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Magma and Igneous
RocksRock: A coherent, naturally occurring, aggregate of mineralsor glass
Geologists distinguish three main types of rocks
1- I gneous
Rocks that form by the freezing or solidification of melt
2- Sedimentary
Rocks that form by the cementing of grains or fragments of pre-existing rocks, or by the precipitation of minerals out of a solution
3- Metamorphic
Rocks that form when pre-existing rocks change due to temperature or pressure,
and/or as a result of squashing or shearing.
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Thin Sections
• To study rocksin detail,geologists cutthin slices of
rock so that theyare translucent
• Geologists can
then look atthem through petrologicmicroscopes
Geologists cut rocks with a rotating saw
and then grind them into thin sections
A hand sample of granite A magnified thin section of granite
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Igneous Rocks- The Basics• Solidified molten rock
(which freezes at hightemp).
– 1,100°C to 650°C.
– Depends on composition.
• Earth is mostly igneousrock.
– Magma: Subsurface melt.
– Lava: Melt at the surface.
• Magma erupts via
volcanoes.
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Igneous Rock Types• In general, there are
two basic types ofigneous rocks
– Extrusive/Volcanic: Igneous rocks that
form due to thefreezing of meltsabove the surface ofthe Earth
• Includes rocks madeof volcanic ash
(pyroclastics)
– Intrusive/Plutonic: Form by freezing ofmelts below the
surface of the Earth.
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Formation of Magma•
Remember that the tectonic plates don’t really float on aliquid asthenosphere, rather the asthenosphere is a ductile
solid and is only melted in specific locations.
• Most magma/lava is not 100% liquid. – Magma/Lava is made of many compounds, all of which have
different melting temps. Analogy: a slushy or frozen margarita
– Only a few percent of liquid is required to make a melt.
• Other than a rise in temperature, what causesmelting of rock within the Earth?Melting happens because of:
– Decrease in pressure (decompression)
– Addition of volatiles (H2O, CO2, etc…)
– Heat transfer from rising magma
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The Earth gets hotter
with increasing depth
due to primordial heatand radioactive decay of
elements near the core.
The rate at which
temperature increases
with depth is called thegeothermal gradient , or
geotherm
Liquids have no
organized structure, so to
melt a rock, the mineral bonds must be broken
(animated gif of atoms)
Melting due to Decompression
The geotherm of the Earth
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At depth, confining
pressure prevents atoms
from breaking free ofcrystals
Solidus: The temperature
when a rock first begins
to melt
Liquidus: The
temperature where the
last solid particle melts
The asthenosphere cools
only slightly as it rises
(convection) because it is
a good insulator (high
specific heat)
The solidus and liquidus of peridotite (ultramafic mantle rock)
Melting due to Decompression
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Melting due to the Addition of Volatiles
• Volatiles: A substance that can easily change into a gas at relatively low
temperatures (H2O, CO2, etc…).
• The addition of volatiles at depth (mainly H2O) seeps into rocks andhelps break bonds (aids in melting).
• Analogy: Think of putting salt onto ice to lower the melting temperature.
Likewise, adding water to rocks changes the melting point of rocks just
like adding salt to water.
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Melting due to the Addition of Volatiles
• The addition of H2O into basalt, for example,
drastically changes its
melting temperature
• In this case, basalts at60km depth beneath the
continents could begin to
melt only if they were
volatile rich.
The geotherm beneath a continent and the solidus
of wet and dry basalt
D e p t h
( k m )
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Melting Due To Heat Transfer
• Melting can also occur
when rising bodies ofhot material essentially
bake the nearby rock
• Analogy: Think of
pouring hot fudge intoice cream. The hot fudge
transfers heat to the ice
cream and melts it
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What is Magma Made of ?• All magmas contain Si and O
– Upon cooling, bond together into si l icon-oxygen tetrahedrons
• More silica (i.e. felsic), more viscous (harder to flow, thicker)• Also contain varying amounts of other elements like Na, K, Al,
Ca, Mg, Fe, etc…
• Dry magmas – no volatiles
•Wet Magmas
– up to 15% volatiles• Volatile content strongly effects the viscosity (ability to flow)
– More volatiles, less viscous (easier to flow or more fluid)
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Types of Magma - CompositionLike rocks, not all magma is made of the same stuff
• We divide magmas into groups by their composition – Felsic (Silicic): 66-76% Silica (SiO2)
• Most viscous, Least dense (~2.5 gm/cm3), melting point 650-800oC
– Intermediate: 52-66% SiO2
– Mafic: 45-52% SiO2, lots of MgO, FeO, and Fe2O3
– Ultramafic: 38-45% SiO2, abundant MgO, FeO, and Fe2O3• Least viscous, Most dense (~3.5 gm/cm3), melting point up to 1300oC I
n c r e a s i n g S i O 2
I n c r e a s i n g F e , M g
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Magma Compositions• Composition controls density, T, and viscosity.
– Most important is the content of silica (SiO2).• Silica-rich magmas are thick and viscous.
