Geology Home Page
Roger Weller, geology instructor
by Geri Norris
Minerals and rocks are stable only under the conditions at which they
Change the conditions and the rocks will change to adapt to the new conditions.
This is the underlying principle to understanding all things geological.
rock becomes buried in the Earth under layers of other rock, metamorphism
occurs. The deeper the rock goes into the Earth the hotter it gets and the
pressure goes higher. Sooner or later the rock will have to conform to the new
conditions. In the process of being squeezed and baked it becomes a metamorphic
tectonics which are driven by Earth’s internal heat along with surface processes
such as weathering, erosion, and deposition accounts for the recycling of
Earth’s materials in the rock cycle.
a good example of the principle that minerals and rocks are stable only under
the condition at which they form. Take the igneous rock granite, expose it at
the Earth’s surface where it becomes unstable and weathers to form, among other
things, clay, a mineral that is stable at Earth’s surface. Clay is deposited to
form the sedimentary rock, shale.
courtesy Roger Weller
goes by and more sediment comes down from the mountains, the shale goes deeper
the Earth where the temperature and pressure rise to the point where the clay
that composes the shale is no longer shale. The clay must adjust and it does so
by recrystalization, into a new mineral, chlorite. Chlorite is a metamorphic
mineral and composes the metamorphic rock, slate.
courtesy Roger Weller)
Chlorite (picture courtesy Serene Shadow)
temperature keeps rising, the slate will transform into phyllite, then a schist,
then to a gneiss and finally melt back to its molten state which cools to form
igneous rocks. This is the rock cycle that says all rocks can be transformed
into other rocks as conditions change.
metamorphism runs its’ entire course, a sedimentary rock is heated into a
metamorphic rock then eventually melted into an igneous rock.
Heat and Pressure- The Mechanics of Metamorphism
In the process of metamorphism there are two sources of heat. The geothermal
gradient calculates at approximately 1 degree C for every 30 meters of depth
with some variance.
second source of heat is from intrusive bodies of magma such as batholiths. As
they cool, they release heat to the surrounding country rocks. This leads to
melting temperature for a rock ranges from a low 700-800 degrees C to as high as
1000 degrees C. The composition of rocks, available fluids, and pressure
control the temperature at which rocks melt. Fluids are important because they
allow the chemicals to move more quickly and easily. The increased mobility
makes for easier melting.
types of pressure involved in metamorphism are confining pressure and directed
Confining pressure, also known as hydrostatic, is equal in all directions and is
generated from the weight of the overlying rock.
pressure, static, is not equal in all directions. It is associated with
mountain building processes when rock is squeezed, crumpled, and stretched as
one continent slides on the edge of another.
not only influences the degree and rate of metamorphism it also causes textural
changes in the rock. For instance, how large the crystals are and their
Types of Metamorphism
metamorphism occurs when heat and chemical fluids from an igneous body alter
adjacent rocks. The rocks in contact with batholiths may be heated to 1000
degrees C and the fluids from the magma also bring changes.
Most metamorphic rocks result from regional metamorphism which takes place over large, elongated areas as the result of tremendous pressure, elevated temperature, and fluid activity. This kind is most obvious along convergent plate boundaries where the rocks are intensely deformed during convergence and subduction.
metamorphism is more restricted in its’ extent being confined to zones adjacent
to faults where high levels of differential pressure develop.
Much of Earth’s ancient crust has been metamorphosed, recycled, deformed, or otherwise disrupted. Except for meteorites, the oldest rocks on Earth are located in the Northwest Territory of Canada. This rock outcrop is called the Acasta Gneiss and is 4.03 to 4.055 BYO (billion years old.) Because the rocks are metamorphic their parent rocks must have been even older.
Acasta Gneiss (picture courtesy alpine.com/images/tn/Big-Red-Hill-big.jpg)
Schreiner, D., Humphries, W., Rocks and Resources; NWT Educational Publication 2007