What Are Extrusive Igneous Rocks

Extrusive igneous rocks are rocks that form when lava cools and solidifies at Earth's surface. These rocks are different from intrusive igneous rocks, which form deep underground. The word "extrusive" means "pushed out," which describes how molten rock is pushed out of a volcano or crack in Earth's crust. When lava reaches the surface, it encounters much cooler temperatures than it experienced underground, causing it to cool rapidly.

The cooling rate of lava has a major effect on the texture of extrusive igneous rocks. When lava cools quickly at the surface, there is not enough time for large crystals to grow. Instead, the rock develops a fine-grained texture, meaning the crystals are so small they cannot be seen without a microscope. In some cases, lava cools so rapidly that crystals do not form at all, creating a smooth, glassy texture. This is similar to what happens when you freeze water quickly to make ice—the faster it freezes, the smaller the ice crystals become.

Basalt is the most common extrusive igneous rock on Earth. It forms from lava that is low in silica and flows easily from volcanoes. Basalt has a dark color, usually black or dark gray, and contains minerals like pyroxene and plagioclase feldspar. Because basalt cools quickly, its crystals are too small to see with the naked eye. Ocean floors are made mostly of basalt, and many volcanic islands, including Hawaii, are built from layers of basalt that erupted underwater and on land.

Another important extrusive rock is rhyolite, which forms from lava that is high in silica. This type of lava is thick and sticky, so it does not flow as easily as basalt lava. Rhyolite is usually light in color, ranging from pink to gray to white. Like basalt, rhyolite has a fine-grained texture because it cools rapidly at the surface. Rhyolite eruptions can be explosive because the thick lava traps gases that build up pressure inside the volcano.

Obsidian is a unique extrusive igneous rock that looks like dark glass. It forms when lava cools so quickly that crystals have no time to grow at all. Obsidian is usually black or dark brown and breaks with sharp edges. Ancient people used obsidian to make cutting tools and arrowheads because of its sharp fracture. Obsidian forms in the same volcanic environments as rhyolite, but only when cooling happens extremely fast.

The environment where extrusive rocks form is always connected to volcanic activity. Volcanoes bring molten rock from deep within Earth to the surface. When this material erupts, it can flow as lava or explode into the air as volcanic ash and fragments. All of these materials cool quickly in the air or water, forming extrusive igneous rocks. The type of extrusive rock that forms depends on the chemical composition of the lava and how quickly it cools. Understanding extrusive igneous rocks helps scientists learn about volcanic processes and the history of Earth's surface.

Interesting Fact: Some volcanic glass, like obsidian, can be so sharp that it is used in modern surgical scalpels because it cuts more precisely than steel blades.