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Volcano
A volcano is generally a conical shaped hill or mountain built by accumulations of lava
flows, tephra, and volcanic ash. About 95% of active volcanoes occur at the plate subduction
zones and at the mid-oceanic ridges. The other 5% occur in areas associated with lithospheric
hot spots. These hot spots have no direct relationships with areas of crustal creation or
subduction zones. It is believed that hot spots are caused by plumes of rising magma that
have their origin within the asthenosphere. Over the last 2 million years, volcanoes have been
depositing lava, tephra, and ash in particular areas of the globe. These areas occur at hot
spots, rift zones, and along plate boundaries where tectonic subduction is taking place.
Not all volcanoes are the same. Geologists have classified five different types of volcanoes.
This classification is based on the geomorphic form, magma chemistry, and the explosiveness
of the eruption. The least explosive type of volcano is called a basalt plateau. These
volcanoes produce a very fluid basaltic magma with horizontal flows as shown in Fig 3.4.
The form of these volcanoes is flat to gently sloping and they can occupy an area from
100,000 to 1,000,000 square kilometres. Deposits of these volcanoes can be as thick as 1800
meters. Large basalt plateaus are found in the Columbia River Plateau, western India,
northern Australia, Iceland, Brazil, Argentina, and Antarctica. Some basaltic magmas can
produce very large slightly sloping volcanoes, 6 to 12°, that have gently flowing magmas
called shield volcanoes. Shield volcanoes can be up to 9000 meters tall. The volcanoes of the
Hawaiian Islands are typical of this type. Extruded materials from this type of volcano mainly
consist of low viscosity basaltic lava flows.
Fig. 3.4: Low viscosity basaltic lava flow
a. A cinder cone is a small volcano, between 100 and 400 meters tall, made up of
exploded rock blasted out of a central vent at a high velocity. These volcanoes
develop from magma of basaltic to intermediate composition (andesite). They form
Lecture Delivered by: Dr. Shahnawaz Ah. Baba
Volcano
when large amounts of gas accumulate within rising magma. Examples of cider cones
include Little Lake Volcano in California and Paricutin Volcano in Mexico (Fig. 3.5
).
Fig. 3.5: Cinder cone volcano
b. Composite volcanoes are made from alternate layers of lava flows and exploded
rock. Their height ranges from 100 to 3500 meters tall. The chemistry of the magma
of these volcanoes is quite variable ranging from basalt to granite. Magmas that are
more granitic tend to be very explosive because of their relatively higher water
content. Water at high temperatures and pressures is extremely volatile. Examples of
composite volcanoes include Italy's Vesuvius, Japan's Mount Fuji, and Washington
State's Mount Rainier and Mount St. Helens.
c.
Caldera the most explosive type of volcano is the caldera, The cataclysmic explosion
of these volcanoes leaves a huge circular depression at the Earth's surface. This
depression is usually less than 40 kilometres in diameter. These volcanoes form when
"wet" granitic magma quickly rises to the surface of the Earth. When it gets to within
a few kilometres of the surface the top of the magma cools to form a dome. Beneath
this dome the gaseous water in the magma creates extreme pressures because of
expansion. When the pressure becomes too great the dome and magma are sent into
the Earth's atmosphere in a tremendous explosion. On the island of Krakatau, a
caldera type volcano exploded in 1883 ejecting 75 cubic kilometres of material in the
air and left a depression in the ground some 7 kilometres in diameter.
Effects of volcanoes
Lecture Delivered by: Dr. Shahnawaz Ah. Baba
Volcano
There are many different types of volcanic eruptions and associated activity: phreatic
eruptions (steam-generated eruptions), explosive eruption of high-silica lava (e.g., rhyolite),
effusive eruption of low-silica lava (e.g., basalt), pyroclastic flows, lahars (debris flow) and
carbon dioxide emission (Fig.3.6). All of these activities can pose a hazard to humans.
Earthquakes, hot springs, fumaroles, mud pots and geysers often accompany volcanic
activity.
Fig.3.6: Volcanic effects
1. Large, explosive volcanic eruptions inject water vapour (H2O), carbon dioxide (CO2),
sulphur dioxide (SO2), hydrogen chloride (HCl), hydrogen fluoride (HF) and ash
(pulverized rock and pumice) into the stratosphere to heights of 16–32 kilometres (10–
20 mi) above the Earth's surface.
2. The most significant impacts from these injections come from the conversion of sulphur
dioxide to sulphuric acid (H2SO4), which condenses rapidly in the stratosphere to form
fine sulphate aerosols. The aerosols increase the Earth's albedo—its reflection of radiation
from the Sun back into space – and thus cool the Earth's lower atmosphere or
troposphere; however, they also absorb heat radiated up from the Earth, thereby warming
the stratosphere.
3. The sulphate aerosols also promote complex chemical reactions on their surfaces that
alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with
increased stratospheric chlorine levels from chlorofluorocarbon pollution, generates
chlorine monoxide (ClO), which destroys ozone (O3). As the aerosols grow and coagulate,
they settle down into the upper troposphere where they serve as nuclei for cirrus clouds
and further modify the Earth's radiation balance
Lecture Delivered by: Dr. Shahnawaz Ah. Baba
Volcano
4. Gas emissions from volcanoes are a natural contributor to acid rain. Volcanic activity
releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon
dioxide each year.
5. Ash thrown into the air by eruptions can present a hazard to aircraft, especially jet aircraft
where the particles can be melted by the high operating temperature; the melted particles
then adhere to the turbine blades and alter their shape, disrupting the operation of the
turbine
Lecture Delivered by: Dr. Shahnawaz Ah. Baba