Metamorphic Rocks

Metamorphic rocks are really igneous or sedimentary rocks that have undergone extremely physical or chemical changes. In most cases, these changes are so severe that the rocks’ original identity is obscured.

If subjected to high temperatures or pressures, the original constituents of a rock can be transformed into new minerals that are chemically stable at these higher temperatures and pressures. These new mineral particles readjust themselves into parallel and flattened patterns to conform to the pattern of pressure affecting the rock. It is this laminated or banded texture that characterizes most metamorphic rocks. While at first glance this banding might suggest a sedimentary origin, closer examination will usually reveal a distinct interlocking texture, unlike the cemented texture of a sedimentary rock.

A common and useful concept is metamorphic grade. This expresses the degree to which metamorphic heat and pressure have changed a rock. Low-grade metamorphism typically changes the mineral composition slightly. At higher grades, more and more of the original minerals react chemically with each other, re-crystallizing to form new minerals, releasing water, carbon dioxide and other volatile constituents. At the highest metamorphic grades, the volatiles are driven off completely and the rock may begin to melt.

Frequently, metamorphic rocks are names for their parent rock types. For examples, a metamorphosed volcanic rock may be called a metavolcanic or a metamorphosed sedimentary rock a metasediment.

Gneiss (pronounced “nice”) is the name of a common group of metamorphic rocks. When used alone, the term generally signifies a rock displaying the distinctive laminated features of high-grade metamorphic rocks. Such rocks are typically composed of quartz, feldspar and mica. Squeezing a granite will cause the light and dark crystals to be dragged into more or less parallel bands. It is the called a granite gneiss.

The common medium-grade metamorphic rock type is schist. The rock has a laminated structure in which the re-crystallized minerals have been oriented in parallel bands. This makes the rock tend to break along its layers, unlike gneiss, which is more completely re-crystallized. A schist is usually named for the most common minerals it contains, like talc schist, mica schist and chlorite schist. Shale, a common sedimentary rock, when subject to high pressures, changes into a rock called slate. This rock is often used as a roofing material as well as a base for pool tables.

Quartzite is an extremely hard rock formed by the metamorphism of pure rock quartz sandstone.

Marble is another metamorphic rock, formed from limestone which has been re-crystallized. It is used extensively as a building stone.