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Geology 101
Lecture Outline - Metamorphic Rocks
1.
Definition of Metamorphic rocks
a) formed by alteration of preexisting rocks
b) alteration produced by heat, pressure and/or chemically
active fluids
c) alteration must take place while rock remains solid (NO
MELTING)
d) using happens from a few kilometers deep to the
crust/mantle boundary
e) changes are in texture and/or mineral composition
2.
Metamorphic Grade - measure of severity of conditions
a) low grade - small amount of change; characteristics of
original rock may still be present (e.g. bedding planes,
fossils, etc.)
b) high grade - rock completely altered; original rock
(protolith) can only be assumed
3.
Zones of Metamorphism - can occur wherever increased
heat and/or pressure and/or chemically active fluids are
present
a) Regional Metamorphic Zone
i) Convergent plate boundary
ii) more of everything is here
b) Contact Metamorphism
i) occurs adjacent to magma chamber
ii) implies and increase in heat only
c) Fault Zone Metamorphism (Dynamic)
i) implies additional pressure
d) Hydrothermal Metamorphism
i) involves hot water - particularly effective near
undersea volcanic centers
e) Shock Metamorphism
i) the result of an instantaneous pressure wave
a) from extraterrestrial impacts or possibly
volcanic eruption
Agents of Metamorphism
a) Heat - changing temperature causes minerals to alter
into forms stable at the new temperature
i) clays to micas at fairly shallow depths
ii) different minerals (rock types) begin to
metamorphose at very different temps. (e.g. shale
changes at low grade conditions, while granite
won’t change until high grade conditions
b) Pressure - two types, each having a different effect on
the alteration
i) Confining pressure (lithostatic)- caused by weight
of overlying materials
a) acts equally in all direction; therefore it simply
tends to decrease the volume, resulting in
dense minerals (garnet)
ii) Directional stress - when pressure is greater in one
direction than in others
a) causes minerals to pack more tightly in one
direction
4.
Lecture 12 & 13
b) this alignment, called foliation, of grains can
be used to tell which direction the pressure was
coming from - perpendicular to alignment
c) Chemically-active fluids - mostly just hot water, or
water vapor
i) What they do…
a) act as pathways to allow rock to alter most
quickly - like the volatiles in a cooling magma
b) “foreign” elements may be present in the fluid
that can be added to the protolith elements to
make “new” minerals
ii) Where they come from…
a) released as the result of mineralogical changes;
e.g. clays are water rich, mica are not
b) escape from cooling magmas - may carry low
melting point elements (gold, etc.)
5.
Mineral changes during metamorphism: Index Minerals
a) Index Mineral - one that is stable over a limited range of
temperatures and pressure and can therefore be used to
tell what the ultimate conditions were during
metamorphism
b) Example sequence; low grade to high grade conditions
i) chlorite, epidote, muscovite, biotite, hornblende,
stauroloite, garnet, pyroxene
ii) NOTE - quartz and feldspars are not on this list,
they remain stable over a wide range of temps and
pressures
6.
Summary of possible metamorphic alterations
a) increase density - packing minerals tighter together, or
packing elements tighter into a new mineral
b) growth of larger crystals - more efficient packing
c) reorientation of mineral grains to form alignments from directed stress
d) conversion of low temp to high temp minerals
e) creation of new minerals caused by injection of fluids
7.
Classification of Metamorphic rocks
a) Foliated - has some type of alignment of grain
i) rock cleavage - aligned micas; rock is Slate
ii) schistosity - aligned platy grains that are large
enough to see; rock is Schist
iii) gneissosity - segregation of minerals(often
granular) into bands; rock is Gneiss
b) Nonfoliated - no alignments
i) Why no alignment?
a) Created in an environment with no directed
stress, or
b) Minerals are equidimensionally shaped - they
cannot show alignment (quartz and calcite are
this way)
(1) Pure quartz makes Quartzite
(2) Pure calcite (limestone) makes Marble
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