How are Volcanoes Formed?

A volcano is a vent through which molten rock known as magma rises, from inside the Earth, out onto its surface. Magma erupting from a volcano is called lava. When the spilled lava cools down and solidifies, it forms the mound-like cone around the vent of volcano. Many people think that a volcano is in fact a mountain from which lava erupts. However, now you know that the 'mountain' is in fact nothing but a cone of solidified lava from previous volcanic eruptions.

Volcanoes are usually termed as active, dormant or extinct. An active volcano is one that is erupting lava, generating seismic activity or releasing gas. On the other hand, a dormant volcano is one that has not erupted for a very long time, but could erupt again in the future. Finally, a volcano that has been dormant for more than 10,000 years is called extinct volcano. For volcanoes to be categorized this way requires extensive research and record-keeping. This was difficult in the old times, but is now easy due to computerization.

Volcanoes are quite different in appearance, with some having perfect cone shapes while others having deep depressions filled with water. The form of a volcano points to the type and size of its eruption; this in turn is controlled by characteristics of the magma, particularly its composition.

Formation of Volcanoes

Before we take a look at how volcanoes form, let us first try to understand, in brief, the structure of the Earth.

The Earth and What's Inside It
Beginning from the innermost, the Earth is made up of following layers - inner core, outer core, mantle, asthenosphere (upper mantle) and Earth's crust. The Earth's crust is like a huge jigsaw puzzle with pieces of different shapes and sizes fitting into each other perfectly. These pieces are called tectonic plates. The crust (and hence the tectonic plates) rest on the asthenosphere; and most volcanic activity can be attributed to characteristics of the asthenosphere.

The asthenosphere is the upper part of the mantle; it is highly viscous, highly plastic, and mechanically weak. Hence, the asthenosphere, in effect, acts like a layer of lubricant for the tectonic plates. The tectonic plates are hence always in relative motion - they are never still. They 'ride' on the asthenosphere. The movement of the tectonic plates is of two main types - convergent (when two tectonic plates move towards each other) and divergent (when two tectonic plates move away from each other).

Tectonic Plates and Formation of Volcanoes
Most volcanoes occur where two tectonic plates meet. When two tectonic plates diverge, they create a gap, or fissure, and hot molten rock rises up through the fissure. This type of volcano is mainly found on the ocean floor and is mostly invisible. If a big enough fissure is formed, a lot of lava may ooze out of it, sometimes even enough to rise to the surface of the ocean and form an island.

If two tectonic plates converge, it causes one plate to be forced beneath the other. The resulting friction generates a lot of heat, causing the magma to liquefy, and rise up in the form of lava. Only a few volcanoes on earth were formed like this, but their eruptions are the most violent and dangerous ones. This is on account of the fact that the friction greatly increases temperature, causing the gases to expand. This in turn generates a huge amount of pressure, literally throwing the lava out with great force, and to great heights.

Finally, the last type of volcanoes form in the middle of a tectonic plate. In the course of normal tectonic movement, magma is pushed up little by little, till it enters crevices in the rock of the lithosphere. A volcano does not form every time this happens. However, if the overlying rocks are brittle, they give away to allow the magma to flow out onto the Earth's surface in the form of lava. This process is called a diapir, and such volcanoes are commonly called 'hotspots'. These are places that are connected by channels to the hot mantle of the earth.

Hope that has given you some basic clarity about how volcanoes are formed. The different processes occurring in nature have and will always continue to intrigue mankind. all we humans can do is just try and understand Mother Nature, for understanding her completely is much beyond our comprehension!