Metamorphic rocks
Pubdate:5/30/2015 SaturdayCategory:Science Popularization

       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.



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