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An Example of Igneous Rocks
The following are examples of igneous rocks. It is not an all inclusive list but a brief pictorial list of some common igneous rocks. Igneous rocks get their name from the latin word for fire “igneus”. The name is appropriate because these rocks are born of fire. Beneath the thin rocky crust of the earth is the inferno of the mantle! The mantle is the origin of this rock type.
Igneous rocks are either intrusive or extrusive. Extrusive igneous rocks come from the mantle and reach the earth's surface usually in a volcanic eruption. Intrusive igneous rocks also originate in the mantle but cool before they reach the surface. These are called Plutonic igneous rocks.
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Andesite is a gray to black volcanic rock. It is generally erupted from stratovolcanoes as thick lava flows. It can also generate strong explosive eruptions to form pyroclastic flows.
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Basalt- Basalt is a hard, black volcanic rock. Less than ½ of the weight of basalt is silica (SiO2). Because of basalt's low silica content, it has a low viscosity (resistance to flow). This enables basaltic lava to flow quickly and allows volcanic gases to escape without explosive events.
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Dacite lava is most often light gray, but can be dark gray to black. It is one of the most common rock types associated with enormous Plinian-style eruptions.
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Pumice is light and porous. It forms during explosive eruptions. Pumice is full of holes caused by expanding volcanic gases. It is composed of volcanic glass and minerals, usually forms in rhyolite magmas but it can also come from basalt magmas. |  |
Obsidian is usually black in color though it can also be red or have a greenish tint. It is a dense volcanic glass, usually composed of rhyolite, rich in iron and magnesium. Obsidian is formed when the lava cools so quickly that crystals do not have time to grow. Obsidian fractures with very sharp edges. It was used by Stone Age cultures for making knives, arrowheads, and other tools where sharp edges are important. |
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Rhyolite is a light-colored volcanic rock. It has a high silica content which makes it very viscous. This prevents gases from escaping causing rhyolite eruptions to be explosive.
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Pictures of Andesite, Dacite, and Basalt are courtesy of the USGS - United States Geological Survey.
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