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Igneous Processes I: Igneous Rock
Formation, Compositions, and Textures
Crustal Abundances of Rock Types
Igneous Rocks • Form by the cooling and hardening
(crystallization/glassification) of
magma.
• Most magma crystallizes before it
can reach the surface, producing
bodies called plutons made of
intrusive (plutonic) igneous rock.
• Some magma (known as lava)
reaches the surface while still at
least partially molten, producing
volcanic eruptions and extrusive
(volcanic) igneous rocks.
Classifying Igneous Rocks A magma is a multi-component material with a bulk
composition which almost always changes as it
moves and cools.
• Composition: types and abundances of different
minerals and non-minerals
• Texture: sizes, shapes, and boundary relationships
of the mineral grains and other components (i.e.
flow patterns
• Method of Cooling: Temperature at eruption and/or
rate of cooling in a magma chamber
• Magmatic Sources and Pathways: determines final
product that appears on Earth’s surface
Igneous Composition Various igneous environments will produce
magmas which differ in silica content and the
abundances of metals such as Fe, Mg, Ca, Na,
and K.
• Mafic: poor in silica (~50%), rich in Fe, Mg, Ca,
poor in Na and K
• Felsic: rich in silica (~70%), poor in Fe, Mg, Ca,
rich in Na and K
• Intermediate: between mafic and felsic (50-70%
silica)
• Ultramafic: “beyond mafic,” even more mafic
than mafic (<50% silica).
Magma (or lava if erupted to
the surface) is composed of
liquid, solid (mineral crystals)
and gas. Its composition is
largely controlled by its
source.
Obsidian flow, Oregon
Pahoehoe flow, Hawaii
Glassy Scoria
Composition
• Magmas are subdivided largely by silica (SiO2)content. As
silica content increases, iron (Fe), magnesium (Mg), and
calcium (Ca) content decreases.
• Lighter elements, such as sodium (Na) and potassium (K)
content follow the silica trends. Chemical compositions are
often described in terms of oxides.
Recognizing Igneous Composition
• Need to be able to identify the common
minerals in igneous rocks: olivine,
pyroxene, amphibole, micas, feldspars,
and quartz.
• If grains are not apparent, can fall back
on the observation that mafic minerals
tend to be dark or green, whereas felsic
minerals tend to be light grey or pink.
• Note that the above point applies to
minerals, not glasses, which can be
strongly colored by submicroscopic
inclusions. Obsidian is felsic, but is
usually black in color.
Silicate Behavior
Bowen (1925) recognized that mafic minerals
tend to have higher melting points and less
polymerization (chain-forming) between
silicate tetrahedra.
Bowen’s Reaction Series summarizes these
trends, along with the effects of dissolution
(dissolving), precipitation (forming crystals),
and solid-state diffusion (of elements
between or within crystals) in determining
which minerals will be produced for a
magma of a given bulk composition.
As magma cools, minerals form at different temperatures. Along the
discontinuous series, there are distinct “steps” at which minerals will
begin crystallizing (and perhaps later dissolving). Along the continuous
series, the composition of the plagioclase shifts from Ca-rich to Na-rich.
The steps described by
Bowen’s Reaction Series
may end up interrupted if
temperatures fall too
quickly. Olivine, for
example, may only be
partially dissolved before
the texture and
composition becomes
“frozen” when the
reaction rates are too
slow.
Such features are
themselves useful in
determining the conditions
under which the rock
formed.
The “continuous” replacement of high-temperature Ca-spar by low-
temperature Na-spar often is incomplete, since it relies upon very
slow diffusion of atoms through already-solid crystals. The result is
“zoned” plagioclase feldspar, with Ca-rich centers and Na-rich
rims.
Changes in Bulk Chemistry
• Further complications arise if materials are
removed during solidification.
• Several fractionation processes:
1) Gravitational settling of initial solids
2) Flow segregation as the magma moves
3) Filter pressing of residual fluid
4) Loss of volatiles (water, gases) along with
readily-dissolved elements which don’t fit
well in the crystallizing silicate minerals
Differentiation of magma can occur from fractional crystallization involving the
removal of crystals as they accumulate. The solid phase will have a composition
that is relatively more mafic than the remaining melt phase.
