volcano

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An opening in the earth's crust from which lava, ash, and hot gases flow or are ejected during an eruption.
• noun A similar opening on the surface of another planet.
• noun A usually cone-shaped mountain formed from the materials issuing from such an opening.

from The Century Dictionary.

• noun A mountain or other elevation having at or near its apex an opening in the earth's crust from which heated materials are expelled either continuously or at regular or irregular intervals.
• noun A kind of firework. See fizgig, See submarine.
• noun Volcanoes originate either in the development of a flssure in the earth's crust which releases pent-up forces within, or in the bursting of these accumulated forces through a less elongated passage and the consequent establishment of the vent. Once released these forces build up about the vent a conical heap of ejected materials which in the end may yield a mountain of great altitude and extent. Cones usually consist both of loose materials and of solid flows and dikes which have come forth as molten rock; but some cones are known which are almost if not entirely the former (cinder cones, tuff cones), and others which are chiefly the latter. The loose materials roll down from the rim both outwardly and inwardly and eventually establish themselves at their angles of repose. Thus the cross-section of a cone exhibits layers of which the outer dip away from the vent and the inner toward it. The coarser fragments are necessarily nearer the vent, and yield agglomerates and breccias. The finer materials (tuffs) lie farther out and sift down at flatter angles until gradually the slope dies out in the general surrounding level. Around the immediate vent there is thus developed a space like an inverted cone, or a bowl, the crater, which is prolonged downward in the vent or chimney, the whole being funnel-shaped in outline. The upper edge of the annular mountain surrounding the crater is called the rim; the outer portions are the slopes or flanks. The loose materials are also much carved and modified by the rains, but where they predominate they yield the symmetrical volcanic peaks such as Fuji-yama in Japan. When outbreaks of molten rock (lava) are superadded to the fragmental materials, they seldom pour out over the rim of the crater, but burst through the flanks. If they enter cracks and congeal in them, they furnish dikes, which serve like ribs to stiffen the loose materials. If they pour forth as a flood down the sides they furnish surface flows or sheets. All these become afterward buried in later outbreaks of fragmental materials until the structure of the cone is very complex. The activity of Mont Pelée, in Martinique, in 1902, was at first explosive but by March, 1903, a columnar mass of hot rock had been protruded as a great spine 500 meters above the vent, evidently starting below as a viscous mass, cooled as it emerged until it practically yielded a solid eruption called a pelélith. It disintegrated in the course of months and fell away. When lava enters very largely into the materials of the mountain, the outline is affected in a notable degree. Some lavas which have high percentages of silica (rhyolites, trachytes) are relatively infusible and are at most ropy and viscous. They well up and congeal with steep slopes and do not move far from the vent. Others which have low percentages (basalts) are very fusible and flow like water for miles. The Hawaiian cones are good examples of the latter and in consequence have very flat slopes; whereas in the Auvergne the ‘puys,’ which belong under the former, are very steep and may have no crater at all. Volcanic vents break out through the floor of the ocean (submarine eruptions) no less than on the land, and are a fruitful cause of oceanic islands. The activity of the cones is variable, and on the basis of this they are classified under several types as follows: continuously active but of corresponding moderation; intermittently active, with quiescent periods of relatively short duration and with outbreaks of notable but not maximum violence; intermittently active, with long periods of rest, followed by excessively violent eruptions. Volcanoes exhibit a marked linear distribution upon the earth's surface, and they favor continental borders more than the interiors. The greatest series of vents encircles the Pacific Ocean and reaches its culmination in Java. A location near the sea is, however, not essential, as was once the prevailing opinion, since the great Mexican cones are on the central plateau, and Kilimanjaro, an active volcano, has been discovered in Africa. Volcanic areas have shifted from time to time. Old and long extinct centers may be detected, as in Maine and southeastern Pennsylvania, which were active before the Cambrian period, while the Hebrides were the scene of enormous outbreaks in the Tertiary. The cause of volcanoes is very obscure. They are obviously connected with the internal heat of the earth. Some refer this to heat still retained from the early nebulous condition of the earth (nebular hypothesis); others to heat produced by mechanical pressure in a globe of accumulating small, cold particles (planetesimal hypothesis); while still others are increasingly inclined to look with favor upon radioactive phenomena below the surface. The localized outbreaks have been referred to contractions of the crust through loss of heat; to readjustments from the shifting of sediments; and to strains caused by the attractions of the sun and moon when in positions favorable to deformation of the globe. In a vent once established there is reason to think that the last named causes affect the periodicity of eruptions. As volcanic activity expires many important after effects are manifested, such as fumaroles, solfataras, hot springs, geysere, and the formation of mineral deposits.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geol.) A mountain or hill, usually more or less conical in form, from which lava, cinders, steam, sulphur gases, and the like, are ejected; -- often popularly called a burning mountain.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A vent or fissure on the surface of a planet (usually in a mountainous form) with a magma chamber attached to the mantle of a planet or moon, periodically erupting forth lava and volcanic gases onto the surface.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a fissure in the earth's crust (or in the surface of some other planet) through which molten lava and gases erupt
• noun a mountain formed by volcanic material