Igneous Rocks

v 0047 of 'Igneous Rocks' by Greg Pouch at 2011-08-24 13:59:29 LastSavedBeforeThis 2011-01-19 17:38:01Geo101\03IgneousRocks.ppt on 'GWPOUCHDELL1720'

Igneous Rocks3 Overview4 Definitions 15 Texture

6 Phaneritic7 Aphanitic8 Porphyritic9 Glassy10 Vesicular11 Tuffaceous

12 Minerals13 Minerals14 Minerals

15 Bowen’s Reaction Series16 Types of igneous rocks

17 Bowen's Reaction Series/Classification18 Telling Granite and Granodiorite apart

19 Caveat Emptor

Overview•Igneous rocks formed from molten rock (magma). •Igneous rocks are characterized by an interlocking crystalline texture without preferred orientations.

•Igneous rocks are classified according to their texture and composition. •The composition tells where the source magma came from.•The texture tells how it cooled.•A few compositions of igneous rock are rather common, and most possible compositions are fairly rare.

•Textures and composition are related.

Definitions 1• Descriptive vs. Genetic terms

–Descriptive terms describe observable properties and should be used for observations.

–Genetic terms talk about the origin of a property, and are conclusions. In some slides, I highlight the distinction by showing genetic implications in gray.

• Magma melted rock=molten rock below the surface, or above surface.

• Lava melted rock at the surface.

• The phrase "lava rock" is a contradiction.

• Country rock the surrounding rock into which an intrusion occurs

• Xenolith a “foreign rock” refers to a chunk of some other rock (from the source region or the country rock

• Extrusive = volcanic formed at the surface (Gen)

• Intrusive = plutonic formed inside the earth (Gen)

Magma

Lava

TextureTexture refers to the size of individual particles. Texture terms are Descriptive, but have Genetic implications.• Phaneritic crystals/grains large enough to be seen. Genetic interpretation is that large crystals indicates slow

cooling, probably at depth.• Aphanitic crystals too small to be seen Indicates rapid cooling, probably at surface or shallow subsurface.• Porphyry two (or more) sets of grain sizes

–Phenocrysts are the larger crystals/grains in a porphyry Usually have Xtl (Crystal) faces.–Groundmass is the smaller crystals in a porphyry.Porphyritic texture indicates two generations of cooling, one much faster than the other. This could be due to change in cooling rate (slowly cooling intrusive moving into near-surface region and cooling quickly) or to change in melting conditions (sudden loss of water leading to rapid crystallization).

• Glassy No crystals at all: the rock is actually a glass rather than a true solid. Obsidian Usually indicates very rapid cooling or a magma that can't crystallize easily.

• Vesicular Contains bubbles, but not enough to make up most of the volume. If the vesicles are filled with minerals, this becomes amygdaloidal (the "nodules" are amygdules). If the vesicles make up a lot of the rock, it can be scoria (still denser than water, but lots of bubbles) or pumice (less dense than water)

• Tuff is an aphanitic igneous rock that occurs in beds with sedimentary features that indicate that it fell as ash rather than flowed as a liquid. Consists of ash and dust that were blown out of a volcano and welded themselves together like caramel corn. It grades into being a sedimentary rock.

Phaneritic•Crystals visible to naked eye, or under hand lens

•Implies Plutonic: cooled slowly at depth

Aphanitic•Crystals are too small to be seen, even under a hand lens. Grainy appearance with small particles, like bread, cardboard, or wood. Often has a few phenocrysts.

•Implies volcanic; implies cooled quickly, maybe at surface or at shallow depth

Porphyritic•At least two distinct sizes of crystals. The large crystals are called phenocrysts, the finer minerals are called groundmass.

•Implies slow cooling for the larger size (phenocrysts) followed by faster cooling for the groundmass.

–Might be due to slow cooling at depth, followed by eruption, or

–Might be related to change of melting temperature due to loss of volatiles.

PhenocrystsGroundmass

Glassy

•No crystals at all, really: the rock is actually a glass rather than a true solid. obsidian

•Usually indicates very rapid cooling or a magma that can't crystallize easily.

Vesicular

• Vesicular Contains scattered bubbles/voids, but not enough to make up most of the volume. If the vesicles are filled with minerals, the "nodules" are called amygdules and the texture is amygdaloidal. If the vesicles make up a lot of the rock, it can be scoria (still denser than water, but lots of bubbles) or pumice (less dense than water)

• The vesicles/amygdules are bubbles frozen in place when the lava “foamed”

Tuffaceous

•Tuff is igneous rock that consists of ash and dust that were blown out of a volcano and welded themselves together. It grades into being a sedimentary rock at one end, and rhyolite or other aphanitic rock at the other.

•It is very hard to tell tuff from rhyolite or other aphanitic rock in hand specimen. Mostly, it’s determined by how it occurs in the field.

From http://www.geolab.unc.edu/Petunia/IgMetAtlas/volcanic-micro/tuff1.UX.html

From http://geology.about.com/od/more_igrocks/ig/extrusives/tuffsalvador.htm

MineralsCertain minerals are common in igneous rocks. These include • Olivine• Hornblende (amphibole)• Augite (pyroxene)• Biotite• Muscovite• Quartz (clear, looks dark)• K-spar• Plagioclase(These are all composed of the 8+2 common elements) Other minerals

are often present, and can give valuable clues to the origin of a rock, and can be valuable in and of themselves.

Igneous rock classification uses abundance of Qtz, KSpar, Plag (and Na vs Ca content), and, if those are rare, olivine and types of pyroxene.

