pahoehoe


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lava

lava (läˈvə), molten rock that erupts on the earth's surface, either on land or under the ocean, by a volcano or through a fissure. It solidifies into igneous rock that is also called lava. Before reaching the earth's surface, the mixture of solid and liquid rock, and gases, is known as magma. Lavas are composed chiefly of silica and the oxides of aluminum, iron, magnesium, calcium, sodium, and potassium. Silica, with soda and potash, predominates in the light-colored, acid felsites; iron oxides, lime, and magnesia, in the dark-colored, basic basalts. Rock froth forms on the upper part of a lava flow if bubbles solidify before the gas can escape. Light-colored, glassy froth is pumice; dark, cindery or slaggy froth, of a coarser texture than pumice, forms what is known as scoriae. Lava flows which solidify as a mass of blocks and fragments with a rough surface are called block lava, or aa; those which solidify with a smooth, ropy, billowy surface are known as corded lava, or pahoehoe. Lava can sometimes cover wide regions through great fissures in the earth's surface, as in the ancient Columbia River plateau of the NW United States, where it is spread over 30,000 sq mi (77,700 sq km) and is up to 5,000 ft (1,524 m) deep. Other such regions are found in the Deccan plateau of India, in E Brazil, and in Iceland. Submarine lavas develop through volcanic activity along the mid-oceanic ridges and plate boundaries, where the mid-oceanic ridges produce more lava than any continental eruptions. Such underwater eruptions also harbor rich fauna unique to the vent area, such as red tube worms and giant clams, whose food supply is based on the hydrogen sulfide abundant in the vent waters. Unique features include black smokers, or hot springs of mineral-rich water that belch out from the ocean ridge where it is most active. In many instances the reasons for the heat and liquidity of magma, its exact source, and the causes of its rise in the earth are not clearly known, though the volcanic activity is often related to seafloor spreading. Other volcanic areas also lie along colliding plate boundaries and around hot spots believed to result from a plume of hot magma rising from the core-mantle boundary. See plate tectonics.
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pahoehoe

[pə′hō·ē‚hō·ē]
(geology)
A type of lava flow whose surface is glassy, smooth, and undulating; the lava is basaltic, glassy, and porous. Also known as ropy lava.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The quarry operators at Wagholi have sought to exploit the aa sill as intensively as possible while not wasting money and time on the underlying pahoehoe unit.
Pahoehoe lava flows appear to look like shiny, smooth ribbons of cake batter.
Pahoehoe is the hottest and most fluid of any type of lava; it has been compared to hot fudge poured from a pan.
This hypothesis is supported by data of Chadwick and colleagues who observed changes in soil mineralogy across a steep precipitation gradient on a 170 000-yr-old pahoehoe flow of Hawi Volcanics (Chadwick et al.
High temperature pahoehoe flows near vents have smooth "ropey" surfaces (Williams and McBirney, 1979, pp.
Some poorly crystallized specimens have a habit resembling ropy lava or pahoehoe, with well-developed crystals extruding out along the ridges of the "flow."
Hawaiian volcanoes produce two kinds of lava, a smooth lava called Pahoehoe (pronounced pah HOH ee hoh ee) and a rough form called `A'a (pronounced AH ah).
Fountaingrass establishes locally in depressions in the pahoehoe lava where well-drained lithosolic soil has developed, and along small cracks and fissures.
Photo: Orange "toes' of pahoehoe lava ooze past ranger (yellow shirt) and visitor where flows cross Chain of Craters Road
Emplacement and inflation of pahoehoe sheet flows: observations and measurements of active lava flows on Kilauea Volcano, Hawaii.