Igneous rocks
Define: crystallization from molten (liquid) state (called Magma)
Magma is liquid rock
Occurs in "Magma Chambers"
Kind of a nebulous concept
Do magma chambers have walls? tops? bottoms?
Poorly understood
The earth is a hot planet
Lots of magma below the surface
Only cooled to the solid state where exposed to space
Magma vs. lava
Earth has been subject to igneous activity throughout its history
Differentiation is the key!!
Heavies to the center, scum to the crust
Therefore, most magmas are relatively light (specific gravity <3.5)
Classification based on composition and texture
All magmas are rich in silica and oxygen
Together called "silica" but includes the oxygen
Varying amounts of other elements and stuff determines magma type
Volatiles (H2O, CO2, HCl, H2SO4)
Misc. elements: iron, magnesium, aluminum, potassium, calcium, sodium
Remember the list of the 8 most abundant elements?
Relative proportions of the elements and volatiles leads to compositional differences
Felsic magma and rocks (Granitic)
Light colored
Silica/Oxygen: >65%
High in potassium, aluminum, sodium
Low in (or don't have) iron, magnesium, calcium
High volatile content
Low temperature (600 deg. C. - 900 deg. C.)
High viscosity
Intermediate magma and rocks (Andesitic)
Medium colored
Silica/Oxygen: 53% to 65%
Varying amounts of all the major elements
Medium volatile content
Medium temperature
Mafic magma and rocks (Basaltic)
Dark colored
Silica/Oxygen: 45% to 52%
High in iron, magnesium, calcium
Low in (or don't have) potassium, aluminum, sodium
Low volatile content
High temperature (>1000 deg. C.)
Low viscosity - flow easily
Ultramafic magma and rocks
Dark colored
Silica/Oxygen: <45%
Very high in iron, magnesium
Small amounts of aluminum, calcium
Generally don't have sodium, potassium
Essentially olivine and pyroxene - little or no plagioclase feldspar
Very low volatile content
Very high temperature (>1600 deg. C.)
Order of crystallization - related to temperature
Describe in detail
Each mineral is stable within a specific temperature range
Several factors can affect magma composition
Compositional Zoning within magma chamber
Felsic on top, mafic on bottom
Order of crystallization
Early-formed minerals can settle out of melt
Remaining magma more felsic
Assimilated country rock
Can melt and add new materials to the melt
DEFINE: How big are the crystals which form upon cooling
Related to cooling history
Slug analogy
What affects rate of cooling
Blanket analogy
Grain size also affected by volatile content
Pegmatites
Appropriate terms
Glassy - nearly instantaneous cooling (obsidian)
Aphanitic - fast cooling, extrusive
Phaneritic - slow cooling, intrusive
Porphyritic - multi-stage cooling history
Chocolate chip cookie rocks
Mixed cooling history
Example: Starts as intrusive and then erupts
Phenocrysts vs. groundmass
Relative size differential
Vesicular - trapped gas and/or water
Church rocks
Pyroclastic (fragmental) - explosive volcanic activity
Usually related to more felsic magmas (WHY?)
Put it all together
Review the igneous rock chart
Pluton - any intrusive igneous rock
Can be further divided by size and orientation
Batholith - >100 km2 exposure
Stock - small than a batholith
Laccolith - concordant with flat bottom
Dike - discordant
Sill - concordant
Volcanic neck - eroded remnant of volcano
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