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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How did Half Dome, the massive rock monument in Yosemite National Park, acquire its unique shape?
At the head of the valley in Yosemite National Park - as if on a pedestal - stands Half Dome, the most colossal and recognizable rock monument in the Sierra Nevada, smoothly rounded on three sides and a sheer vertical face on the fourth. Half Dome, which stands nearly 8,800 feet (2,682 meters) above sea level, is composed
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Is glacier ice a type of rock?
Yes – glacier ice, like granite, is a type of rock. Glacier ice is actually a mono-mineralic rock (a rock made of only one mineral, like limestone which is composed of the mineral calcite). The mineral ice is the crystalline form of water (H2O). It forms through the metamorphism of tens of thousands of individual
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What are sedimentary rocks?
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock
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Are there geologic maps or publications for where I live?
Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database.
Some geologic maps can be purchased in hard copy through
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Where can I find information about the geology and natural history of many National Parks?
Our National Parks are the showcases of our nation's geological heritage. The websites for most individual parks have detailed information about their geology and natural history.
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What is the difference between a rock and a mineral?
A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties.
A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral matter.
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What are metamorphic rocks?
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form
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Bedrock Map Debuts for the Granite State
As fall foliage begins to blanket New Hampshire, pleasantly diverting the attention of residents and visitors, scientists are preparing to unveil some of the geologic secrets of the famous yet not-well-known rocks that lie beneath the fiery cover.
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
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
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.
A granite cliff, looking east into the Gulf of Maine, at Thunder Hole in Acadia National Park
A specimen of gabbro, an igneous mineral that was used as a purifying agent in the iron smelting process at the Saugus Iron Works.
This photo shows the ropey texture of a recently crusted pahoehoe flow, as well as its superior strength as a natural insulator. The crack is still glowing hot but the lava is no longer moving under the crust.