• Silica- poor magmas and thin and “runny.”
– These characteristics govern eruptive style.
Type Density Temperature Viscosity
Felsic Very low Very low (600 to 850°C) Very High: Explosive eruptions.
Intermediate Low Low High: Explosive eruptions.
Mafic High High Low: Thin, hot runny eruptions.
Ultramafic Very high Very high (up to 1,300°C) Very low
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Bowen’s Reaction Series • In order to understand the melting and solidifying of magma we need to understand
Bowen’s reaction series. – Bowen figured this out by melting rocks in an oven, lettingthem cool, and watching what minerals crystallized
• This series outlines the order in which minerals form in a cooling melt• Also applies in reverse order to rocks that are partially melted
• Discontinuous series (different minerals form) and Continuous series (Plagioclase only)
• So, a melt gets less mafic as it cools; In heating, the first minerals to melt are felsic.
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Why are Magmas so Variable in Composition?
Di ff erences in M agma composition occur due
to 5 main reasons…
1. Different source rock compositionsmelt a felsic rock = felsic magma
2. Magma mixingmix felsic magma with mafic magma
= intermediate magma
3. Partial melting
4. Assimilation
5. Fractional crystallization
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Partial Melting• Most magmas are not 100% liquid
– Commonly 2-30% melt; called a crystal mush
• According to Bowen’s reaction series, rocks that are partiallymelted become more mafic, because the silica-rich felsic mineralsare melted first.
• The melted part of the partial melt is thus more felsic than theremaining rock.
The felsicmineral,
quartz, is a
common
cement in
many rocks
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Assimilation
• As magma sits in its chamber,
it may incorporate minerals
from the surrounding wall rock
– Called assimilation
• Occurs when wall rocks fall
into the magma and melt
(stoping) or when the magma
partially melts minerals from
the wall rock
• Degree of assimilation depends
on composition of wall rock,
temp of magma, amount of
H20 present, amount fractures
in and strength of the wall
rock, and residence time
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Stoping & Xenoliths• Stoping: The process of incorporating chunks of wall rock into a magma body
• Xenolith: A non-melted chunk of wall rock incorporated into a magma body
– May have a very different composition than the magma
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Xenolith
• A xenolith ingranite in theMojave desert
• Usually recognized because they mayhave a differenttexture (grain size)and compositionthan the rest of therock
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Magma Movement• If magma did not move, no extrusive/volcanic rocks would
ever have formed
• Magma rises because: – hotter and less dense than the surrounding rock and therefore
buoyantly rises.
– the weight of the overlying rock (lithostatic pressure) literallysqueezes the magma out.
• Analogy: Think of stepping on a tube of toothpaste to force it out, or mud squishing through your toes when you step in a puddle
• Viscosity affects a magma or lava’s ability to flow
– Controlled by:• Temperature (high temp - low viscosity)
• Volatile content (more volatiles – less viscous)
• Silica content – silica tends to form silica-oxygen tetrahedrons that bond with each other to make long chains that ultimately resist flow(more silica – more viscous)
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Extrusive Igneous Rock Environments• Explosive eruptions generally occur
when source magma is:
– High in silica (felsic-intermediate)
– Low temp
– High in volatiles
• These volcanoes form
– Lava domes
– Ash clouds and ash flows
• Ef fusive eruptions generally occur when
source magma is:
– Low in silica (mafic)
– High temp
– Low in volatiles
• These volcanoes form
– Fluid lava flows
– Fire fountains (if volatiles), lava tubes
Hawaii
Cascades NW USA
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Intrusive Igneous Rock Environments• Magma rises by percolating between grains and/or by forcing open cracks in the
subsurface
• The magma that doesn’t reach the surface of the Earth cools into intrusiveigneous rocks
– Country rock or wall rock: The pre-existing rock that magma intrudes into
– I ntr usive contact: The boundary between the igneous intrusion and the wall rock
• Tabular intrusions: Dike, Sil l, L accoli th (pseudo-tabular, or sheet-like)
• Non-tabular intrusions: Pluton, Batholi th, Stock
Mt.
Rushmore is
carved out of
a granitic
igneous
intrusion
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• Dikes: igneousintrusions that cut
across layering, i.e.discordant
• Sills: igneousintrusions that
follow layering, i.e.concordant
Dikes and Sills
Dik i h Si N d B h li h
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Dikes in the Sierra Nevada Batholith
• Near Ruby Lake, CA @ ~12,000 ft
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• Laccolith: a dome-like sill that bends the layers above it into a
dome shape
Laccoliths
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Non-Tabular Intrusions: Plutons
• Pluton: Irregular blob-shaped
discordant intrusions that rangein size from 10’s of m, to 100’s
of km
• Batholith: A pluton that is 100 km2
in surface exposure
• Stock: A pluton that is
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The Sierra Nevada Batholith
h Si d h li h
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The Sierra Nevada Batholith
• At ~100 Ma the
west coast of theUS, was a
subduction zone
with numerous
volcanoes
• The magma
chambers cooled
and the rocks
above were
eroded away
leaving a large
batholith
exposed.
