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The Winter 2010 issue of The Earth Scientist includes a variety of educational resources, ranging from astronomy to glaciers. Check out the other publications and classroom materials in our online store.
This picture shows A'a' flowing over pahoehoe on Kilauea Volcano, Hawai'i.
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Courtesy of USGS.

Basalt Rocks

Basalt is an extrusive igneous rock that is very dark in color. It is the most common type of rock in the Earth's crust and it makes up most of the ocean floor.

It is made of many dark colored minerals such as pyroxene and olivine. Basalt also contains some light colored minerals such as feldspar and quartz, but the amounts are small. Typically, you can't see most of the mineral crystals without using a microscope because quick cooling prevents large crystals from forming.

Basalt forms when lava reaches the Earth's surface at a volcano or mid ocean ridge. The lava is between 1100 to 1250° C when it gets to the surface. It cools quickly, within a few days or a couple weeks, forming solid rock. Very thick lava flows may take many years to become completely solid.

Two Hawaiian words are used to describe the two types of volcanic basalt: 'a'a and pahoehoe. 'A'a basalts have rough surfaces (that make barefoot people cry, "Ah! Ah!" as they walk across it). They form from fast flowing lava. Pahoehoe basalts have a smooth glassy surface that looks like many ropes. The "ropes" form when the surface cools, becoming solid rock while lava flows beneath it.


Last modified November 1, 2005 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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