• Continue to use your mineral identification skills

• Learn to describe, classify, interpret rock hand samples:

Assignments:1. You get a lab handout with each set of rocks

Spend 2 lab periods working on the questionsTurn in at beginning of next lab

2. Formal rock descriptions: one per lab handout (= 4 total)– One-page detailed description of one rock sample

sedimentary

metamorphic

How 2nd half labs will work

volcanicplutonic

(1 week)(1 wk)

Igneous

Volcanic vs. PlutonicDoesn’t make it to the surface(= cools slowly underground)

“Intrusive”

Erupts onto the surface(=cools fast)

“Extrusive”

Our goal: identify, classify, interpret these rocks

Igneous rocks

Mt. Etna firefountain

Igneous Petrology: study of melts (magmas) and the rocks thatcrystallize from them

Lake Ann stock, Mt Baker

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1

Textures in igneous rocks controlled by:

Magma Composition

Cooling Rate

How we classify rocks: (1) mineralogical composition(2) texture

Importance of TexturesTexture is:

Size, shape and arrangement of crystals (+- glass)

Helps us understand processes that formed a rock:its origin and history

Nucleation Rate

How fast are new, tiny mineral “seeds” born

Growth Rate

How fast do the “seeds” grow bigger

Cooling Rates Control:

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1

Expected to begin at liquidus….but doesn’t

Always starts below the liquidus because:

NucleationGrowthDiffusion of elements

How do minerals know when to startnucleating & growing?

These factors controlcrystallization, andcannot keep up withtemperature drop

2

3

1

The degree to which the temperature of a melt falls below thetrue crystallization temperature before minerals begin to form

= “Undercooling”

rate

or

Slow cooling results in minor undercooling (at Ta)and crystals form here

Result:• Rapid growth rate

• Slow nucleation rate

Few coarse-grained(large, >5mm)

crystals

molten

liqui

dus

Situation #1:slow cooling (= plutonic rocks)

Where crystalsshould ideallybegin to form

rate

or

Result:• Slower growth rate• Rapid nucleation

rate

Many fine-grained(small,

rate

or

Result:• Little, if any,

nucleation or growth

No crystals:The magma quenches

to a glass

Where crystalsshould ideallybegin to form

molten

liqui

dus

Situation #3:Super fast cooling

(= volcanic rocks like obsidian orthe rinds on pillow basalt)

Extremely rapid cooling causes the mostundercooling (at Tc) but crystals don’t grow

Why?

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1

Textures in igneous rocks controlledby:

Magma Composition

Cooling Rate

How Magma Composition Affects Texture:Through Viscosity

Weight % SiO2 in magma

Log Viscosity

50 60 70

Basalt Andesite Dacite Rhyolite

Hi temp

Low tem

p

Viscosity causes undercooling

Volcanic vs. PlutonicDoesn’t make it to the surface(= cools slowly underground)

“Intrusive”

Erupts onto the surface(=cools fast)

“Extrusive”

Igneous rocks

Many fine-grained crystals toNo crystals (totally glass)

Few coarse-grained crystals

Glass forms easiest inSiO2-rich magmas (likerhyolites), which areviscous. This inhibitsdiffusion of elements andprevents crystals fromforming & growing. Picture of obsidian lava flow

Big Obsidian FlowNewberry Volcano, OR

Igneous Textures3 major ones

Aphanitic- Crystals (if any) too smallto see with naked eye

Phaneritic- Crystals visible with naked eye

Fragmental- Accumulated chunks of igneous rocks, crystal fragments, glass, pumice…

- Forms in violent eruptions- Collectively known as “Pyroclastic”

1

2

3

Igneous Textures3 major ones

Aphanitic- Crystals (if any) too smallto see with naked eye

Phaneritic- Crystals visible with naked eye

Fragmental- Accumulated chunks of igneous rocks, crystal fragments, glass, pumice…

- Forms in violent eruptions- Collectively known as “Pyroclastic”

1

2

3

Either may also be described as“Porphyritic”

= contains phenocrysts

“Phenocrysts”Crystals that are conspicuously larger

than others

“Groundmass” or “Matrix”Fine-grained part of rock thatsurrounds the phenocrysts

Many other common textures: See handout

Porphyritic?

Aphanitic groundmass (=the dough)

Yes

3 types of phenocrysts:Chocolate chipsWalnutsRaisins

This rock, cookiite, is porphyritic with aphanitic groundmass

Groundmass: glassy or fine-grained?Hopefully not glassy!

Crystal Habits (aka Shape)

EquantTabularBladed

ColumnarAcicular

PrismaticPlaty

Look in your lab handout

clear

clear

white

Quartz, OlivineEquant habit (equidimensional)Look for conchoidal fracture (no cleavage)Quartz – translucent white to grayOlivine – translucent yellow-green

Plagioclase, Sanidine, Orthoclase(Microcline usu. metamorphic only)

Usually Tabular or Bladed habitLook for flat faces (due to cleavages)

whitepolysynthetic (=albite) twinning (finestriations on faces: use hand lens)

plutonic rocks only pink (usually) or (rarer)

volcanic rocks only

Pyroxene:

Prismatic or Columnar habitOrthopyroxene – brownClinopyroxene – greenCleavages at 90 deg

Amphibole: (usually black hornblende)Prismatic or Acicular (pencil-like) habitCleavages at 60/120

Micas:Biotite – blackPhlogopite – gold – tanMuscovite –

Platy habitSuper-shiny flat cleavage surfacesPlagioclase

Albite +Anorthite

Orthoclase

Sanidine

K-spars:

Volcanic vs. PlutonicDoesn’t make it to the surface(= cools slowly underground)

“Intrusive”

Erupts onto the surface(=cools fast)

“Extrusive”

Igneous rocks

Spectrum ranging from:Many fine-grained crystals toNo crystals (totally glass)

Aphanitic or porphyritic-aphanitic orfragmental

Hypocrystalline, Holohyaline, orHolocrystalline

Few coarse-grained crystals

Phaneritic or porphyritic-phaneritic

Holocrystalline

Determine the mode: volume percent of each phenocryst mineral in the rock= MODE

Example: Mode of some average granite might be

25% quartz 30% plagioclase 5% biotite30% kspar 10% hornblende

Volcanic Rocks

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1

“Felsic” –light-colored minerals

“Mafic” –dark-colored mineralsmagnesium ferric iron

feldspar silicaFelsic rocks: light color; has mostly felsic minerals

Mafic rocks: dark color; mostly mafic minerals

Ultramafic rocks >90% mafic minerals

Apply terms to rocks:

(except for anorthite!)

Color of rock: felsic vs. mafic

Finally, put it all together:

Rhyolite: Light-colored Can be porphyritic or be Plag, qtz, sanidinetotally glassy (obsidian) G’mass: glassy or very fine-grained

*often occurs as pyroclastic rocks (tuffs, ignimbrites, pumice)*

No quartzHas quartz

No olivineMaybe olivine

Basalt: Dark-colored Fine grained (± porphyritic) ~ equal amounts plag + cpx

*subdivided into tholeiitic, alkalic, calc-alkaline*

Andesite: Grayish Commonly porphyritic Plag, opx, cpx, hornblendeG’mass: aphanitic or glassy

Color Grain Size Phenocrysts + Groundmass

Dacite: Light-colored Commonly porphyritic Plag, qtz, sanidine, hbl, bioG’mass: fine-grained or glassy