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Volcanoes in Eruption - Set 2
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The word volcano is used to refer to the opening from which molten rock and gas issue
from Earth's interior onto the surface, and also to the cone, hill, or mountain built
up around the opening by the eruptive products. This slide set depicts ash clouds,
fire fountains, lava flows, spatter cones, glowing avalanches, and steam eruptions
from 18 volcanoes in 13 countries. Volcano types include strato, cinder cone, basaltic
shield, complex, and island-forming. Perhaps no force of nature arouses more awe and
wonder than that of a volcanic eruption. Volcanoes can be ruthless destroyers. Primitive
people offered sacrifices to stem the tide of such eruptions and many of their legends
were centered around volcanic activity. Volcanoes are also benefactors. Volcanic processes
have liberated gases of the atmosphere and water in our lakes and oceans from the
rocks deep beneath Earth's surface. The fertility of the soil is greatly enhanced
by volcanic eruptive products. Land masses such as islands and large sections of continents
may owe their existence entirely to volcanic activity. The "volcano" is used to refer
to the opening from which molten rock and gas issue from Earth's interior onto the
surface, and also to the cone, hill, or mountain built up around the opening by the
eruptive products. The molten rock material generated within Earth that feeds volcanoes
is called magma and the storage reservoir near the surface is called the magmachamber.
Eruptive products include lava (fluid rock material) and pyroclastics or tephra (fragmentary
solid or liquid rock material). Tephra includes volcanic ash, lapilli (fragments between
2 and 64 mm), blocks, and bombs. Low viscosity lava can spread great distances from
the vent. Higher viscosity produces thicker lava flows that cover less area. Lava
may formlava lakes of fluid rock in summit craters or in pit craters on the flanks
of shield volcanoes. When the lava issues vertically from a central vent or a fissure
in a rhythmic, jet-like eruption, it produces a lava fountain. Pyroclastic (fire-broken)
rocks and rock fragments are products of explosive eruptions. These may be ejected
more or less vertically, thenfall back to Earth in the form of ash fall deposits.
Pyroclastic flows result when the eruptive fragments follow the contours of the volcano
and surrounding terrain. They are of three main types: glowing ash clouds, ash flows,
and mudflows. A glowing ash cloud (nuee ardente) consists of an avalanche of incandescent
volcanic fragments suspended on a cushion of air or expanding volcanic gas. This cloud
forms from the collapse of a vertical ash eruption, from a directed blast, or is the
result of the disintegration of a lava dome. Temperatures in the glowing cloud can
reach 1,000 deg C and velocities of 150 km per hour. Ash flows resemble glowing ash
clouds; however, their temperatures are much lower. Mudflows (lahars) consist of solid
volcanic rock fragments held in water suspension. Some may be hot, but most occur
as cold flows. They may reach speeds of 92 km per hour and extend to distances of
several tens of kilometers. Large snow-covered volcanoes that erupt explosively are
the principal sources of mud flows. Explosions can give rise to air shock waves and
base surges. Air shock waves are generated as a result of the explosive introduction
of volcanic ejecta into the atmosphere. A base surge may carry air, water, and solid
debris outward from the volcano at the base of the vertical explosion column. Volcanic
structures can take many forms. A few of the smaller structures built directly around
vents include cinder, spatter, and lava cones. Thick lavas may pile up over their
vents to form lava domes. Larger structures produced by low viscosity lava flows include
lava plains and gently sloping cones known as a shield volcanoes. A stratovolcano
(also known as a composite volcano) is built of successive layers of ash and lava.
A volcano may consist of two or more cones side by side and is referred to as compound
or complex. Sometimes a violent eruption will partially empty the underground reservoir
of magma. The roof of the magma chamber may thenpartially or totally collapse. The
resulting caldera may be filled by water. The volcanic structure tells us much about
the nature of the eruptions.
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