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Igneous

Igneous Rocks

Igneous rocks (from the Greek word for "fire") form when hot, molten rock (magma) crystallizes and solidifies. Magma originates deep within the Earth, near active plate boundaries or hot spots. Magma that rises to the surface is called lava. Igneous rocks are classified into two groups depending upon where the molten rock solidifies: extrusive or intrusive.

Key Terms

• Felsic: Derived from the words feldspar and silica to describe an igneous rock having abundant light-colored minerals such as quartz, feldspars, or muscovite.
• Mafic: Derived from the words magnesium and ferric (Fe is the chemical symbol for iron) to describe an igneous rock having abundant dark-colored, magnesium- or iron-rich minerals such as biotite, pyroxene, or olivine.

Igneous Rock Types

Molten lava cooling and forming igneous rock at Hawai'i Volcanoes National Park.

NPS Photo/Dale Pate

Extrusive Igneous Rock

Extrusive, or volcanic, igneous rock is produced when magma exits and cools as lava at or near the Earth's surface. Exposed to the relatively cool temperatures of the atmosphere, the lava cools quickly meaning that mineral crystals don't have much time to grow. This results in rocks with a very fine-grained or even glassy texture. Hot gasses are often trapped in the quenched lava, forming bubbles (vesicles). Types of extrusive igneous rocks include: pumice, obsidian, andesite, rhyolite, and basalt. Volcanic processes has shaped the extrusive igneous rock formations at these parks: Hawai'i Volcanoes National Park, Sunset Crater Volcano National Monument, and Crater Lake National Park.

Dark colored diabase dikes intrude through light colored granite at Acadia National Park.

NPS Photo/Georgia Hybels.

Intrusive Igneous Rock

Intrusive, or plutonic, igneous rock forms when magma remains inside the Earth's crust where it cools and solidifies in chambers within pre-existing rock. The magma cools very slowly over many thousands or millions of years until is solidifies. Slow cooling means the individual mineral grains have a very long time to grow, forming a rock with large, visible crystals. National parks with excellent examples of intrusive igneous rocks include Yosemite National Park, and Acadia National Park.

Igneous Rock Textures

Felsic rhyolite with an aphanitic texture.

Photo courtesy of Tina Kuhn.

Aphanitic

Describes the texture of fine-grained igneous rock in which different components are not distinguishable by the unaided eye.

Obsidian is a volcanic glass with a conchoidal fracture.

Photo courtesy of Tina Kuhn

Glassy

Describes the texture of certain extrusive (volcanic) igneous rocks that is similar to broken glass and developed as a result of rapid cooling of the lava without distinctive crystallization. Synonymous with "vitreous."

Diorite is a classic "salt and pepper" rock with a phaneritic texture.

Photo courtesy of Tina Kuhn

Phaneritic

Describes an igneous rock texture in which mineral grains are large enough to be seen with the unaided eye and are of approximately equal size.

In this specimen of rhyolite porphyry, phenocrysts of obsidian are surrounded by a fine matrix.

Photo courtesy of Tina Kuhn

Porphyritic

Describes an igneous rock of any composition that contains conspicuous phenocrysts (larger crystals) in a fine-grained groundmass.