Geography of Igneous Rocks in the United States

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When heated, molten rock (magma) crystallizes and solidifies, igneous rocks are formed. Igneous stems from the Latin word ignis which means fire.

About 15% of the Earth’s present land surface is made up of igneous rocks.

Textures of Igneous Rocks

When examining an igneous rock, the texture can be described in one of four ways.

Aphanitic: texture of fine-grained igneous rock in which distinct components cannot be distinguished by the naked eye.

Glassy (or vitreous): when lava is rapidly cooled without noticeable crystallization, the texture of some extrusive (volcanic) igneous rocks is comparable to fractured glass.

Phaneritic: mineral grains that can be seen with the naked eye and are about similar in size.

Porphyritic: a fine-grained igneous rock with prominent phenocrysts (larger crystals) 

Types of Igneous Rocks

There are two types of igneous rocks: intrusive and extrusive.

Intrusive Rock

Also known as platonic, intrusive igneous rocks form the majority of igneous rocks on Earth. Intrusive rocks form when magma cools within the crust of planet Earth.

Slow cooling allows individual mineral grains to grow millions of years, resulting in a rock with large and visible crystals. Intrusive ingenious rocks are hard and slow to erode.

Some common types of intrusive igneous rocks are: granite, gabbro, and diorite.

Diabase is a mafic (meaning high in magnesium and iron), igneous rock that usually forms below the Earth’s surface.

An igneous rock on display.
This intrusive igneous rock has large amounts of augite and labradorite minerals, and comes from the Lucke Quarry, which is southeast of Leesburg, VA. Photo: USGS, public domain.

What are Some U.S. National Parks That Have Intrusive Igneous Rocks?

  • Acadia National Park, Maine
  • Joshua Tree National Park, California
  • Yosemite National Park, California

Largest Granite Monolith in the World

Ove millions of years, erosion exposes the hard features of platonic rocks such as granite. The United States has large granite landforms in over 30 states.

One of the most iconic granite landform in the United State is El Capitan in Yosemite National Park. At 3,000 feet (900 meters) tall, El Capitan is the largest granite monolith in the world.

El Capitan in Yosemite National Park.
El Capitan in Yosemite National Park. Photo: Alex Demas, USGS. Public domain.

Intrusive Igneous Landforms

The United States is home to many examples of intrusive igneous landforms.

Batholiths

Batholiths are igneous rocks that have been exposed at the surface of the Earth by erosion and uplift.

Basket Dome is part of the Sierra Nevada batholith. Basket dome is a granite dome located in Yosemite National Park in the Yosemite Valley.

Basket Dome, Yosemite National Park.
Basket Dome, Yosemite National Park. Alex Demas , USGS. Public domain.

Sills and Dikes

Sills and Dikes are magma bodies that are tabular in shape and fill fractures.

Igneous dikes are formed when magma fills fractures in older rocks.

Light grey dikes visible among the dark gray, fine-grained sedimentary rocks of the Kaguyak Formation in the hills above Big River, Alaska.  Photo: Katmai National Park and Preserve, NPS, public domain.
Light grey dikes visible among the dark gray, fine-grained sedimentary rocks of the Kaguyak Formation in the hills above Big River, Alaska. Photo: Katmai National Park and Preserve, NPS, public domain.

An intruded dike located in Hance’s Creek, Grand Canyon National Park, in the Hakatai Shale (Yh) sedimentary rock strata can be seen in the photo below. The dike is made up of gabbro, which is the plutonic equivalent of basalt.

Photograph of a blocky dike of brown igneous rock that intruded into the red, layered sedimentary rock of the Hakatai Shale at Hance Creek.
Blocky dike of brown igneous rock that intruded into the red, layered sedimentary rock of the Hakatai Shale at Hance Creek, Grand Canyon National Park, NPS. Public domain photograph.

In contrast to dikes, sills follow the bedding planes. A sill, also known as sheet, is a flat igneous rock intrusion that occurs between preexisting strata of rock.

Dark colored diabase dikes intrude through light colored granite at Acadia National Park, Maine.
Dark colored diabase dikes intrude through light colored granite at Acadia National Park, Maine. Photo: NPS/Georgia Hybels, public domain.

Extrusive Rock

When magma cools as lava on or near the Earth’s surface, it forms extrusive, or volcanic, igneous rock.

Lava cools quickly when exposed to the relatively cool temperatures of the environment, limiting the amount of time mineral crystals have to form. In the end, you’re left with rocks that are almost glassy in texture.

Bubbles that form in extrusive rock due to trapped gas bubbles are called vesicles.

Types of extrusive igneous rocks are: pumice, obsidian, andesite, rhyolite, and basalt. 

Example of Rhyolite Rock

This rock was created by the slow cooling of lava as it flowed. The friction that occurs when viscous lava meets solid rock is what causes the banding. Rhyolite is a felsic extrusive rock.  As a result of the high silica content in rhyolite, the lava formed from it is exceedingly viscous which means it flows slowly.

Rhyolite banding, Pinnacles National Park, 2014.
Rhyolite banding, Pinnacles National Park, 2014. Photo: NPS, public domain.

Example of Basalt Rock

Columns of basalt rock breaking off of a cliff.

Columnar basalt, Overhanging Cliff.
Columns of basalt rock. Photo: Jim Peaco/NPS, June 1989, public domain.

Example of Pumice Rock

Flow banding of Panum pumice and obsidian of the same composition. Photo: USGS, public domain.
Flow banding of Panum pumice and obsidian of the same composition. Photo: USGS, public domain.

Example of Perlite Rock

Perlite is volcanic glass with a high water content. Bands of white perlite can be seen in this photo taken in Yellowstone National Park.

Boulders with strands of white perlite, Yellowstone National Park.
Boulders with bands of white perlite, Yellowstone National Park. Photo: NPS/RG Johnsson, public domain.

What are Some U.S. National Parks That Have Extrusive Igneous Rocks?

  • Hawai’i Volcanoes National Park, Hawai’i:
    Hawai’i Volcanoes National Park includes two of the world’s most active volcanoes, Kīlauea and Mauna Loa, and extends from sea level to 13,677 feet.
  • Sunset Crater Volcano National Monument, Arizona
  • Crater Lake National Park, Oregon

What are Large Igneous Provinces?

For geologists, the term “large igneous province” (LIP) refers to an exceptionally large accumulation of igneous rocks that form when magma moves through the crust and rises to the surface. These provinces can include both intrusive and extrusive igneous rocks.

Siletzia is a large basaltic Paleocene and Eocene igneous province that accreted to North America in the early Eocene in coastal Oregon, Washington, and southern Vancouver Island.

References

Volcanic landforms: Intrusive igneous. (n.d.). NPS.gov. https://www.nps.gov/subjects/geology/intrusive-igneous-landforms.htm

Wells, R., Bukry, D., Friedman, R., Pyle, D., Duncan, R., Haeussler, P., & Wooden, J. (2014). Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot. Geosphere10(4), 692-719. https://doi.org/10.1130/GES01018.1

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