Introduction
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
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Felsic: Derived from the words feldspar and silica to describe an igneous rock having abundant light-colored minerals such as quartz, feldspars, or muscovite.
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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.
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:
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Hawai'i Volcanoes National Park, Hawai'i [Geodiversity Atlas] [Park Home]
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Sunset Crater Volcano National Monument, Arizona [Geodiversity Atlas] [Park Home]
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Crater Lake National Park, Oregon [Geodiversity Atlas] [Park Home]
Also see, NPS—Volcanic Landforms: Extrusive Igneous
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:
- Acadia National Park, Maine [Geodiversity Atlas] [Park Home]
- Joshua Tree National Park, California [Geodiversity Atlas] [Park Home]
- Yosemite National Park, California [Geodiversity Atlas] [Park Home]
Also see, NPS—Volcanic Landforms: Intrusive Igneous
Featured Video—Yosemite Granite
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Yosemite Nature Notes 20: Granite
Many national parks were founded for their geology, and Yosemite is known throughout the world for its exceptional high cliffs and rounded domes. Visitors to the park, from hikers to rock climbers, experience a landscape dominated by granite.
- Duration:
- 7 minutes, 11 seconds
Igneous Rock Textures
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.
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."
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.
Photo courtesy of Tina Kuhn
Porphyritic
Describes an igneous rock of any composition that contains conspicuous phenocrysts (larger crystals) in a fine-grained groundmass.
Last updated: April 25, 2019