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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.
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.
Fissure 21 pāhoehoe lava flows ooze onto Kaupili Street, in the Leilani Estates Subdivision. Video 1 shows the flows on May 25, around 2:15 AM, HST. Video 2 shows lava oozing over a berm on May 24, around 6:00 PM, HST. This pāhoehoe flow advances as continuous lava outbreaks from beneath its cooled crust
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...
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 close-up view showing the ropy texture that forms on the surface of pāhoehoe lava.
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.
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This outcrop is exposed in a rock quarry in Chalfant Valley about 25 km (15 mi) southwest of Long Valley Caldera. The two main units of the Bishop Tuff deposit are visible here: (1) the lower 5 m (16 f) of the section consists of the pumice that fell to the ground (airfall pumice) downwind from the eruption; and (2) the upper 5-6 m (16-20 f) of the section consists of the...