Lesson 5: Magma Generation and Igneous Rocks Flashcards
Igneous rocks form when:
a melt solidifies at or below the Earth’s surface
The molten rock beneath the Earth’s surface is called ____.
Magma
The greatest amount of magma is created at
Divergent plate boundaries
How is magma generated at subduction zones?
Addition of volatiles
What type of magma contains the lowest amount of silica?
a. Felsic
b. Intermediate
c. Mafic
d. All magmas contain the same amount of silica
c. Mafic
_______ is a mafic extrusive igneous rock.
Basalt
A _________ is a concordant igneous rock body which runs parallel to pre-existing bedrock.
Sill
Magma rises toward the Earth’s surface because:
it is less dense than the surrounding rocks
Why are intrusive igneous rocks from a batholith generally coarser-grained than those from volcanic vents?
They cooled more slowly and thus had more time to grow
A _______ texture indicates two stages of cooling.
Porphyritic
Magma
molten/partially molten material found beneath the surface of the Earth
Lava
What we call magma that reaches the Earth’s surface.
Where does magma form in terms of plate tectonics?
Mostly at divergent plate boundaries
- mid-ocean ridges
- Continental rifts
Few at convergent plate boundaries
- subduction zones
What are the three ways to generate magma
- Decompression melting
- Addition of volatiles
- Addition of heat
What is decompression melting?
a rapid decrease in pressure that can cause the mantle to melt without an increase in temperature.
Where do we see decompression melting in terms of plate tectonics?
Divergent plate boundaries
What is addition of volatiles?
flux melting that occur due to chemical compounds (water) lowering the melting temperature of the rock
Where do we see addition of volatiles in terms of plate tectonics?
Subduction zones
What is addition of heat?
Inducing melting if temperature exceeds melting temperature
Where do we see addition of heat in terms of plate tectonics?
Mantle plumes/hotspots
How does magma evolve?
- Fractionation
- Assimilation
- Magma mixing
What is fractionation?
The composition of magmas change as the magma cools
What is assimilation?
The chemical evolution of magma is influenced by its reaction with, and digestion of the wall rock that it encounters during its ascent.
What is magma mixing?
during the ascent of two chemically different magma bodies, the more buoyant mass may overtake the slower-rising body.
Compare intrusive and extrusive igneous rocks.
Intrusive: below the surface of the earth; slow cooling of magma
Extrusive: on the surface of the earth; rapid cooling
What are the results of intrusive enviornments? (5)
- Dike
- Sill
- Lacccolith
- Plutons
- Batholiths
What is the difference between a dike and a sill?
Dike is a vertical intrusion and cuts across sills
Sill is a horizontal intrusion that form parallel to existing features instead of cutting through it
Describe laccoliths.
A mushroom-shaped intrusive body that has intruded between layers of sedimentary rock.
Rising magma forces the overlying layers to rise into a dome
Describe plutons.
A blob-like igneous intrusion
Describe batholiths.
A huge blob-like igneous intrusion , formed as composite of many plutons
What are the different compositions of magma (igneous rocks)?
- Felsic
- Intermediate
- Mafic
Describe the composition of felsic rocks
High in silica
Low in iron and magnesium
Lighter in color
Describe the composition of intermediate rocks
Lighter than felsics
Average silica, iron, and magnesium
Describe the composition of mafic rocks
Low in silica
High in iron and magnesium
Darker in color than intermediate and felsic rocks.
What are the textures of igneous rocks?
- Aphantic - fine-grained
- Phaneritic- course-grained
- Porphyritic - 2 sizes of crystals
- Glassy
- Pyroclastic - various fragments
- Pegmatitic - exceptionally coarse-grained
- Vesicular - bubbles
How are aphanitic textures characterized? (3)
No visible grains to the naked eye
Rapid cooling of lava/magma
May contain vesicles
How are phaneritic textures characterized? (3)
Crystals large enough to see with the naked eye
Slow cooling of magma
Intrusive origin
How are porphyritic textures characterized? (3)
Two distinct sizes of crystals (large and small)
Minerals form at different temperatures and rates
Usually indicates two stages of cooling
Larger crystals are phenocrysts
Finer crystals are ground-pass
How are glassy textures characterized?
Very rapid cooling of molten rock
Extrusive origin
How are pyroclastic textures characterized? (2)
various fragments
Extrusive origin
Tuff if the largest fragments are less than 2.5 inches
Volcanic breccia if fragments are larger than 2.5 inches
How are pegmatitic textures characterized?
Exceptionally coarse-grained
Forms in the late stages of crystallization of granitic magmas
How is vesicular texture characterized?
Gas bubbles trapped in colling lava
Extrusive origin
