Igneous Petrology Exam Review
What are 2 types of physical processes of crystal-liquid separation? Describe Each. Which process occurs at the fastest rate?
crystal settling, compaction
What are 2 types of rock that might remain after melting the mantle?
Harzburgite and dunite
What is anorthosite?
A mafic rock consisting of almost all plagioclase
What is peridotite?
A mantle rock with pyroxene and a high percentage of olivine
What is the asthenosphere?
A plastic layer of the mantle
What is trachyte?
A quartz poor , K-feldspar rich volcanic rock
What is rhyolite?
A quartz rich volcanic rock
What is a sedimentary rock?
A rock type that makes up 66% of exposed rock on land
What is phonolite?
A rock with 15wt% total alkalies and 60wt% SiO2
What is dacite?
A rock with 3wt% total alkalies and 60wt% SiO2
What is hornblendite?
An ultramafic rock consisting of almost all hornblende
What is pyroxenite?
An ultramafic rock consisting of almost all pyroxene
What type of meteorite represents the undifferentiated Earth?
Carbonaceous Chrondites (contains pyroxene and olivine globules)
Explain how we can locate an ancient volcanic vent by using the size of lithic clasts
Bigger clasts are near the vent because they can't travel very far, smaller clasts are farther away
What is a lithic?
Debris that has been caught in a volcanic rock
Where does a magma begin to fragment and produce pumice and ash?
During explosion, glass transition into pyroclastic flow/hot ash
What mechanical process of crystal-liquid separation would you expect to be occurring in the mantle?
Gravity settling, compaction, and flow segregation
Name 5 types of textures that are most closely associated with a high degree of undercooling in felsic magmas?
Glassy, flow banding, graphic, granophyritic, dendritic
What key observation led B. Marsh to suggest that the rheological locking point for mist magma was when they reached ~50% crystals?
He didnt find volcanic rock that had >50% crystals. When magma reaches 50% crystals, it reaches the rheological locking point at which it is uneruptable
High pressure mantle melting favors the production of quartz-saturated or under-saturated melts?
High pressure favors under-saturated melts because high pressure means greater depth where quartz does not exist as a crystal
Name 3 ways by which the mantle melts and give an example of the tectonic settings
Increase temperature, lower pressure, add volatiles (H2O). Subduction zones(add volatiles), mid ocean ridges(lower pressure), hot spots (temp increase)
What is a Moho?
The separation between crust and mantle
Name 3 ways we can generate compositional diversity in magmas
Melt of the crust combining with magma, liquid crystal separation (crystal fractionization), and magma mixing
What is a metamorphic rock?
Mantle rock type
Describe 3 ways by which we have determined the composition of the mantle
Meteorites, volcanic xenoliths, komatiites, opheolites (oceanic thrust)
What are the requirements for melting in the mantle system as seen in the diagram below?
Must cross (be on the right side) the purple lines to dehydrate the mineral and create melt
Will mantle melting occur in a phlogopite-bearing peridotite along the ocean geotherm at ~1000C and 130km depth? Explain.
No because it is not past (on the right side) the Phlogopite line
Explain why heating of amphibole or phlogopite bearing peridotite is not an effective way of producing mantle melts. See diagram.
Not enough water, they will never be heated enough to overcome the pressure due to depth in order to melt
What ultramafic rock best represents the upper mantle in composition/mineralogy?
Peridotite
By what proposed mechanism do magmas ascend through the brittle crust? Ductile crust?
Plume vs. stoping
If magma undergoes a slow rate of cooling followed by a very fast rate, what rock texture is likely to develop?
Porphyritic
What key features in an igneous rock will allow one to differentiate between pyroclastic debris and lava flow
Pyroclastic debris would accumulate lithics and be very large and blocky as it has a high viscosity. Lava flows have low viscosity so they are more consistent and have smaller crystals
What is a Transform Fault?
Shear boundaries
Explain how the geothermal gradient changes from oceanic to continental lithosphere
Steep geothermal gradient when the crust is thin (oceanic) shallow geothermal gradient for a thicker crust because conduction is harder through a thick crust
Describe 3 different volcano morphologies and the composition of magma associated with each
Strato = rhyolitic, alternating layers of lava and tephra Cinder come = basalt, conical piles of tephra Shield = basalt, runny lava flows
What is differentiation?
The process of enriching elements in Si, K, and Al
What is the lithosphere?
The rigid layer of the mantle
Describe how the dihedral angle influences melt migration
This influences the ability to form an interconnected film. The lower the angle, the higher the permeability
Which 2 magma types are subalkaline?
Tholeiitic and calc-alkaline
What tectonic setting are tholeiitic, calc-alkaline, and alkaline basalts typically associated with?
Tholeiitic is associated with any environment, calc-alkaline is associated with subduction zones, alkaline is associated with plume magma, convergent margins, oceanic, and continental environments
What types of igneous rocks formed the early continental crust?
Tronjamite, Tonalite, Granodiorite, (NOT GREENSTONE)
What factors play the most important role in determining the explosive potential in magma?
Viscosity(Building pressure), water(bubble coalescence)
Explain what a vitrophere and vapor phase alteration is
Vitrophere = a glassy rock with phenocrysts on the bottom VPA = When hot gas alters the vitrophere by baking it on top. Causes welding of the rock, gas streamed out and froze in place
Name 3 factors that affect the movement of magma
Volatiles(H2O), Temp, Silica content