Animation From Pearson ebook
• file:///C:/Users/Patty%20weston/Desktop/C
lass%20Docs%202013-
2014/ESS%20101/Pearson%20Animation
s/resources/anim/FractionalCrystallization
_GL.html
• Fractional crystallization
Magmatic differentiation of magma by fractional crystallization. Note how the
composition of the magma changes as more mineral crystals form. Think of the
yellow atoms forming to Fe-Mg silicate minerals that crystallize first during the
differentiation process. Think of the red atoms comprising the silica-rich melt.
As earlier formed minerals are removed from the magma by fractional crystallization, a
greater proportion of the denser elements (Fe and Mg) are removed leaving a residual
melt that is more enriched in silica and lighter elements. Minerals and rocks that form
later will have a greater proportion of the lighter elements (SiO, Al, Na and K).
Several metals of economic interest, such as gold, silver, and copper, do not
“fit” well in the growing silicate minerals. Instead, they often are carried
away from the magma in aqueous fluids and become deposited in cracks
(veins) as pressures and temperatures decrease towards the surface. Silica
also is carried this way, precipitating as quartz.
Gold ore in a
quartz vein
Igneous Rock Classification
• High silica rocks are light in color (pale grey to pink)
• Low silica rocks are dark (due to more dark
minerals containing Mg and Fe)
Low Silica Medium Silica High Silica
Silica Content and Color
Basalt Andesite Rhyolite
Gabbro Diorite
Granite
Extr
usiv
e
Intr
usiv
e
• Even when molten, the silica
tetrahedra will polymerize into
chains. These will become
entangled and thereby inhibit
flow.
• Over the range of 50-70% silica
content, this extent of tangling
results in a change of about 7
orders of magnitude in
viscosity:10,000,000 times!
• Mafic (basaltic) magmas can
flow almost like water. Felsic
(rhyolitic) magmas are far more
sluggish than toothpaste!
Silica Content and Viscosity
Mafic lavas often erupt in a gentle fashion. Their low viscosities
make it less likely that gas pressure will build to the point of
explosiveness.
Due to their low viscosities, basaltic composition magma (lava)
will flow great distances from its vent.
Intermediate (andesitic) and felsic (rhyolitic) lavas often erupt with
great violence (as at Pinatubo above) in large part because gases
cannot easily escape them. When they do not explode, they
instead ooze slowly and do not travel far.
Rhyolite/dacite flows will retain steep slope fronts because of their high viscosity.
• High silica volcanoes are explosive, due to build-up of pressure within volcano. Viscous lava won’t flow far, so volcanoes are tall and pointy (stratovolcanoes).
• Low silica volcanoes are non-explosive. Lava is runny, so volcanoes are broad and non-pointy (shield shape)
Silica content and Volcano Type
Summary of Trends with Composition
Mafic (Basalt/Gabbro)
• Density about 3.3 g/cm3
• Crystallization ~1200°C
• Low Silica
• Rock color = dark grey to
black
• Low viscosity
• Typically mild eruptions
• Shield Volcanoes (low,
wide)
Felsic (Rhyolite/Granite)
• Density about 2.7 g/cm3
• Crystallization ~700°C
• High Silica
• Rock color = pale
grey/pink
• High viscosity
• Typically violent eruptions
• Stratovolcanoes (tall,
pointy)
Igneous Textures
• Slow cooling produces large grains, rapid
cooling produces small (or no) grains.
• Terms for Crystal Size:
• Phaneritic: visible to unaided eye, also called
coarse-grained. Usually intrusive.
• Aphanitic: crystalline, but not visible, also called
fine-grained. Usually extrusive.
• Glassy: not crystalline. Extrusive.
• Porphyritic: coarse grains (phenocrysts)
surrounded by fine grains (groundmass). Began
crystallizing underground, then erupted and finished
solidifying on surface. Extrusive.
Phaneritic igneous rocks crystallize slowly (usually underground). Chemical
composition also plays a role in determining the specific rock type.
Gabbro Diorite Granite
Phaneritic grains are distinguishable to the unaided eye. This rock contains
quartz (light gray), plagioclase feldspar (white) and biotite (black) crystals.
