Courtesy of the USGS

Kilauea Volcano in Hawaii is currently erupting, sending molten rock across the landscape and into the ocean. Every day, new rocks are forming as the lava cools and solidifies.

Sometimes magma spills out onto Earth's surface, most commonly from a volcanic eruption. As the magma moves quickly from the super-hot conditions of Earth's interior to the much cooler environment at the surface, it cools and solidifies rapidly. The rocks that result are called EXTRUSIVE IGNEOUS ROCKS.

Darker minerals such as olivine, biotite, and amphibole make up mafic igneous rocks.

But sometimes magma does not come to the surface. Instead, it forces its way into cracks and spaces between other rocks already inside the Earth and solidifies into rock before ever reaching the surface. Magma that never makes it to the surface solidifies relatively slowly because it takes it a long time to cool inside the hot Earth. The result is called INTRUSIVE IGNEOUS ROCK.

In addition to being categorized as extrusive and intrusive, igneous rocks are also classified according to their mineral content. Those rich in quartz are classified as FELSIC, or acidic. Felsic igneous rocks are light colored. Conversely, igneous rocks rich in iron and magnesium-bearing minerals (such as amphibole and pyroxene) are darker and are classified as MAFIC, or basic. Both felsic and mafic igneous rocks can contain feldspar minerals.

Extrusive Igneous Rocks

Courtesy of the USGS

Basalt is a black volcanic rock that forms when lava cools on the surface.

Another common extrusive igneous rock that forms under conditions similar to those for basalt is rhyolite. But rhyolite is a felsic extrusive rock, because its main mineral constituents are quartz, feldspar, and light-colored micas. Like basaltic magmas, rhyolitic magmas are often associated with volcanic eruptions of great violence. Also like basalt, rhyolite crystals cannot be observed with the naked eye.

Intrusive Igneous Rocks

Mount Rushmore is carved from granite in the Black Hills of South Dakota.

Although intrusive rocks develop from the same types of magma as extrusive rocks, intrusive rocks look quite a bit different because they cool more slowly and thus have larger crystals. In fact, the large mineral crystals of intrusive igneous rocks can be seen without a lens or microscope. They are often coarse textured and have a rough feel, unlike extrusive rocks, which feel smooth.

One type of intrusive igneous rock that forms from the same type of magma as basalt is called gabbro. Gabbro contains no quartz. Rather, it is a mafic rock whose main mineral constituents are pyroxene, feldspar, and some olivine.

Granite is made up primarily of quartz, mica and feldspar.

The best known of the igneous rocks is granite, an intrusive rock that forms from the same type of magma as rhyolite. Like rhyolite, granite is a felsic rock, and is composed almost entirely of feldspar and quartz. But unlike rhyolite, granite is rough textured and has obvious crystals. In certain cases where the granite forms very slowly, crystals can be up to several feet long. Because granite is a hard and tough rock, it is used for many construction projects.

Both extrusive and intrusive igneous rocks can become exposed to Earth's surface after they form. There, they are vulnerable to attack from environmental agents such as wind, water, ice, and gravity. These forces of nature break rocks into pieces that are later deposited and cemented to form sedimentary rocks, the next type of rock to be considered.

Igneous Rocks Instructions: This is the classic game Concentration. Click a question-mark tile to turn it over. Try to find its match by clicking another tile. If the tiles match, they will turn yellow and stay face up. If they don't match, they will flip over and you must try again. Click Here!