What Makes a Volcano Erupt?

The earth's crust is made up of the lithosphere and asthenosphere. The lithosphere comprises the upper crust and uppermost mantle region. Below this layer is the asthenosphere, a solid layer of low viscosity. The lithosphere's composition is identified as tectonic plates. There are eight major and a number of minor plates that float on the asthenosphere. The tectonic plates move along collisional boundaries, centers that spread and stretch and/or what are called transform boundaries. Volcanic eruptions occur along the boundaries of these plates.

How Does a Volcano Erupt?

Most volcanoes are observed at places of convergence or divergence of tectonic plates. These plates are surface areas that are joined and connected via a single surface slab. It is not necessary that they form when two tectonic plates slide by each other. In the case of a mid-oceanic ridge, they are caused by tectonic plates that pull apart or diverge. Then, there are volcanic eruptions that are caused due to the exact opposite; the coming together or converging of the tectonic plates.

Volcanoes also form in event of extensive thinning of the crust due to stretching of the tectonic plates. Some classic examples of this kind of activity include Wells Gray-Clearwater volcanic field, Rio Grande Rift and the Eifel volcanoes in Europe. They are also triggered by mantle plumes or hotspots. These regions are ideally away from the tectonic plate boundaries, like Hawaii. Similar conditions and subsequent volcanic eruptions are observed on other rocky planets.

Divergent plates: At regions along ridges that are mid-oceanic, tectonic plates diverge and a new crust is formed when the hot molten lava cools and solidifies. Since the crust in this region is very thin, the 'pull' of the plates results in pressure and leads to expansion that is adiabatic in nature. Subsequently, this causes the mantle to melt partially, leading to a volcanic eruption. Most volcanic activity is observed on ocean beds since the divergent plate boundaries are mostly present in that region.

Convergent plates: The zone where an oceanic and continental plates collide is called a subduction. When the oceanic plate submerges beneath the continental plate, an ocean trench is formed, creating magma on account of the lowered melting temperature of the mantle above. The magma thus created is high in silica and most of the time, cools at depth. However, when it does make it to the surface, there is a volcanic eruption.

Hotspots: These regions are usually located above mantle plumes. In these region, the convection surface of the earth's mantle generates a plume of magma or lava that rushes to the crust. The temperature results in the melting of the crust and a divergence of the vent. They remain dormant and when there is renewed activity beneath, the tectonic plate move on to the 'hotspot'.

Volcanic structure and behavior depends on many factors. On cooling of the magma, rugged peaks, lava domes, summit craters and plateaus are observed. Volcanic material comprises ash and magmatic gases, accompanied by steam. Cryovolcanoes have been observed on the moons of Jupiter, Neptune and Saturn. Mud volcanoes are those that are the result of temperatures lower than igneous volcanoes. Then, there are shield volcanoes and fissure vents. In the latter, the vents are linear cracks formed while the magma rushes towards the earth's surface, while the former displays low-viscosity and lava movement that is at a great distance, but not accompanied by a wild explosion. The other types of volcanoes include lava domes, cinder cones, stratovolcanoes, supervolcanoes, submarine and subglacial volcanoes and strombolian lava bombs.