Rocks are classified according to their origin or how they are formed. They are igneous, sedimentary, and metamorphic. Each type will be described in order to enable you to classify some of the common types that are abundant in your locality.
Classification of Rocks:
1. Igneous rocks – are formed by the solidification or hardening of molten materials upon cooling. Some common examples of igneous rocks are Granite, Basalt, Obsidian and Pumice.
The Granite is usually light colored with large crystals, able to reflect light falling on its surface, and has medium weight for its size.
The Basalt is in dark color, composed of very fine crystals, dull, unable to reflect light, heavy for its size.
The Obsidian is dark in color, composed of small crystals, glassy, and of medium weight.
The Pumice may be light or dark in color, dull, sponge like in appearance and very light in weight.
2. Sedimentary rocks – are formed by the accumulation of mud, sand, and gravel that are deposited and cemented. They may also be formed from materials that precipitated from solutions in water.
These are the following examples of sedimentary rocks; the sandstone, limestone and Shale.
The Sandstone is usually yellow, red or brown. They are made of sand-sized grains and are generally not tightly cemented together. They are of medium weight and appear glassy under a magnifying glass.
The Limestone is generally gray in color and made of pieces smaller than sand. Unlike sandstone, the limestone particles are tightly cemented together. Limestone is heavy for its size.
The Shale is gray or brown in color. It is composed of very fine dust like pieces. It appears dull and may break into layers.
3. Metamorphic rocks – are formed from either igneous or both due to great heat and pressure.
Here are some common examples of metamorphic rocks and their characteristics:
Gneiss – looks similar to granite streaks or bands. Its color and weight are the same as granite.
Slate – usually dark in color and occurs in sheets or layers.
Marble – very light in color, with very fine crystals, and like fine sugar in appearance when light strikes the surface.