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What are Igneous Rocks

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

Igneous rocks (from the Greek phrase for "fire") structure when hot, molten rock (magma) crystallizes and solidifies. Magma originates deep inside the Earth, close to lively plate boundaries or warm spots. Magma that rises to the floor is referred to as lava. Igneous rocks are labeled into two companies relying upon the place the molten rock solidifies: Extrusive or Intrusive.

Key Terms

  • Felsic: Derived from the phrases feldspar and silica to describe an igneous rock having plentiful light-colored minerals such as quartz, feldspars, or muscovite.
  • Mafic: Derived from the phrases magnesium and ferric (Fe is the chemical image for iron) to describe an igneous rock having ample dark-colored, magnesium- or iron-rich minerals such as biotite, pyroxene, or olivine.
Molten lava cooling and forming igneous rock at Hawai'i Volcanoes National Park, Hawai'i. NPS image by using Janice Wei.

Extrusive Igneous Rock

Extrusive, or volcanic, igneous rock is produced when magma exits and cools as lava at or close to the Earth's surface. Exposed to the tremendously cool temperatures of the atmosphere, the lava cools rapidly which means that mineral crystals do not have a good deal of time to grow. This results in rocks with a very fine-grained or even glassy texture.

Related: What are Sedimentary Rocks

Hot gasses are frequently trapped in the quenched lava, forming bubbles (vesicles). Types of extrusive igneous rocks include: pumice, obsidian, andesite, rhyolite, and basalt. Volcanic approaches has fashioned the extrusive igneous rock formations at these parks:

  • Hawai'i Volcanoes National Park, Hawai'i 
  • Sunset Crater Volcano National Monument, Arizona
  • Crater Lake National Park, Oregon
Dark coloured diabase dikes intervene via mild coloured granite at Acadia National Park, Maine. NPS image through Georgia Hybels.

Intrusive Igneous Rock

Intrusive, or plutonic, igneous rock varieties when magma stays inside the Earth's crust the place it cools and solidifies in chambers inside pre-existing rock. The magma cools very slowly over many lots or tens of millions of years till it solidifies. Slow cooling capacity the mineral grains have a very lengthy time to grow, forming a rock with large, seen crystals. National parks with high-quality 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]

Igneous Rock Textures

Many country wide parks had been headquartered for their geology, and Yosemite is recognized at some stage in the world for its extremely good excessive cliffs and rounded domes. Visitors to the park, from hikers to rock climbers, ride a panorama dominated via granite.


What are Igneous Rocks

Describes the texture of fine-grained igneous rock in which exclusive elements are no longer distinguishable by means of the unaided eye.

Obsidian is a volcanic glass with a conchoidal fracture.


What are Igneous Rocks

Describes the texture of sure extrusive (volcanic) igneous rocks that is comparable to damaged glass and developed as a end result of speedy cooling of the lava besides one-of-a-kind crystallization. Synonymous with "vitreous."

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

Photo courtesy of Tina Kuhn


What are Igneous Rocks

Describes an igneous rock texture in which mineral grains are massive sufficient to be viewed with the unaided eye and are of about equal size.

In this specimen of rhyolite porphyry, phenocrysts of obsidian are surrounded with the aid of an excellent matrix.
Photo courtesy of Tina Kuhn


What are Igneous Rocks

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

In this specimen of rhyolite porphyry, phenocrysts of obsidian are surrounded by using a quality matrix.
Photo courtesy of Tina Kuhn