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.

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.

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.

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.

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.

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.

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.

Foliated Metamorphic Rocks

Gneiss Phyllite

Schist Slate

Non-Foliated Metamorphic Rock

MarbleQuartzite

Hornfels