Magma and Igneous
Rocks
Rock: A coherent, naturally occurring, aggregate of minerals
or glass
Geologists distinguish three main types of rocks
1- Igneous
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.
Thin Sections
• To study rocks
in detail,
geologists cut
thin slices of
rock so that they
are translucent
Geologists cut rocks with a rotating saw
• Geologists can
then look at
them through
petrologic
microscopes
and then grind them into thin sections
A hand sample of granite
A magnified thin section of granite
Igneous Rocks- The Basics
• Solidified molten rock
(which freezes at high
temp).
– 1,100°C to 650°C.
– Depends on composition.
• Earth is mostly igneous
rock.
– Magma: Subsurface melt.
– Lava: Melt at the surface.
• Magma erupts via
volcanoes.
Igneous Rock Types
• In general, there are
two basic types of
igneous rocks
– Extrusive/Volcanic:
Igneous rocks that
form due to the
freezing of melts
above the surface of
the Earth
• Includes rocks made
of volcanic ash
(pyroclastics)
– Intrusive/Plutonic:
Form by freezing of
melts below the
surface of the Earth.
Formation of Magma
• Remember that the tectonic plates don’t really float on a
liquid 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 causes
melting of rock within the Earth?
Melting happens because of:
– Decrease in pressure (decompression)
– Addition of volatiles (H2O, CO2, etc…)
– Heat transfer from rising magma
Melting due to Decompression
The Earth gets hotter
with increasing depth
due to primordial heat
and radioactive decay of
elements near the core.
The rate at which
temperature increases
with depth is called the
geothermal gradient, or
geotherm
Liquids have no
organized structure, so to
melt a rock, the mineral
bonds must be broken
(animated gif of atoms)
The geotherm of the Earth
Melting due to Decompression
At depth, confining
pressure prevents atoms
from breaking free of
crystals
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 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 and
helps 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.
Melting due to the Addition of Volatiles
Depth (km)
• The addition of H2O into
basalt, for example,
drastically changes its
melting temperature
• In this case, basalts at
60km 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
Melting Due To Heat Transfer
• Melting can also occur
when rising bodies of
hot material essentially
bake the nearby rock
• Analogy: Think of
pouring hot fudge into
ice cream. The hot fudge
transfers heat to the ice
cream and melts it
What is Magma Made of ?
• All magmas contain Si and O
– Upon cooling, bond together into silicon-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)
Increasing Fe, Mg
Increasing SiO2
Types of Magma - Composition
Like 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
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
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, letting
them 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.
Why are Magmas so Variable in Composition?
Differences in Magma composition occur due
to 5 main reasons…
1.
Different source rock compositions
melt a felsic rock = felsic magma
2.
Magma mixing
mix felsic magma with mafic magma
= intermediate magma
3.
Partial melting
4.
Assimilation
5.
Fractional crystallization
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 partially
melted become more mafic, because the silica-rich felsic minerals
are melted first.
• The melted part of the partial melt is thus more felsic than the
remaining rock.
The felsic
mineral,
quartz, is a
common
cement in
many rocks
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
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
Xenolith
• A xenolith in
granite in the
Mojave desert
• Usually recognized
because they may
have a different
texture (grain size)
and composition
than the rest of the
rock
Fractional Crystallization
• Not all minerals crystallize at
the same temperature – This is
fractional crystallization
• As magmas cool, they become
more felsic.
• Mafic minerals crystallize first
and are more dense than the
melt, so they sink to the bottom
Bowen’s reaction
series is an example
of fractional
crystallization
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) literally
squeezes 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)
Extrusive Igneous Rock Environments
•
Explosive eruptions generally occur
when source magma is:
–
–
–
•
Effusive eruptions generally occur when
source magma is:
–
–
–
High in silica (felsic-intermediate)
Low temp
High in volatiles
•
These volcanoes form
–
–
•
These volcanoes form
–
–
Lava domes
Ash clouds and ash flows
Hawaii
Cascades
NW USA
Low in silica (mafic)
High temp
Low in volatiles
Fluid lava flows
Fire fountains (if volatiles), lava tubes
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 intrusive
igneous rocks
– Country rock or wall rock: The pre-existing rock that magma intrudes into
– Intrusive contact: The boundary between the igneous intrusion and the wall rock
• Tabular intrusions: Dike, Sill, Laccolith (pseudo-tabular, or sheet-like)
• Non-tabular intrusions: Pluton, Batholith, Stock
Mt.
Rushmore is
carved out of
a granitic
igneous
intrusion
Dikes and Sills
• Dikes: igneous
intrusions that cut
across layering, i.e.
discordant
• Sills: igneous
intrusions that
follow layering, i.e.
concordant
Dikes in the Sierra Nevada Batholith
•
Near Ruby Lake, CA @ ~12,000 ft
Laccoliths
• Laccolith: a dome-like sill that bends the layers above it into a
dome shape
Non-Tabular Intrusions: Plutons
• Pluton: Irregular blob-shaped
discordant intrusions that range
in 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 <100 km2 in
surface exposure
The Sierra Nevada Batholith
The Sierra Nevada Batholith
•
At ~100 Ma the
west coast of the
US, was a
subduction zone
with numerous
volcanoes
•
The magma
chambers cooled
and the rocks
above were
eroded away
leaving a large
batholith
exposed.
Effects of Intrusions
• Dikes form in regions of
crustal stretching
• Sills may cause uplift at the
surface of the Earth
Effects of Intrusions
• Dikes form in regions of
crustal stretching
Scotland was stretched during the Cenozoic
• Sills may cause uplift at the
surface of the Earth
La Sal Mountains, Utah were uplifted by a laccolith
Effects of Intrusions
• Plutons disrupt the
surrounding layers of rock
and may cause crustal
stretching above
• Plutons grow by stoping:
opening cracks and
assimilating xenolithic
blocks in the melt
Cooling of Magma and Lava
• Magma cools for
several reasons
– Removal of
volatiles
– It rises to a cooler
location and has
time to cool
• Cooling depends
very much on the
geometry (surface
area) of the
intrusion.
Tabular-shape = fast
cooling
Spherical shape =
slow cooling
– Cooling times vary
from days minutes
to millions of years
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)
• Interlocking Texture (Phaneritic):
Rock made of interlocking crystals that
grew as the melt solidified. Commonly
called crystalline igneous rocks
– Crystals fit together like pieces of a puzzle
Igneous Textures
• Fragmental Texture:
Volcanic rocks that are made
of various types of fragments
that form from volcanic
eruptions.
– Fragments can be:
• Crystals
• Xenoliths (from volcano walls)
• Glass
Volcanic
Breccia –
angular pieces
of fragments
entrained in the
eruption
A Welded Tuff – white specks are fragments, grey is ash
Crystalline Igneous Rocks
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 less
dense 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
{ 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 tuff
•
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?
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
Subduction creates
volcanism
1- The down-going slab has
lots 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.
Hot Spots and Volcanism
• There are many hot spots throughout the world, including
Hawaii and Yellowstone.
• Many pacific islands are or were hot spots
Large Igneous Provinces (LIPs)
• LIP: a region of particularly voluminous eruptions of magma/lava
– May be a consequence of a super plume
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 Deccan
Traps (India) are two examples.
Formation of Igneous Rocks at Mid Ocean Ridges
• ~70% of the Earth’s surface
(including the underwater
surface) is oceanic crust, so
most igneous rocks form at mid
ocean ridges
• Mid Ocean Ridge lavas are
compositionally similar to
Oceanic Hot Spots (basalt,
mafic)
• Underwater flows for Pillow
Basalts
– Pillows have glassy outer rim and
more crystalline center