This post discusses about what causes volcanoes to occur and its various effect. In this article, we will also discuss volcanoes and the landforms it forms. So, lets start with what a volcano is?

Volcanoes are nothing but a vent (outlet) through which gases, ashes and/or molten rock material escape to the ground from the interior of the earth.

Material that reaches the ground from the interior of the earth as a result of volcanic eruption: lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, Sulphur compounds and minor amounts of chlorine, hydrogen and argon.

We all know that mantle has a high density than the crust. Mantle contains a weaker zone called asthenosphere. It is from this that the molten rock materials find their way to the surface.

The material in the upper mantle portion (within the earth’s crust) is called magma.

Once it starts moving towards the crust or it reaches the surface, it is referred to as lava.

Types of lava:

Basic Lava:

• These are the hottest lava and highly fluid.
  These are dark coloured, rich in iron and magnesium, poor in silica.
  They flow quietly and are not explosive.
  They affect extensivas, spreading out asz thin sheets over great distances before they solidify

Acid Lava:

• These are highly viscous with a high melting point.
  These are light coloured and have low density with high percentage of silica.
  They flow slowly and seldom travel far before solidifying.
  They result in loud explosions, throwing out many volcanic bombs or pyroclasts.

The next question that comes to mind is what causes volcanoes or why these volcanic eruptions happen?

This is because of the following reasons:

• The first and foremost reason is the crustal disturbance in the zone of weakness due to deep faulting or mountain folding.
• Heat generated by degeneration of radioactive elements inside the earth which causes an increase in temperature inside the earth, thus causing eruptions of the inside materials.
• Reduction in pressure of rock due to formation of fractures causing the formation of magma.
• Formation of gases due to high temperature.

Plate Tectonics and Volcanoes

Divergent plate boundaries

At the mid-oceanic ridges, two tectonic plates move apart from each other.

The release of pressure leads to partial melting of the mantle causing volcanism and creating new oceanic crust.

Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor.

Convergent plate boundaries

These occurs where two plates (oceanic and continental) slide toward each other to form a subduction zone.

In this case, the oceanic plate subducts or submerges under the continental plate forming a deep ocean trench just offshore.

Water released from the subducting plate lowers the melting temperature of the overlying mantle
wedge, creating magma.

This magma tends to be very viscous due to its high silica content so that it often does not reach the surface and cools at depth.

When it does reach the surface, a volcano is formed.

Example: Mount Etna

Hotspots

It occurs within tectonic plates (and not plate boundaries/ plate movements)

When the warm and light mantle plumes bodies material within the mantle rises by convection , it result in hotspot.

The temperature of the plume causes the crust to melt and form pipes, which can vent magma and thus cause volcanism.

Distribution of Volcano:

Volcanoes generally occur along the coastal mountain ranges, as off-shore islands and in the midst of ocean.

The greatest concentration is in the Circum Pacific region termed as ‘Pacific Ring of Fire’ due to the existence of active volcanoes in this area.

Atlantic coasts have comparatively few active volcanoes but many dormant/extinct volcanoes. Example: Canary Island.

There are no volcanoes in Australia.

Iceland, Sicily and Japan are the biggest volcanic islands in the world.

The Himalayas have no active volcanoes at all.

References: NCERT Geography