What is Basalt? - Definition, Uses & Composition

Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about basalt, an extrusive volcanic igneous rock. It is extremely common, not only on Earth, but elsewhere in the solar system, including on the moon and Mars.

Definition of Basalt

With a name derived from the Latin for 'very hard stone', basalt is indeed a very hard, black igneous rock found all over Earth and our solar system. It can be found not only on Earth, but also on Earth's moon and Mars. It is an extrusive igneous rock, which forms from volcanic lava that cools rapidly at Earth's surface. It is generally black in color due to the high amounts of magnesium oxide and calcium oxide, and very low amounts of lighter colored silicate minerals. It has several industrial applications, including building materials and thermal insulators.

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  • 0:02 Definition of Basalt
  • 0:41 Chemistry of Basalt
  • 1:37 Formation of Basalt
  • 2:28 Uses for Basalt
  • 3:03 Basalt on Other Planets
  • 4:03 Lesson Summary
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Chemistry of Basalt

Basalts are composed primarily of magnesium oxide (MgO) and calcium oxide (CaO). They are very low in silicon (SiO2), usually less than 50 percent. They also contain some iron oxide (FeO) and aluminum oxide (Al2O3). As a result, they are very dark in color, and usually appear as shades of black and black-green. Occasionally, some red or orange colors can appear in basalts and are due to that particular basalt containing rusting iron compounds. Basalt is an igneous rock that forms from the cooling and crystallization of magma. And usually, magma is formed from the melting of dark colored peridotite and pyroxene-rich rocks. There are some lighter colored basalts found, which form when a basalt magma contains a higher than normal amount of calcium or sodium minerals.

Formation of Basalt

Basalt is an extrusive igneous rock. All igneous rocks form as the result of the cooling and crystallization of magmas, either at Earth's surface or below the surface. If the rock hardens below the surface, it forms large crystals and is referred to as intrusive. If it hardens at the surface, it forms small crystals (because it doesn't have sufficient time for crystals to grow in size) and is called an extrusive rock. Basalt forms at the surface and has small crystal grains, and is, therefore, an extrusive rock. When basalts form at the surface, it does not have to be on a dry surface, as many basalts form in the water. Pillow basalts form when basalt lavas erupt into ocean waters, and basalt is also formed at the earth's mid-ocean ridges (where new ocean crust is formed).

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