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GEOLOGY 12
CHAPTER 3 IGNEOUS ROCKS
QUESTIONS FOR REVIEW
1.
ANSWERS
What is an igneous rock?
• An igneous rock is one that has formed from a magma, a hot silicate melt.
2.
3.
Explain briefly the effect of the following on the melting temperature of rock:
a. changes in pressure;
b. presence of water vapor.
• a.
Increased pressure tends to raise melting temperatures,
and vice versa.
• b.
The presence of water vapor or other volatiles tends to
lower the temperatures at which rocks melt.
Cite three possible heat sources that might contribute to melting.
Three possible heat sources are:
• increase of temperature with depth;
4.
5.
•
heat released during radioactive decay;
•
the heat of magma brought from deep in the mantle up into the crust.
What is Bowen's Reaction Series? How do the continuous and discontinuous
branches differ?
•
The Reaction Series describes the sequence in which minerals crystallize from a
cooling magma.
•
On the continuous side (PLAGIOCLASE FELDSPARS), the crystals react continuously
with the melt during cooling, their compositions gradually changing without a change
in mineralogy.
•
On the discontinuous (FERROMAGNESIAN) side, the early-formed minerals may at
some point become unstable with respect to the melt, so that they react with it to be
transformed into a different mineral--for example, the olivine may all be converted to
pyroxene.
Describe two ways in which the composition of a magma can be modified.
(More than two possible answers exist. Possibilities include:)
•
ASSIMILATION: the magma incorporates bits of wallrock that are melted and mixed
into the magma.
•
FRACTIONAL CRYSTALLIZATION: early-formed crystals float or settle out of the melt
causing the composition of the magma to change.
•
MAGMA MIXING: two different magmas combine.
6.
7.
8.
Assimilation of felsic rocks by mafic magmas is more common than the reverse.
Why?
•
Mafic magmas are generally hotter; felsic minerals melt at lower temperatures.
•
It is much easier for a hot mafic magma to melt a felsic rock than for a granitic
(felsic) magma to melt the high-temperature ferromagnesian (mafic) minerals.
How is the grain size of an igneous rock related to its cooling rate? What does a
porphyritic texture indicate?
•
In general, the slower the cooling (the Longer it takes to cool), the more time
available for Larger crystals to grow, and the coarser the grain size.
•
A porphyritic rock suggests an early period of slow cooling followed by rapid cooling
and crystallization of the fine matrix.
Plutonic rocks may be more readily identifiable in handsample than volcanic
rocks. Why?
•
9.
Plutonic rocks, because they are more slowly cooled, they are generally coarsergrained. The individual minerals are therefore easier to see and identify.
Most volcanic glasses are rhyolitic in composition. Compositional layering is
more often observed in mafic than in felsic plutons. What property of a magma
may have a bearing on both these observations? Explain.
•
The property is viscosity. Mafic magmas are generally less viscous than felsic
magmas therefore crystals grow more slowly in viscous (thick) melts.
•
Rhyolitic (felsic) glasses cool rapidly and do not have sufficient time to crystallize
before solidification.
•
Layered plutons form by crystal settling or flotation; crystals can move more easily
in the less-viscous mafic melts while they tend to stay suspended in more viscous
silicic melts.
10. What is a discordant pluton? Give two examples.
•
A discordant pluton is one that cuts across any structure in the country rocks.
•
Examples include dikes, pipes and stocks.
Geology 12
Chapter 3
Questions For Review
Page 2
NOTES
11. What is a batholith? Suggest two ways in which the necessary granitic magma
might be formed and discuss any problem with each idea.
NOTE: The “problem” for geologists is that there are huge granite batholiths in the
Earth’s crust. How did they get there? Where did the source of magma come from and
what was the heat source for melting such large amounts of rock?
•
A batholith is a very large intrusion and usually granitic to granodioritic in
composition. (Granitic (felsic) magma is cooler than mafic magma.) There are at
least four possible magma sources.
1.
Granitic magma can form by FRACTIONAL CRYSTALLIZATION of a basaltic (mafic) magma
from the mantle, but one would have to start with an immense volume of basaltic melt
to end up with the large volume of granite that a batholith represents.
2.
Very limited PARTIAL MELTING of mantle material could produce granitic melt, but again,
a huge volume of mantle would have to be involved to produce a large batholith of
granite.
3.
A MAFIC MELT from the mantle could ASSIMILATE (granitic) continental crust to produce a
granodioritic composition, but a great deal of heat would be required to melt that crust.
4.
The batholith could have formed from MELTED CONTINENTAL CRUST; however, a source
of heat sufficient to cause that much melting of the crust is not usually available.
Geology 12
Chapter 3
Questions For Review
Page 3