Types of Rocks: Characteristics and Uses
Crystalline Rocks
The most common rocks in the Earth system consist of crystals. They are heterogeneous rocks that comprise heterogeneous crystals, all visible, and are called plutonic. The most important are:
- Granite: A coherent rock, usually gray but sometimes pink. The following are distinguished:
- Quartz: Gray, almost transparent crystals with a vitreous luster.
- Feldspars: The most abundant crystals, sometimes white with pink.
- Mica: Black crystals that are very bright.
Granite is the most abundant rock in continents. Its coherence and ability to withstand heavy loads without deformation make it suitable for construction.
- Syenite: A pink rock with characteristics very similar to granite. Its main component is feldspar.
- Gabbro: A dark rock. The minerals that compose it are difficult to distinguish with the naked eye. Among them are plagioclase and biotite.
These belong to the group of plutonic rocks.
Volcanic Rocks
Volcanic rocks are rocks that formed from materials ejected by volcanoes. The most notable are:
- Basalt: A black rock with a matte gloss. Its texture is very different from that of plutonic rocks. It only shows some of its crystals.
- Pumice: A light-colored and porous rock that floats in water. This rock is unmistakable due to its vacuolar texture.
- Obsidian: A shiny black rock formed from a vitreous paste.
These are volcanic rocks.
Rocks Formed by Fragments
These rocks are not formed by crystals but by sedimentary fragments and are called sedimentary rocks. There are three kinds:
- Conglomerates: Rocks that have grains larger than 2 mm. These grains are called clasts. Clasts are joined by finer materials that constitute the matrix.
- Sandstone: Rocks that have a fragment size of less than 2 mm but noticeable to the naked eye.
- Clay or Argillite: Rocks consisting of fragments so small that you need a magnifying glass to see them.
These belong to the group of sedimentary rocks.
Limestone
Limestones are not composed of fragments of other rocks. The most important is limestone. Limestones have very different appearances, but they all show bubbles when hydrochloric acid is added.
Rocks Formed by Plates
The arrangement of plates is called foliation, and it is a characteristic of metamorphic rocks. These rocks are composed of minerals such as micas, which show a preferential orientation along parallel layers. The following are notable:
- Slate: A rock formed by very small, flat crystals that are not visible. Its color is variable but predominantly dark gray.
- Shale: Presents wavy lamination and forms crystals that, unlike slate, are visible to the eye.
- Gneiss: Made up of crystals larger than those of shale. Its minerals are arranged in wavy bands in which micas are altered to quartz and feldspars.
They belong to the group of metamorphic rocks.
Building Rocks
Granite, sandstone, and basalt are very suitable for building construction due to some features:
- They are consistent, meaning they do not easily break down into fragments.
- They are resistant to pressure and therefore carry much weight without deforming.
- They are not easily altered by the attack of atmospheric agents.
Ornamental Rocks
Ornamental rocks have some features that allow them to be cut and polished for greater brightness and beauty. They are used to coat walls and countertops and in sculptures and decorative objects. Since antiquity, the ornamental rock par excellence has been marble. Currently, many rocks are given this use, including some traditionally used for construction, such as granite or limestone.
Coal
Coal is a sedimentary rock used as a fossil fuel. It is black and originates from the decomposition of terrestrial plants that accumulate in dead marshes. The dead plants accumulate at the bottom of a basin. Water covers them and therefore protects them from the atmosphere that would destroy them. A slow transformation begins due to the action of anaerobic microorganisms that cannot live in the presence of oxygen. Over time, a progressive enrichment in carbon occurs.
Oil
Oil is a complex chemical compound of solids, liquids, and gases. Its color varies from amber to black. The etymological meaning of the word “petroleum” is “stone oil.” It has the texture of an oil and is found in sedimentary rock deposits. Factors for its formation include:
- Absence of air
- Remains of plants and animals (especially marine plankton)
- Great pressure from the layers of soil
- Action of bacteria
- High temperatures