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. There are 2 categories:

• __________ (___________): Most magma crystallizes before it can reach the surface, producing large rock deposits called __________.

• ___________ (____________):Some magma (aka lava) reaches the surface at least partially molten, producing eruptions and volcanoes.

Classifying Igneous RocksClassification of igneous rocks is based on the following four factors:1. _________________: types and abundances of different

minerals and non-minerals. A magma evolves and changes composition as it moves and cools.

2. ___________: sizes, shapes, and boundary relationships of the mineral grains and other components (i.e. flow patterns)

3. Method of ___________: Temperature at eruption and/or rate of cooling in a magma chamber. This will mostly affect the texture and types of minerals present.

4. Magmatic ____________________: determines final product that appears on Earth’s surface

Magma (or lava if erupted to the surface) is composed of liquid, solid (mineral crystals) and gas. Its composition is largely controlled by its ______________.

Obsidian flow, Oregon

Pahoehoe flow, Hawaii

Glassy Scoria

1. Composition

Igneous CompositionVarious 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 (SiO2) (~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(rare at Earth’s surface)

Mafic Felsic

Igneous Composition

• Magmas are subdivided largely by _____________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.

(Mafic) (Intermediate) (Felsic)

Recognizing Igneous Composition• Common minerals in igneous rocks:

olivine, pyroxene, amphibole, micas, feldspars, and quartz.

• Mafic minerals tend to be ________________, whereas felsic minerals tend to be _______________.– Exception: volcanic ________, such

as obsidian. The color of the glass is misleading. Obsidian is felsic, but is usually black in color.

Silicate BehaviorBowen (1925) recognized

that mafic minerals tend tohave higher ___________points and less polymerization (__________________) 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 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 be 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.

Microscopic vies of an olivine crystal with partially dissolved edges. This indicates that the temperature of the magma at the time this lava was erupted was too low for olivine to be stable.

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

• Several fractionation processes:1) Gravitational ____________ of

initial solids2) Flow _________________ as the

magma moves3) Filter pressing of residual fluid4) Loss of ______________ (water,

gases) along with readily-dissolved elements which don’t fit well in the crystallizing silicate minerals

5) _______________ that partially empty the magma chamber

• Fractionation: materials are _________________during solidification. This can change the composition of the melts in addition to simple cooling.

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.

• This makes the remaining melt more ____________, while the settled crystals at the bottom of the chamber are more ____________. More Mafic

More Felsic

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.

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 ______________________ and become deposited in cracks (_________) as pressures and temperatures decrease towards the surface. Silica also is carried this way, precipitating as _____________.

Gold ore in a quartz vein

Igneous Rock Classification

Ultra-Mafic

Mafic Felsic

• ____________silica rocks (felsic) are light in color (pale grey to pink).

• ________ silica rocks (mafic) are dark (due to more dark minerals containing Mg and Fe)

Low Silica Medium Silica High Silica

Silica Content and Color

Basalt Andesite Rhyolite

GabbroDiorite

Granite

Extr

usiv

eIn

trus

ive

• Even when molten, the silicate tetrahedra will form __________. These will become entangled and inhibit ______.

• Over the range of ______________silica content, this extent of tangling results in a change of about seven orders of magnitude in viscosity (10,000,000x).

• Mafic (basaltic) magmas can flow almost like water. Felsic (rhyolitic) magmas are far more sluggish than toothpaste.

Silica Content and Viscosity

Silica Content and Viscosity

• High silica volcanoes are ____________, 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 (_________ shape)

Silica content and Volcano Type

Mafic lavas often erupt in a gentle fashion. Their low viscosities make it less likely that _______________ will build to the point of explosiveness.

Due to their low viscosities, basaltic composition magma (lava) can 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.

Summary of Trends with CompositionMafic (Basalt/Gabbro)• Density about 3.3 g/cm3

• Crystallization ~_____oC

• ________Silica (SiO2)

• Rock color = dark grey to black

• ________viscosity

• Typically _____ eruptions

• ____________Volcanoes (low, wide)

Felsic (Rhyolite/Granite)• Density about 2.7 g/cm3

• Crystallization ~______oC

• ________ Silica

• Rock color = pale grey/pink

• ________ viscosity

• Typically _________ eruptions

• _________________ (tall, pointy)

2. Igneous Textures• Slow cooling produces large grains. Rapid

cooling produces small (or no) grains.• Terms for Crystal Size:

• Phaneritic: visible to unaided eye, coarse-grained. Usually _______________.

• Aphanitic: crystalline, but crystals not visible, fine-grained. Usually ___________.

• Glassy: not crystalline. _____________.• Porphyritic: coarse grains (phenocrysts)

surrounded by fine grains (groundmass). Began crystallizing underground, then erupted and finished solidifying on surface. ____________.

