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Types of Igneous Rocks

Rocks are of three main types, viz., sedimentary rocks, metamorphic rocks and igneous. Here, we will have a discussion on the types of igneous rocks known to man.
Sedimentary rocks, as the name suggests, evolve through sedimentation of material on the Earth's surface and within different water bodies. Metamorphic rocks are those which have morphed from either sedimentary rocks or igneous rocks.

Formation of igneous rocks takes place under the ground or above the ground. Under the ground, the magma or melted rock gets trapped in small pockets and a gradual cooling process of these pockets transforms the magma into igneous rocks, also known as intrusive or plutonic rocks . When a volcano erupts, the magma rises above the surface of the earth and it is known as lava. Cooling of this lava results in the formation of igneous rocks above the ground, which are known as extrusive (volcanic rocks). Another type of igneous rock, which is less common in occurrence is known as the hypabyssal igneous rocks. These rocks form at a depth, that lies between the extrusive and the intrusive rocks.

Various Textures of Igneous Rock

Igneous rock types bear mainly six kinds of textures. They include phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. Here is a briefing on each of these types.

Phaneritic
This texture is borne by most intrusive rocks. It is clearly visible to the naked eyes. Phaneritic texture comes into being by a slow crystallization of the rocks deep underground. Gabbro, diorite and granite are common examples.

Aphanitic
The crystals of this texture is not visible to the naked eye; not even hand lens. This texture is typical of extrusive rocks which form from the rapid cooling process of lava near the Earth's surface. Examples are basalt, andesite and rhyolite.

Porphyritic
Porphyritic texture consists of two different mineral types. They are the phenocrysts (the larger grains) and the matrix (the finer grains). Rocks with porphyritic are believed to have undergone two stages of cooling. In the first stage of cooling, the phenocrysts must have formed at a depth, while, in the second stage, the matrix must have gotten crystallized near the surface.

Glassy
Igneous rocks which contain a glassy texture, are the ones which do not contain any mineral grains. It means, such rocks undergo a cooling which is so rapid that the minerals have no scope of getting crystallized. This process usually results when some erupted lava comes in contact with materials, which are much cooler. Obsidian and pumice are rocks bearing a glassy texture.

Pyroclastic
Igneous rocks obtain a pyroclastic texture, when lava is blasted into the air and comes down in the form of fragmental materials, volcanic ash, lapilli (a size classification term for tephra, which is material that falls out of the air during a volcanic eruption) and volcanic bombs.

Pegmatitic
Rocks with a pegmatitic texture comprises mineral grains which grow to become exceptionally large. The texture is typically found in intrusive rocks. Granite and diorite pegmatites are examples of this texture.

Igneous Rock Types and Their Uses
  • The first kind we would consider would be granite, which is an intrusive or a plutonic rock. Its high content of silica, potassium, sodium and quartz makes it usable for architectural construction, ornamental stone, flooring, paving, facing stones, worktops, gravestones and monuments.
  • Then comes pumice, another intrusive rock which finds wide applications as an abrasive material in hand soaps, soaps, cleansers, and dental products, emery board, etc. It is also used by industries in concrete, insulation, acoustic tile, stucco and plaster.
  • Basalt is a common extrusive igneous rock which is used in floor tiles and for aggregate in construction projects. The crushed form of this rock is extensively used for concrete aggregate, asphalt sidewalk aggregate, railroad ballast, filter stone in drain fields and many other purposes.
  • A massive plutonic rock known as gabbro is utilized in road metal, railroad ballast, etc.
Rocks are one of the most important components of the Earth. Knowing about them not only involves theoretical study, but extensive practical approach. So why don't you seek help from a petrolgist, and know about all kinds of rocks in an elaborate manner.
By Rajib Singha
Published: 1/6/2010
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