Extrusive Igneous Rocks

This photograph shows a fountain of liquid magma during the eruption of Stromboli volcano in Italy. Click on image for full size (128K JPG ) Image from Dr. J. Alean, Eglisau, Switzerland

Extrusive igneous rocks, or volcanics, are formed when magma makes its way to Earth's surface, erupts or flows above the surface as lava, and then cools forming rock. The lava that erupts onto the Earth's surface can come from different levels of the Earth's upper mantle, between 50 km and 150 km below the surface. It is under high pressure because of the weight of the rocks above and the pressure will allow it to come to the surface if it can find a route up.

When lava erupts onto the Earth's surface, it cools quickly because the temperature above the surface is much lower than below. If the lava cools very quickly, in less than a day or two, there is no time for elements to bond forming minerals. Instead, elements are frozen in place within volcanic glass. Often, lava cools over a few days to weeks and minerals have enough time to form but not time to grow into large crystals. These rocks with tiny crystals are called aphanitic or fine-grained.

Basalt is the most common type of extrusive igneous rock and the most common rock type at the Earth's surface.

This photograph shows a fountain of liquid magma during the eruption of Stromboli volcano in Italy. Click on image for full size (128K JPG) Image from Dr. J. Alean, Eglisau, Switzerland

Extrusive igneous rocks, or volcanics, form when magma makes its way to Earth's surface. The molten rock erupts or flows above the surface as lava, and then cools forming rock. The lava comes from the upper mantle layer, between 50 km and 150 km below the Earth's surface.

When lava erupts onto the Earth's surface, it cools quickly. If the lava cools in less than a day or two, there is no time for elements to form minerals. Instead, elements are frozen in place within volcanic glass. Often, lava cools over a few days to weeks and minerals have enough time to form but not time to grow into large crystals.

Basalt is the most common type of extrusive igneous rock and the most common rock type at the Earth's surface.

This photograph shows a fountain of liquid magma during the eruption of Stromboli volcano in Italy. Click on image for full size (128K JPG) Image from Dr. J. Alean, Eglisau, Switzerland

These types of igneous rocks are made at places like volcanoes. There, red-hot lava spills above the surface of the Earth and cools pretty quickly. Quick cooling makes rocks with very little mineral crystals.

Take a look at the most common type of igneous rock! It is called basalt.

Find out more about extrusive igneous rocks by visiting our intermediate level page!