What is basalt?
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What is Basalt?

Basalt

August 2016 - Basalt is a fine-grained, hard rock that forms when bits of lava shoot out of volcanoes, so it's one kind of igneous rock.

When the lava cools quickly, it turns into basalt. Usually basalt is black or gray. Basalt is a mixture of feldspar and pyroxene, a rock made mainly of silica and oxygen. Pyroxene is what jade is before metamorphosis.

gray stone curved grindstone with cylindrical stone on top
Basalt grindstone (Niger, ca. 5000 BC)

Basalt is pretty common on Earth, and there is also a lot of basalt on the Moon, and on other planets including Mars and Venus.

Like granite, basalt is a very hard rock. Because it is a common rock and so hard, people used basalt for early choppers and for grinding stones for grinding grains like millet and barley.

Asphalt pavement
Asphalt pavement made with basalt

Even in the Stone Age, people carried basalt grinding stones (along with igneous obsidian for tools) long distances by boat in order to sell them to people who lived in places where there wasn't any basalt. Roman engineers paved a lot of Roman roads with basalt, and today engineers still use a lot of ground-up basalt to make asphalt to pave roads.

Learn by doing: finding different kinds of rocks
More about basalt
More about igneous rocks

Bibliography and further reading about rocks:

More about Basalt
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Karen Carr is Associate Professor Emerita, Department of History, Portland State University. She holds a doctorate in Classical Art and Archaeology from the University of Michigan. Follow her on Instagram or Twitter.
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