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
What are Xenoliths?
chunks of wall rock incorporated into the magma.
What is fractional crystallization?
crystals that form early settle out of the melt by gravity. The composition of the remaining melt changes as a result.
Who discovered that fractional crystallization leads to changes in magma composition?
N.L. Bowen
What minerals crystallize first? at what temperature?
Mafic minerals crystallize first at high temperatures (Olivine)
What are the last minerals to crystallize? at what temperature?
Quartz, muscovite, and K-feldspar. At Low temperatures
What are the 4 types of magmas? what are they based on?
- Felsic 2. Intermediate 3. Mafic 4. Ultramafic
Based on % silica content
Felsic magma has (density, temperature, SiO2 content)
LOW density, LOW temperature, HIGH SiO2 content
Mafic magma has (density, temperature, SiO2 content)
HIGH density, HIGH temperature, LOW SiO2 content
Magma composition is determined by a number of factors. The most important is…
The composition of the initial rock
A mantle source will produce what kind of magma?
Mafic and ultramafic
A crustal source will produce what kind of magma?
Mafic, intermediate, and felsic magmas
What kind of minerals melt first?
Silica rich minerals melt first
Complete melting of xenoliths into the magma is called?
Assimilation
Igneous rocks are described by
Color and texture
Texture includes these three things
Interlocking (crystalline), Fragmental, and Glassy
What texture is used to describe coarse and fine?
A porphyritic texture
What kind of crystals are fine-grained and grow fast?
Aphanitic
Phaneritic crystals…
develop by cooling slow and are coarse-grained
Phaneritic crystals are usually found in….
Aphanitic crystals are usually found in…
Intrusive settings
Extrusive settings
Name 4 ways to describe rock composition
- Basaltic
- Diotoric
- Rhyolitic
- Peridotitic
Si, Na, K and Al are all associated with
Felsic composition
What is pressure relief melting?
What causes hot rock to melt when it convects upwards
