EOS 316 Midterm 1

Question 1

Where are igneous rocks sourced from?

Answer 1

Mantle-derived melts

Question 2

How does the mantle melt? (3 ways)

Answer 2

(1) Increasing temperature
(2) Lower the pressure (adiabatic rise)
(3) Add volatiles (H2O)

Question 3

What are the characteristics of the ocean crust (thickness, composition)?

Answer 3

10km thick, uniform stratigraphy, mafic, ophiolite suite top down (sediments, pillow basalt, sheeted dikes, massive gabbro)

Question 4

What are the characteristics of the CC (thickness, composition)

Answer 4

~35km thick, highly variable composition (average: granodiorite), buoyant - unsubductable

Question 5

Describe the composition and rheology of the mantle

Answer 5

ultramafic, mainly solid, viscous

Question 6

Describe the composition and rheology of the earth’s core

Answer 6

Fe-Ni metallic alloy, outer core liquid, inner core solid

Question 7

describe the pressure gradient as we move deeper inside the earth

Answer 7

Pressure increases linearly through the mantle (~1kbar every 3 km), increases more rapidly inside the core since the alloy is more dense

Question 8

How does temperature change as we move deeper inside the earth?

Answer 8

-Thermal energy dominantly comes from the decay of radioactive elements in the mantle

-The lithosphere geotherm is approximately ~25ºC/km

-Mantle: small dT/dz - stays hot!
Lithosphere: Large dT/dz - temp drops fast!

Question 9

how does the mantle melt to produce magmas at (a) divergent boundaries, (b) convergent boundaries, (c) hotspots?

Answer 9

a) decompression melting
b) flux melting
c) decompression melting

Question 10

How do we classify igneous rocks? (3)

Answer 10

(1) Texturally: grain, crystallinity
(2) Modally: naming an igneous sock based on the percentage of each mineral present
(3) Chemically

Question 11

What’s the difference between holocrystalline, hypo crystalline, and holohyaline?

Answer 11

holocrystalline: all crystal
hypocrystalline: part crystal, part glass
holohyaline: all glass

Question 12

What is a porphyritic / phyric rock texture?

Answer 12

visibly large crystals in a finer-grained groundmass.

Question 13

how-to QAP

Answer 13

(1) Analyze all minerals within the rock
(2) Normalize out any non-QAP (mafic / ultramafic)
(3) use new QAP minerals for analysis and name using chart

Question 14

how-to non-QAP

Answer 14

(1) Analyze all minerals within the rock
(2) Normalize out any QAP (felsics)
(3) use new QAP minerals for analysis and naming scheme

Question 15

When is normative mineralogy useful?

Answer 15

a) fine-grained, extrusive rocks: ~hypothetical mineral assemblage
b) helps distinguish between magma types

Question 16

What are the types of silica saturation from normative minerals - Si:(Fe + Mg) ? (ratio and name)

Answer 16

(1) Quartz 1:n/a - ‘Si-oversaturated’
(2) Opx (hypersthene): 1:1 ‘Si-saturated’
(3) Olivine: 1:2 ‘Si-undersaturated’

Question 17

What are Si-saturated rocks?

Answer 17

(1) Normative hypersthene, normative olivine
(2) no quartz, no feldspathoids

Question 18

What are the types of silica saturation from normative minerals - Si:(Na + K) ? (ratio and name)

Answer 18

(1) Quartz
(2) Feldspathoids
(2a) Leucite: KAlSi2O6
(2b) Nepheline: (Na,K)AlSiO4

Question 19

What are Si-oversaturated rocks?

Answer 19

(1) Normative quartz, normative hypersthene
(2) No feldspathoids, no olivine

Question 20

What are Si-undersaturated rocks?

Answer 20

(1) Normative feldspathoids, normative olivine
(2) No quartz, no hypersthene

Question 21

How can we subdivide rocks in the alkaline field? (3)

Answer 21

(1) Hi-K series
(2) K-Series
(3) Na-Series

Question 22

How can we subdivide rocks in the sub alkaline field? (2)

Answer 22

Tholeiitic series
Calc-alkaline series

Question 23

At what setting can you find alkaline magmas?

Answer 23

Convergent plate margins, oceanic plates, continental plates

Question 24

At what setting do you find tholeiitic magmas?

Answer 24

Convergent plate margins, divergent plate margins, oceanic plates, continental plates

Question 25

Ate what setting can you find calc-alkaline magmas?

Answer 25

convergent plate margins

Question 26

What components are (/can be) in a magma? (4)

Answer 26

(A) melt (liquid)
(B) Crystals (solid)
(C) Volatiles (fluid = dissolved or exsolved gas)
(D) Possibly xenoliths or xenocrysts

Question 27

Describe the melt of a magma

Answer 27

-linked silica tetrahedra
-can have non-binded oxygen making it impure

Question 28

How do NBO’s affect the melt of a magma?

Answer 28

more NBO’s = depolymerized melt
less NBO’s = more H2O can dissolve

Question 29

What are the most important volatiles in a magma (2)?

Answer 29

(1) H2O
(2) Sulphur

Question 30

What controls magma behaviour? (4)

Answer 30

(1) Viscosity / composition
(2) Temperature
(3) Density
(4) Exsolution of dissolved volatiles

Question 31

What variables affect viscosity? (4)

Answer 31

(1) composition (wt% SiO2)
(2) Temperature
(3) % Crystals
(4) % Dissolved volatiles (eg. H2O)

Question 32

What is the top –> down order for an ophiolite sequence?

Answer 32

Pelagic sediments –> pillow basalts –> sheeted dikes –> gabbros

Question 33

How do hydrothermal fluids interact with ocean magma?( 4)

Answer 33

(1) Hot upwelling fluids reacting with rock
(2) Hydrothermal metamorphism
(3) Some areas concentrate large volumes of upwelling fluids
(4) Metasomatism = rock changes compositionally and mineralogically via fluids

Question 34

What is a phase?

Answer 34

A physically distinct, mechanically separable phase (liquid, solid, gas)