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.

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• 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.