Petro Exam 1

Question 1

Describe three ways by which we have determined the composition of the mantle

Answer 1

Volcanic Xenoliths, Meteorites, Komatiites, Ophiolites

Question 2

What type of meteorite represents the undifferentiated Earth?

Answer 2

Carbonaceous chondrites

Question 3

Explain how the geothermal gradient changes from oceanic to continental lithosphere.

Answer 3

Heat transfers more efficiently through oceanic crust (steep for thin)

Question 4

What types of igneous rocks formed the early continental crust?

Answer 4

tonalite-trondhjemite-granodiorite (TTG)

Question 5

Name three ways by which the mantle melts and give an example of the tectonic settings

Answer 5

Increase temperature, lower pressure, adding volatiles (H20)

Question 6

What tectonic setting(s) are tholeiitic basalts typically associated with?

Answer 6

Tholeiitic - convergent and divergent boundaries or within oceanic or continental plates (everywhere)

Question 7

What tectonic setting(s) are calc-alkaline basalts associated with?

Answer 7

ONLY at convergent plate boundaries (subduction zones)

Question 8

What tectonic setting(s) are alkaline basalts typically associated with?

Answer 8

convergent plate boundaries and within oceanic and continental plate boundaries NOT at divergent plate boundaries

Question 9

High pressure mantle melting favors the production of quartz-saturated or under-saturated melts?

Answer 9

High pressure = under saturated quartz magmas

Question 10

What ultramafic rock best represents the upper mantle in composition/mineralogy?

Answer 10

Peridotite

Question 11

Explain why heating of amphibole- or phlogopite-bearing peridotite is not an effective way of
producing mantle melts

Answer 11

there usually isn’t enough water and there is not a lot of amphibole or phlogopite in the mantle.

Question 12

What are the requirements for melting in the mantle system as seen in the diagram below?

Answer 12

free H2O unbounded in minerals and proper temp/pressure conditions (fraction melt is limited by water)

Question 13

Will mantle melting occur in a phlogopite-bearing peridotite along the ocean geotherm at
~1100°C and 80 km depth? Explain.

Answer 13

No, it is not past the phlogopite liquidus

Question 14

Will mantle melting occur in an amphibole-bearing peridotite along the shield geotherm at
~1000°C and 130 km depth? Explain

Answer 14

No, because it is not past the H2O solidus, so there won’t be enough water.

Question 15

What mechanical process of crystal-liquid separation would you expect to be occurring in the
mantle?

Answer 15

compaction → deforming the crystal, squeezing it out

Question 16

Which two magma types are subalkaline?

Answer 16

Tholeitic and calc-alkaline.

Question 17

What are two types of rock that might remain after melting the mantle?

Answer 17

Harzburgite and dunite

Question 18

Describe how the dihedral angle influences melt migration

Answer 18

Low angles allow melt to be able to squeeze through grains, higher permeability (dihedral angles)

Question 19

Name three factors that affect the movement of magma.

Answer 19

Viscosity, composition of melt (specifically amount of silica (it is sticky!!)), water content, temperature.

Question 20

What key observation led B. Marsh to suggest that the rheological locking point for most magma was when they reached ~50vol.% crystals?

Answer 20

Once magma reaches 50% crystals, it is essentially a solid (really dense, sticky!!). It can’t erupt anymore

Question 21

By what proposed mechanism do magmas ascend through the brittle crust? The ductile crust?

Answer 21

Brittle - stoping, crust is breaking off into the magma chamber. The magma forces its way up, kind of like steps
Ductile - think mantle plume

Question 22

Name three ways we can generate compositional diversity in magmas.

Answer 22

Crystal fractionation (M&M magma chamber), partial melting, magma mixing

Question 23

What are two types of physical processes of crystal-liquid separation? Describe each. Which
process occurs at the fastest rate?

Answer 23

Crystal settling= density and gravity settle the more dense crystals to the bottom. The fastest settling
Compaction= assume crystals form a deformable, viscous network. The melt and crystals are compacted to form a solid.

Question 24

Name three types of textures that are most closely associated with a high degree of undercooling in felsic magmas?

Answer 24

Glassy, aphanitic, fine-grained, chaotic, elongated (acicular), porphyritic

Question 25

If a magma undergoes a slow rate of cooling followed by a very fast rate, what rock texture is likely to develop?

Answer 25

Porphyritic, big crystals form, then very fine-grained or aphanitic groundmass forms around the big crystals

Question 26

Describe three different volcano morphologies and the composition of magma associated with each.

Answer 26

Shield volcanoes - typically basaltic magma, runny & broad, slight dome shape

Cinder Cone - intermediate or felsic (andesite-dacite), conical piles of tephra, smallest type

Stratovolcanoes - usually intermediate or felsic magmas (rhyolite, dacite, granodiorite), built on lava flow and eruptions, large cone shape

Question 27

What factor(s) play(s) the most important role in determining the explosive potential in a magma?

Answer 27

silica content and water

Question 28

Where does a magma begin to fragment and produce pumice and ash?

Answer 28

In the conduit

Question 29

Explain how can we locate an ancient volcanic vent by using the size of lithic clasts?

Answer 29

Larger lithics land closer to the vent

Question 30

What is a lithic?

Answer 30

Rock and glass pieces, similar to breccia (debris), carried up in ash groundmass (i.e. tuff)

Question 31

What determines the height of a volcanic plume?

Answer 31

The mass flux or the amount of material and magma being erupted per secound

Question 32

Explain what a vitrophyre and vapour phase alteration are?

Answer 32

Vitrophyre: a porphyritic rock with crystals in a glassy groundmass (origin: volcanic eruption), composition dependent on crystals, not the same as spherulitic obsidian.
Vapour Phase Alteration: high-temperature quartz and alkali feldspar fill void
spaces in pumice and matrix, which acts like a cement. Occurs under extremely hot conditions.

Question 33

What key feature(s) in an igneous rock will allow one to differentiate between pyroclastic debris and lava flow.

Answer 33

Lava flows tend to be more glassy or vitrophyre, whereas pyroclastic debris is more of a conglomerate of pieces from the volcano.