A pink granite is dominated by potassium feldspar (pink crystals), quartz (gray
glassy appearance), plagioclase (porcelain white mineral) and biotite (black
sheets).
Aphanitic rocks contain mineral grains which are too small to distinguish
clearly with the unaided eye. Same magnification as the previous image.
Obsidian has a glassy texture. It may contain a few isolated mineral grains
or even an abundance of submicroscopic crystal “seeds” (crystallites), but
it is mostly amorphous, lacking the long-range order of crystal structure. .
Note the characteristic concoidal fracture diagnostic of obsidian.
Porphyritic rock is partially coarse and partially fine. The large
phenocrysts formed first, slowly, in the subsurface, whereas the
groundmass crystallized quickly after eruption onto the surface. This is
often referred to a two-stage cooling process
Other Igneous Textures
Pyroclastic “Broken by Fire”:
• Violent volcanic eruptions produce an explosive
spray of lava which hardens (at least partially)
while in flight.
• The resulting fragments may or may not weld to
one another upon landing, but usually retain the
outlines of their initial crusts.
• Individual particles range from dust-sized,
called ash, to building-sized, called bombs, and
are typically a mixture of minerals and glass.
A large pyroclastic eruption of Mount Pinatubo in the Philippines (1992).
The ash and other volcanic derived clasts can become welded together
to form fine-grained tuff or coarse-grained volcanic breccia.
Volcanic ash (tephra) derived from the Mount Mazama
(Crater Lake, Oregon) eruption 6800 years ago.
Welded tuffs in thin section: The triangular
fragments are created when the magma between
gas bubbles is blown apart. The fragments then get
flattened and welded together from the heat and
weight of the flow.
Volcanic breccia forms from a welded, mixture of large, angular volcanic
clasts within a matrix of fine ash. This photo was taken on Lipari Island,
Italy by Raymond Coveney.
Hand Sample
Volcanic Bombs: molten
rock aerodynamically
shaped due lava freezing
while in flight.
Vesicular: As a magma
approaches the surface, it
undergoes decompression and
cooling. This decreases its
ability to hold various gases
(H2O, CO, CO2, etc.) in solution.
These gases will separate as bubbles which will
either escape or remain trapped as the
magma hardens around them. Trapped
bubbles are called vesicles.
Other Igneous Textures
Pumice (shown) or scoria (darker) form when gas bubbles are trapped in
rapidly cooling pyroclastic materials. The rocks are glassy and frothy.
Scoria often forms in basaltic magmas where gases are escaping—
often near the tops of flows. Bubble size can get quite large, since the
lower viscosity lavas allow gases to coalesce into larger bubbles
compared to a felsic lava (which will form pumice)
Scorias can be a deep red when the iron in the mafic lava is oxidized by
the escaping gases.
Pahoehoe (ropey textured) basalt flows have a lower viscosity than aa (blocky
textured) flows, which have degassed and cooled.
Aa Flow (Think about what
you would say if you had to
walk on this aa flow (ah, ah).
Pahoehoe Flow (Smooth word, smooth flow).
Other Igneous Textures
Other Igneous Textures
Pillow Basalts: when basaltic lava
erupts underwater or flows into
water, it will form into pillow-like
shapes, often with a glassy rind,
since the exterior of the pillow is in
contact with cold water and freezes
rapidly.
Other Igneous Textures
Columnar Jointing: fracture
pattern into the shape of
hexagonal columns that happens
when lava (usually basaltic) cools
and contracts. The columns will
be perpendicular to the cooling
surfaces, such as the air and
ground.
Columnar Jointing at Devil’s Postpile, near
Mammoth Lakes, CA. The direction of the columns
changes near the front of the flow
Typical Magmatic Sources
• The mantle is ultramafic. Unusually extensive
melting will produce ultramafic magmas, but
“routine” partial melting produces mafic
magmas.
• Partial melting of subducting oceanic crust
(mafic) and its associated sediments
produces mafic and intermediate magmas.
• Interaction with continental material is
required for the production of felsic magmas.
Sources of Magma
• In nearly all of the crust and mantle,
temperatures are too low for melting to occur
at the surrounding pressures.
