the public
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Igneous Rocks
Melting Earth Materials
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Igneous rocks are those formed from partially molten
(melted) materials
Partially molten rock is called magma as it moves
through the Earth, and lava when it is extruded at the
surface.
From where does magma originate?
Partial melting of the mantle and crust due to
•Influxes of heat
•Decreasing pressure
•The movement of water or other fluids
Magma
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Pressure & Temperature
Most materials
have positive
slopes between
their states
For rocks, we
need to consider
the solid-liquid
changes. Adding
water changes
melting
relationship
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SiO2 and melting
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Partial melting in S.W. Maine
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When plates move away from each other, it
stretches and thins the crust, reducing
pressure on the mantle.
Tectonic thinning of the crust
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Tectonic increase of
crustal thickness
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Plume theory
Earth is hottest at its
core, the release of heat
may be convective -
mass transferred. Some
masses may move as
buoyant ductile bodies.
Diagram by J. Tarney
Thermochemical plumes
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End of the ride… if such plumes exist, they would
become more or less stalled at the transition to
brittle mantle.
Diagram by J. Tarney
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Four crystals growing
in melt. Note angular
faces typical of the 2/m
crystals as seen from
the 001 plane.
Planes (facets) result from
energy minimization along
a crystallographic plane -
depends on T, P, and X.
Hornblende
Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH, F)2
Crystal-liquid interfaces
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On this phase
diagram, there
are two phases,
a solid and a
liquid.
The line
represents the
conditions
where both will
be present at
equilibrium
Crystallization
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From Blackburn & Dennen,
1998
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Growth interface
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Crystal growth results from diffusion of components
to the crystal surface
Crystals can grow in
any medium - solids,
liquids, gasses,
supercritical fluids.
Liquids and fluids may
be melted rocks, C-O-
H or aqueous fluids, or
a mixture. All are
contingent on
component transport.
Diffusional growth
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Reactive growth
New minerals may form from
recrystallization of reaction of
preexisitng grains.
Overgrowth, mantling, coronas
This is a diffusion driven process.
This may result from a change in
conditions or composition during
growth
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Compositional zoning
Changes in physical
conditions or in
surrounding composition
may be reflected in
crystals
Ti variations as an
impurity in quartz (CL
image by D. Wark)
REE variations as an
impurity in zircon (BSE &
CL images by E.B.
Watson)
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Bowen’s reaction series
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Mineral names carry
a certain mystique -
rock names seem
more utilitarian
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Igneous nomenclature“archaic” texture discrimination
boundary at 1mm.
An alkali feldspar and quartzdominant rock with an averagegrainsize >1 mm is a granite, <1mm is a rhyolite.
> 1mm Phaneritic
< 1 mm Aphanitic
Glassy - vitrophyric
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Rhyolite
Dacite
Andesite
Basalt
Phonolite
Granite
Diorite
Tonalite
Gabbro
Anorthosite
Foid Syenite
After Strekeisen, 1978
Mean grain size < 1mmMean grain size > 1mm
Common rock names
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SiO2
Quartz | Coesite | Stishovite
mineral.galleries.com
De minuscules cristaux de quartz ont une
disposition radiale autour de la coesite : ceci
montre que le quartz se forme au d師riment de la
coesite (LPA)
High birefringence Stishovite in coesite, synthetically grown
by J. Mosenfelder, CalTech.
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Feldspars
Plagioclase Feldspar
Anorthite
(CaAl2Si2O8)
Alkali Feldspar
Albite (NaAlSi3O8)
Orthoclase (KAlSi3O8)
Images from mineral.galleries.com
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Feldspathoids
Leucite (KAlSi2O6)
Nepheline (NaAlSiO4)
Sodalite (Na4Al3Si3O12Cl)
Image from mineral.galleries.com
dkimages
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Grain size origins
Surface extrusion (volcanism)
forces molten material to cool
rapidly in the air. Grain sizes
are small.
Intrusion (plutonism)
forces molten material
to cool slowly deep
underground. Grain
sizes are large.
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Cooling rapidly can limit the organization of atoms into
lattices. Fast = glass, slower = crystalline, slower still =
bigger crystals
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From Blackburn & Dennen,
1998
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Coarse: Dunite, Lherzolite, Harzburgite, Periodotite
Fine: Kimberlite
Minerals: these are rocks largely comprised of olivine
(isolated silicate) and pyroxene (single-chain silicate)
Origin: solidification of early Earth
Location: the mantle
Univ. North Carolina, Atlas of rocks,
minerals, and textures website
Ultramafic (very Mg-Fe rich) rocks
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Classification
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NesosilicatesOlivine
Forsterite Mg2SiO4
Fayalite Fe2SiO4
Image from mineral.galleries.com
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Inosilicates (singles)Pyroxene
Orthopyroxene - hypersthene
Enstatite Mg2Si2O6
Orthoferrosilite Fe2Si2O6
Clinopyroxene - Augite
Diopside CaMgSi2O6
Hedenbergite CaFeSi2O6
Image from mineral.galleries.com
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Univ. North Carolina, Atlas of rocks,
minerals, and textures website
Kimberlite, lamprophyre
Volcanic ultramafic
rock. Very rare, and
sourced straight from
the mantle - eruption
rates must be
impressive. Some are
diamondiferous.
