This picture shows A'a' flowing over pahoehoe on Kilauea Volcano, Hawai'i.
Click on image for full size
Courtesy of USGS.
Basalt is a hard, black extrusive igneous rock. It is the most common type of rock in the Earth's crust and it makes up most of the ocean floor.
The prevalence of dark minerals such as pyroxene and olivine cause basalt to have a dark gray to black color. Basalt includes smaller amounts of light colored minerals such as feldspar and quartz. By weight basalt contains less than 52% quartz (SiO2). Typically, most of the crystals of these minerals are too small to be seen without a microscope because the high rate of cooling prevents large crystals from forming.
Basaltic magma is formed when parts of the mantle layer melt due to high temperatures deep within the Earth. The magma is at temperatures between 1100 to 1250° C when it reaches the Earth's surface as lava at a volcano or mid ocean ridge, yet it cools quickly, in a few days or a couple weeks, forming basalt.
Shield volcanoes, are also composed almost entirely of basalt. Other types of volcanoes are made partially of basalt as well as other types of volcanic rocks. Flood basalts are found in many places around the globe including the Columbia River basalts in the northwest United States and the Deccan Traps basalts in India. They form when huge quantities of lava pour out onto a continent.
Two Hawaiian words are used to describe the two types of volcanic basalt: 'a'a and pahoehoe. 'A'a basalts are thicker with 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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