Download Igneous Rock Formation, Compositions, and Textures

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
_________ (plutonic) igneous rock.
• Some magma (known as lava)
reaches the surface while still at
least partially molten, producing
volcanic eruptions and _________
(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.
• ______________: types and abundances of
different minerals and non-minerals
• ____________: sizes, shapes, and boundary
relationships of the mineral grains and other
components (i.e. flow patterns
• ___________________: Temperature at eruption
and/or rate of cooling in a magma chamber
• ______________________: 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.
• ________: poor in silica (~50%), rich in Fe, Mg,
Ca, poor in Na and K
• ________: rich in silica (~70%), poor in Fe, Mg,
Ca, rich in Na and K
• _____________: between mafic and felsic
• ______________: “beyond mafic,” even more
mafic than mafic
Composition
Pahoehoe flow, Hawaii
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.
Glassy Scoria
Obsidian flow, Oregon
• Magmas are subdivided largely by ___________________.
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 ______________
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 ___________, whereas felsic
minerals tend to be _______________.
• Note that the above point applies to
minerals, not glasses, which can be
strongly colored by submicroscopic
inclusions. Obsidian is ______, but is
usually black in color.
Silicate Behavior
Bowen (1925) recognized that mafic minerals
tend to have __________________ and
less polymerization (chain-forming) between
silicate tetrahedra.
Bowen’s Reaction Series summarizes these
trends, along with the effects of dissolution,
precipitation, and solid-state diffusion in
determining which minerals will be produced
for a magma of a given bulk composition.
As magma cools, minerals form at different _______________. Along the
_______________ series, there are distinct “steps” at which minerals will
begin crystallizing (and perhaps later dissolving). Along the ___________
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 lowtemperature 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) ________________________of initial solids
2) __________________as the magma moves
3) ___________________of residual fluid
4) _____________(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.
Magmatic differentiation of magma by __________________. 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).
Gold ore in a
quartz vein
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.
Igneous Rock Classification
Silica Content and Color
• High silica rocks are light in color (pale grey to pink)
• Low silica rocks are dark (due to more dark
minerals containing ______________)
Intrusive
Extrusive
Low Silica
Basalt
Medium Silica
High Silica
Rhyolite
Andesite
Granite
Gabbro
Diorite
Silica Content and Viscosity
Even when molten, the silicate
tetrahedra will polymerize into
chains. These will become
entangled and thereby
____________.
Over the seemingly modest range
of 50% to 70% silica content, this
extent of tangling results in a
change of about _____ orders of
magnitude in viscosity, or a factor
of around ________________.
____________magmas can flow
almost like water. ___________
___________ magmas are far
more sluggish than toothpaste.
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 and felsic lavas often erupt with great violence 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.
Silica content and Volcano Type
• 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 (________________).
• Low silica volcanoes are non-explosive.
Lava is runny, so volcanoes are broad
and non-pointy (______________)
Summary of Trends with Composition
Mafic (Basalt/Gabbro)
Felsic (Rhyolite/Granite)
• Density about 3.3 g/cm3
• Density about 2.7 g/cm3
• Crystallization ~1200°C
• Crystallization ~700°C
• Low Silica
• High Silica
• Rock color = dark grey to
black
• Rock color = pale
grey/pink
• Low viscosity
• High viscosity
• Typically mild eruptions
• Typically violent eruptions
• Shield Volcanoes (low,
wide)
• Stratovolcanoes (tall,
pointy)
Igneous Textures
• Slow cooling produces large grains, rapid
cooling produces small (or no) grains.
• Terms for Crystal Size:
• ______________: visible to unaided eye, also
called coarse-grained. Usually ____________.
• ____________: crystalline, but not visible, also
called fine-grained. Usually ____________.
• _____________: not crystalline. _____________.
• ____________: coarse grains (phenocrysts)
surrounded by fine grains (groundmass). Began
crystallizing underground, then erupted and finished
solidifying on surface. _____________.
Gabbro
Diorite
Granite
_____________ igneous rocks crystallize slowly (usually underground). Chemical
composition also plays a role in determining the specific rock type.
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).
_________ 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 ______________.
Note the characteristic conchoidal fracture diagnostic of obsidian.
__________________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
______________ “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 ____, to building-sized, called ________,
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 _______ or coarse-grained volcanic ____________.
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.
Hand Sample
____________________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.
Volcanic ________:
molten rock
aerodynamically shaped
due lava freezing while in
flight.
Other Igneous Textures
___________: 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.
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 _______ in the mafic lava is
oxidized by the escaping gases.
Other Igneous Textures
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).
Pahoehoe (ropey textured) basalt flows have a _________ viscosity than aa
(blocky textured) flows, which have degassed and __________.
Other Igneous Textures
________________: 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 _____________.The
columns will be ______________
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 __________. Unusually
extensive melting will produce ____________
magmas, but “routine” partial melting
produces _________ magmas.
• Partial melting of subducting oceanic crust
(mafic) and its associated sediments
produces mafic and _____________magmas.
• Interaction with _________________is
required for the production of felsic magmas.
Sources of Magma
• In nearly all of the crust and mantle,
temperatures are too _______ for melting to
occur at the surrounding pressures.
• Magma production occurs when:
– warm rock travels ___________ (decompression
melting), as at divergent zones and hotspots, or
– cold rock is forced _____________ and absorbs
heat from its new surroundings, as at subduction
zones
Mafic Magma Formation
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 ______________while
the melting temperature curve is
dependent upon pressure (depth) and
_____________ 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 (______) 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 _______________.
Mafic magma forms at four different tectonic settings. Mafic (basaltic) magma
is always derived from a partial melt of the ultramafic asthenosphere.
Felsic Magma Formation
Felsic (granitic) magma forms from a
partial melt of continental crust, which
contains dissolved water. Dissolved water
content in a magma __________ its
melting temperature with ____________
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 3545 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 _________________
margins. As the continental crust
thickens it begins to partially melt at
depth. Igneous intrusions (plutons)
form below the mountain belts.
______________ 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.
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 Magma Formation
Intermediate (andesitic) composition
magma can crystallize below the
surface beneath _________________
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
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 crust. In
an ocean-continental convergent margin it may mix with partially melted continental
crust, increasing the magma’s _____________(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.