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basalt

From The Columbia Encyclopedia
(bӘsôlt', băs'ôlt), fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state. Basaltic magma is derived by partial melting of the peridotite that is found in the asthenosphere which reaches the mid-ocean ridges, such as the Mid-Atlantic Ridge, and forms the new oceanic crust, the uppermost layer of the lithosphere . Because molten basalt is lighter than peridotite, it rises more rapidly. Basaltic magmas contain around 50% silica; they are the most common extrusive rocks and comprise more than 90% of all volcanic rock. It forms mostly lava flows, including present-day Hawaiian flows, and the ancient Columbia River plateau of the NW United States. Basalt dominates the mid-ocean islands and surrounding regions of the Hawaiian Islands and Iceland, as found by samples of lava flows found in drill cores recovered by vessels of the Deep Sea Drilling Project and the now defunct…
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Full text Article basalt

From Britannica Concise Encyclopedia
Dark igneous rock that is low in silica content and comparatively rich in iron and magnesium. Some basalts are glassy (have no visible crystals), and many are very fine-grained and compact. Basaltic lavas may be spongy or pumice-like. Olivine and augite are the most common minerals in basalts; …
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Full text Article basalt

From The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide
Commonest volcanic igneous rock in the Solar System. Basalt is an extrusive rock , created by the outpouring of volcanic magma. The magma cools quickly, allowing only small crystals to form. Much of the surfaces of the terrestrial planets Mercury, Venus, Earth, and Mars, as well as the Moon, are…
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Full text Article Basalt

From Guide to Minerals, Rocks and Fossils Full text Article Igneous rocks
Columnar jointing in basalt
Color When fresh it is black or greyish black; often weathers to a reddish or greenish crust. Grain size Fine. Texture Usually dense with no minerals identifiable in hand specimen; a freshly broken surface is dull in appearance. May be porphyritic. Structure Often vesicular and/or amygdaloidal. …
| 343 words , 6 images
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Full text Article basalt

From The Columbia Encyclopedia
(bӘsôlt', băs'ôlt), fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state. Basaltic magma is derived by partial melting of the peridotite that is found in the asthenosphere which…
| 431 words
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Full text Article basalt

From The American Heritage(R) Dictionary of the English Language
basalt columnar basalt at Sheepeater Cliff,...
A hard, dense, dark igneous rock composed chiefly of plagioclase feldspar, pyroxene, and olivine, and often having a glassy appearance. A kind of hard unglazed pottery. [From New Latin basaltēn, a misreading (in manuscripts of Pliny) of Latin basanītēn, accusative of basanītēs, a word Pliny uses of…
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Full text Article basalt

From Philip's Encyclopedia
Hard, fine-grained, basic igneous rock , which may be intrusive or extrusive. Its colour can be dark green, brown, dark grey or black. It can have a glassy appearance. There are many types of basalt with different proportions of elements. It may be compact or vesicular (porous) because of gas…
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Maps of volcanic fields on Earth and...
Abstract Basaltic volcanic fields consist of one or more volcanoes within a defined area that is separate from other volcanic areas and that has an internally consistent tectonic/structural setting. Most of the volcanoes are monogenetic, each having only a single eruptive episode lasting from weeks…
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Large igneous provinces (LIPs) of the...
Abstract Large igneous provinces (LIPs) represent huge volumes of erupted and intruded magma and are exceptional volcanic events in Earth history. They were erupted over a brief period of geologic time. The volume of magma emitted during each individual eruption that makes up an LIP (frequently 10 3…
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Full text Article basalt

From The American Heritage Student Science Dictionary
basalt
basalt columns at Yellowstone National...
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Full text Article basalt

From The Macquarie Dictionary
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