Geology Reference
In-Depth Information
In this section I describe the various stages of metamorphic change, characteristic tex-
tures of metamorphic rocks, and how they are classified.
And you thought you were stressed! Heat, pres-
sure, and metamorphism
Metamorphism changes an original or
parent
rock into a new type of rock.
This change happens in one of three ways:
Contact with heat:
Contact metamorphism
occurs when magma moves up through
crustal rock and brings with it high levels of heat. The surrounding rock (the
aure-
ole
) is heated enough to cause changes in the mineral structures. This process is
illustrated in Figure 7-13. Contact metamorphism may also be called
thermal meta-
morphism.
Sometimes water is heated by magma in this way and then enters a
rock, causing changes to its mineral structure; this process is called
hydrothermal
metamorphism.
Figure 7-13:
Contact meta-
morphism.
Burial under rocks and sediment:
Burial metamorphism
affects rocks buried at
great depth (usually more than 6 miles). The rocks are exposed to heat and pres-
sure that cause changes to the minerals in the parent rock.
Direct pressure and heat from plate collisions:
When two crustal plates collide,
the result is mountain building or subduction (which I describe in Chapter 10).
