Igneous Rocks

 

General Statements

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

 

Magma composition - elements in the liquid state

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

Bowen's Reaction Series

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

 

Texture: the other factor affecting classification

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

 

Igneous structures

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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