igneous rocks Flashcards
what are igneous rocks
form from a melt e.g. granite, basalt
in what way is Earth mostly igneous rock
= Magma – Subsurface melt
= Lava – Melt at the surface
2 types go igneous rocks
Intrusive igneous rocks – Cool slowly underground
Extrusive igneous rocks – Cool quickly at the surface
4 reasons earth is hot inside
Planetesimal and meteorite accretion
Gravitational compression
Differentiation
Radioactive decay
what is the Partial melting in crust / upper mantle due to
= Pressure release (A drop in pressure initiates “decompressional melting”)
= Volatile addition
= Heat transfer (Rising magma carries mantle heat)
what happens when volatiles are added
e.g. water, co2
- cause rocks to melt at much lower T
- Important in subduction process
what 3 components is magma made of
- Solid – solidified minerals carried by the liquid
- Liquid – the melt itself comprised of mobile ions - Dominantly Si and O; Ca, Fe, Mg, Al, Na, K
- Gas - volatiles (e.g., water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide gas (rotten egg smell))
4 major magma types based on % silica (SiO2)
= Felsic (or silicic) 66 - 76% silica
= Intermediate 52 - 66% silica
= Mafic (Mg and Fe-rich) 45 - 52% silica
= Ultramafic 38 - 45% silica
difference between silica rich and silica poor magma
Silica-rich magmas: thick and viscous
Silica-poor magmas: thin and “runny”
what does Magma compositions vary chemically due to
= Initial source rock compositions
= Partial melting
= Assimilation
= Fractional crystallization
how does source rock compositions influence magma composition
= Mantle source – Ultramafic and mafic magmas.
= Crustal source – Mafic, intermediate, and felsic magmas
how does Partial melting influence magma composition
- Upon heating, silica-rich minerals melt first
- Partial melting yields a silica-rich magma
- Removing a partial melt from its source creates:
= a magma richer in Si
= a residue poorer in Si
how does Assimilation influence magma composition
Assimilated materials change magma composition
how does Fractional crystallization influence magma composition
- As magma cools, early crystals (e.g., olivine, pyroxene, Ca-felspar) settle by gravity
- Melt composition changes as a result
= Fe, Mg, and Ca are removed in early settled solids
= Si, Al, Na, and K remain in melt and increase - Felsic magma can evolve from mafic magma
how does magma move upward
by…
= Injection into cracks
= Melting overlying rocks
what happens to magma pressure with upward migration
pressure decreases - releases volatiles -> increasing viscosity
what does viscosity depend on
= Temperature:
Hotter - Lower viscosity
Cooler – Higher viscosity
= Volatile content:
More volatiles – Lower viscosity
Less volatiles – Higher viscosity
= Silica (SiO2) content:
Less SiO2 (Mafic) – Lower viscosity
More SiO2 (Felsic) – Higher viscosity
how does igneous environments affect the cooling rates of magma
- Extrusive settings – Cool at or near the surface
Cool too fast to grow big crystals - Intrusive settings – Cool at depth
Lose heat slowly
Crystals grow large
what does Magma invading colder country rock initiate
= Thermal (heat) metamorphism and melting
= Inflation of fractures
= Incorporation of country rock fragments (xenoliths)
= Hydrothermal (hot water) alteration
2 ways Magma intrudes into other rocks as part of Igneous Activity
Magma intrudes into other rocks in 2 ways
= dikes and sills (planar, tabular bodies)
= Plutons
difference between dikes and sills
dikes:
- Crosscut rock fabric
- Dominate in extensional settings (e.g., MORs)
sills:
- Parallels rock fabric
- Lift entire landscapes skyward
- Usually intruded near the surface
what’s a pluton
A large, deep igneous rock body
- Plutonic intrusions modify the crust
- a larger batholith
- Created at subduction zones
- Long subduction history linked to large batholiths
how fast is heat lost based on depth
Deep plutons cool slowly
Shallow flows cool rapidly
how fast is heat lost based on shape
Surface to volume ratio
- Spherical bodies cool slowly
- Tabular bodies cool faster
