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Video:What Are Igneous Rocks?

with Jen D'Amore

Igneous Rocks are formed from magma cooling and hardening over time. This video explains the origin of these varied rocks and the minerals that compose them.See Transcript

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Transcript:What Are Igneous Rocks?

Hi, I'm Jen D'Amore for About.com, and this video is all about the origin of igneous rocks.

Igneous Rocks Come from Magma Cooling

Igneous, from the latin word for fire, is the first great class of rock. Igneous rocks originate from hot fluid material, like lava or magma, that has erupted at or below the earth's surface. Magma is a liquid mush filled with a variety of mineral crystals. As the magma cools, some minerals crystallize sooner than others, so it evolves throughout the process, and continues to move through the crust pulling other minerals into it's magma.

Plutonic Rock

Magma cools at a slower rate deep inside the earth than it does at the surface, so much larger grains are formed. Rock formed from deep magma is called "plutonic." Plutonic rock takes millions of years to cool, and is sometimes discovered after years of erosion reveals it.

Intrusive, Extrusive Igneous Rock and Obsidian

Shallow magma creates "intrusive" igneous rock, that takes thousands of years to cool and have small to medium sized grains, a phaneritic texture. Rock formed of lava that can take from mere seconds, to months to cool is "extrusive." Since extrusive rocks cool relatively quickly, they have invisible or very small grain, considered an aphanitic texture. Obsidian is formed when lava hardens quickly and has a glassy texture without crystals. The bubbling release of carbon dioxide and water vapor is what creates the holes you see in igneous rocks.

Equigranular Texture

Like a piece of bread, igneous rocks have an equigranular (or equal grain) texture, where mineral grains are packed together tightly, rather than being made of layers. Since it takes a lot of heat to form magma or lava, igneous rocks form where plates come together, where continental crust is pushed together, or where lithospheric plates pull apart at mid-ocean ridges.

Minerals in Igneous Rocks

Igneous rocks are classified by the minerals they contain including dark minerals like feldspar, quartz, amphiboles, and pyroxenes, and softer minerals like olivine and mica. One well known igneous rock is basalt, which is rich in magnesium and iron and is created by either lava flows or magma intrusions, making it either extrusive or intrusive.

Common Igneous Rocks: Granite and Basalt

Probably the igneous rock you'll most often encounter is granite, which has become a popular choice for kitchen countertops. Granite is a light, coarse-grained plutonic rock rich in feldspar and quartz, though it's common to hear people refer to any plutonic rock as "granite." Basalt is found on the deep sea floor, in vocanic island arcs, and along the edges of continents, while granitic rocks can be found on the continents.

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