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Roger Weller, geology instructor

by Geri Norris
Physical Geology
Spring 2010


     Minerals and rocks are stable only under the conditions at which they form. 

    Change the conditions and the rocks will change to adapt to the new conditions.

     This is the underlying principle to understanding all things geological.

     When a 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 rock.

     Plate 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.

     Here is 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.







Pictures courtesy Roger Weller

     As time 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.



(picture courtesy Roger Weller)


chlorite-zoom-75.jpg Chlorite picture by SereneShadow


                                                                        Chlorite (picture courtesy Serene Shadow)



     If the 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.

     When 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.

     The 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 metamorphism.

     The 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.

     The two types of pressure involved in metamorphism are confining pressure and directed pressure.

     Confining pressure, also known as hydrostatic, is equal in all directions and is generated from the weight of the overlying rock.

     Directed 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.

     Pressure 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 orientation.

Types of Metamorphism

     Contact 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.


     Dynamic 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




Schreiner, D., Humphries, W., Rocks and Resources; NWT Educational Publication 2007