Metamorphic Rocks
Metamorphic Rocks
• Metamorphism occurs when any
previously existing rock, the parent rock,
is buried in the earth under layers of other
rock. The deeper the rock is buried the
hotter it gets, and the higher the pressure
becomes. Eventually, rock must adjust to
the new conditions, whether it is baked, or
squeezed, or both, and in the process
becomes a metamorphic rock.
Temperature
• The temperature is going up because the
center of the earth is molten, and the
closer we get to that heat the hotter it gets.
This rise in temperature with depth is
called the geothermal gradient.
Pressure
• The pressure is going up because, . . .
well, imagine having a pile of rocks ten
miles thick sitting on your head. This is
hydrostatic pressure. It is equal in all
directions. But, there is also directed
pressure, squeezing pressure that
happens when two continents collide at
subduction zones of plate boundaries.
Metamorphism - The mineralogical and
textural changes that occur in a rock in
response to changes in temperature and
pressure, short of melting
Two Types of Metamorphic Rocks
• There are two basic types of metamorphic
rocks
– Foliated Rocks can be observed in the field by
parallel bands within the rocks or it can often
be observed on a smaller scale in the hand
specimen.
– Non Foliated Rocks as the name implies,
does not have any parallel orientation of the
grains within the metamorphic rock. Non
foliated rocks have recrystallized without
producing parallel structures.
Contact Metamorphism
• When magma intrudes into host rock, localized
contact metamorphism occurs along the
contact between the pre-existing rock mass and
the cooling magma.
• Contact metamorphism occurs at shallower
levels of the crust, where the pressure is
relatively low.
• Contact metamorphism commonly produces
fine-grained rocks that lack foliation.
Regional Metamorphism
• Regional metamorphism occurs over broad
areas of the crust.
• This occurs in deep basins where sediments or
sedimentary rocks have accumulated. At a depth
of about 10 kilometers, the confining pressure of
the overlying material combined with geothermal
heat is great enough to metamorphose rocks.
• Because the compression does not impose a
directed pressure, metamorphic rocks formed
from burial are non foliated and lack any
banding of minerals.
Formation of Foliated Rocks
• Along the boundary of tectonic plates where
collision or subduction is occurring, directed
pressure is exerted on rock.
• Under great pressure, mineral grains may be
smeared or partially melted and
recrystallized into bands aligned perpendicular
to the direction of greatest pressure.
• This creates foliated metamorphic rocks with
minerals in distinct bands.
Amphibolite is a non-foliated metamorphic rock
that forms through recrystallization under
conditions of high viscosity and directed pressure.
It is composed primarily of amphibole and
plagioclase, usually with very little quartz.
Gneiss is foliated metamorphic rock that has
a banded appearance and is made up of
granular mineral grains. It typically contains
abundant quartz or feldspar minerals.
Hornfels is a fine-grained nonfoliated
metamorphic rock with no specific composition. It
is produced by contact metamorphism. Hornfels is
a rock that was "baked" while near a heat source
such as a magma chamber, sill or dike.
Marble is a non-foliated metamorphic rock
that is produced from the metamorphism of
limestone. It is composed primarily of
calcium carbonate.
Quartzite is a non-foliated metamorphic
rock that is produced by the metamorphism
of sandstone. It is composed primarily of
quartz.
Schist is metamorphic rock with well developed foliation.
It often contains significant amounts of mica which allow
the rock to split into thin pieces. The specimen shown
above is a "muscovite schist" because it contains a
significant amount of muscovite mica.