Metamorphic rocks originate from ingenious, sedimentary, or even other metamorphic rocks. The rocks are formed when there is a change in texture and/or mineral composition in the surrounding area where the rocks exist. This change or “metamorphosis” happens in a solid state, reacting to high pressure and temperature, or hot circulating fluids. These variables add new minerals and textured to the existing ingenious, sedimentary, or metamorphic rock. The variables are added by ordinary methods of metamorphism like regional and contact methods (
In order to understand the contact methods it is important to first understand the two different categories of metamorphic rocks, foliated and non-foliated. The first category, foliated, includes gneiss, phyllite, schist and slate. Foliated rocks have a layered or banded look, which is created by exposure to heat or direct pressure. The second category non-foliated includes quartzite and marble. Non-foliated rocks differ from foliated rocks in they do not have a layered or banded appearance (
Consequently, regional metamorphosis happens in large areas and created gneiss and schist. Both gneiss and schist are foliated, however gneiss is coarse-grained with dark mineral bands of feldspar and quartz. Where schist is predominately composed of mica minerals and quartz. This is all dependant on the composition of the original rock, the temperature, and pressure when the metamorphic rock is formed. A key example of this can be seen when limestone changes into marble. Pure limestone changes into marble, but when limestone has the variable mineral aluminum present, the aluminum can change the marble to a marble with mineral grains. These grains are called poephyroblasts, of corundum (aluminum oxide) or spinel (magnesium aluminum oxide). Another example is when schist holds a variable and changes into garnet (
Apart from regional methods, metamorphic rocks can also occur with contact methods. The contact method happens when igneous rock intrudes into rock and baking the surrounding rock with high temperatures. If hot fluids are present it may introduce new minerals into the igneous rock. This process is known as metasomatism and possible new minerals from this could include garnet, diopside, epidotre, and many other new rock formations (
In conclusion, metamorphic rocks form through the process of change when variables are introduced into the area where metamorphic rocks exist. The process can only happen when the right variables exist, thus creating a process that can only happen within this genius.
Aber, S.W. (1999). An introduction to gem origin and occurance. Retrieved May 20, 2009 from
Bryant, A. (2005). Metamorphic rocks. Retrieved May 20, 2009 from