Revised 8 / 06 (Monroe 6th ed.)

Magma and Igneous Rocks - Chapter 4




Magma composition


Put it all together: Classification

Click here for online mineral and rock ID charts

Igneous structures



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

Plate margins

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


Magma composition

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?)


Put it all together: Classification

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


Igneous structures

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


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


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