Igneous Rocks Flashcards
Igneous activity occurs in four settings:
- isolated mantle plume hot spots
- volcanic arcs bordering deep ocean trenches
- mid-ocean ridges
- continental rifts.
Where does most igneous activity on planet Earth take place?
Mid-ocean ridges
What is Decompression melting?
Rocks that would otherwise melt are kept solid by high pressure. When the pressure decreases, melting ensues.
These hot spots are a mix of mafic and felsic (basalt and rhyolite)
Continental hot spots
Oceanic hot spots are:
mafic (basaltic)
The base of the crust is hot enough
to melt rock. Why doesn’t the rock melt?
Because the pressure is too high
When does pressure drop?
When the rock is carried to shallower depths
When does Flux Melting occur?
Flux melting occurs when volatiles are introduced into the hot mantle. Volatiles lower the melting T of a hot rock.
What is heat-transfer melting?
When rising magma carries mantle heat with it. This raises the T in nearby crustal rock, which melts.
What are the three components of magma?
Solids
liquids (composed of Si and O)
gases
How is magma movement governed?
By viscosity, which is affected by temperature, volatile content, and % silica content.
Magma with less SiO2 is…
mafic (low viscosity)
Mama with more SiO2 is…
Felsic (high viscosity)
There are two types of igneous rock environments
- Intrusive
2. Extrusive
In extrusive settings…
- Rock cools fast
- at/near the surface
- crystals do not have time to grow large
In intrusive settings…
- Rock cools slow
- at depth
- crystallization occurs over a long period of time
These rocks are usually more resistant to erosion, and often stand high on the landscape.
Intrusive rocks
What are Plutons? What do they form?
Large blobs of magma that cool deep underground. They form batholiths.
What are batholiths?
made of immense volumes of intrusive rock. Batholiths often remain as evidence of fossil subduction zones.
What are sills?
a tabular sheet of igneous rock intruded between and parallel with the existing strata.
What are continental rifts?
places where continental lithosphere is being faulted, stretched, and thinned.
Why are mafic magmas more likely to be extrusive?
- higher temperature
- lower viscosity
- lower SiO2 content
What is a common feature of extrusive settings?
Lava flows
What are LIPs?
Large Igneous Provinces - unusually large outpourings
of magma that have appeared over time
