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Glossary - Basalt

Basalt

Volcanic rock (or lava) that characteristically is dark in color (gray to black), contains 45 to 53 percent silica, and is rich in iron and magnesium. Basaltic lavas are more fluid than andesites or dacites, which contain more silica.
  • Basalt is the most common rock type in the Earth's crust (the outer 10 to 50 km). In fact, most of the ocean floor is made of basalt.
  • Huge outpourings of lava called "flood basalts" are found on many continents. The Columbia River basalts, erupted 15 to 17 million years ago, cover most of southeastern Washington and regions of adjacent Oregon and Idaho.
  • Basaltic magma is commonly produced by direct melting of the Earth's mantle, the region of the Earth below the outer crust. On continents, the mantle begins at depths of 30 to 50 km.
  • Shield volcanoes, such as those that make up the Islands of Hawaiʻi, are composed almost entirely of basalt.

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