Volcanoes
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Long
ago there was a city called Pompeii(see ruins next) in 79 a catastrophic
volcanic eruption of Mt. Vesuvius buried the city and obscured the sun on a mild
winter afternoon (August was a winter month in the version of the
Roman calendar then in use).
The only reliable eyewitness account of the event was
recorded by
Pliny the Younger in a letter to the historian
Tacitus. Pliny saw a strange phenomenon occurring over Mt. Vesuvius: a large
dark cloud shaped rather like a pine tree emanating from the mouth of the
mountain. After some time the cloud rushed down the flanks of the mountain and
covered everything around it including the surrounding sea.
The "cloud" that Pliny the Younger wrote about is known
today as a pyroclastic flow, which is a cloud
of superheated gas, ash, and rock that erupts from a volcano. Pliny stated that
several earth tremors were felt at the time of the eruption and were followed by
a very violent shaking of the ground. He also noted that ash was falling in very
thick sheets and the village he was in had to be evacuated. Also, the sea was
sucked away and forced back by an "earthquake", a phenomenon which modern
geologists call a tsunami (a series of massive waves occurring
after seismic activity).
His description then turned to the fact that the sun was
blocked out by the eruption and the daylight hours were left in darkness. His
uncle
Pliny the Elder had already taken several ships to investigate the
phenomenon. On the other shore, Pliny the Elder apparently died from
carbon dioxide asphyxiation after lying on the ground.
So what exactly is a volcano?
A volcano is a
geological landform formed where
magma comes close to the surface of a planet. On earth, this phenomenon
tends to occur near the boundaries of the
continental plates (see, however,
hotspot volcanoes).
Like most of the interior of the Earth, the movements
and dynamics of magma is poorly understood. However, it is known that the
volcanic process begins when magma rises to near the surface deep beneath a
volcano, occupying a magma chamber. Magma in the chamber is forced upwards and
flows from a vent as
lava,
or can react with water in the surrounding landform and cause explosive
discharges of steam, escaping gases from the magma, and ejection of rocks,
cinders,
volcanic glass, and
volcanic ash.
The study of volcanoes is called vulcanology (or
volcanology in some spellings).
Formation
Most volcanoes are formed at destructive plate margins,
where oceanic crust sinks below the continental crust because oceanic crust is
denser than its continental counterpart. Friction will cause the oceanic crust
to melt, and the reduced density will force the newly formed magma to rise. As
the magma rises it pushes through the continental crust, erupting as volcanoes.
For example, Mount St. Helens is found inland from the margin between the
oceanic Juan de Fuca Plate and the continental North American Plate.
A volcano generally presents itself to the imagination
as a mountain sending forth from its summit great clouds of smoke with vast
sheets of flame, and it is not infrequently so described. The truth is that a
volcano seldom emits either smoke or flame. What is mistaken for smoke consists
of vast volumes of fine dust, mingled with steam and other vapors — chiefly
sulphurous. What appears to be flames is the glare from the erupting materials,
glowing because of their high temperature — this glare reflects off the clouds
of dust and steam, resembling fire.
Perhaps the most conspicuous part of a volcano is the
crater, a basin, roughly of a circular form, within which occurs a vent
(or vents) from which magma erupts as gases, lava, and ejecta. A crater can be
of large dimensions, and sometimes of vast depth. Very large features of this
sort are termed
calderas. Some volcanoes consist of a crater alone, with scarcely any
mountain at all; but in the majority of cases the crater is situated on top of a
mountain (the volcano), which can tower to an enormous height. Volcanoes that
terminate in a principal crater are usually of a
conical form.
Volcanic cones are usually smaller features composed of loose ash and
cinder, with occasional masses of stone which have been tossed violently into
the air by the eruptive forces (and are thus called ejecta). Within the
crater of a volcano there may be numerous cones from which vapors are
continually issuing, with occasional volleys of ashes and stones. In some
volcanoes these cones form lower down the mountain, along rift zones.
Volcano types and structural components
One way of classifying volcanoes is by the type of material erupted, which
affects the shape of the volcano:
Shield volcano: Hawaii and Iceland are examples of volcanoes which
extrude huge quantities of lava that gradually builds to form a wide mountain.
Their lava is generally very fluid and solidifies in long flows as basalt. The
largest lava shield on Earth, Mauna Loa, is 30,000 feet high (it sits on the sea
floor) and 75 miles in diameter. Olympus Mons is a shield volcano on Mars, and
the tallest mountain in the solar system.
Smaller versions of the lava shield include the Lava Dome, Cone, and Mound.
