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Central volcanoes have a single vertical lava pipe and tend to develop a conical profile. The volcanic cone is generally built up of a succession of lavas, ignimbrites, and welded tuffs (porous rock formed by the cementation of solidified volcanic ash and dust particles). Lava flows from the throat of a central volcano following the easiest path downhill, its flow pattern strongly influenced by the topography.
The shape of any given volcanic landform depends on a variety of local circumstances and on the relative abundances of lavas, tuffs, and ignimbrites. This in turn depends on the composition of the magma arriving at the surface. The lower the viscosity, the more readily the lava flows away from the throat or fissure. There is, as a consequence, relatively little tendency to build up a steep-sided cone. The more viscous the magma, however, the greater the tendency to chill and solidify close to the source and to form a cone. In many cases, highly viscous lava also may clog the throat of the volcano, causing a pressure buildup that can only be relieved by violent explosions and nuées ardentes. Such eruptions, exemplified by those of Mount St. Helens in southwestern Washington state during the early 1980s and Vesuvius in AD 79, may completely remove the top of a volcanic cone and occasionally part of the interior of the cone as well. The resultant roughly circular hollow is called a caldera. Further eruption may lead to the formation of a lava lake within the cone, and if the lava cools and solidifies, the inward drainage of rainwater may produce a water lake on the surface of the lava lake. A caldera may also form without an explosion by the collapse of the top of the cone into an underlying accumulation of magma. Kilauea on southeastern Hawaii Island is an excellent example of a large volcanic cone with a well-developed caldera produced by collapse. |
Volcanic
eruptions are perhaps the most spectacular of our planet's
extreme behaviour. They are the crucibles of creation. There
are 550 known active volcanoes, with 500 million people
living close to them. Up to two dozen of these could erupt
at any moment.Fissure volcanoes are much more common than
those of the central type. They occur along fractures in the
crust and may extend for many kilometres. Lava, usually of
basaltic composition, is ejected relatively quietly and
continuously from the fissures and forms enormous plains or
plateaus of volcanic rock. Submarine fissure eruptions are
common along the crests of mid-ocean ridges and are pivotal
in seafloor spreading (see seafloor spreading hypothesis).
When molten rock is extruded under water, pillow lava--piles
of sack-shaped rock masses measuring up to several metres in
diameter--are often formed.