1. DEFINATION 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 have 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.
How are Volcanoes a Result of Plate Tectonics?
Earth is made of four concentric layers - inner core, outer core, mantle, and crust. Outermost layer
is the crust, on which we live. This layer is actually made up of seven major tectonic plates. These
plates float on the mantle - a semi-solid layer made of molten rock, which we call magma.
Convection currents are formed in the mantle as a result of radioactive decay in the Earth's crust.
These currents cause the plates in the crust to either move towards each other, away from each
other or slide past each other. This movement of plates is called plate tectonics.

2. Formation of Volcanoes through Convergence
A convergent or destructive boundary is where two plates (usually a continental and oceanic) move
toward each other, forcing the denser (oceanic) plate to go under the lighter (continental) plate and
sink into the mantle. This is called subduction, and the place where this happens is the subduction
zone. When water and rocks from the denser, subjugating plate, come in contact with the hot
mantle, they transform into magma. This magma, due to the various gases present, is lighter than
the surrounding material and might break through faults in the crust in an violent explosion. These
volcanic eruptions are violent and can cause immense damage to all living things in their proximity.
Mt. Fuji, in Japan is a volcano found at convergent boundary.
Formation of Volcanoes through Divergence
Divergence is another cause of volcano formation. At a divergent or constructive boundary, two
plates diverge i.e. they move away from each other, forming a gap between them. Hot magma rises
from the mantle to fill this gap, cools on the surface and forms a new crust of igneous rock. This type
of lava flow, though accompanied by earthquakes, is mostly placid, and usually occurs on the ocean
bed. These type of eruptions can continue for years resulting in the formation of oceanic ridges like
the mid-Atlantic ridge. The ridges can break the surface of water to form volcanic islands. Iceland,
world's second largest volcanic island, is a part of mid-Atlantic oceanic ridge.

3. What Causes a Volcano to Erupt?
Volcanic eruptions and earthquakes are a way for Earth to release pressure and heat, much like a
safety valve. There are three dominating theories to explain what causes a volcano to erupt -
Due to Density-difference in Magma
As per the first theory, due to heat and pressure in the Earth's mantle, solid rocks melt, to form
magma. Magma has the same mass as the solid rock, but more volume, making it lighter and more
buoyant. So, it will attempt to rise, if this magma continues to encounter high-density material till it
reaches the Earth's crust, volcanic eruption occurs. It can either be in the form of a lava-flow or may
be explosive.
Pressure of Released Gases
According to the second theory, magma contains dissolved substances such as water, sulfur dioxide
and carbon dioxide. The solubility of magma decreases with the decrease in pressure as it rises up
towards the crust, and the gases get released in the form of bubbles. When the volume of the gas
bubbles in magma reaches around 75%, magma disintegrates into pyroclasts, a mixture of partially
molten and solid fragments. The bursting of pyroclasts is very explosive and the cause of some of the
most violent eruptions on the surface of Earth.
Injection of New Magma
The third theory says that when new magma enters a chamber already brimming with magma, the
volcano erupts due to the additional pressure exerted by the injection of new magma. This type of
eruption can be placid or violent.
The intensity of eruption usually depends on the viscosity of magma and its gas content. High-
viscosity magma usually results in bigger, more intense eruptions, whereas magma that flows easily
will have lower pressure build-up, so a less violent eruption. Highly viscous magma is characterized
by the presence of more silicates and contains less dissolved water. Another important factor is the
amount of gases present in the magma. Magma containing large amount of trapped gases will lead
to a violent eruption, and less gases in magma will result in an effusive flow.
EFFECTS OF A VOLCANO

4. The problems caused by volcanoes are earthquakes,tsunami and mudslides. Mudslides happen in
California. Environmental effects caused by the volcanoes can kill crops and animals that are near to
it's location the ash falls that spread when a volcanoe errupts may harm not only the environment
but also the humans as well. environmental problems consist such as
mudslide,tsunami,earthquakes,floods,drought and death to people,animals and plants.Volcanoes
can also cause distructon of buildings,roads but they also bring minerals like gold,silver,iron,etc.