GEOLOGY
IGNEOUS ROCKS UNIT
Chapter 4
Reading: Chapter 4
Learning Objectives (I can…)
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Explain the difference between lava and magma.
Explain how igneous rocks form.
Demonstrate how igneous rocks are classified.
Explain where magmas originate.
Explain how magmas evolve.
Relate how temperature, pressure and water content influence
the formation of magma.
Analyze texture differences of igneous rocks.
Identify the minerals specific to chemical equivalents of igneous
rocks.
Igneous Rock Comparisons
 Magma vs. lava
 Volcanic vs. plutonic igneous rocks
 Extrusive vs. intrusive igneous rocks
 Acidic vs. basic igneous rocks
 Less viscous vs. more viscous igneous rocks
 Aphanitic vs. phaneritic igneous rocks
 Felsic vs. Mafic igneous rocks
 Dark vs. light colored igneous rocks
 Coarse-grained vs. fine-grained igneous rocks
Key Terms
Igneous Rocks
Plutonic
Granite
Pegmatite
Diorite
Gabbro
Peridotite
Volcanic
Rhyolite
Andesite
Basalt
Komatiite
Tuff
Breccia
ash
obsidian
pumice
welded tuff
scoria
Descriptive Terms
Aphanitic
mafic
Acidic
Volcanic
Vesicular
Phaneritic
ultra-mafic
Basic
Plutonic
Phenocryst
Felsic
Magma
Extrusive
Viscous
Pyroclastic
Magma Generation
Partial melting and fractional crystallization
Crystal settling
Assimilation of surrounding rock
Magma mixing
Large-scale Intrusive Features
Pluton (concordant--sill vs. discordant--dike)
Batholith
Lacolith
Volcanic pipes and necks
Stock
Intermediate
Lava
Intrusive
Porphyritic
glassy
Intrusive
ultra-mafic
Intermediate
Aphanitic
Phaneritic
Felsic
Plutonic
mafic
Magma
Acidic
Volcanic
Lava
Basic
Extrusive
Viscous
Intrusive
ultra-mafic
Intermediate
Aphanitic
Phaneritic
Felsic
Plutonic
mafic
Magma
Acidic
Volcanic
Lava
Basic
Extrusive
Viscous
Explain the difference between lava and magma.
1. Explain how igneous rocks form.
Igneous rocks from as they solidify from a magma or lava. Slow-cooled
form beneath the surface, rapid-cooled form near or at the surface
Plutonic
Volcanic
-lava
-ejecta
2. Demonstrate how igneous rocks are classified.
Classified by texture, color and chemistry (see block diagram)
3. Explain where magmas originate.
a.
Subduction zones (convergent plate boundary—oceanic/oceanic
or oceanic/continental crust)
b.
Spreading zones (divergent plate boundary—rift zones)
c.
Hot spots in mantle (plates move over hot spot)
4. Explain how magmas evolve.
The purest magma would be melted mantle (ultra-mafic).
The magma changes composition
a) as different minerals crystallize or melt at different temperatures.
 Fractional crystallization and crystal settling
 Partial melting
b) As water mixes with the oceanic crust, causing a partial melt of the
minerals that melt at a lower temperature (i.e., felsic quartz, kfeldspar).
c) Assimilation: Inclusion of country rock
d) Magma mixing
5. Relate how temperature, pressure and water content influence
the formation of magma.
The addition of water and the release of pressure lowers the melting point
of the magma, such that some of the minerals in the crust or mantle
preferentially melt. High temperatures also cause melting to occur.
6. Analyze texture differences of igneous rocks.
See igneous rock comparisons and textures.
7. Identify the minerals specific to chemical equivalents of igneous
rocks.
Felsic: quartz, k-feldspar, plagioclase, biotite
Intermediate: plagioclase, amphibole (hornblende)
Mafic: plagioclase, amphibole, pyroxene, olivene