How volcanoes are formed

Volcanoes have been an interesting phenomenon of Earth to study, and apart from that, this has been a major occurrence which has inspired film makers. However, it is a known fact that these can turn out extremely deadly and it would be worthwhile to know how these are formed although we are not prone to volcanic eruptions in our country. We have dealt with this topic in one of our earlier issues too.

Generally speaking, volcanoes are a natural way for the Earth to cool off. Whatever the cause maybe, there are few basic steps which can be seen when a volcano is born.

First point to note is that magma, a semi-solid hot rock, which lies in the centre of the globe, is responsible for all this chaos. Pieces of rocks break off from the rocky layer above the magma and mix into the magma.

This produces a gas, and the gas filled magma then rises towards the Earth's surface from the upper mantle and goes into the magma chamber about two miles from the surface, which is like a huge bucket that holds all the hot materials.

If you are wondering what the mantle is, it is the region between the core and crust of the Earth and the Earth plates reside above it. The hot magma continues to put pressure on the Earth's crust and finally finds a weak point, and comes out of the weakest point.

The lava flows out, and when it is cooled over time, a cone shaped volcano is created.

How volcanoes are triggered

Generally there are three main phenomena which trigger volcanoes. They can happen in subduction zones and divergent boundaries. These are related to the plates of the Earth and we looked at these in some of our earlier editions. They can also take place in regions known as Hot Spots.

Subduction zones

A volcano is formed when Earth plates hit each other. In a subduction zone, one of the plates goes under the other. This causes the Earth to heat, and it melts the rock into magma and gases.

This can happen as deep as 100 miles below the Earth's surface. The magma produced, settles in pools or chambers near the surface. lThis causes pressure to build up due to heat and an eruption can take place.

Divergent boundaries

Apart from plates hitting against each other, volcanoes can also be formed when the plates go away from each other. This makes weak regions in the surface and as a result magma pushes up and lava forms. These types of volcanoes are found on the ocean floor.

Hot spots

The majority of earthquakes and volcanic eruptions occur near plate boundaries, but there are some exceptions. For example, the Hawaiian Islands, which are entirely volcanic, have formed in the middle of the Pacific Ocean more than 3,200 km away from the nearest plate boundary. How do the Hawaiian Islands have volcanoes being in the middle of a plate?

The volcanoes around Hawaii are not created by tectonic plates, but due to something called hot spots. Hot spots are places that have extreme heat in the mantle. This heat causes magma to bubble and move to the surface. Hawaii, Iceland, and Yellowstone are examples of hot spots.

Interestingly, hot spots also record changes in plate motions because these spots do not move since they are below the plates and therefore remain in fixed locations for tens of millions of years.

The Hawaiian volcanic ridges are in a line since the plate has moved over the hot spot over millions of years and the extreme heat has given birth to volcanoes in regions above it.

The current theory

There was no proper theory for a long time, but now a mathematical model has been made by geologists and geophysicists and they have come up with a set of really close facts and figures to predict volcanic behaviour. Researchers have developed this model, focusing more on how the gas filled magma would behave near the Earth's surface and whether lava would pour out on the surface.

All the steps which we considered above take up rather a long time and depend on many factors, and researchers have tried to take everything into account. According to their calculations, the gas filled magma rises up from the depth of 100 to 200 kilometres at the rate of 1.2 to 2.4 centimetres per year, and it can burst out to the surface from the depth of about 30 kilometres.

Therefore if things happen this way, it would take from 2.9 to 14.2 million years for a volcano to form on the Earth's surface since its birth within the Earth, depending on the depth and the speed. This suggests that volcanoes we have today have a very old history.

Thanks to these latest discoveries, predictions can be made of the birth of volcanoes several million years in advance.

Aravinda Dassanayake