Eff t f I t i
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Effects of Intrusions
• Dikes form in regions of
crustal stretching
• Sills may cause uplift at the
surface of the Earth
Eff t f I t i
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Effects of Intrusions
• Dikes form in regions of
crustal stretching
• Sills may cause uplift at the
surface of the Earth
La Sal Mountains, Utah were uplifted by a laccolithScotland was stretched during the Cenozoic
Eff t f I t i
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Effects of Intrusions
• Plutons disrupt the
surrounding layers of rockand may cause crustal
stretching above
• Plutons grow by stoping:
opening cracks andassimilating xenolithic
blocks in the melt
C li f M d L
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Cooling of Magma and Lava
• Magma cools forseveral reasons – Removal of
volatiles
– It rises to a coolerlocation and hastime to cool
• Cooling dependsvery much on thegeometry (surfacearea) of theintrusion.
Tabular-shape = fastcooling
Spherical shape =slow cooling
– Cooling times varyfrom days minutes
to millions of years
I T
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Igneous Textures• Glassy Texture : A solid mass of glass
or tiny crystals surrounded by a glass
matrix – Matrix: the smaller stuff in a rock
(relative term)
• I nter locking Texture (Phaneritic) :Rock made of interlocking crystals thatgrew as the melt solidified. Commonlycalled crystall ine igneous rocks
– Crystals fit together like pieces of a puzzle
I T t
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Igneous Textures
• Fragmental Texture :
Volcanic rocks that are madeof various types of fragments
that form from volcanic
eruptions.
– Fragments can be:• Crystals
• Xenoliths (from volcano walls)
• Glass
A Welded Tuf f – white specks are fragments, grey is ash
VolcanicBreccia –
angular pieces
of fragments
entrained in the
eruption
Crystalline Igneous Rocks
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Crystalline Igneous Rocks
Glassy Igneous Rocks
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Glassy Igneous Rocks
• Obsidian: Mass of solid felsic glass; conchoidal fracture
• Tachylite: mafic, bubble-free mass of >80% glass (very rare)
• Pumice: glassy felsic volcanic rock that contains abundant open
pores called vesicles (lt grey to tan in color). Occasionally lessdense than water (it floats!)
– Vesicle: a open space left over from a gas bubble in a lava or magma
• Scoria: glassy mafic volcanic rock with abundant vesicles(>30%). Grey, black, or red in color.
– Typically has larger and rounder vesicles than pumice
Fragmental Igneous Rocks
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Fragmental Igneous Rocks
{ Rocks blasted out of volcanoes…commonly called pyroclastic rocks }
• Tuff: Fine-grained rock, composed of lithified volcanic ash and/or fragmented
lava and pumice. Formed from ash fall from the air, or from hot material that
avalanches down the side of a volcano.
– If material is still very hot (gooey) it may get squished upon landing and weld with
other particles forming a welded tuf f
• Volcanic Breccia: Large angular chunks of material from either volcanic
debris flows (blocky lava flow) or air fall (bombs).
• Hyaloclasite: formed when lava erupts under ice of water and cools so quickly
that it shatters into fragments that weld or cement together.
Where Does Igneous Activity Occur?
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Where Does Igneous Activity Occur?
Most volcanoes occur at plate boundaries or Hot Spots
Most subaerial (above sea level) occur in volcanic arcs
Subduction-related volcanic arcs are responsible for “the ring of fire”
Subduction and Volcanism
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Subduction and Volcanism Subduction creates
volcanism
1- The down-going slab haslots of volatiles (e.g. H2O).
At depth, these volatiles are
heated and are squeezed
from the rock and migrate
into the asthenosphere
above the plate.2- The addition of volatiles,
as we now know, changes
the melting point of rocks
and causes the
asthenosphere to melt above
the sinking plate.3- The sinking plate may
partially melt too, but most
melting occurs in the
asthenosphere above the
slab.
H t S t d V l i
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Hot Spots and Volcanism
• There are many hot spots throughout the world, includingHawaii and Yellowstone.
• Many pacific islands are or were hot spots
Large Igneous Provinces (LIPs)
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Large Igneous Provinces (LIPs)
• LIP: a region of particularly voluminous eruptions of magma/lava
– May be a consequence of a super plume
Flood Basalts
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Flood Basalts
• Flood basalts are a type of LIP that emits a large amount of basalt flows.
• The Columbia River flood basalts (above) and the DeccanTraps (India) are two examples.
Formation of Igneous Rocks at Mid Ocean Ridges
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Formation of Igneous Rocks at Mid Ocean Ridges
• ~70% of the Earth’s surface(including the underwater
surface) is oceanic crust, somost igneous rocks form at midocean ridges
• Mid Ocean Ridge lavas arecompositionally similar toOceanic Hot Spots (basalt,mafic)
• Underwater flows for PillowBasalts
– Pillows have glassy outer rim andmore crystalline center