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 slate.
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
Schist
Phyllite
Slate
Non-Foliated Metamorphic Rock
Quartzite
Marble
Hornfels