Igneous Rocks

Igneous rocks are one of the most abundant rocks found on earth. These rocks are one of the main types of rocks, along with sedimentary and metamorphic rocks. Igneous rocks form when the hot molten rock solidifies. Igneous rocks can be found above or below the earth’s surface. Igneous rocks are classified into two main categories.

Intrusive Igneous Rocks: These rocks crystallize below the earth’s surface and make up most of the earth’s mantle. Intrusive rocks are formed due to slow cooling that may take up to many years. These rocks have large crystals due to a slow cooling rate. Common intrusive igneous rocks are diabase, diorite, granite, gabbro, pegmatite, and peridotite.

Extrusive Igneous Rocks: These rocks are found above the earth’s surface. These rocks are formed after the rapid cooling of molten lava. The cooling process is so quick that these rocks don’t have large crystals but an amorphous glass structure. Andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff are common extrusive igneous rocks.

Igneous Rocks Classification

Igneous rocks are classified based on their formation, texture, mineralogy, chemical composition, and geometry of the igneous body.

Texture

Texture helps name rocks. The rock’s texture, shape, size, orientation, mineral grain distribution, and inter-grain relationship determine if the rock is tuff, pyroclastic lava, or simple lava. Texture information is only a secondary part of naming a rock.

Mineralogical Classification

The International Union of Geological Sciences (IUGS) (https://www.iugs.org/) recommends classifying igneous rocks based on their mineral composition whenever possible. The difficulty of studying minerals inside the rock depends on its coarseness. Coarse-grained rocks are easier to explore than fine-grained rocks. It is impossible to study this for glassy volcanic rocks.

Chemical Classification

If a rock is not classified by mineralogy, chemical classification is used. Chemical classification identifies the elements present in the rock. Most igneous rocks are made up of silica, sodium, potassium, calcium, iron, and magnesium. All these elements combine to form silicate minerals. These minerals are present in more than 90% of all igneous rocks. The ratio of major and minor minerals elements are expressed in weight percent oxides.

Igneous Rocks Uses

Igneous rocks have a wide variety of use. Many igneous rocks are used in construction, flooring, and landscaping. Some rocks are also used to obtain precious gemstones. Polished granite is used to make countertops, stairs, and is used in high-traffic areas due to its strength and durability.

Pumice is used as an abrasive in the medical and cosmetic industry. It is also used to remove dead skin cells.

Igneous rocks are also a valuable source of minerals. Pegmatite is the main source of commercial minerals like feldspar, mica, and lithium.

Basalt is used as a railroad track ballast, flooring, cobblestone, and countertops. Gabbro is used to make work surfaces, floor tiles, facing stones, and cemetery markers. Diorite is a hard rock, and it is used in landscaping due to its weather resistance. Obsidian is volcanic glass, and it is used to make scalpel blades, ornamental stones, and decorative specimens.