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IUGS Crystaline
Coarse
crystaline rock
classification -
note that the
scheme
precludes the
coexistence of
foids and SiO2.
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Fine grained
rock
classification -
compare with
the previous
slide.
This is a nice
diagram, but
the fine-grained
nature of these
rocks can
impede its use.
IUGS Fine grained
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IUGS TAS
If you really want to classify a volcanic rock - powder
it and analyze its composition. How does this relate
to the previous slide? What of plutonic rocks?
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Coarse: Gabbro, Anorthosite Fine: Basalt
Minerals: Largely olivine (isolated), pyroxene (single-
chain), and feldspar (framework).
Origin: Partial melting of the mantle
Oceanic crust, oceanic islands, the moon.
Univ. North Carolina, Atlas of rocks, minerals,
and textures website
AnorthositeAnorthosite
Univ. North Dakota, Plutonic Images
Mafic (Mg-Fe rich) Rocks
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BasaltBasalt
GabbroGabbro
Univ. North Carolina, Atlas of rocks, minerals,
and textures website
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If most of the heat needed for
melting is deep within the earth,
why are there volcanoes?
Magma is driven upwards.
Parameters - density, viscosity
Recall density = mass / volume
Viscosity is a measure of resistance to flow -
maple syrup is more viscous than water
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Hogan et al., 1998
Ascent of Magmas
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Kilauea, Hawaii
March, 1996
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Hawaiian magmatism
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Two plumes?
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20-406Figure 20.19
A cross section of a shield volcano
Generally low Si
Low volatile
High rate
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Mauna Kea, Hawaii
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Pu’u Hulu
Mauna Loa, Hawaii
Pahoehoe
Viscosity and Lava
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Compare water to maple syrup - the latter is many times
more viscous.
What makes for runny (less viscous) magma
Temperature
Viscocity is almost always reduced in hotter fluids
Lower silica content
Lower silica content means fewer tetrahedra sharing
oxygens - think about the minerals in mafic rocks
Volatile (H2O, CO2, HF, H2SO4) content
Many of these components work to depolymerize the
magma.
Viscosity - Resistance to flow
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Fluorine and evolution
High -T crystalslargely cannotincorporate largeamounts ofcomponents like H2O,OH, F, Cl, and SO2.
“To many petrologists avolatile component…does just what onewants it to do.”
N.L. Bowen, 1928
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Oceanic rifting
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Example: the East African Rift
http://jsc.nasa.gov
STS-32
Continental Rift
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Diagram by J. Tarney
Rifting - Birth of an ocean
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Question 4: how does fluorine influence riftmagmatism?
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Diagram by J. Tarney
Cartoon to left shows
some general attributes
of rift formation
Presumably, the tension
that drives rifting is not
well understood.
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The minimum liquid fractions at each temperatureand pressure give the melting curve that we usefor mantle rocks. DJ DePaolo
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By adding a small amount of water, thedegree of melting increases significantly.
With water - melting starts much deeper
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Coarse: Granodiorite, Tonalite Fine: Andesite, Dacite
Minerals: Feldspar (framework), pyroxene (single
chain), amphibole (double chain), and quartz
(framework)
Origin: liquid fraction of crystallizing mafic liquids.
Location: Convergent tectonic settings
TonaliteTonalite
Intermediate rocks (less Fe-Mg, more Na-K-Si-Al)
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Example: the Aleutians
Subduction: Island Arcs
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Example: the Cascades
Subduction: Continental Arcs
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Mount Saint Helens: Mount Saint Helens:
May 18, 1980 0832May 18, 1980 0832
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Q: What is a
volcanic dome?
View from South Rim, June 1991View from South Rim, June 1991
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Crater LakeCrater Lake
OregonOregon
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Coarse: Granite
Fine: Rhyolite
Minerals: Feldspar and quartz (framework), with small
amounts of amphibole (double chain) and mica (sheet).
Origins: Partial melting of crustal rocks and liquid
fraction of crystallizing intermediate liquids.
Location: continental magmatism
GraniteGranite
Felsic (K, Na, and Si rich) rocks
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Mount Scott Granite
An intergrowth ofquartz and feldspar
Likely result of too fewnucleation sites
Undercooling
Viscosity contrasts
Rapid diffusion
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Photo by Mike Brown (UMD)
Metamorphic
materials that
exceed the
solidus for
the system
start to melt
Migmatite
Making granites?
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Univ. North Carolina, Atlas of rocks,
minerals, and textures website
RhyoliteRhyolite
VitrophyreVitrophyre
©Pam Gore, GPC
Felsic rocks
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Pegmatite Pegmatites are
typically coarsely
crystalline - may
contain rare
minerals. Most
result from
extreme
fractionation.
Slow cooling
Low nucleation
Lots of H2O