Definition
A volcano is a rupture in the crust of
a planetary-mass object, such as Earth, that allows hot lava, volcanic
ash, and gases to escape from a magma chamber below the
surface. On Earth, volcanoes are most often found where tectonic plates are diverging or converging
and most are found underwater.
Plate
Tectonic and Volcano
According to the theory of
plate tectonics, the Earth's lithosphere, its rigid outer shell, is broken
into sixteen larger plates and several smaller plates. These are in slow
motion, due to convection in the underlying ductile mantle, and
most volcanic activity on Earth takes place along plate boundaries, where
plates are converging (and lithosphere is being destroyed) or are diverging
(and new lithosphere is being created). Stratovolcanoes tend to form at
subduction zones, or convergent plate margins, where an oceanic plate slides
beneath a continental plate and contributes to the rise of magma to the surface.
At rift zones, or divergent margins, shield volcanoes tend to form as two
oceanic plates pull slowly apart and magma effuses upward through the gap.
Volcanoes are not generally found at strike-slip zones, where two plates slide
laterally past each other. “Hot spot” volcanoes may form where plumes of lava
rise from deep within the mantle to Earth's crust far from any plate margins.
Mechanism
of Volcano
Deep within the Earth it is
so hot that some rocks slowly melt and become a thick flowing substance
called magma. Since it is lighter than the solid rock around it,
magma rises and collects in magma chambers. Eventually, some of the magma pushes
through vents and fissures to the Earth's surface. Magma that has erupted
is called lava.
Some volcanic eruptions are
explosive and others are not. The explosivity of an eruption depends on
the composition of the magma. If magma is thin and runny, gases can escape
easily from it. When this type of magma erupts, it flows out of the volcano. A
good example is the eruptions at Hawaii’s volcanoes. Lava flows rarely
kill people because they move slowly enough for people to get out of their way.
If magma is thick and sticky, gases cannot escape easily. Pressure builds up
until the gases escape violently and explode. A good example is the eruption of
Washington’s Mount St. Helens. In this type of eruption, the magma blasts
into the air and breaks apart into pieces called tephra. Tephra can range
in size from tiny particles of ash to house-size boulders.
Explosive volcanic
eruptions can be dangerous and deadly. They can blast out clouds of hot
tephra from the side or top of a volcano. These fiery clouds race down
mountainsides destroying almost everything in their path. Ash erupted into the
sky falls back to Earth like powdery snow. If thick enough, blankets of ash can
suffocate plants, animals, and humans. When hot volcanic materials mix with
water from streams or melted snow and ice, mudflows form. Mudflows have buried
entire communities located near erupting volcanoes.
Benefits
of Volcano
Clearly the destructive
potential of volcanoes is tremendous. But the risk to people living nearby can
be reduced significantly by assessing volcanic hazards, monitoring volcanic
activity and forecasting eruptions, and instituting procedures for evacuating
populations. In addition, volcanism affects humankind in beneficial ways.
Volcanism provides beautiful scenery, fertile soils, valuable mineral deposits,
and geothermal energy. Over geologic time, volcanoes recycle
Earth’s hydrosphere and atmosphere.
Volcanic ash and weathered
basalt produce some of the most fertile soil in the world, rich in nutrients
such as iron, magnesium, potassium, calcium, and phosphorus.
Tuff formed from volcanic
ash is a relatively soft rock, and it has been used for construction since
ancient times. The Romans often used tuff, which is abundant in Italy, for
construction. The Rapa Nui people used tuff to make most of
the moai statues in Easter Island.
Volcanic activity is
responsible for emplacing valuable mineral resources, such as metal ores.
Volcanic activity is accompanied by high rates of heat flow from the Earth's interior. These can be tapped as geothermal power.
