Metamorphic Rocks

Metamorphic rocks are actually sedimentary or igneous rocks that have undergone extreme chemical and physical changes. In the majority of the cases, these changes are so drastic that the original identity of the rocks is obscured.

When subjected to high pressures or temperatures, the original constituents of the rocks can be transformed into new minerals that are pretty stabled chemically at these higher pressures and temperatures. These new particles of minerals adjust themselves into flattened and parallel patterns to be able to conform to the pattern of pressure which is affecting the rock. It is this banded or laminated texture the gives most metamorphic rocks their characteristics. While at first impression this banding might suggest a sedimentary origin, if we examine them closer we will usually find that it reveals a distinct interlocking texture, this is not like the cemented texture of a sedimentary rock.

Metamorphic grade is a common and useful concept. This shows the degree to which the metamorphic pressure and heat have changed a rock. Low-grade metamorphism typically changes the mineral composition only slightly. More and more of the original minerals react chemically with each other at higher grades, recrystallizing to form new minerals, releasing carbon dioxide, water and other volatile constituents. The volatiles are driven off completely at the highest metamorphic grades and the rock may start melting.

Metramorphic rocks are frequently named for their parent rock types. An example of this is: a metamorphosed volcanic rock can be called a metavolcanic or a metamorphosed sedimentary rock can be called a metasediment.

Gneiss (which is pronounced “nice”) is the name of a group of metamorphic rocks. When it is used alone, the term most of the time means a rock displaying the distinctive laminated feature of high-grade metamorphic rocks. These rocks are typically composed of mica, feldspar and quartz. If we squeeze a grantie we will cause the light and the dark crystals to be dragged into somewhat parallel bands. We then call it granite gneiss.

The schist is the most common medium-grade metamorphic rock type. This rock has a structure which is laminated in which the recrystallized minerals have been oriented in bands that are paralleled. These bands make the rock tend to break along its layers, different from the gneiss which is more completely recrystallized. The schist is obviously named for the most common minerals it contains, like chlorite schist, mica schist and talc schist.

Another common sedimentary rock is shale, when it is subjected to high pressures it changes into a rock named slate. This rock is most of the time used as a material for roofing and also for the bases of pool tables.

An extremely hard rock which is formed by the metamorphism of pure quartz sandstone is the quartzite.

Another metamorphic rock which is formed from limestone which has been recrystalized is marble. It is extensively used as a stone for building.