Nature Gallery

Volcanoes

The build-up of molten rock in a volcano before it
erupts is like the gases in a shaken bottle of champagne. If the amount of
gas in a volcano’s magma is high, the
inevitable release leads to massive explosions.

The amount of gas inside
magma—molten rock—is one of the most important indicators determining how
violent an eruption will be. The viscosity, or thickness, of magma is
another important factor. Under ground, gases remain suspended under
pressure in the magma, but when magma rises to the lower pressures of the
surface, the gases expand. Volcanoes with less gaseous and more fluid
magma usually have less violent eruptions because the small amount of gas
easily escapes from the lava into the air.

Thick, sticky magma, on the other hand, slows down
the escape of gases and may also block a volcano’s main vent. When the
gases are finally released, they burst out of the lava in furious and
turbulent blasts. These explosive eruptions are characterized by large
clouds of flying rock particles, rather than lava flows.

Volcanic
Products

Volcanoes emit a variety of
substances, with varying degrees of force. These substances are lava,
pyroclastic material, ash, and gases.

Lava is magma that reaches the surface. This
liquefied rock is many times hotter than boiling water and glows bright
yellow, orange, and red. Lava may erupt in explosive bursts, like giant
fountains, or flow gently down the slopes
of a mountain. Lava can leave a volcano
from the top vent or emerge from vents along the sides.

Except for the molten rock that lands back inside
the main crater to continue bubbling, all
lava eventually cools and solidifies. Some lava cools quickly, on or near
the volcano, but more fluid lava may travel for miles before slowly
congealing into rock. Over time, solidified lava from different eruptions
steadily increases the size and height of the volcano.

All fragments thrown into
the air by a volcanic eruption are called pyroclastics. During a more
violent eruption, the force of the blast sends superhot gas and millions
of pieces of lava into the air. These particles are classified as bombs,
cinders, or ash, depending on their size and shape. Small pieces of lava,
which solidify almost immediately, form slivers of volcanic glass.

Together with rocks blown from the sides of a
volcano, the entire collection of ejected material becomes a hot,
fast-moving cloud of rock and ash. These flows can travel at great speed
down the flanks of a volcano and into surrounding areas, causing extensive
destruction. In 1902 the eruption of Mount
Pelée, on the island of Martinique, created this type of pyroclastic blast
and destroyed the town of Saint-Pierre,
killing about 30,000 people.

Like lava, pyroclastic material raining down on a
volcano eventually compacts into solid layers that build up the volcano’s
bulk. Some eruptions actually reduce the height of a volcano, because they
are so powerful that they literally blow the top of the volcano off. In
1883 the cataclysmic explosion of Krakatau in Indonesia destroyed most of the island, which
had been formed by the volcano.

Volcanoes often spew great quantities of ash many
kilometres into the air. This fine ash can drift for thousands of
kilometres, falling on distant lands, yet the smallest particles of dust
may remain suspended in the atmosphere
for months. The uprush of gas and vapours from the Krakatau eruption
reportedly carried fine ash to a height of 27 kilometres (17 miles). In
addition to creating colourful sunsets for many months afterwards, the
vapour and ash clouds can have long-lasting effects on the atmosphere and
climate.

Steam and other gases such as carbon dioxide,
hydrogen, carbon monoxide, and sulphur dioxide continuously escape from
the surface of lava. Volcanic areas can emit harmful gases in immense
quantities. In 1986 a volcanic lake in
northern Cameroon released toxic gases that
killed more than 1,700 people.

The danger to life posed by active volcanoes is not
limited to the eruption of molten rock or showers of ash and cinders.
Disastrous mudflows are an equally serious hazard. One triggered by a
small eruption that melted ice and snow on Ruiz
Peak volcano in Colombia claimed more
than 25,000 lives in 1985, one of the worst volcanic disasters in the 20th
century. Some mudflows may occur long after an eruption is over, when
heavy rains saturate loose volcanic debris. In addition, eruptions near
glaciers can melt vast quantities of ice,
resulting in damaging floods. Iceland occasionally suffers these massive floods,
known there as Jökulhlaup.

Volcanic
Landforms

The shapes of volcanoes vary according
to the types of particles thrown from the volcano during eruptions. The
beautifully symmetrical cones of Mount Fuji in Japan and Mayon in
the Philippines are examples of strato-volcanoes, or
composite volcanoes. This type of volcano emits a combination of lava and
pyroclastic material. The mixture allows the successive layers to solidify
and support additional mass. Strato-volcanoes are the highest and steepest
volcanoes in the world.

Volcanoes that consist
predominantly of pyroclastic materials are called cinder cones. These
mountains, such as Capulin Mountain in New Mexico (USA), are easily eroded and usually do not reach great heights.
Shield volcanoes, on the other hand, are
predominantly lava-based landforms that have gradual slopes and wide
bases, because they release fluid lava slowly. These volcanoes can create
huge landforms. Mauna Loa and Mauna Kea on the island of Hawaii (The Big Island) are classic examples:
Mauna Kea has a base on the ocean floor more than 200 kilometres (120
miles) wide.

