Petro Exam 1 Flashcards
Describe three ways by which we have determined the composition of the mantle
Volcanic Xenoliths, Meteorites, Komatiites, Ophiolites
What type of meteorite represents the undifferentiated Earth?
Carbonaceous chondrites
Explain how the geothermal gradient changes from oceanic to continental lithosphere.
Heat transfers more efficiently through oceanic crust (steep for thin)
What types of igneous rocks formed the early continental crust?
tonalite-trondhjemite-granodiorite (TTG)
Name three ways by which the mantle melts and give an example of the tectonic settings
Increase temperature, lower pressure, adding volatiles (H20)
What tectonic setting(s) are tholeiitic basalts typically associated with?
Tholeiitic - convergent and divergent boundaries or within oceanic or continental plates (everywhere)
What tectonic setting(s) are calc-alkaline basalts associated with?
ONLY at convergent plate boundaries (subduction zones)
What tectonic setting(s) are alkaline basalts typically associated with?
convergent plate boundaries and within oceanic and continental plate boundaries NOT at divergent plate boundaries
High pressure mantle melting favors the production of quartz-saturated or under-saturated melts?
High pressure = under saturated quartz magmas
What ultramafic rock best represents the upper mantle in composition/mineralogy?
Peridotite
Explain why heating of amphibole- or phlogopite-bearing peridotite is not an effective way of
producing mantle melts
there usually isn’t enough water and there is not a lot of amphibole or phlogopite in the mantle.
What are the requirements for melting in the mantle system as seen in the diagram below?
free H2O unbounded in minerals and proper temp/pressure conditions (fraction melt is limited by water)
Will mantle melting occur in a phlogopite-bearing peridotite along the ocean geotherm at
~1100°C and 80 km depth? Explain.
No, it is not past the phlogopite liquidus
Will mantle melting occur in an amphibole-bearing peridotite along the shield geotherm at
~1000°C and 130 km depth? Explain
No, because it is not past the H2O solidus, so there won’t be enough water.
What mechanical process of crystal-liquid separation would you expect to be occurring in the
mantle?
compaction → deforming the crystal, squeezing it out
Which two magma types are subalkaline?
Tholeitic and calc-alkaline.
What are two types of rock that might remain after melting the mantle?
Harzburgite and dunite
Describe how the dihedral angle influences melt migration
Low angles allow melt to be able to squeeze through grains, higher permeability (dihedral angles)
Name three factors that affect the movement of magma.
Viscosity, composition of melt (specifically amount of silica (it is sticky!!)), water content, temperature.
What key observation led B. Marsh to suggest that the rheological locking point for most magma was when they reached ~50vol.% crystals?
Once magma reaches 50% crystals, it is essentially a solid (really dense, sticky!!). It can’t erupt anymore
By what proposed mechanism do magmas ascend through the brittle crust? The ductile crust?
Brittle - stoping, crust is breaking off into the magma chamber. The magma forces its way up, kind of like steps
Ductile - think mantle plume
Name three ways we can generate compositional diversity in magmas.
Crystal fractionation (M&M magma chamber), partial melting, magma mixing
What are two types of physical processes of crystal-liquid separation? Describe each. Which
process occurs at the fastest rate?
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.
Name three types of textures that are most closely associated with a high degree of undercooling in felsic magmas?
Glassy, aphanitic, fine-grained, chaotic, elongated (acicular), porphyritic
If a magma undergoes a slow rate of cooling followed by a very fast rate, what rock texture is likely to develop?
Porphyritic, big crystals form, then very fine-grained or aphanitic groundmass forms around the big crystals
Describe three different volcano morphologies and the composition of magma associated with each.
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
What factor(s) play(s) the most important role in determining the explosive potential in a magma?
silica content and water
Where does a magma begin to fragment and produce pumice and ash?
In the conduit
Explain how can we locate an ancient volcanic vent by using the size of lithic clasts?
Larger lithics land closer to the vent
What is a lithic?
Rock and glass pieces, similar to breccia (debris), carried up in ash groundmass (i.e. tuff)
What determines the height of a volcanic plume?
The mass flux or the amount of material and magma being erupted per secound
Explain what a vitrophyre and vapour phase alteration are?
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.
What key feature(s) in an igneous rock will allow one to differentiate between pyroclastic debris and lava flow.
Lava flows tend to be more glassy or vitrophyre, whereas pyroclastic debris is more of a conglomerate of pieces from the volcano.
