Slides 4c: Igneous Flashcards
What are the major types of magma and how are they classified?
There are four major magma types based on % silica (SiO2).
- Felsic (feldspar and silica) 66–76% SiO2
- Intermediate 52–66% SiO2
- Mafic (Mg- and Fe-rich) 45–52% SiO2
- Ultramafic 38–45% SiO2
How are igneous rocks classified?
based on their viscosity aka. silicate content
mafic —-(increasing silica content)—–> felsic
How do intrusive and extrusive igneous rocks differ in terms of their grain characteristics?
intrusive –> Coarse-grained
extrusive –> fine-grained
Magmas vary chemically due to ____
- initial source rock compositions.
- partial melting.
- assimilation.
- magma mixing.
_______ dictates initial magma composition.
Source rock
What type of magma is formed from mantle source?
ultra-mafic and mafic magmas
What type of magma is formed from crustal source?
mafic, intermediate, and felsic magmas.
What is partial melting?
Upon melting, rocks rarely dissolve completely.
Instead, only a portion of the rock melts.
How does the composition of the rock affects the melting time?
Si-rich minerals melt first;
Si-poor minerals melt last.
Partial melting, therefore, yields a silica-rich magma.
Removing a partial melt from its source creates____
- felsic magma.
- mafic residue.
What is assimilation?
- Magma melts the wall rock it passes through.
- Blocks of wall rock (xenoliths) fall into magma.
- Assimilation of these rocks alters magma composition.
What is magma mixing?
- Different magmas may blend in a magma chamber.
* The result combines the characteristics of the two.
What happens when magma mixing is not complete?
•Often magma mixing is incomplete, resulting in blobs of one rock type (xenolith) suspended within the other.
What is fractional crystallization?
When a magma cools, mafic minerals that have a greater melting point will form first. As a result, the remaining magma becomes more felsic (Si-rich) as it cools –> fractional crystallization
What are the changes that occur with cooling during igneous rock formation and what are the outcomes?
Fractional crystallization—early crystals settle by gravity.
Melt composition changes as a result.
- Fe, Mg, Ca are removed as early mafic minerals settle out.
- Remaining melt becomes enriched in Si, Al, Na, and K.
What is Bowens Reaction Series?
N. L. Bowen—devised experiments cooling melts (1920s).
- Early crystals settled out, removing Fe, Mg, and Ca.
- Remaining melt progressively enriched in Si, Al, and Na.
Which is the correct order of adjectives describing magma composition?
←more silica less silica→
A.felsic / intermediate / ultramafic / mafic.
B.ultramafic / mafic / felsic / intermediate.
C.intermediate / felsic / mafic / ultramafic.
D.felsic / intermediate / mafic / ultramafic.
D
Why are there different magma compositions?
- initial source rock compositions.
- partial melting.
- assimilation.
- magma mixing.
What are the two major categories of the igneous environments? What are the based on? What are their characteristics?
Two major categories—based on cooling locale.
Extrusive settings—cool at or near the surface.
- Cool rapidly.
- Chill too fast to grow big crystals.
Intrusive settings—cool at depth.
- Lose heat slowly.
- Crystals often grow large.
What are the steps of formation of igneous rocks in extrusive settings?
- Lava flows cool as blankets that often stack vertically.
- Lava flows exit volcanic vents and spread outward.
- Low-viscosity lava (basalt) can flow long distances.
- Lava cools as it flows, eventually solidifying.
What kind of eruptions occur in extrusive settings? What are its characteristics?
Explosive ash eruptions
- High-viscosity felsic magma erupts explosively.
- Yield huge volumes of ash that can cover large regions
- Pyroclastic flow—volcanic ash and debris avalanche Races down the volcanic slope as a density current
Often deadly
In intrusive settings magma invades preexisting wall rock by _______
- percolating upward between grains
- forcing open cracks.
What is the wall rock and what are its regions?
magma-intrusive contact reveals high heat.
- Baked zone—rim of heat-altered wall rock
- Chill margin—rim of quenched magma at contact
Geologists categorize intrusions by shape and these categories are _____
- Tabular (sheet)—planar with uniform thickness
- Blister-shaped—a sill that domes upward
- Balloon-shaped—blobs of melted rock
