|Volcanoes in Eruption - Set 1
||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 explosive eruptions,
lava fountains and flows, stream eruptions, and fissure eruptions from 19 volcanoes
in 13 countries. Volcano types represented in this set include strato, cinder cone,
complex, fissure vent, lava dome, shield, 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 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. The molten rock
material generated within Earth that feeds volcanoes is called magma and the storage
reservoir near the surface is called the magma chamber. 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 form lava
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,
then fall 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 (nue 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 then partially or totally collapse. The resulting
caldera may be filled by water. The volcanic structure tells us much about the nature
of the eruptions.