• Magma production occurs when:
– warm rock travels upwards (decompression
melting), as at divergent zones and hotspots, or
– cold rock is forced downwards and absorbs heat
from its new surroundings, as at subduction
zones
Mafic magma forms from a partial melt
of the asthenosphere, which occurs at a
depth (100-350 km) where the
geothermal gradient intersects the
melting temperature curve for upper
mantle rock (garnet peridotite).
•Note that the geothermal gradient is
dependent upon pressure (depth), while
the melting temperature curve is
dependent upon pressure (depth) and
composition of the rock involved. The
curve is for a “dry” melt, with no water
involved.
•Even in the region of melting, only a
small fraction (1-5%) of the rock actually
melts– this is the portion with the lowest
melting point. The product is a
relatively low-density mafic magma from
an ultramafic starting material. This
magma will tend to be displaced
upwards by buoyancy.
Mafic Magma Formation
Mafic magma forms at four different tectonic settings. Mafic (basaltic) magma
is always derived from a partial melt of the ultramafic asthenosphere.
Felsic (granitic) magma forms from a
partial melt of continental crust, which
contains dissolved water. Dissolved water
content in a magma reduces its melting
temperature with increasing pressure
(water molecules will inhibit the silicate
tetrahedra from forming bonds).
Note that the melting temperature curve
for a wet granitic melt increases with
decreasing pressure (opposite of basaltic
dry melt). Melting occurs at a depth of 35-
45 km within continental crust.
As granitic magma rises it solidifies (point
X) as its melting temperature increases
while the geothermal gradient (actual
temperature) decreases. Granitic
composition magmas rarely reach the
surface as volcanic rhyolite flows because
of the high water content and
corresponding increase in melting
temperature as it rises towards the
surface.
Felsic Magma Formation
Granitic composition magma is
produced at continental collision
margins. As the continental crust
thickens it begins to partially melt at
depth. Igneous intrusions (plutons)
form below the mountain belts.
Volcanism is rare in continental
collision boundaries.
As collisional tectonic mountain
ranges are uplifted the overlying
marine sedimentary and metamorphic
rocks are eroded exposing the
underlying granitic plutons.
The granitic rocks of New Hampshire
and Vermont represent old granitic
plutons that were intruded when the
Appalachian Mountains formed 300
million years ago as North American
continent collided with proto-European
continent.
Felsic Magma Formation
Granitic rock excavated from a quarry in Barre, Vermont formed as plutons
beneath the Appalachian Mountains when North Africa collided with
eastern North America 300 million years ago.
Roof pendant of remnant “country rock” (dark metamorphic rock) lies
above the intruded Sierra Nevada Batholith (light colored granodiorite).
Granitic composition magma reaches to the surface in
Yellowstone Park because the continental crust is being
heated closer to the surface by upwelling magma
generated from a hotspot in the asthenosphere.
The Yellowstone Caldera (Wyoming) formed following a very large eruption
~600,000 years ago. The rhyolite flows are very viscous and internal gas
pressures can be very high
Intermediate (andesitic) composition
magma can crystallize below the
surface beneath subduction zones and
create large coarse-grained plutonic
bodies.
Compositions can range from granite
to diorite.
El Capitan shown on the left is part of
the Sierra Nevada intrusive complex
that formed over 90 million years ago
when a subduction zone existed along
the margin of California.
The plutonic bodies comprising the
Sierra Nevada are similar in origin to
the plutonic bodies forming under the
modern Cascades.
Grano-diorite
rock from the
Sierra Nevada
Intermediate Magma Formation
Andesitic magma is produced from a partial melt of oceanic crust along subduction
zones. Introduction of water forced out of the subducting plate lowers the melting
temperature of the upper mantle, which rises and partially melts the overlying
asthenosphere. In an ocean-continental convergent margin it may mix with partially
melted continental crust, increasing the magma’s silica content (becomes more felsic).
Mount St. Helens dacites are more silica rich than Mt. Rainier andesite, likely due to
continental source.
Mt. St. Helens is composed of intermediate composition dacitic flows. Dacite is
slightly more felsic (has greater silica content) than andesite, but more mafic
(higher Fe and Mg content) than rhyolite.
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