METAMORPHIC ROCK
HOW METAMORPHIC ROCKS FORM
The term metamorphic means to change and that is exactly what metamorphic rocks have done. Similar to a
caterpillar changing into a butterfly or a tadpole turning into a frog, rocks can change into a new type of rock due to
intense heat and pressure. When a sedimentary or igneous rock is subjected to intense heat or pressure, the mineral
grains in the rock are flattened and realigned. The original rock is not melted, merely heated and exposed to
pressure causing the change in minerals. This causes a new rock to form, a metamorphic rock.
REGIONAL vs. CONTACT METAMORPHISM
All metamorphic are formed by heat and pressure but the source of the heat and pressure can come from two
different situations. The first source is regional metamorphism. Most of the metamorphic rocks on Earth are
formed this way. It is called regional metamorphism due to the large areas of rock that are changed into
metamorphic rock. The intense heat and pressure for regional metamorphism is caused by plate tectonics. The
movement of the plates sliding past each other causes immense heat and pressure. This results in large area of rock
being changed into metamorphic rock.
The second source of metamorphism is a result of magma. When pre-existing rock come into contact with magma,
II heat from the magma causes the mineral in the rock to realign converting it into a metamorphic rock.
thePart
extreme
This process is referred to as contact metamorphism.
FOLIATED vs. UNFOLIATED
When the minerals within certain rocks are flattened and realigned, they form stripes, layers or bands on the rock.
Metamorphic rocks with these visible bands are called foliated metamorphic rocks. Gniess and Slate are two
examples of foliated metamorphic rocks. Metamorphic rocks that lack these bands or stripes are known as
unfoliated or non-foliated metamorphic rocks. Quartzite and Marble are two unfoliated metamorphic rocks
EXAMPLES OF METAMORPHISM
When the igneous rock granite is exposed to intense heat and pressure, it changes to a metamorphic rock called
gneiss (pronounced nice). Limestone, which is a sedimentary rock, is converted to marble when it undergoes
metamorphism.