Click
on the link below to hear a mini-lecture audio segment.
Introduction
Metamorphic rocks are the result of the application of heat, pressure and directed stress, or some combination of these effects applied to pre-existing rock of any type. The process by which metamorphic rocks are produced is called metamorphism.
There are three types of metamorphism, which are characterized by the nature of the conditions that cause them. They are; Regional, Contact, and Dynamic metamorphism
Regional metamorphism occurs when heat, pressure and directed stress are applied to pre-existing rock.
- Regional metamorphism involves the recrystallization and reorientation of the minerals in the existing rock by a process called solid state recrystallization. In solid state recrystallization the minerals in the rock are recrystallized and reoriented without being melted.
- The result is a rock that has pronounced foliation and layering. The foliation is usually perpendicular to the direction of the applied stress. The amount of recrystallization in a regionally metamorphosed rock is a function of the magnitudes of the heat and pressure at which the process occurs.
- When metamorphism occurs at high temperature and pressure levels it is said to be high grade metamorphism, and it results in highly foliated and reorganized rocks with compositional layering separating layers of different minerals.
- It is difficult to visualize how recrystallization without significant melting can result in a metamorphic rock with such pronounced foliation or "fabric". this process is shown in the movie link below.
- If metamorphism occurs under lower temperature and pressure levels, it is said to be low grade metamorphism, and results in a rock which is foliated but not segregated or layered in terms of its mineral distribution.
- It is difficult to visualize how recrystallization, even at low temperature and pressure, can result in a metamorphic rock with such pronounced foliation or "fabric". this process is shown in the movie link below.
- Some metamorphic rocks are formed from rocks which contain "equant" minerals (same dimensions in all directions). The resulting metamorphic rock does not display foliation, as shown in the movie link below.
- Contact metamorphism occurs when country rock is exposed to high heat. In most cases proximity to a magma produces contact metamorphism. All contact metamorphosed rocks are called hornfels. Rocks produced by contact metamorphism do not have a foliated texture (unless the original rock was foliated) and appear sugary. Contact metamorphism accounts for less than 2% of the metamorphic rocks on the earth.
-Contact metamorphic rocks display a non-foliated, baked sort of a fabric, as shown in the movie link below.
- Dynamic metamorphism occurs in fault zones when country rock is ground up and partially recrystallized. Rocks formed by dynamic metamorphism display a sugary texture, moderate foliated texture, and small round rock fragments within the foliation. Dynamic metamorphism accounts for less than 2% of the metamorphic rocks on the earth.
- Dynamic metamorphic rocks develop their foliation by the grinding and fusing of minerals in the fault zone and display a foliated but sugary appearance, as shown in the movie link below.
Tectonic Environs
All types of metamorphism are indirectly or directly related to plate tectonics.
Metamorphic rocks clearly related to tectonic environment. In the exercise below you will be given an opportunity to test your understanding of that relationship.
Let's take a virtual reality field trip to a tectonic environment in which metamorphism has and is occurring. We will be going to the Golden Gate National Park near San Francisco where two plates are sliding past each other and in the past were in subduction. Can you spot the location of the boundary between one plate and another? What evidence can you see of deformation and metamorphism? To move around in the virtual reality panorama, simply place your cursor in the scene, hold your mouse down, and move your mouse. To zoom in or out, use the + and - buttons.
Metamorphic Rock Classification
Metamorphic rocks are classified in terms of their mineral content and texture. The textural classification has to do with the presence or absence of foliation and the intensity of the foliation.
Put
on your 3D glasses and click on the link below.