If the magma contains a lot (>65%) of silica the lava is called acidic and is
very viscous (not very fluid) and is pushed up in a blob which will then
solidify, Lassen Peak in California is an example. This type of volcano has a
tendency to explode because it easily plugs. Mt. Pelée on the island of
Martinique is another example.
If, on the other hand the magma contains relatively small (<52%) amounts of
silica, the lava is called basic, and it will be very fluid, capable of flowing
like water for long distances. A good example of this is the Great Ţjórsárhraun
(Thjórsárhraun) lava flow which was produced by an eruptive fissure almost in
the geographical center of Iceland roughly 8.000 years ago, and it flowed all
the way down to the sea, a distance of 130 kilometers, and covered an area of
800 square km.
Volcanic cones result from eruptions that throw out mostly small pieces
of rock that build up around the vent. These can be relatively short-lived
eruptions that produce a cone-shaped hill perhaps 100 to 1000 feet high.
Stratovolcanoes or composite volcanoes such as Mt. Fuji in Japan, Vesuvius in
Italy, Mount Erebus in Antarctica, and Mount Rainier in the northwestern United
States are tall conical mountains composed of both lava and rocks.
Supervolcanoes are a class of volcanoes that have a large caldera and can
potentially produce devastation on a continental scale and cause major global
weather pattern changes. Potential candidates include Yellowstone National Park
and Lake Toba, but are very hard to define given that there is no minimum
requirement to be categorized as a supervolcano.
Volcanoes are usually situated either at the boundaries between tectonic plates
or over hot spots. Volcanoes may be either dormant (having no activity) or
active (near constant expulsion and occasional eruptions), and change state
unpredictably.
Volcanoes on land often take the form of flat cones, as the expulsions build up
over the years, or in short-lived cinder cones. Under water, volcanoes often
form rather steep pillars and in due time break the ocean surface in new
islands.
Seismicity
Seismic activity (small earthquakes and tremors) always occurs as volcanoes
awaken and prepare to erupt. Some volcanoes normally have continuing low-level
seismic activity, but an increase can signify an eruption. The types of
earthquakes that occur and where they start and end are also key signs. Volcanic
seismicity has three major forms: short-period earthquakes, long-period
earthquakes, and harmonic tremor.
Short-period earthquakes are like normal fault-related earthquakes. They
are related to the fracturing of brittle rock as the magma forces its way
upward. These short-period earthquakes signify the growth of a magma body near
the surface.
Long-period earthquakes are believed to indicate increased gas pressure
in a volcano's "plumbing system." They are similar to the clanging sometimes
heard in your home's plumbing system. These oscillations are the equivalent of
acoustic vibrations in a chamber, in the context of magma chambers within the
volcanic dome.
Harmonic tremor occurs when there is sustained movement of magma below
the surface.
Patterns of seismicity are complex and often difficult to interpret. However,
increasing activity is very worrisome, especially if long-period events become
dominant and episodes of harmonic tremor appear.
In December 2000 scientists at the National Center for Prevention of Disasters
in Mexico City predicted an eruption within two days from Popocatépetl, on the
outskirts of Mexico City. Their prediction used research done by M. Chouet, a
Swiss vulacanologist, into increasing long-period oscillations as an indicator
of an imminent eruption. The government evacuated tens of thousands of people.
Forty eight hours later, bang on time, the volcano erupted spectacularly. It was
Popocatépetl's largest eruption for a thousand years and yet no one was hurt.
Gas emissions
As magma nears the surface and its pressure decreases, gases escape. This
process is much like what happens when you open a bottle of soda and carbon
dioxide escapes. Sulfur dioxide is one of the main components of volcanic gases,
and increasing amounts of it herald the arrival of more and more magma near the
surface. For example, on May 13, 1991, 500 tons of sulfur dioxide were released
from Mount Pinatubo in the Philippines. On May 28--just two weeks later--sulfur
dioxide emissions had increased to 5,000 tons, ten times the earlier amount.
Mount Pinatubo erupted on June 12, 1991. On several occasions, such as before
the Mount Pinatubo eruption, sulfur dioxide emissions have dropped to low levels
prior to eruptions. Most scientists believe that this drop in gas levels is
caused by the sealing of gas passages by hardened magma. Such an event leads to
increased pressure in the volcano's plumbing system and an increased chance of
an explosive eruption.
Ground deformation
Swelling of the volcano signals that magma has accumulated near the surface.
Scientists monitoring an active volcano will often measure the tilt of the slope
and track changes in the rate of swelling. An increased rate of
swelling--especially if accompanied by an increase in sulfur dioxide emissions
and harmonic tremors--is a high probability sign of an impending event.
