easc 116 ch. 3 igneous rx
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CH. 3 – IGNEOUS ROCKS
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Igneous Rocks
Ignis = “fire”
Form from cooling & crystallization of magma (molten rock)
Lava = molten rock at surface
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Igneous Rocks
Classified by texture & composition
Texture = crystal size of minerals
Composition = minerals present
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Changes of State of Matter
Add heat (melting):
Rx Magma (atoms faster)
Remove heat (crystallize):
Rx (atoms slower) Magma
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Texture of Igneous Rocks
Depends on magma’s cooling rate
1) Cools slowly
Atoms can move through liquid
- crystals grow (large, small)
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Magma’s Cooling Rate
2) Cools quickly
Forms a solid faster than atoms can move around
- crystals are (large, small)
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Texture (Crystal Size)
Two environments where magma cools:
1) At surface or shallow burial
2) Deep underground
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Types of Texture
1) Aphanitic
- crystals invisible to naked eye
- magma cooled ___________
- extrusive/volcanic
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Types of Texture
2) Vesicular
- aphanitic rock w/many holes (vesicles) in it
- gas trapped as magma cools very quick
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Types of Texture
3) Phaneritic
- crystals easily visible
- magma cooled ___________
- intrusive/plutonic
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Types of Texture
4) Porphyritic
- two size crystals in same rock
Large crystals = phenocrysts
Small crystals = groundmass
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Porphyritic Texture
2-stage cooling process:
1st stage = slow cooling at depth
_______________ are formed
2nd stage = rapid cooling of remaining magma
_______________ are formed
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Types of Texture
5) Glassy
- no crystalline structure
Ex: Obsidian; pumice
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Types of Texture
6) Pegmatic
All minerals larger than 1 cm
Forms during later stage of crystallization
- large percentage of liquid allows ions to migrate easier
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Types of Texture
7) Pyroclastic
“pyro” = fire; “clastic” = pieces
Consolidated rock fragments ejected during violent volcanic eruptions
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Pyroclastic texture
Fragments can be volcanic ash, blobs of magma, or angular blocks
- not composed of interlocking crystals!
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Igneous Rock Composition
The most abundant elements of Earth’s crust are _____ & ______
Magma is mostly silica with varying amounts of other six elements
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Composition
Other elements collect in water at top of magma chamber
Creates hydrothermal solution
- cools in fractures as highly concentrated ore deposits
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Igneous Compositions
Mafic
- 45-50% silica (“silica-poor”)- rich in magnesium, iron (Fe), &
calcium
- dark-colored: black, dark gray, brownish-red
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Igneous Compositions
Felsic (feldspar & silica)
- 60-75% silica (“silica rich”)- rich in aluminum, potassium, &
sodium
- light-colored: light gray, pinkish
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Igneous Compositions
Intermediate
Aphanitic: medium gray
Phaneritic: black & white crystals
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Importance of Igneous Rocks
Compose 90% of Earth’s crust
Two types of crust:
oceanic crust = basalt
continental crust = granite
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Question:
Why different igneous rock compositions if only 8 elements?
N.L. Bowen experimented with mafic magmas
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Results:
** Same sequence of minerals crystallized from magma
Referred to as Bowen’s Reaction Series (Fig. 3.16)
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Bowen’s Reaction Series
Discontinuous series (left) vs. continuous series (right)
Discontinuous series:
- each mineral has a unique silicate structure
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Continuous series
Different compositions of plagioclase feldspars
- ions of Ca2+ and Na+ can interchange within crystalline structure
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Bowen’s Reaction Series
As magma crystallizes:
First mineral to form = mafic
Composition of remaining magma changes
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Bowen’s Reaction Series
Biotite uses last mafic elements in magma
- remaining magma becomes more “silica rich”
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Bowen’s Reaction Series
Minerals that form in same temp. conditions are found together in same igneous rock
QUESTION: Which mineral would you expect to form phenocrysts in a porphyritic basalt?
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Phenocryst Mineral:
a) Quartz
b) Potassium feldspar
c) Biotite
d) Calcium-rich plagioclase f-spar
e) Sodium-rich plagioclase f-spar
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Felsic magma
Felsic minerals melt at cooler temp’s
- magma not as hot as mafic magma
- felsic magma not as buoyant
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Felsic magma
Magma doesn’t reach surface
Ex: Continental crust = granite
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Important Concepts
A mineral is stable if the environmental conditions are similar to where it formed
Ex: temperature, pressure, water- content
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Example:
Mafic minerals are stable in ____ temperatures
Felsic minerals are stable in ____ temperatures
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Important Concepts
If a mineral is put under different environmental conditions, it becomes unstable
- mineral has to react to new environment by changing
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New Environmental Conditions:
If at the surface, weathering occurs
If within the crust, metamorphism occurs