Metamorphic rocks are the masters of change! Because of intense heat and
pressure deep within the earth, these rocks have undergone a total
transformation from their original form. Unlike igneous rocks, metamorphic
rocks never truly melt. Instead all of the change happens in the solid state.
In some cases the pressure comes from the original rocks getting buried. More
often than not, the pressure comes from powerful forces caused by movements
within the earth crust. Under these conditions, solid rock can actually flow,
just like toothpaste. This results in rocks that have been twisted and bent,
often with a new set of minerals in place of the old ones.
Most metamorphic rocks once formed the roots of ancient mountains.
Because of erosion, the mountains have long since disappeared, exposing the
rocks. Some typical metamorphic rocks include gneiss, schist, and slate.
Another important metamorphic rock is marble, which is used for buildings,
monuments, and sculpture.
There are two basic types of
metamorphic rocks: 1) foliated metamorphic rocks such as gneiss, phyllite,
schist and slate which have a layered or banded appearance that is produced by
exposure to heat and directed pressure; and, 2) non-foliated metamorphic rocks
such as marble and quartzite which do not have a layered or banded appearance.
Schist is metamorphic rock with well
developed foliation. It often contains significant amounts of mica which allow
the rock to split into thin pieces. It is a rock of intermediate metamorphic
grade between phyllite and gneiss. The specimen shown above is a "chlorite
schist" because it contains a significant amount of the mineral chlorite.
Gneiss is foliated metamorphic rock
that has a banded appearance and is made up of granular mineral grains. It
typically contains abundant quartz or feldspar minerals that results from high
grade metamorphism.
Slate is a foliated metamorphic rock
that is formed through the metamorphism of shale. It is a low grade metamorphic
rock that splits into thin pieces.
Marble is a non-foliated metamorphic
rock that is produced from the metamorphism of limestone. It is composed
primarily of calcium carbonate.
