Geology Test: Igneous Rocks
Very early in Earth's history, it was so hot that the surface was entirely molten
True
A volcano emits ash, which falls from the sky, settles in layers, and is eventually cemented. The resultant rock is termed
Tuff
Which type of magma has the greatest iron and magnetic content?
Ultramafic
Important volatiles in magmas include
Water and carbon dioxide
A dike is
a tabular intrusion that cuts across preexisting layers
A sill is
a tabular intrusion that lies parallel to surrounding layers of sedimentary rock
If a body of magma becomes more felsic, its viscosity will
increase
If a body of magma cools, its viscosity will
increase
Typically, as magma ascends through the crust, its viscosity is expected to
increase
Coarse-grained diorite is most similar in mineral composition to fine-grained
Andesite
Most commonly, felsic igneous rocks
Are lighter in color than mafic rocks
Radioactive isotopes, differentiation of the Earth's iron core, gravity driven compression, and meteoric bombardment all caused early Earth to
Be much hotter than at present
All other factors being equal, intrusive rocks that form deep within Earth ______________ than intrusive rocks that cool near the surface
Cool more slowly
The formation of magma within Earth is not caused by which of the following Processes
Decompression, addition of volatiles, and transfer of heat from adjacent magma or very hot rocks (all of the above)
If the volatile content of magma is increased, its viscosity will
Decrease
All minerals within igneous rocks have a melting temperature of 750C
False
When rock is melted within Earth, typically the entire volume of rock is transformed from solid state to a liquid.
False
Which type of magma has the greatest silica content?
Felsic
Which type of magma is produced at the coolest temperatures?
Felsic
The grain size of an igneous rock is determined primarily by
How rapidly it cooled during crystallization
When magma crystallizes, _____________ are formed.
Intrusive igneous rock
Obsidian
Is volcanic glass, possesses conchoidal fracture, and typically is felsic in composition (all of the above)
If a body of igneous (source) rock is subjected to partial melting, the magma that is produced is expected to be
More felsic than the source rock
If a body of magma is subjected to fractional crystallization, the rock that results is expected to be
More mafic than the magma
Bombs, ash, and cinders are all examples of
Pyroclastic Debris
Igneous rocks
are formed through the freezing of melt, can be produced at the surface of the Earth as well as deep below the surface, and are the most common type of rocks within Earth (all of the above)
Close-grained gabbro is most similar in mineral composition to fine-grained
basalt
As compared to aphanitic igneous rocks, phaneritic rocks are
coarser grained
Volatiles refer to substances that
have a tendency to evaporate and are stable as gases
Geologically, melts are equivalent to both
lavas and magmas
The difference between lava and magma is that
magma is found beneath the Earth's surface, whereas lava has reached the surface
When very hot rock rises rapidly to the surface, it
may melt due to a decrease in pressure
The distinction between a pluton and a batholith is
numerous plutons make up batholith
A blob-like igneous rock body that has cooled beneath the surface of the Earth is
pluton
An igneous rock with a mixed texture of coarse grains (phenocrysts) surrounded by fine crystals (groundmass) is termed
porphyritic
Pegmatites, which occur in dikes, are unusual among shallow intrusive rocks in that they
possess exceptionally coarse grains
Coarse-grained granite is most similar in mineral composition to fine-grained
rhyolite
Recently formed magmas at depth have a tendency to
rise upward toward the surface
Pieces of country rock absorbed by an intrusive body and that do not melt form
xenoliths
