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What is Igneous Rock?

By S. Mithra
Updated Jun 04, 2024
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Igneous refers to one of the three major types of rock, with metamorphic and sedimentary being the other two. Although it can form either above or below ground, it is always created when molten material from the inner layers of the Earth cool and harden. In fact, the label comes from the word “ignis,” meaning “of fire.” Broadly grouped by whether this process happens above or below the Earth’s surface or both, these types of rocks also can be classified by composition. They have significant scientific and everyday uses.

Formation

The Earth has three major layers, including the crust, mantle and core. Scientists divide these into smaller subsections, such as the lithosphere — the outer crust and upper mantle — and asthenosphere — the lower, fluid part of the mantle. The inner layers are under incredibly intense pressure and are extremely hot.

When minerals are close enough to the center of the Earth, they get heated to between 1,100 – 2,400° Fahrenheit (590 – 1,300° Celsius) and change from a solid to a liquid. The resulting material is called magma. Sometimes, it gets trapped in pockets, where it cools and becomes solid again. In other cases, forces such as convection currents bring magma to the surface, and it escapes through volcanic eruptions as lava before losing heat and stiffening. In either instance, the hardened substance is igneous rock.

Intrusive Rock

The melted magma that hardens underneath the surface of the Earth is known as intrusive, internal or plutonic igneous rock, because it forms in hollow spots underground. The term “plutonic” has its history in mythology, with the Roman god Pluto — known in Greece as Hades — ruling the underworld where the spirits of all the dead supposedly dwell. It generally is easy to identify this type of rock because magma cools very slowly under the Earth’s surface, allowing crystals to grow big enough to view with the naked eye. Some intrusive rocks are granite, diorite, rhyolite and gabbro.

Extrusive Rock

When magma escapes as lava and hardens, experts refer to it as extrusive igneous rock, which simply means it flowed or was thrust out of the deeper layers of the planet. This type usually cools much more quickly, so larger lumps of mineral or crystals typically don’t have time to form. In fact, many volcanic rocks are primarily silica, a kind of glassy sand. It often contains air bubbles, as well. A good example in this category is pumice, which has so many spaces from where air was trapped that it can float. Other kinds in the extrusive group are basalt, andesite, scoria and obsidian.

Porphyry

Porphyry is igneous rock that cools in two different stages instead of just one. The process starts in the mantle with the formation of large crystals. The material then moves closer to the Earth’s surface, where it loses heat very quickly in the upper crust or comes out of a volcano. During the second stage, the rapid loss of heat usually keeps the crystals that form much smaller. It is a mixture of the intrusive and extrusive types overall as a result.

Compositional Classification

Although scientists group these rocks by their texture or grain size, they also consider composition. They use three major groups for this system: mafic, felsic and intermediate. Those in the mafic category are made of the minerals pyroxene, olivine and feldspar. Like flaked obsidian, they have dark colors like green and black. Combinations of feldspar and quartz create felsic rocks in much lighter colors, such as white or pink, that sparkle in the light. Intermediate types lie somewhere in the middle, with medium shades of grey and green made of amphibole, feldspar and biotite.

Scientific Importance

Geologists and other professionals who study the Earth are interested in all types of igneous rocks because they provide some clues about what it is like deep in the planet, including temperature and pressure conditions. The chemical makeup of each rock tells scientists what elements are present and what reactions are happening underground. Through a method called radiometric dating, those who study these materials often can figure out the age of the rocks, which then can be used to create a timeline of the Earth’s geological history.

By studying the formation of these rocks and other physical processes, people have learned that the Earth is constantly changing. Even though it might take thousands of years for igneous material to form and make its way to the surface, the process is always ongoing. This puts a much different perspective on the world, teaching individuals to see development and metamorphoses as natural.

Everyday Uses

People generally use various types of these rocks in architecture, furniture or decorations. Granite countertops, for example, are popular in contemporary homes because of their attractive, natural appearance and durability. Many artists who sculpt choose forms of igneous material as a medium, and some people like to collect different kinds for their beauty and uniqueness. Individuals also have used them in jewelry, handbags, shoes and other accessories, although the weight of the material often is a concern in these cases. Some even find their way into beauty care, such as using pumice stones to get rid of calluses.

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Link to Sources
Discussion Comments
By anon253502 — On Mar 09, 2012

How much is it worth?

By bfree — On May 31, 2011

My gardener recommended that I start using some type of igneous volcanic rock potting soil in my flower gardens to help retain moisture. I've heard of lava rocks before but never a lava based potting soil.

Anyway I found it on the internet and I've been using it now for about two months. It's a little more expensive than your average bag of soil but I must say it's well worth it.

My garden has never been this beautiful before with such big colorful blooms. My gardener was right, those little pumice stones are like tiny sponges. And the good thing is I don't have to water as often.

By lavaboy — On Dec 25, 2009

Some pumice will float because of trapped air bubbles.

By anon4045 — On Sep 30, 2007

what is a igneous rock that is so light that it floats?

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