Igneous Rocks

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granite boulders
Granite boulders at Joshua Tree National Park, California. Photo past Eva DiDonato.

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 chosen 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 draw an igneous rock having arable light-colored minerals such as quartz, feldspars, or muscovite.

  • Mafic: Derived from the words magnesium and ferric (Atomic number 26 is the chemical symbol for iron) to draw an igneous rock having abundant dark-colored, magnesium- or iron-rich minerals such as biotite, pyroxene, or olivine.

Molten Lava
Molten lava cooling and forming igneous rock at Hawai'i Volcanoes National Park, Hawai'i. NPS photo by Janice Wei.

Extrusive Igneous Rock

Extrusive, or volcanic, igneous rock is produced when magma exits and cools equally lava at or virtually the Earth's surface. Exposed to the relatively cool temperatures of the atmosphere, the lava cools rapidly meaning that mineral crystals don't have much time to grow. This results in rocks with a very fine-grained or fifty-fifty 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, Hawai'i [Geodiversity Atlas] [Park Home]

  • Dusk Crater Volcano National Monument, Arizona [Geodiversity Atlas] [Park Abode]

  • Crater Lake National Park, Oregon [Geodiversity Atlas] [Park Dwelling house]

Likewise see, NPS—Volcanic Landforms: Extrusive Igneous

diabase dike
Dark colored diabase dikes intrude through lite colored granite at Acadia National Park, Maine. NPS photograph by Georgia Hybels.

Intrusive Igneous Rock

Intrusive, or plutonic, igneous stone forms when magma remains inside the Earth'south 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 ways the private mineral grains accept 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 Habitation]
  • Yosemite National Park, California [Geodiversity Atlas] [Park Habitation]

Likewise see, NPS—Volcanic Landforms: Intrusive Igneous


Featured Video—Yosemite Granite

  • Alaskan Elders: Timeless Tales From the Trail (Open Captioned)

    Yosemite Nature Notes 20: Granite

    Many national parks were founded for their geology, and Yosemite is known throughout the earth for its exceptional high cliffs and rounded domes. Visitors to the park, from hikers to rock climbers, feel a landscape dominated by granite.

    Duration:
    7 minutes, 11 seconds


Igneous Stone Textures

Rhyolite
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
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
Diorite is a archetype "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.

rhyolite porphyry
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 whatever limerick that contains conspicuous phenocrysts (larger crystals) in a fine-grained groundmass.

Last updated: September xi, 2019