2. Heat and Pressure Change 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 parent rock.  The resulting (new) rock is called a metamorphic rock.  It is also possible for the parent rock to be a metamorphic rock.  Rocks do not melt when they under go metamorphism.

3. Heat and Pressure Change Rocks  The process by which bonds between atoms in minerals break and re-form in new ways during metamorphism is called recrystallization.  During this time, atoms can combine in different ways, and new minerals can form in place of older ones.  For example, during recrystallization shale that forms from silt and clay can form garnet.

4. Metamorphic Changes Occur Over Large Areas  When both high temperatures and high pressure are present metamorphic changes can occur over large areas.  When only one of these conditions is present, changes tend to occur over smaller areas.  An example of changes over a large area is a region where large blocks of rock are pressing together and pushing up mountain ranges.

5. Metamorphic Changes Occur Over Large Areas  This process can affect an area hundreds of kilometers wide and tens of kilometers deep.  Typically, the deeper below the surface the rocks are the greater the metamorphic changes that occur in them.  For example, a sedimentary rock may change to slate near the surface but become gneiss deep inside a mountain.

6. Metamorphic Changes Can Occur Over Small Areas  Magma can push into rocks underground, or surface rocks can be covered by lava flow.  The magma or lava heats the rocks causing metamorphic change.  The rocks are heated but not squeezed.  This heat can cause change in rocks from less than one meter to several hundred meters.

7. Most Metamorphic Rocks Develop Bands of Minerals  A common property of metamorphic rock is called foliation.  Foliation is the arrangement of minerals in flat or wavy parallel bands.  Foliation develops when rocks are under pressure.  Foliation occurs when minerals flatten out or line up in bands.

8. Nonfoliated Rocks  Metamorphic rocks that do not show foliation are called nonfoliated.  Metamorphic rocks may not show foliation because it is made up of only one type of mineral. Therefore, different minerals cannot separate and line up in layers.  Marble is one common nonfoliated metamorphic rock.  The second reason a metamorphic rock may be nonfoliated is because it has not been subjected to high pressure.

9. Foliated Metamorphic Rocks GneissPhyllite SchistSlate

10. Non-Foliated Metamorphic Rock Marble Quartzite Hornfels