Igneous Petrology

Question 1

What is magma and what determines its properties?

Answer 1

A multiphase mixture of melt and volatiles with a liquid-like texture due to structure of network formers. The amount of network formers determine solidity and malleability

Question 2

What are the most common magmas from high to low viscosity?

Answer 2

Rhyolite, dacite, andesite, basalt, komatiite

Question 3

What is the isobaric phase rule?

Answer 3

Phases present + degrees of freedom = components present + 1

Question 4

What is the lever rule?

Answer 4

The proportion of A in a substance = A/A+B
A is measured from solid to bulk for liquid, as the solid line is 0% liquid

Question 5

How does cooling affect mineral size & texture?

Answer 5

Faster cooling creates more, smaller crystals
Slower cooling creates coarse grains
Rapid cooling creates fine grains

Question 6

What is a congruent system?

Answer 6

A system involving the melting of a solid into a liquid with the exact same composition

Question 7

What is an incongruent system?

Answer 7

A system involving the multi-stage transition of solid to liquid, with changes in composition along the way so that compounds don’t reach the liquidus directly

Question 8

What happens at the peritectic point?

Answer 8

A solid reacts with a liquid to produce a different solid phase and a new liquid

Question 9

How do crystals separate in a closed incongruent system?

Answer 9

Gravity settles crystals at the bottom of the melt due to density, or buoyancy lifts melt above the minerals

Question 10

What is the difference between magma mixing and mingling?

Answer 10

In magma mixing, 2 liquids coexist and exchange mass without becoming homogenised
In magma mingling, 2 liquids are present without interacting

Question 11

What are cumulates and on which part of the liquid line of descent do they form?

Answer 11

Cumulates are crystals formed from excess minerals in a parent melt. They form to the right of the LLD

Question 13

What does the partition coefficient represent?

Answer 13

The ratio of concentration in coexisting phases for any trace element and 2 phases

Question 14

How is the partition coefficient interpreted?

Answer 14

When Kd > 1, the crystal phase is preferred
When Kd < 1, the melt phase is preferred
When multiple Kd > 1, the highest value is the most compatible

Question 15

What causes the flux of basalt dominated magma from the mantle?

Answer 15

Either the total melting of basaltic rock (eclogites) or consistent partial melting of another type (peridotites)

Question 16

What are lherzolites and hazburgites?

Answer 16

Lherzolites contain high CO2 levels and partially melt to form Mid Ocean Ridge Basalts, while Hazburgites contain low CO2

Question 17

In what 3 conditions do volcanoes form?

Answer 17

When the geothermal moves to higher temperatures, mass is removed from the mantle, or the composition of the mantle changes the solidus

Question 18

Where and how does decompression melting occur?

Answer 18

At divergent plate boundaries as the lithosphere fills the place left by the crust

Question 19

What are Benioff-Wadati zones?

Answer 19

Planar zones of seismic activity corresponding with the down-going slab of a subduction zone

Question 20

What are granitoid intrusions?

Answer 20

Intrusions with the general composition and texture of granite, forming in abundant terrains, that are built up in pulses between 100 to 100,000 years apart

Question 21

What is vapour present melting? How is it self-limiting?

Answer 21

Melting that occurs in rocks in equilibrium with a discrete fluid phase; when pressure is high, solubility increases to dehydrate surrounding rocks and lower their chance of melting

Question 22

What is vapour absent melting? How does it make melt easier to abstract?

Answer 22

Melting that occurs in anhydrous rocks and hydrous rocks with hydrous components that have been released as melt; the produced melt volume is larger than the solid assemblage, forcing open potential pathways and making melt easier to extract

Question 23

What are the 2 principal magma series of ocean magmatism?

Answer 23

Tholeiitic series: dominant in crust, with low alkali, silica saturated content and a similar composition to MORBs. Formed by submarine volcanism from low-alkaline mafic magma
Alkaline series: subordinate in crust, with a higher alkali content. Undersaturated in silica and formed at areas of continental rifting and subduction from alkaline mafic magma

Question 24

What are the 2 main controls on ocean magmatism?

Answer 24

Pressure: increasing pressure drives the eutectic upwards, towards undersaturation
Volatiles: higher CO2 content drives eutectic up; high H2O content drives eutectic down

Question 25

Why is ocean magmatism melt quantity lower than those of MORBs?

Answer 25

Oceanic crust limits upwelling, cutting off the melting gradient

Question 26

What are the 3 main ocean magmatism sources?

Answer 26

The depleted mantle, bulk silicate earth, and primitive helium

Question 27

When does a melt behave compatibly?

Answer 27

When the concentration rises significantly in the first 20% of fractional crystallisation