Revised 8 / 06 (Monroe 6th ed.)
Including...
Put it all together: Classification
Click here for online mineral and rock ID charts
Click here for a summary of the 3 major rock types
The earth is just a big ball of magma
Crystallized to solid state where exposed to the cold of space
To summarize solid matter:
who knows what --> subatomic particles --> p/n/e --> atoms --> elements --> minerals --> rocks
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 still a hot planet
Lots of hot rocks below the surface
But it needs more than just heat for the hot rocks to melt
Must also have a reduction in pressure so the rock can expand enough to become a liquid
Probably restricted to areas of large scale crustal breakage
Magma vs. lava
Intrusive vs. extrusive
Earth has been subject to igneous activity throughout its history
It seems certain that igneous rocks were the first to form
Differentiation is the key!!
Heavies to the center, scum to the crust
Therefore, most crustal magmas have a relatively low density (S.G. <3.5)
Classification based on composition and texture
Elements in the liquid state
All crustal (and mantle?) magmas are rich in oxygen and silicon
See Monroe; Table 4-2, pg. 110 - together called "silica" but includes the oxygen
REVIEW the importance of oxygen and silicon
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? (Monroe; fig. 3-11, pg. 80)
Relative proportions of the elements and volatiles leads to compositional differences
Click here for background information on the terms "mafic" and "felsic"
Felsic magma and rocks (Granitic) (Monroe; fig. 4-21, pg. 120)
Light colored
Relatively low specific gravity (generally less than 3.0)
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
Partially due to low temperature
Also due to higher proportion of silica
Forms long molecular chains which interfere with each other and make the magma "sticky"
DIGRESS TO: Are there Ultra-felsic magmas?
Possibly: obsidian and related pumice deposits
Very high silica content (up to 80%)
Extremely sticky magma - does not flow!
Also very low temperature, cools quickly
Intermediate magma and rocks (Andesitic) (Monroe; fig. 4-20, pg. 119)
Medium colored
Medium specific gravity (plus or minus 3.0)
Silica/Oxygen: 53% to 65%
Varying amounts of all of them
Medium volatile content
Medium temperature
Mafic magma and rocks (Basaltic) (Monroe; fig. 4-19, pg. 119)
Dark colored
When fresh - for several reasons they tend to rapidly weather at the surface and lighten in color
Relatively high specific gravity (generally greater than 3.0)
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 (Monroe; fig. 4-17, pg. 118)
Dark colored
When fresh - for several reasons they tend to rapidly weather at the surface and lighten in color
They also easily metamorphose to serpentinite, which also lightens the color
Relatively high specific gravity (generally greater than 3.3)
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.)
These magmas are so hot that current conditions do not allow the formation of extrusive ultramafic rocks
Order of crystallization - related to temperature
Bowen's Reaction Series (Monroe; fig. 4-9, pg. 111)
Describe in detail - this is very important!
Each mineral is stable within a specific temperature range
And can change to a lower temperature mineral as the magma cools
Plagioclase feldspar is a special problem (the Continuous Branch)
We will return to Bowen later in the course:
Metamorphic rocks and the formation of granitic magma
Rates of surface weathering and soil development
Several factors can affect magma composition
Compositional Zoning within magma chamber
Felsic on top, mafic on bottom
Seem to be very common in extrusive environments
Felsic/intermediate pyroclastic blasts followed by intermediate/mafic flows
Order of crystallization
Early-formed minerals can settle out of melt (Monroe; fig. 4-10, pg. 112)
Remaining magma more felsic
Assimilated country rock (DEFINE)
Can melt and add new materials to the magma
Results in compositional differences
Some pieces do not completely melt
Inclusions (Monroe; fig. 4-7, pg. 109)
Magma mixing (Monroe; fig. 4-11, pg. 113)
Rising magmas of different compositions mix
How is this recognized?
The other factor affecting classification
DEFINE Texture: How big the mineral grains are
Related to cooling history (Monroe; fig. 4-14, pg. 116)
DIGRESS TO: Human/cow/slug analogy
What affects rate of cooling?
DIGRESS TO: Blanket analogy
Put both together: intrusive vs. extrusive igneous rocks
Grain size also affected by volatile content
Pegmatites - a special case (Monroe; fig. 4-23, pg. 121)
Very coarse texture, but in a dike (which should cool relatively quick)
DIGRESS TO: Pala pegmatites
Appropriate terms
Glassy - nearly instantaneous cooling (obsidian)
Aphanitic - fast cooling, extrusive
Phaneritic - slow cooling, intrusive
Porphyritic - multi-stage cooling history
Mixed cooling history
Example: Starts as intrusive and then erupts
Phenocrysts vs. groundmass
Relative size differential
Vesicular - trapped gas and/or water
Like a loaf of bread
Frothy - mixed with the atmosphere (pumice, scoria)
Pyroclastic (fragmental) - explosive volcanic activity
Usually related to more felsic magmas (WHY?)
Igneous rock chart (Monroe; fig. 4-16, pg. 117)
Click here for a summary of the major divisions of igneous rocks
Click here for online mineral and rock ID charts
Pluton - any intrusive igneous rock (Monroe; fig. 4-24, pg. 123)
Batholith - >100 square kilometers exposure
Complex formation, poorly understood
Commonly felsic in composition
Granitization - formed in place by the melting of pre-existing rock
One possible end result of complete regional metamorphism
Complex igneous/metamorphic transitions support this method for many batholiths
Intrusion
Forced up "from below" and injected into overlying country rock
Pressure at depth and density considerations support this method for others
Stock - smaller than a batholith
Commonly a small exposed piece of a larger batholith
Volcanic neck - eroded remnant of volcano (Monroe; Front cover)
Devil's Tower
Ship Rock
Several local examples
Pilot Rock, Rabbit Ears, Mt. Thielsen
Dike - discordant (cuts across regional stratigraphy)
Can lead to volcanic activity if the dike extends to the surface
Sill - concordant (conformable with regional stratigraphy)
Laccolith - concordant with flat bottom
Domed top due to deformation of overlying sedimentary layers
Click here for more on elements and minerals common to the major magma types
Click here for more on plate tectonics and the formation of magma
GeoMan's Home Page | RCC Index | High School Geology Index
You are GeoManiac number since April 1, 1997