Phaneritic igneous rocks crystallize slowly (usually underground). The rock name depends on the chemical composition and whether it is intrusive or extrusive.

Gabbro Diorite Granite

Mafic Intermediate FelsicComposition:

Rock Name:

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 because it cools _______________ to form crystals. It may contain isolated mineral grains (i.e. snowflake obsidian) or even an abundance of submicroscopic crystal “seeds”(crystallites), but it is mostly amorphous, lacking the long-range __________of crystal structure.

• Obsidian usually exhibits conchoidal fracture (Obsidian has often been used to make arrowheads).

Snowflake obsidian

Obsidian with conchoidal fracture

• Porphyritic rock is partially coarse and partially fine. The large ____________________ (crystals within the fine-grained matrix) formed first, slowly, in the magma chamber. Then the rock was erupted before it fully crystallized, so the matrix (groundmass) is very fine-grained.

• This is often referred to a _________________ cooling. process

Sample Igneous Textures and Where they Form.

Other Igneous TexturesPyroclastic “Broken by Fire”:

• Violent volcanic eruptions produce an explosive spray of lava which hardens (at least partially) while in _____________.

• The resulting fragments may or may not _________ 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.

Volcanic ash (tephra) derived from the Mount Mazama(Crater Lake, Oregon) eruption 6800 years ago.

The ash and other volcanic derived clasts can become welded together by heat to form fine-grained tuff or coarse-grained volcanic breccia.

thin section of a welded tuff (ash)

Hand Sample of a breccia

Volcanic Breccia

Volcanic Bombs: molten rock aerodynamically shaped due lava _____________ while in flight. Can be any size

• Vesicular: As a magma approaches the surface, it undergoes decompression andcooling. This ______________ its ability to hold various gases (H2O, CO, CO2, etc.) in solution.

• These gases will separate as ____________ which will either escape or remain trapped as the magma hardens around them. Trapped bubbles are called vesicles.

• If the frozen bubbles are large enough, they can later contain ______________, which can crystallize and form geodes.

Other Igneous Textures

______________ 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 volatiles.

Pumice or scoria (darker) form when gas bubbles are ____________ in rapidly cooling pyroclastic materials. The rocks are glassy and frothy.

Pahoehoe (ropey textured) basalt flows have a _________viscosity than aa (blocky textured) flows, which have degassed and ______________.

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

Pele's _________ is abundant around Halema'uma'u Crater, and originates from the active, spattering lava lake. When the lava spatters into the air, some of it stretches into thin fibers, which quickly cool into amber strands of glass, and can be carried off by the wind. (USGS/Hawaiian Volcano Observatory)

Other Igneous TexturesPillow Basalts: when basaltic lava erupts underwater or flows into water, it will form into pillow-like shapes, often with a ____________ rind, since the exterior of the pillow is in contact with cold water and ____________ rapidly.

Other Igneous TexturesColumnar 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.

4. Typical Magmatic Sources• The mantle is ultramafic.

Extensive melting can produce ultramafic magmas, but this is fairly rare. “Routine”_____________melting produces _____________magmas.

• Partial melting of subductingoceanic crust (mafic) and its associated sediments produces mafic and __________________ magmas.

• Interaction with continental material is required for the production of ____________ magmas.

Typical Magmatic Sources

Sources of Magma• In nearly all of the crust and mantle, temperatures are too

________ for melting to occur at the surrounding ______________.

• Magma production occurs when:– Warm rock travels _______________ (decompression

melting), as at divergent zones and hotspots, or– Cold rock is forced downwards and absorbs_______from

its new surroundings, as at subduction zones

• Mafic magma forms from a partial melt of the ultra-mafic asthenosphere, which occurs at a depth (100-350 km).

• Melting temperature is dependent upon pressure (_________) and______________of the rock involved.

• Only a small fraction (____) of the rock actually melts– the portion with the lowest melting point. The melt is a __________-density mafic magma from an ultramafic starting material.

• This magma will tend to rise due to buoyancy.

• ____________in the melt will lower the melting temperature of the rock.

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 ______________ 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 _____________).

• Melting occurs at a depth of __________km within continental crust.

Felsic Magma Formation

• Granitic composition magma is produced at continental collision margins. As the continental crust thickens it begins to partially melt at depth. Granite intrusions (felsic _____________) form below the mountain belts.

• As collisional tectonic mountain ranges are uplifted, the overlying marine sedimentary and metamorphic rocks are erodedexposingthe underlying granitic plutons.

Felsic Magma Formation at Continent-Continent Boundaries

Granitic composition magma reaches the surface in Yellowstone Park because the continental crust is being melted closer to the surface by upwelling magma generated from a _______________in the asthenosphere.

Intermediate (_______________) 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_______________crust along _______________zones.

• Introduction of __________ 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 silica content (becomes more felsic). Mount St. Helens dacites are more silica rich than Mt. Rainier andesite, likely due to continental source.