Types
Igneous rocks fall into two basic categories. Extrusive, or volcanic, is the name given to rocks that form at or near the Earth's surface through volcanic activity. Often found with a fine texture, extrusive rocks types include basalt, scoria, obsidian, pumice, rhyolite and andesite. Intrusive rocks, also known as plutonic rocks, are those that form beneath the surface from magma that cools after it becomes trapped. They usually have a coarse texture and include granite, diorite, gabbro and pegmatite.
Granite
Granite is used to make monuments, floor tiles and kitchen counter tops. It is a light-colored rock found on all of the continents. In addition to its uses in daily life, granite is well known because of its natural occurrence in famous landmarks in the United States, including Mount Rushmore in South Dakota and Stone Mountain in Georgia.
Basalt
A dark-colored igneous rock, basalt is found on the ocean floor and in lava flows. It the most common type found under the Earth's surface. Construction projects often use crushed basalt for the bases of roads and in concrete and asphalt pavement aggregates. Basalt is also found on large quantities on the moon.
Obsidian
Obsidian is a smooth volcanic glass that forms on the Earth's surface. It can be found wherever lava cools such as along the edges of a lava flow and areas where lava meets water. Although black is the most common color, it also can be found in brown or green, and in rare cases, blue, red, orange or yellow. In early societies, obsidian was used to make cutting tools. Today, it is found in jewelry and surgical tools.
Gabbro
Gabbro is an igneous rock similar in composition to basalt, but it is formed when magma cools slowly, instead of rapidly like basalt. It is coarse and black to very dark green in color. When polished, it is used to make cemetery markers, floor tiles and kitchen counter tops. It is a durable rock that holds up to wear and weathering.