This picture shows A'a' flowing over pahoehoe on Kilauea Volcano, Hawai'i. Click on image for full size (123K JPG ) 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.

This picture shows A'a' flowing over pahoehoe on Kilauea Volcano, Hawai'i. Click on image for full size (123K JPG) 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.

This picture shows A'a' flowing over pahoehoe on Kilauea Volcano, Hawai'i. Click on image for full size (123K JPG) 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.