Volcanoes by Dr. Gaurav Garg
What is Volcano?
A volcano is a vent, or opening on the earth’s surface, through which heated materials consisting of gases,
water, liquid lava and fragments of rocks are ejected from the highly heated interior of the earth.
Volcanic materials discharged during eruptions include gases and vapour, lavas, fragmental materials and ashes.
1. Vapour and gases - Steam and vapour constitute 60 to 90 per cent of the total gases discharged during a volcanic
eruption. Carbon dioxide, nitrogen oxides, sulphur dioxide, hydrogen are the main components of the gases which are
ejected out of the volcano.
2. Magma and lava - Molten rock materials, till the time they remain below the surface, are called ‘Magma’. But when they
come to the earth’s surface, they are called ‘Lava’.
Lavas and magmas is divided into two groups -
Basic lava/mafic - Mostly it is Basaltic lava. It is characterised by
minimum silica content and least viscosity, maximum fluidity. It
spreads with maximum flow speed due to high fluidity. It is also
the hottest lava. It is mostly dark coloured.
Acidic lava/felsic - It is characterised by high silica content, high
viscosity, minimum fluidity. It spreads relatively slowly on the
surface. It is mostly light coloured.
World Distribution of Volcanoes
1. Circum Pacific Belt (Ring of Fire)
2. Mid-Continental Belt
3. Mid-Atlantic Belt
● Plate tectonics is the theory that Earth's outer shell is divided into several plates. The plates act like a hard and rigid
shell compared to Earth's mantle.
● The lithosphere is broken up into very large continental and ocean-sized plates, medium-sized regional plates, and
several small ones.
● These plates interact along their boundaries, where they converge, diverge, or slip past one another.
● Such interactions are thought to be responsible for most of Earth’s seismic and volcanic activity, although earthquakes
and volcanoes can occur in plate interiors.
Why don't the Himalayas have Volcanoes?
● The Himalayas are a special case, because they are forming due to Continental collision between Indian plate and
Eurasian Plate.
● In the Himalayas, a continental plate is pushed under another continental plate, and since both crusts have low density,
the surface is not heated up as much and does not sear through the layers above, whereas in the ring of fire, oceanic
plates are submerged, much heavier, and sink deeper causing higher pressure, and thus melt easier.
● The making of a volcano requires magma – molten rock. The composition of the material, including a lot of water,
then makes the molten surface of the submerged oceanic plate rise up and form volcanoes.
● In short, no water, no molten rock, no magma, no volcanoes.
Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under
another and is forced to sink due to high gravitational potential energy into the mantle. Subduction zones are sites that
usually have a high rate of volcanism and earthquakes.
Divergent Plate Boundaries
● At a divergent boundary, tectonic plates move apart from one another.
● The Atlantic Ocean is home to a divergent plate boundary, a place called the Mid-Atlantic Ridge. Here, the North
American and Eurasian tectonic plates are moving in opposite directions.
● Along the Mid-Atlantic Ridge, hot magma swells upward and becomes part of the North American and Eurasian
plates. The upward movement and eventual cooling of this buoyant magma creates high ridges on the ocean floor.
Ring of Fire
● The Ring of Fire (also referred as Circum-Pacific belt) is a path along the Pacific Ocean characterized by active volcanoes
and frequent earthquakes. The majority of Earth’s volcanoes and earthquakes take place along the Ring of Fire.
● The abundance of volcanoes and earthquakes along the Ring of Fire is caused by the amount of movement of tectonic
plates in the area.
Mid Continental belt: where volcanoes mainly arise from convergence of continental plates. Ex- Mount Etna (Italy).
Mid oceanic ridge belt: where volcanic activities due to splitting of plates. Ex- Helena Mountains (Iceland).
Volcanic islands in Atlantic Ocean
● Jan Mayen (Norway)
● Iceland
● Azores (Portugal)
● St Paul’s rock (Brazil)
● Ascension Island (UK)
● St Helena (UK)
● Tristan da Cunha (UK)
● Gough Island (UK)
● Bouvet Island (Norway)
● Himalayan Mountain range was formed due to Continent-Continent plates collision of the Indian and Eurasian Plates.
● Because the rock making up continental plates is generally lighter and less dense than oceanic rock, it is too light to
get pulled under the earth and turned into magma.
● Instead, a collision between two continental plates crunches
and folds the rock at the boundary, lifting it up and leading to
the formation of mountains and mountain ranges.
● When an oceanic plate collides with a continental plate .
Oceanic crust tends to be denser and thinner than continental
crust, so the denser oceanic crust gets bent and pulled under,
or subducted, beneath the lighter and thicker continental
crust. This forms what is called a subduction zone. As the
oceanic crust sinks, a deep oceanic trench, or valley, is
formed at the edge of the continent.
● The crust continues to be forced deeper into the earth, where
high heat and pressure cause trapped water and other gasses
to be released from it. This, in turn, makes the base of the
crust melt, forming magma.
0 Comments