Rock Types

Rocks are usually classified (divided) into three groups: (a) igneous, (b) sedimentary, and (c) metamorphic.

This classification is according to the origin of the rocks.

It should be understood that all three rock types are still being formed.

Igneous rocks

Igneous rocks are usually divided into two groups-intrusive (plutonic) and extrusive (volcanic) rocks. Granite is the most common of the intrusive rocks and it underlies most of the continents. Basalt is the most common of the extrusive rocks and it underlies most of the oceans. Granite was formed when magma intruded into the earth’s crust. This cooled slowly over a long period of time and formed a coarse grained rock that is very resistant to erosion. You need to know about the colour, the constituent minerals and the uses of granite. Basalt was formed when lava was exposed at the surface and cooled quickly. It is a fine-grained, dull coloured rock and it is noted for its resistance (hardness). Sometimes, if it cools rapidly, it can split into hexagonal columns. Basalt often results in the formation of a plateau---good examples are the Antrim-Derry Plateau and the Deccan of India. Granite and Basalt are two examples of igneous rocks.

Granite: was forced up near the earth’s surface. It cooled slowly and formed large crystals. It is usually grey in colour but other colours are possible. It was formed many millions of years ago and the rocks that once covered it have been eroded to expose the granite.

It is used as a building material and in the construction of monuments.

Basalt: was formed when magma cooled quickly on the earth’s surface. This magma was forced out through fissures (cracks) and since it cooled quickly it formed very small crystals. It is usually very dark (often black) in colour.

It is also used as a building material and for road foundations.

Sedimentary rocks

These rocks are formed by compressed sediments or fragments and often form on lakebeds or under the sea. There are three basic categories of sedimentary rocks. Sedimentary rocks form in layers (bedding planes). The three groups of sedimentary rocks are:

1. Organic sedimentary rocks - these are formed from the compacted remains of living things. Limestone and Peat are obvious examples.
2. Inorganic (Mechanical) sedimentary rock - these are formed from the compacted remains of the broken down particles of other rocks; Sandstone and Shale are examples.
3. Chemically Derived sedimentary rocks - these include gypsum and rock salt.  However, the most prized deposits associated with chemical deposition are oil and gas.

Metamorphic rocks

Metamorphic rocks are those which have been metamorphosed (changed). They are originally either igneous or sedimentary rocks but, due to the application of great heat and/or pressure, they were changed. The heat was caused by contact with magma and the pressure was usually caused during the creation of fold mountains.

Typical metamorphic rocks would include marble (originally limestone) and quartzite (originally sandstone).