Under certain circumstances, instead of issuing
from a central vent, lava pours out along cracks, or fissures, that may
extend for several kilometres across the land surface. Flows of this sort
have created thick sheets of basalt
covering thousands of square kilometres. The Deccan
Plateau in India, which covers more
than 500,000 square kilometres (200,000 square miles), was formed in this
way. The Columbia Plateau in the northwest United States
is another example. In modern times, fissure eruptions on a smaller scale
have been observed in Iceland and Hawaii.

Some enormous, craterous basins called calderas, at the top of long-dormant or extinct volcanoes, form when a massive
explosion forces the upper part of a volcano to collapse. Some of these
calderas eventually fill with water, forming deep lakes, such as the
picturesque Crater Lake in the northwest
US.

States of Volcanic
Activity

Volcanoes can be active, dormant, or
extinct. Active volcanoes have erupted in a
relatively recent period. There are more than 500 active volcanoes on
continents or islands; thousands more
exist under the oceans. Many active
volcanoes are in the Ring of Fire, a zone of seismic and volcanic activity
that encircles the Pacific Ocean. Izalco
Volcano, in El Salvador, has been
erupting since 1770. Other active volcanoes include Stromboli in the Aeolian
Islands near Sicily, and Cotopaxi in the Andes of
Ecuador.

Dormant volcanoes are those
that have not erupted for many years, but have the potential to erupt
again. The eruption that follows prolonged dormancy is usually violent, as
was the explosion in 1980 of Mount Saint Helens in the northwest US, after 123 years of inactivity. The massive
eruption in 1991 of Mount Pinatubo, in the Philippines, came after six centuries of
dormancy.

Extinct volcanoes have not erupted in thousands of
years and show no signs of doing so in the future. Mount Kenya, the second highest mountain in Africa, is an extinct volcano. Edinburgh Castle, in
Scotland, sits on top of an extinct
volcano.

Creation of Volcanoes

Most active volcanoes ultimately derive their
energy from processes associated with the theory of plate tectonics. Volcanoes tend to coincide
with major plate boundaries, though some,
like the Hawaiian Islands, formed over hot spots in the earth's surface far from plate
boundaries.

At subduction zones, where one plate moves beneath
the other, the subducted plate is dragged downwards into the earth's mantle until it reaches a depth where high temperatures partially melt the rock. The
resulting magma then rises along vertical fissures and reaches the surface
through a volcanic vent. Volcanoes along the Andes in South America and the Cascade Range in North America are examples of volcanoes that
formed on continental crust overlying
subduction zones. When fissures open up on the seafloor, volcanic islands
form as a result, such as Japan and the Philippines.

At divergent plate boundaries, where two plates
move away from each other, magma wells up along the linear boundary.
Iceland is a volcanic land mass on top the Mid-Atlantic Ridge, a divergent plate boundary. New
additions along this ridge, such as the
island of Surtsey, still continue to be
created. A third type, known as transform
boundaries, exists when two plates slide alongside each other. The
interaction of plates at a transform boundary, such as the San Andreas Fault in the western United States, does not normally lead to
volcanic activity.

Hot Spots

Hundreds of hot spots exist around the world. These
are areas in the lithosphere that are
underlain by unusually hot magma. This
heat causes partial melting of the lithosphere, eventually leading to
volcanic activity. The Hawaiian
Islands are a classic example of a volcanic grouping formed over one
hot spot.

Over thousands of years, as
the Pacific Plate inched its way in a
northwest direction, the stationary hot spot underneath the plate
successively created volcanoes above it. Several of these volcanoes
reached the ocean’s surface, forming the Hawaiian Islands.

As the plate continued to move, volcanoes, embedded
in the plate, travelled away from the source of magma and eventually
became extinct. This hot spot still continues to create new volcanoes.
Thus, the islands are progressively younger from the northwest to the
southeast. Several volcanoes in the chain are still quite active, and new
underwater volcanoes are forming to the southeast of Hawaii as the Pacific Plate continues to move
over the hot spot.

Humans and Volcanoes

Volcanoes are an important
aspect of many cultures. One of the most famous and beautiful volcanoes in
the world is Mount Fuji in Japan, which last erupted in 1708. According to
legend, Mount Fuji arose from the plain
during a single night in 286 BC. Geologically, the mountain is much older than
the legend asserts. Certain religious sects regard the mountain as a
sacred place. Thousands of pilgrims from all parts of the country visit
Mount Fuji annually, and numerous shrines and temples are on its slopes.
Mount Fuji is also revered in Japanese literature and art.

Volcanoes, when not causing mass destruction, can
actually benefit humans. For example, they may provide extremely fertile
land for crops and forests. Vineyards and orchards now cover the lower
slopes of Mount Vesuvius, which destroyed the
town of Pompeii in AD
79 in a pyroclastic explosion. Higher up, oaks and chestnut trees grow.
Volcanoes, when inactive, can also provide areas for sightseeing, hiking,
and camping, and many have become parks. Tourism often results from
continuous or recent volcanic eruptions. Many people visit Hawaii Volcanoes National Park to view the
spectacular lava flows from a safe distance.

Scientific Inquiry

Geologists and
volcanologists, who specifically study volcanoes, attempt to increase our
knowledge of volcanoes and try to predict when eruptions will occur.
Volcanic earthquakes and changes in the
shape of volcanoes are two signals of impending eruptions. Like
earthquakes, however, volcanoes can be unpredictable, and those who live
in their vicinity are constantly at risk.