VOLCANO
A volcano is a mountain where lava (hot,
liquid rock) comes from a magma chamber under the ground. Most volcanoes have a
volcanic crater at the top. When a volcano is active, materials come out of it.
The materials include lava, steam, gaseous sulfur compounds, ash and broken
rock pieces.When there is enough pressure, the volcano erupts. Some volcanic
eruptions blow off the top of the volcano. The magma comes out, sometimes
quickly and sometimes slowly. Some eruptions come out at a side instead of the
top.
Volcanoes
are found on planets other than Earth. An example is Olympus Mons on Mars
Types of volcanoes
Shield
volcanoes are built out
of layers of lava from continual
eruptions (without explosions). Because the lava is so fluid, it spreads out,
often over a wide area. Shield volcanoes do not grow to a great height, and the
layers of lava spread out to give the volcano gently sloping
sides. Shield
volcanoes can produce huge areas of basalt, which is
usually what lava is when cooled.
The base of the
volcano increases in size over successive eruptions where solidified lava
spreads out and accumulates. Some of the world's largest volcanoes are shield
volcanoes.Even though their sides are not very steep, shield volcanoes can be huge. Mauna Kea in Hawaii is the biggest mountain on Earth if it is measured from its base on the floor of the sea.
Strato volcanoes
Mount Fuji, an active strato volcano in Japan that last erupted in 1707–08
A strato
volcano, also known as
a composite volcano is a tall, conical volcano. It is
built up of many layers of hardened lava, tephra, pumice, and volcanic
ash.
Unlike shield
volcanoes, strato volcanoes have a steep profile and periodic eruptions. The
lava that flows from strato volcanoes cools and hardens before spreading far.
It is sticky, that is, it has high viscosity. The magma forming this lava is often felsic, with
high-to-intermediate levels of silica, and less mafic magma. Big
felsic lava flows are uncommon, but have travelled as far as 15 km
(9.3 mi)
Two famous
strato volcanoes are Japan's Mount
Fuji, and Vesuvius. Both have big
bases and steep sides that get steeper and steeper as it goes near the top.
Vesuvius is famous for its destruction of the towns Pompeii and Herculaneum in 79 AD,
killing thousands.
Shield volcano
When magma is
very hot and runny, gases can escape and eruptions are gentle with considerable
amounts of magma reaching the surface to form lava flows. Shield volcanoes have
a broad, flattened dome-like shape created by layers of runny lava flowing over
its surface and cooling. Because the lava flows easily, it can move down
gradual slopes over great distances from the volcanic vents. The lava flows are
sufficiently slow for humans to outrun or outwalk them. This type of magma has
a temperature between 800°C and 1200°C and is called basaltic magma.
Caldera
A caldera is a
basin-like feature formed by collapse of land after a volcanic eruption. This
happens after a huge strato volcano blows its top off. The base of the crater
then sinks, leaving a caldera where the top of the volcano was before. Krakatoa, best known
for its catastrophic eruption in 1883, is much smaller now
A traditional way to classify or
identify volcanoes is by its pattern of eruptions. Those volcanoes which may
erupt again at any time are called active. Those that are now quiet called
dormant (inactive). Those volcanoes which have not erupted in historical times
are called extinct.
a) Active :-An active volcano is currently erupting, or it has erupted in the last 10,000 years. An example of an active volcano is Mount St. Helens in the United States (US).
b) Dormant :-
A dormant
volcano is "sleeping," but it could awaken in the future. Mount
Rainier in the United States is considered dormant.
c) Extinct :-
An extinct
volcano has not erupted in the past 10,000 years. Edinburgh Castle in Scotland
is located on top of an extinct volcano.
Types
of Volcanoes
Most people have never seen a real
volcano but have learned about them through movies or books. So when most
people think of a volcano, they usually conjure up the Hollywood version: a
huge, menacing conical mountain that explodes and spews out masses of lava
which falls on rampaging dinosaurs, screaming cave people, or fleeing mobs of
betogaed Romans--depending on their favorite volcano disaster movie. While
those types of volcanoes do indeed exist, they represent only one
"species" in a veritable zoo of volcano shapes and sizes.
Some types of volcanoes are easily
recognizable and some are not. The "Hollywood" types are easily
recognized. Many are located in populated areas and have well-known names:
Vesuvius, Krakatoa, Fujiyama, and Mount St. Helens. These volcanoes are
typically tens of miles across and ten thousand or more feet in height. As
illustrated in the figure above, they have moderately steep sides and sometimes
have small craters in their summits. Volcanologists call these
"strato-" or composite volcanoes because they consist of layers of
solid lava flows mixed with layers of sand- or gravel-like volcanic rock called
cinders or volcanic ash.