MineralsCertain minerals are common in igneous rocks. These include • Olivine• Hornblende (amphibole)• Augite (pyroxene)• Biotite• Muscovite• Quartz• K-spar• Plagioclase(These are all composed of the 8+2 common elements) Other minerals

are often present, and can give valuable clues to the origin of a rock, and can be valuable in and of themselves.

Igneous rock classification uses abundance of Qtz, KSpar, Plag (and Na vs Ca content), and, if those are rare, olivine and types of pyroxene.

Minerals•Mafics=Ferromagnesian contain Fe and Mg and are black and green.

– Olivine– Pyroxene– Hornblende– Biotite

•Felsics = Sialic contain Silica and Alumina and are generally white/light colored

–Quartz–Muscovite–Feldspars

•Plagioclase (CaAl, NaSi) AlSi2O8 is a solid solution between two endmembers anorthite CaAl2Si2O8 and albite NaAlSi3O8

•Potassium feldspar K-spar KAlSi3O8 includes three polymorphs (orthoclase, sanidine, microcline) and can incorporate sodium into its crystal structure at high temperatures–Alkali Feldspar (K,Na)AlSi3O8 is a solid solution between KSpar and Albite and is common in felsic rocks.

Bowen’s Reaction SeriesThis started as a theory of how to generate granite from an originally basaltic melt. (That rarely happens) It

is most useful in understanding partial melting and differentiation. Basically, as a melt cools, minerals come out of melt in a certain order. Or, as a rock melts, they melt in the

reverse order.For most situations, a magma only goes down two or three steps in the reaction series before it has

crystallized completely. When partial melting occurs, Bowen's reaction series is traveled from bottom to top, and the first partial melts are usually one or two steps down from the source-rock's bulk composition.

–Continuous Plagioclase feldspar–Discontinuous mafic minerals

• When rock is partially melted, the minerals, and compositions of minerals, with the lowest melting points melt first. This causes the melt to be further down Bowen's reaction series, and the restite further up Bowen's reaction series, than the source rock. If the degree of partial melting is very high, the melt is close to the starting composition. If the degree of partial melting is very low, the melt can be much further down Bowen's reaction series than the source, and the restite can show almost no change.

Types of igneous rocks Igneous rocks are mostly classified on texture (crystal size) and composition (minerals)• The textures glassy, scoriaceous, pumaceous, and tuffaceous override composition, and the rocks obsidian,

scoria, pumice, and tuff are based entirely on texture.• For phaneritic rocks, classification depends on abundances of essential minerals (quartz, plagioclase, K-

spar, and total mafics, or types of mafics if total mafics > 90%), and is aided by accessory minerals (mafics, muscovite…),

• For aphanitic rocks, classification uses observed minerals and overall color (and laboratory analysis)• This is a simplified classification scheme. A more comprehensive scheme is found at http://www.geol.lsu.edu/henry/Geology3041/lectures/02IgneousClassify/IUGS-IgneousClassFlowChart.htm

Felsic Felsic Intermediate

Intermediate Mafic Ultramafic

Minerals quartz, K-spar, sodic plag

biot, musc.

sodic plag, minor K-spar, rare qtz.

amph, biot

int. plag, no K-spar,

amph

calcic plag,

pyr, ol

minor very calcic plag

Major ol, pyr

Appearance (not reliable)

light colored or pink to red

light gray gray, salt-and-pepper like a Dalmatian

black black

Phaneritic Granite Granodiorite, Granitoids

Diorite Gabbro Peridotite

Aphanitic Rhyolite (explosive)

Andesite (depends on eruption)

Basalt(gentle)

Komatiite

Granodiorite

Telling Granite and Granodiorite apart

Rock Granite Granodiorite

Quartz must be Significant (>10%) Must be present but minor (3 to 10%)

(If absent, or very minor, call it diorite.)

Feldspar Lots of K-spar (usually salmon pink).

Tends to look chalky.

Little if any K-spar. Mostly intermediate plagioclase (usually light gray to white).

Tends to look greasy.

Mafics Biotite common. Amphibole often. Amphibole common. Sometimes biot or pyr.

Might contain muscovite

from http://www.geol.umd.edu/~jmerck/geol100/images/08/granite.jpg from http://geology.about.com/library/bl/images/granodiorite.jpg

Caveat Emptor•In the building industry, any phaneritic rock might be called "granite". A "granite" countertop might be made from granite, granodiorite, diorite, or even gabbro.

•Granite (senso stricto, or at least senso geologico) or granodiorite MIGHT, depending on the source, contain uranium or lead. (More on this when we talk about ex-solution and ores.) Think about this when selecting cooking utensils and surfaces.

•Granite has a very specific definition. There are a lot of granite-like rocks that nearly meet the definition of granite and are called granitoids. (For example, a sample might have too little quartz to be a true granite, so we'd call it a granitoid.) The composition can vary in a deposit, so we are sometimes lax with our rock terms.

Summary of Igneous Rocks•Igneous rocks formed from molten rock. Igneous rocks are characterized by an interlocking crystalline texture without preferred orientations. Igneous rocks are classified according to their texture and composition. The composition tells where the source magma came from, the texture tells how it cooled.

•A few compositions of igneous rock are rather common, and most possible compositions are fairly rare.

•The textures pumaceous, scoriaceous, tuffaceous, and glassy immediately determine rock name.

•For phaneritic rocks, you need to determine the mineralogy and use that to find a rock name. For aphanitic rocks, you determine mineralogy of the phenocrysts and overall rock color, and use those to determine a rock name.

•The classification scheme used in this class is a greatly-simplified taxonomy.

• A more comprehensive scheme can be found at http://www.geol.lsu.edu/henry/Geology3041/lectures/02IgneousClassify/IUGS-IgneousClassFlowChart.htm