What are igneous rocks?

Igneous rocks (from the Greek word for fire) form from when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.

Intrusive Igneous Rocks:
Intrusive, or plutonic, igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth's surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies. Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture.

Extrusive Igneous Rocks:
Extrusive, or volcanic, igneous rock is produced when magma exits and cools above (or very near) the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture.

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Geologist Jamey Jones studying and preparing to collect samples of igneous rocks in Lake Clark National Park
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The photo was taken by Erin Todd during helicopter-supported geologic fieldwork in Lake Clark National Park this past summer. The project is funded by the USGS Mineral Resources Program that is focused on investigating the bedrock geology of the national park and surrounding areas through geologic mapping and supporting analytical work such as geochemistry and geochronology. In the photo, Jamey Jones is studying and preparing to collect samples of igneous rocks that make up the core of the northern Aleutian Range in the eastern part of the park. The rocks are Jurassic in age (approximately 180 to 160 million years old) and provide important clues about the geologic evolution of south-central Alaska and the formation and distribution of mineral resources in the region

Close up of Graphite
March 23, 2017

Mineral: Graphite (C) in pegmatite rock
Mineral Origin: Ticonderoga, NY
Primary Mineral Commodity: Graphite
Mineral Commodity Uses: brake linings, foundry operations, heat-resistant lubricants, refractory applications, and steelmaking

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A pink granite monument at the Lyndon Baines Johnson Memorial Grove on the Potomac. It was hewn from a quarry in the Texas Hill Country, where Johnson was born and raised.

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A granite cliff, looking east into the Gulf of Maine, at Thunder Hole in Acadia National Park

Image: Gabbro at Saugus Iron Works
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A specimen of gabbro, an igneous mineral that was used as a purifying agent in the iron smelting process at the Saugus Iron Works. 

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