Metamorphic Rocks

-Are formed when igneous, metamorphic, or heat and / or pressure change sedimentary rocks.

          Heat and pressure on a rock cause the atoms in the rock to rearrange and the minerals then recrystallize. Changes in the temperature or pressure make some minerals unstable and they change into new minerals.  This process is called metamorphism. 

2 types of metamorphism

Contact

-If hot molten rock comes in contact with an existing rock it will change the rock where it touched it into a new kind of rock.

-Less common

-Less pressure on these rocks so they have lower density than regionally metamorphosed rocks

-No foliation

Regional

-If large areas of the earth’s crust are exposed to heat and pressures caused by plate tectonics, pressure from rock layers above them or heat from internal radiation, they will change into metamorphic rocks.

-         Most metamorphic rocks are formed this way.

-         High density

-         Exhibit banding and foliation

Classification

Scientists classify them by their texture, composition of minerals, banding and foliation.

Characteristics of metamorphic rocks

Metamorphic rocks can be foliated or non-foliated.

Foliated (have appearance of layers)

-Slaty foliation=flat, even layers with small crystals (named for slate)

-Schistose foliation=medium sized crystals arranged in uneven layers (named for schist)

-Gneissic foliation (banding)=alternating bands of light and dark colored minerals that can be in straight lines or distorted

 (Named for gneiss)

Nonfoliated

-May exhibit distortion or stretching of crystals or rock fragments but with no apparent layering. 

-Usually created from contact metamorphism