This picture shows A'a' flowing over pahoehoe on Kilauea Volcano, Hawai'i.
Click on image for full size
Courtesy of USGS.
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.
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