Download Questions for Review KEY

GEOLOGY 12
CHAPTER 7 METAMORPHIC ROCKS
QUESTIONS FOR REVIEW
1.
2.
3.
4.
5.
ANSWERS
What is metamorphism? What limits the maximum temperatures possible in
metamorphism?
•
Metamorphism involves changes in CHEMISTRY, MINERALOGY, or TEXTURE resulting
from the application of heat and pressure to rocks.
•
At very high temperatures, rocks begin to melt and the line between
metamorphism and igneous processes is crossed.
Describe two sources of heat for causing metamorphism.
•
The existence of the GEOTHERMAL GRADIENT means that with deeper burial, rocks
will be subjected to higher temperatures.
•
HOT MAGMA rising into the cooler crust may also heat and metamorphose that
crust.
Explain the distinction between confining pressure and directed stress. Which
leads to such deformation as folding or development of foliation?
•
Confining pressure is uniform in all directions; it is the type of pressure imposed
on rocks at depth.
•
Directed stress is not uniform; it leads to the orientation of minerals and the
development of foliation, and production of folds.
Describe two ways in which the presence of fluids can influence
metamorphism.
•
Fluids migrating through permeable rocks may ADD OR REMOVE IONS.
•
The presence of fluids ALLOWS IONS TO MOVE (diffuse) MORE READILY through dry
rock, so recrystallization and changes in mineralogy may occur.
How do rock cleavage and schistosity develop?
•
Both develop in the presence of DIRECTED STRESS.
•
Rock cleavage develops as clays and other platy minerals reorient themselves
into parallel planes (perpendicular to maximum compressive stress).
•
PROGRESSIVE GROWTH OF MICA FLAKES in such parallel planes leads to SCHISTOSE
TEXTURE.
6.
7.
8.
9.
What is metasomatism? Describe one environment in which it might
commonly occur.
•
METASOMATISM is the alteration of a rock by the fluids percolating through it,
introducing additional ions in the solution. (HYDROTHERMAL METAMORPHISM)
•
In the contact zone (AUREOLE) around a cooling pluton or magma chamber, fluids
escaping from the magma might cause metasomatism.
Define the following: (See glossary.)
•
Phyllite: fine-grained, low-grade foliated metamorphic rock, showing cleavage,
and shiny, wavy surfaces
•
Schist: foliated metamorphic rock of low to moderate grade, rich in relatively
coarse micas.
•
Gneiss: metamorphic rock showing compositional banding or textural banding.
•
Amphibolite: regional-metamorphic rock of moderate grade, very rich in
amphiboles.
Explain the difference between contact and regional metamorphism.
•
Contact metamorphism is a LOCALIZED phenomenon characterized by ELEVATED
TEMPERATURE but relatively LOW PRESSURE. It is the metamorphism commonly
observed around PLUTONS.
•
Regional metamorphism may involve ELEVATED TEMPERATURES AND/OR
PRESSURES; as the name implies, it occurs on a larger scale. It may arise as a
consequence of DEEP BURIAL OF ROCKS, or of MOUNTAIN BUILDING.
Outline the progressive metamorphism of rocks buried deeper and deeper in
the crust. Describe the facies through which they might pass and some of the
changes to be expected.
•
The rocks would ordinarily pass through the ZEOLITE, GREENSCHIST, and
AMPHIBOLITE facies into the GRANULITE facies.
•
One would expect BREAKDOWN of clays to form micas, and later the breakdown of
micas, amphiboles, and other minerals.
•
LOSS of water and other volatiles would occur.
•
The rocks become MORE COMPACT and dense with increasing metamorphic grade.
•
If directed stresses are present, FOLIATION and ultimately GNEISSIC (BANDED)
TEXTURE may also develop.
Geology 12
Chapter 7
Questions For Review
Page 2
10.
11.
What is a migmatite?
•
A migmatite is a "MIXED ROCK" formed at the upper temperature limits of
metamorphism, as the lowest-melting-temperature minerals begin to melt.
•
The rock has both IGNEOUS AND METAMORPHIC CHARACTERISTICS.
Explain what characteristics make a mineral useful as an index mineral.
•
The mineral should be SUFFICIENTLY COMMON that it occurs in many rocks.
•
However, it should also be STABLE OVER A LIMITED RANGE OF TEMPERATURE OR
PRESSURE conditions.
Geology 12
Chapter 7
Questions For Review
Page 3