Metamorphic rocks are those which change type over time and under certain conditions. They are proof that rock may not be as stable and permanent as it seems.
Rocks seem to us from the limited vantage point of humans like such permanent things. The kind of things stable, and good to build houses upon. The kinds of things which remain solid and true even as the years pass and everything else changes around them. Rocks can be counted on to remain the same.
For indeed, those same rocks which existed back in the times of the dinosaurs clearly still exist today, and from these immovable creations we can learn about the past.
This all needs to be put into perspective, though. The word metamorphic comes from the Greek word metamorjos (metamorphos) which means “to change form.” It is quite an accurate name to give the type of rock so named due to its formation as a rock beginning as one type, and then metamorphosing into another.
So when rocks are thought of as being large, immovable and permanent, such needs to be taken with a grain of salt. Sure this is true within the scopes of human lifetimes, but one consider the life of a rock throughout the entire history of the Earth, it doesn’t take long to notice that some rocks do indeed undergo great change.
The Formation of Metamorphic Rocks
Metamorphic rocks are indeed the shiftiest of all the rock types. After all, it is this very “changeability” for which they are known and named. And it is this fact which makes them continually interesting.
So, how can a rock simply change from one type to another?
It all starts with those two forces which seem to drive almost all of the geological forces on the Earth (and perhaps, if one looks hard enough, everything else as well) – heat and pressure. This being the case, it should come as no surprise that the vast majority of the metamorphic rock on the Earth exists in the deepest parts of the continental crust of the Earth, where the vast amounts of mass pushing down on it creates enough pressure to change the prolith (the original rock, which can be either igneous or sedimentary) both chemically and physically into altogether new rocks.
The great pressure on these rocks allows metamorphic minerals, which are able to form chemically only under great pressure, to take the place of the previous chemicals in the rocks which tend to break apart chemically under such conditions.
Examples of such minerals (also known as index minerals) are sillimanite, staurolite, and muscovite. Also present in metamorphic rocks are certain minerals such as quartz, micas and feldspars which are not actually formed by the metamorphic activities, but remain as “leftovers” from the pre-metamorphic rock.
These metamorphic rocks which form over wide areas below the Earth (in fact, the vast majority of the lower crust is made up of metamorphic formations, apart from more recent igneous activity here and there) are known as regional metamorphic rocks.
Metamorphic Foliation
The other change that occurs in regional metamorphism is known as foliation. This occurs when pressure is applied to the prolith from a single direction.
When this occurs during metamorphism, crystalline minerals such as mica and chlorite may grow in long lines perpendicular to the direction of force. After a metamorphic rock has been pushed through the surface of the Earth due to seismic activity, forming mountains and cliffs, these lines of foliation are often still visible as parallel lines running through an otherwise solid rock.
The only other type of metamorphic rock that has been classified is the contact metamorphic rock. This type differs from regional rock in that it occurs only in small, localized areas. When magma is pushed through the Earth and injected into the rocks surrounding a volcano (known as country rock), the heat and pressure from this can be so great that the direct contact with the magma causes just that single layer of rock to metamorphose, leaving the rest of the country rock untouched.
The interesting thing about this type of metamorphic activity is that it can occur to any type of rock imaginable. Whatever form of stone happens to be located next to an area of volcanic activity can undergo contact metamorphism – and all different types of rocks respond to the ensuing pressure differently (making life even more interesting for geologists).
Metamorphic Study
There is a great deal of scientific research which has gone into the study of metamorphic activity, and much of this activity is of great interest to chemists and physicists alike, who might use the study of these rocks to better understand how certain elements bond with each other to form more complex structures such as minerals, as well as how all of this behaves under great pressure.
Not to mention the part such studies as this could play in being able to better understand seismic and volcanic activity, and perhaps even to finally begin to understand all of the crazy things going on in the center of the Earth. It’s about time, after all.
So there is a great deal to appreciate about these deceptively complicated rocks. Much more so than could ever be possible to cover in such a limited space as this. There is much more to learn, and the web is full of great resources for anyone so interested.
See Also:
References:
“USGS CMG Infobank Geology School: Metamorphic Rock.”
