Teacher Notes Metamorphic Rocks
Metamorphic Rocks
The process in which an existing rock is changed by heat or pressure, or both,
is called metamorphism.
The original sedimentary or igneous rock is called the parents rock. The
resulting rock is a metamorphic rock.
Even a metamorphic rock can be a parent rock for another metamorphic rock.
Heat and Pressure Change Rocks
During metamorphism, rocks undergo many changes. One type of change
occurs when pressure causes a rock’s minerals to flatten out in one direction.
Rocks do not melt when they undergo metamorphism.
“What would the resulting rock be if temperatures got high enough to melt the
rock?”
Recrystallization
Heat and pressure can break the bonds that join atoms in minerals. The atoms
then can join together differently as new bonds.
Recrystallization has two main results. First, individual mineral crystals can
grow larger as more atoms join their crystal structures.
Second, atoms can combine in different ways, and new minerals can form in
place of older ones. For example, shale is a sedimentary rock that is formed
from silt and clay. During recrystallization, garnet can form from these
materials.
Two Types of Metamorphic Rocks
Foliated Rocks are rocks with layered bands of minerals. This is not to be
confused with sedimentary rocks. Minerals flatten out or line up in bands
when rocks are under pressure.
Examples: Slate, phyllite, schist.
Nonfoliated Rocks are rocks without layered bands. One reason that a
metamorphic rock may not display foliation is that it is made up of entirely one
mineral.
Examples; marble, quartzite.