Expert: Dr Thomas Bell - 12/17/2007

Question
I am taking geology in school and I am having a little trouble with something? The question is how does temperature, differential stress, and water contribute to the metamorphic process?

Answer
Kristy,

When minerals crystallize from a melt (magma) at high temperature and pressure they are in thermodynamic equilibrium.  When a plutonic or volcanic rock is exposed at the surface, none of these minerals are in equilibrium anymore because the pressure and temperature have changed drastically.  

Surface exposure leads to chemical weathering where the minerals undergo partial or complete disintegration.  As the remaining mineral grains are loosened, they may be eroded, transported to a sedimentary basin, and buried.  The deeper they get buried the hotter they get.  

Several important transformations begin as soon as the temperature starts to rise above 100 deg C.  All of these minerals that are thrown together by the sedimentary process are very unlikely to be in thermodynamic equilibrium.  As the temperature increases, the rate of reaction with each other and the water that is trapped in the pore space increases as well.  One of the first things to happen is new crystal growth in the pore space, cementing the grains together to form sedimentary rock.  

As the temperature and pressure increase with deeper burial, the reactions speed up and new minerals are formed while the old ones dissolve.  Most of the chemical components for the new mineral phases growth are carried by water that is trapped along grain boundaries. Water is the all important solvent that facilitates many metamorphic reactions though some can take place in the solid state particularly at higher temperatures.  Other metamorphic fluids such as carbon dioxide and sulfur dioxide can play a role in the redistribution of chemical components for metamorphic reactions.

The distinction between sedimentary and metamorphic rocks is somewhat arbitrary and is best looked at as a continuum of increased reaction rate and degree of mineral dissolution and growth of new minerals.

Differential stress plays a role in the orientation of metamorphic minerals and which minerals will form.  The crystal structure of some minerals is favored over others with the same or very similar compositions where differential stress is high.  This is quite evident in the metamorphic fabric that develops in many areas of moderate to high metamorphic grade.  Elongate and platy minerals are preferentially oriented according to the stress field that existed when they were forming.