Igneous rocks
Formed from magma or lava
Two basic types of igneous rocks
Intrusive (plutonic)
Extrusive (volcanic)
How do we tell igneous rocks apart?
Differentiate between igneous rocks based on two things
Visual textures
Tells the environment they formed in
Composition
Question:
How does the environment the rocks form in change the way the rocks will look?
“Texture” of Igneous Rocks
Intrusive
slow cooling
minerals grow large (can see unaided)
Extrusive
cools quickly
often hard to see minerals unaided
Other differences
Intrusive:
Water retained due to “sealed unit”
Creates element rich solutions
No evidence of flow
Never vesiculated
Extrusive:
Water tends to degas
Often see flow structures…banding, jointing etc…
Often has vesicles
Common “Textures” of igneous rocks
Phaneritic
Pegmatititic
Aphanitic
Porphyritic
Flow Banded
Pyroclastic
amygdaloidal
Composition of Igneous Rocks
Based on TOTAL % of silica in ROCK
Some common rock forming minerals found in igneous rocks
Quartz: SiO2 (all silica)
Plagioclase NaAlSi3O8 to CaAl2Si2O8
Orthoclase: KAlSi3O4
Augite: Ca(Mg, Fe)Si2O3(Al,Fe)2O3
Biotite: K(Mg, Fe)3AlSi3O10(OH)2
Olivine: (Mg,Fe)2SiO4 (low silica)
Felsic: high percentage of silica (SiO2) (>65%)
Intermediate: silica from 55-65%
Mafic: silica from 40-55% and large amounts of magnesium (Mg) and iron (Fe)
Ultramafic: silica <40% and abundant iron and magnesium
Based on total silica in rock…all minerals together
Other compositional differences besides silica
Felsic:
High aluminum, sodium and potassium
Low iron, calcium and magnesium
Ultra mafic:
Low aluminum, sodium and postassium
High iron, calcium and magnesium
Felsic
(often light in color)
Rhyolite
Granite
Intermediate (salt and pepper)
Diorite
Andesite
Mafic
(Dark)
Gabbro
Basalt
Ultramafic
Peridotite
Komatiite
Silicate minerals in Igneous rocks
Bowen’s reaction series:
Shows at what temperature minerals will crystallize from a melt
Shows which minerals are found in which igneous rocks
Continuous vs discontinuous series
How do we get different igneous rock types?
Why are there different compositions of magma?
What type of rock is being melted
If you melt a felsic rock you will get a different composition magma than if you
melt a mafic rock
Partially melting a rock will give a different composition than the original rock…WHY?
Partial melting
Not all minerals melt or are stable at the same temperature
Partially melting a rock removes some minerals and leaves others
Partially melting a rock will always produce a magma that is richer in silica
Partial melting implies that only a portion of the rock is melted.
Which minerals will melt first if only part of the rock is melted….
Partially melting peridotite (ultramafic rock) creates basalt and gabbro
Partially melting Continental material (granites) creates what?
How could magma of intermediate composition be created?
What are other ways to get different compositions of magma?
Assimilation
Incorporate into a melt the surrounding country rock
Magma mixing
Some Igneous rock Names
Intrusive
Granite
Diorite
Gabbro
Periodotite
Extrusive
Rhyolite
Andesite
Basalt
Komatiite
Scoria
Pumice
Obsidian
Tonight’s activity…
Work with the concepts of textures and compositions.
Review minerals
Prepare for exam next week
Next week learn to identify the different igneous rocks.