This picture shows A'a' flowing over pahoehoe on Kilauea Volcano, Hawai'i.
Click on image for full size
Courtesy of USGS.
Basalt is volcanic rock. It is the most common type of rock in the Earth's crust and it makes up most of the ocean floor. It forms when lava reaches the Earth's surface at a volcano or mid ocean ridge. The lava is very hot when it gets to the surface. It cools quickly, often in just a few days, forming solid rock.
Basalt is very dark in color. It is made mostly of dark colored silicate minerals. You usually can't see the mineral crystals without using a microscope because the crystals are so small.
There are two Hawaiian words to describe the two types of basalt from volcanoes. These words are 'a'a and pahoehoe. 'A'a basalts have rough surfaces. Pahoehoe basalts have a smooth and glassy surface that looks like many ropes.
Last modified November 1, 2005 by Lisa Gardiner.
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