Volcanic activity
There are many different kinds of volcanic activity and eruptions:
phreatic (steam) eruptions
explosive eruption of high-silica lava (e.g., rhyolite)
effusive eruption of low-silica lava (e.g., basalt)
pyroclastic flows
lahars (debris flow)
carbon dioxide emission
All of these activities can pose a hazard to humans.
Volcanic activity is often accompanied by earthquakes, hot springs, fumaroles,
mud pots and geysers. Low-magnitude earthquakes often precede eruptions.
Surprisingly, there is no consensus among volcanologists on how to define an
"active" volcano. The lifespan of a volcano can vary from months to several
million years, making such a distinction sometimes meaningless when compared to
the life spans of humans or even civilizations. For example, many of Earth's
volcanoes have erupted dozens of times in the past few thousand years but are
not currently showing signs of activity. Given the long lifespan of such
volcanoes, they are very active. By our life spans, however, they are not.
Complicating the definition are volcanoes that become restless but do not
actually erupt. Are these volcanoes active?
Scientists usually consider a volcano active if it is currently erupting or
showing signs of unrest, such as unusual earthquake activity or significant new
gas emissions. Many scientists also consider a volcano active if it has erupted
in historic time. It is important to note that the span of recorded history
differs from region to region; in the Mediterranean, recorded history reaches
back more than 3,000 years but in the Pacific Northwest of the United States, it
reaches back less than 300 years, and in Hawaii, little more than 200 years.
Dormant volcanoes are those that are not currently active (as defined above),
but could become restless or erupt again.
Extinct volcanoes are those that scientists consider unlikely to erupt again.
Whether a volcano is truly extinct is often difficult to determine. For example,
since calderas have life spans sometimes measured in millions of years, a
caldera that has not produced an eruption in tens of thousands of years is
likely to be considered dormant instead of extinct. Yellowstone caldera in
Yellowstone
National Park is at least 2 million years old and hasn't erupted for 70,000
years, yet scientists do not consider Yellowstone as extinct. In fact, because
the caldera has frequent earthquakes, a very active geothermal system, and rapid
rates of ground uplift, many scientists consider it to be a very active volcano.
Pyroclastic flows are a
common and devastating result of some volcanic eruptions. They are fast moving
fluidized bodies of hot gas, ash and rock (collectively known as tephra) which
can travel away from the vent at up to 150 km/h. The gas is usually at a
temperature of 100-800 degrees Celsius. The flows normally hug the ground and
travel downhill under gravity, their speed depending upon the gradient of the
slope and the size of the flow.
Volumes range from a few hundred cubic meters to more than a thousand cubic
kilometers, and the larger ones can travel for hundreds of kilometer although
none have occurred for several hundred thousand years. Most flows are around one
to ten cubic kilometers and travel for several kilometers. Flows usually consist
of two parts - the basal flow hugs the ground and contains larger, coarse
boulders and rock fragments, whilst an ash cloud rises above it because of the
turbulence between the flow and the overlying air.
Whilst moving, the kinetic energy of the boulders will flatten trees and
buildings in their path. The hot gases and high speed make them particularly
lethal.
From Wikipedia, the free
encyclopedia.
Hawaii Volcanoes by Gayle Olson
The Island of Hawaii is made up of five volcanoes. Each
began to
grow beneath the sea and eventually joined to form a single island. The
volcanoes grew from a hotspot beneath the sea. The chain of volcanoes were
created as the Pacific Plate moved northwestward over the hotspot. Kohala was
the first to emerge from the sea followed by Mauna Kea, Mauna Loa, Kilauea, and
Loihi. Activity has ceased at Kohala and now the most activity has been
occurring at Kilauea and Loihi.
Loihi is still beneath sea level about 20 miles off the southeast flank of the
island. A scientific team has installed underwater cable to Loihi. On October
12, 1997, the cable was laid and scientists discovered that Loihi was in full
eruption. The communication cable provides a direct connection to Loihi.
In the past, instrument experiments had to be dropped off at the seamount and
later physically picked up by a submersible research vessel to recover the data.
What sort of results have scientists received from the communication cable? An
underwater microphone called a hydrophone is sending back sounds of popping,
booming and crashing. This may be the first real-time monitoring of an undersea
eruption. Once the cable was installed and tested, scientists linked up the
communication system to carry data first to the Hawaiian Volcano Observatory and
then to Oahu where it is available for researchers.
Entire article and related Internet resources:
http://scienceforfamilies.allinfo-about.com/features/hawaiivolcanoes.html
About the author:
Author and Internet Content Developer since 1995.
