W4.1&2 Igneous Rocks
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What are 3 types of primordial heat? Describe each one (It’s a “GREAT accent, important difference”)
• Gravitational compression: as earth grew, increasing pressure converted gravitational potential energy into internal heat • Accretion / Impacts: collisions during planet growth converted kinetic energy into heat, driving early melting • Differentiation (core formation): as earth separated into layers, dense liquid metal sank toward the centre to form the core - this gravitational energy dissipated as heat through deformation and viscous/frictional processes
How does the earth generate internal heat?
Through radioactive decay of elements in the crust and mantle
What is the difference between magma and lava? What is a lava flow?
Magma: melt beneath Earth’s surface Lava: when magma reaches the surface Lava flow is lava moving downslope under gravity
What is the difference between intrusive and extrusive rocks?
Extrusive igneous rocks – lava cools quickly at/above Earth’s surface; includes lava flow and pyroclastic material Intrusive igneous rocks – lava cools slowly underground
What rocks are more common, intrusive or extrusive?
Intrusive – most magma solidifies underground
What 3 changes in condition can trigger melting of Earths interior?
• decreased pressure • addition of volatiles (H2O and CO2 – lowers melting temp) • heat transfer (heating surrounding rock)
Is high temp on its own sufficient to produce melting?
It is necessary but not sufficient on its own
Decompression melting: What happens at “solidus”, “liquidus”, and in-between solidus and liquidus?
• solidus – melting begins (first liquid appears) • liquidus – melting is complete (last solid disappears) • between solidus and liquidus – partial melting (mix of crystals and melt)
How does decompression melting happen?
Hot asthenosphere rises, making pressure drop faster than temp
Where does decompression melting occur?
• hot spots • continental rifts • mid-ocean ridges
When does flux melting happen, and what are volatiles?
• when volatiles are added to hot mantle rock • components that can form a fluid/gas at earth’s surface, and can travel through rock at depth
How hot is mafic magma compared to the melting point of felsic continental crust?
mafic magma ~ >1100-1300 Celsius felsic continental crust melts at 650-850 celius
what percent of wet magma is made of volatiles, and mainly which volatile?
• ~15% volatiles • H2O
What 4 reasons cause melts to vary in composition? (DAMS)
• degree of partial melting (small vs large melt fraction produces different melt chemistry) • Assimilation (magma melts/incorporates surrounding wall rock as it rises) • Magma mixing (2 magmas combine, producing intermediate composition) • source rock composition (what is melting? mantle vs crust; basalt vs granite)
What type of melting is the norm on earth, and why?
• partial melting • only 2-30% of a rock melts before the melt migrates away, so the whole rock rarely melts
What are the 2 processes through which assimilation happens?
• dissolving chemicals from the wall rock • engulfing xenoliths that break off, sink, and partially dissolve
What affects viscosity?
• Temp Hotter - lower viscosity Cooler - higher viscosity • Volatile content more volatiles - lower viscosity less volatiles - higher viscosity • Silica content mafic - lower viscosity felsic - higher viscosity
list 3 reasons cooling occurs
• rising magma enters cooler crust • trapped magma in a chamber loses heat to wall rock • lava at surface cools very quickly in air or water
Cooling rate depends on heat transfer; what 4 factors matter?
• depth – deep intrusions cool slowly; shallow intrusions cool quickly • size - small magma bodies cool faster than large ones (surface area/volume increases as size decreases) • shape - thin tabular “pancake” intrusions cool faster than spherical bodies of the same volume • groundwater: circulating cold water in rock wall can carry heat away, speeding cooling
What can result from fractional crystallization? Why?
A magma can become more felsic over time, because as magma cools, minerals crystallize in sequence and may separate from the melt; the first minerals to crystallize in a mafic magma are mafic minerals
What are two main categories of igneous environments?
• extrusive - magma erupts and cools quickly (crystals stay tiny) • intrusive - magma cools slowly underground in chambers/cracks (crystals can grow large)
What are the 3 main igneous textures (size, shape, arrangement)?
• crystalline (interlocking crystals) • fragmental (volcanic fragments) • glassy (quenched melt, few/no crystals)
What does texture reflect?
cooling history
Difference between phaneritic and aphanitic textures?
phaneritic - coarse grained aphanitic - fine grained