how 2nd half labs will work - university of...
TRANSCRIPT
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• 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
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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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2
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
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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
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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
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3
1
The degree to which the temperature of a melt falls below thetrue crystallization temperature before minerals begin to form
= “Undercooling”
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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
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rate
or
Result:• Slower growth rate• Rapid nucleation
rate
Many fine-grained(small,
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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
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Why?
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Textures in igneous rocks controlledby:
Magma Composition
Cooling Rate
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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
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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
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Big Obsidian FlowNewberry Volcano, OR
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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”
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2
3
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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”
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2
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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
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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!
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Crystal Habits (aka Shape)
EquantTabularBladed
ColumnarAcicular
PrismaticPlaty
Look in your lab handout
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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:
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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
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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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“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
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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