Another easily recognized type of
volcano (seen at right) is the "cinder cone." As you might expect
from the name, these volcanoes consist almost entirely of loose, grainy cinders
and almost no lava. They are small volcanoes, usually only about a mile across
and up to about a thousand feet high. They have very steep sides and usually
have a small crater on top.
A third easily recognized volcano
may be familiar to you from news reports from Hawaii: the "shield"
volcano. This type of volcano can be hundreds of miles across and many tens of
thousands of feet high. The individual islands of the state of Hawaii are
simply large shield volcanoes. Mauna Loa, a shield volcano on the
"big" island of Hawaii, is the largest single mountain in the world,
rising over 30,000 feet above the ocean floor and reaching almost 100 miles
across at its base. Shield volcanoes havelow slopes and consist almost entirely
of frozen lavas. They almost always have large craters at their summits.
Geophysical hazards: volcanic eruptions
Definition and
characteristics
volcanic eruptions happen when lava and gas are discharged from a volcanic vent. The most common consequences of this are population movements as large numbers of people are often forced to flee the moving lava flow. Volcanic eruptions often cause temporary food shortages and volcanic ash landslides called Lahar.
The most dangerous type of volcanic eruption is referred to as a 'glowing avalanche'. This is when freshly erupted magma forms hot pyroclastic flow which has temperatures of up to 1,200 degrees. The pyroclastic flow is formed from rock fragments following a volcanic explosion, the flow surges down the flanks of the volcano at speeds of up to several hundred kilometres per hour, to distances often up to 10km and occasionally as far as 40 km from the original disaster site.
volcanic eruptions happen when lava and gas are discharged from a volcanic vent. The most common consequences of this are population movements as large numbers of people are often forced to flee the moving lava flow. Volcanic eruptions often cause temporary food shortages and volcanic ash landslides called Lahar.
The most dangerous type of volcanic eruption is referred to as a 'glowing avalanche'. This is when freshly erupted magma forms hot pyroclastic flow which has temperatures of up to 1,200 degrees. The pyroclastic flow is formed from rock fragments following a volcanic explosion, the flow surges down the flanks of the volcano at speeds of up to several hundred kilometres per hour, to distances often up to 10km and occasionally as far as 40 km from the original disaster site.
The
International Federation response adjusts to meet the needs of each specific
circumstance. As population
movement is often a
consequence, the provision of safe areas, shelter, water, food and health
supplies are primordial. In general response prioritizes temporary shelter
materials; safe water and basic sanitation; food supplies; and the short term
provision of basic health services and supplies.
Effects
of Volcanoes
Volcanic
hazards
Volcanic
hazards include explosions, lava flows, bombs or ballistics, ash or tephra,
pyroclastic flows, pyroclastic surges, mudflows or lahars, landslides,
earthquakes, ground deformation, tsunami, air shocks, lightning, poisonous gas
and glacial outburst flooding known as jökulhlaups. Each hazard has a different
consequence, although not all occur in all eruptions or in association with all
volcanoes.
Volcanic
eruptions are measured using a simple descriptive index known as the Volcano
Explosivity Index which ranges from zero to eight. The index combines the
volume of material ejected with the height of an eruption column and the
duration of the eruption.
Interesting
fact: Volcanic ash clouds can damage aircraft engines but ash is not visible by
radar, the main navigation aid for aircraft. There are nine Volcanic Ash
Advisory Centres around the world which use satellites to help track volcanic
ash clouds and provide warnings for aircraft.
Safety Tips
• Stay away from active volcanoes.
• If you live near an active volcano,
keep goggles and a mask in an emergency kit, along with a flashlight and a
working, battery-operated radio.
• Know your evacuation route. Keep gas
in your car.
If a volcano erupts in your area
• Evacuate only as recommended by authorities to stay clear of
lava, mud flows, and flying rocks and debris.
• Avoid river areas and low-lying
regions.• Before you leave the house, change into long-sleeved shirts and long pants and use goggles or eyeglasses, not contacts. Wear an emergency mask or hold a damp cloth over your face.
• If you are not evacuating, close
windows and doors and block chimneys and other vents, to prevent ash from
coming into the house.
• Be aware that ash may put excess
weight on your roof and need to be swept away. Wear protection during cleanups.• Ash can damage engines and metal parts, so avoid driving. If you must drive, stay below 35 miles (56 kilometers) an hour.
