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This photograph shows a fountain of liquid magma during the eruption of Stromboli volcano in Italy.
Click on image for full size
Image from Dr. J. Alean, Eglisau, Switzerland
For more information, images and video about Stromboli volcano see Stromboli On-Line.

Extrusive Igneous Rocks

Extrusive igneous rocks, or volcanics, form when magma makes its way to Earth's surface. The molten rock erupts or flows above the surface as lava, and then cools forming rock. The lava comes from the upper mantle layer, between 50 km and 150 km below the Earth's surface.

When lava erupts onto the Earth's surface, it cools quickly. If the lava cools in less than a day or two, there is no time for elements to form minerals. Instead, elements are frozen in place within volcanic glass. Often, lava cools over a few days to weeks and minerals have enough time to form but not time to grow into large crystals.

Basalt is the most common type of extrusive igneous rock and the most common rock type at the Earth's surface.


Last modified June 17, 2003 by Lisa Gardiner.

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TES XXVI, 3 fall 2010 The Fall 2010 issue of The Earth Scientist, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!

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