Basalt, one of the most abundant rocks in the earth's crust. It is an igneous rock (one formed by the cooling of molten materials). Basalt is usually dark in color and contains grains of microscopic size. It is heavier than granite (another abundant igneous rock) and contains less silicon and more iron and magnesium. Like granite, basalt is used for monuments and gravel.
Basalt is thought to make up much of the ocean floor, and is known to occur beneath the granite of the continents. Basalt reaches the earth's surface through volcanoes and other openings, as lava. It cools in massive form—as in the great outpourings that formed the Columbia Plateau in the northwestern United States—and also in ropy or billowy shapes. Occasionally, it forms spectacular closely packed columns such as those of the Devils Postpile in California.