Chapter 6—Identification of Metamorphic Rocks
Metamorphism—process by which pre-existing rock is subject to changes below the
zone of surface weathering
-keep in mind: minerals stable in environment of formation, unstable when this
environment changes
-conditions of metamorphism not well-defined because different rocks behave
differently under metamorphic conditions (due to differences in physical properties).
-lower temp limit: 150 degrees F.
-Pressure range: 1000-20,000 atm
-foliation—organization of platy and tabular minerals (think mica, chlorite,
feldspar) into planar structure, so rock appears layered. Pressure is at 90 degrees to
foliation surface
-Some rocks more easily metamorphosed than others.
Agents of metamorphism
Temperature
Pressure
Fluids
Types of metamorphism
Contact—localized; usually temp and fluids
Regional—widespread; pressure, temp, fluids
Contact Metamorphism
-Occurs immediately adjacent to magma intruded into existing rock (draw picture,
refer to Fig. 6.2)
-Heat and fluids from magma cause recrystallization of pure rocks (think qtz
sstone), recrystallization and fm of new minerals in impure rocks (more than one
mineral)
-No foliation (because uniform pressure)
-examples: Pure Limestone—calcite grains melt and grow larger or combine w/
other grains to form larger grains. Coarse-grained Marble is the result
Shale—contains clay, quartz, calcite (cement). Recrystallization causes formation
of qtz, feldspar, hornblende to make new rock called Hornfels
Regional Metamorphism
-Metamorphic effects seen over a wide area
-rocks subjected to elevated temps (because they are closer to the Earth’s interior)
-higher pressures, due to weight of rocks above and converging plates of crust
-circulating fluids due to water being forced out of pore spaces in rock and
hydrous minerals
-rocks become foliated, follow progressive metamorphism
Progressive metamorphism:
ShaleSlatePhylliteSchistGneiss
Example: Begin w/ Shale. Composition: Kaolinite clay, some feldspar and
quartz silt. Raise pressure and temp: clay minerals recrystallize into larger mica
grains and orient themselves in a stable position (mild foliation). The shale is
now a slate.
Raise pressure and temp some more: oriented mica grains grow larger and
become visible to the unaided eye. Noticeable foliation. The slate has now
become a phyllite.
More pressure and temp: all minerals recrystallize and grow larger, including
mica, qtz, feldspar, etc. Strong foliation. The phyllite is now a schist.
Even more pressure and temp: quartz and other non-platy minerals separate from
the mica and produce definite bands of differing minerals. The schist is now a
gneiss.