An Example of Igneous Rocks

The following is an example 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.

Andesite 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.
  • Andesites erupt at temperatures between 900 and 1100° C. Andesite contains crystals composed of plagioclase feldspar, pyroxenes, and hornblende.
  • The word andesite comes from the Andes Mountain Range where andesite is common.
Basalt-lava Basalt Lava - 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.
  • The minerals in basalt include olivine, pyroxene, and plagioclase.
  • Basalt is erupted at temperatures between 1100 to 1250° C.
  • Basalt is the most common rock type in the Earth's crust most of the ocean floor is made of basalt.
  • Basaltic magma is commonly produced by direct melting of the Earth's mantle
Dacite 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.
  • Dacite lava consists of about 63 to 68 percent silica (SiO2).
  • Dacite generally erupts at temperatures between 800 and 1000°C
  • Common minerals include plagioclase feldspar, pyroxene, and amphibole
  • Dacite has a high crystal content. This combined with a high silica content makes it viscous and prone to explosive eruptions.
  • Dacite was erupted from Mount St. Helens 1980-86
Pumice 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, and can form in all types of magma: basalt, andesite, dacite, and rhyolite.
Obsidian 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.
Rhyolite 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.
  • Rhyolite has a silica (SiO2) content greater than about 68 percent by weight.
  • Common mineral types include quartz, feldspar and biotite
  • Rhyolite is erupted at temperatures of 700 to 850° C.
  • Rhyolite eruptions often produce pumice or obsidian

All pictures on this page are courtesy of the USGS - United States Geological Survey. Discriptions are adapted from the USGS.

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