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What Makes a Volcano Erupt?

Gaynor Borade
The eruption of a volcano spews molten ash, rock, and gases from within the Earth's surface. Prolonged activity of this nature results in the formation of mountains. A volcano erupts due to friction between tectonic plates or due to magmatic activity in mantle plumes.
Earth's crust is made up of the lithosphere and asthenosphere. The former comprises the upper and the lower crust, whereas the latter consists of the uppermost mantle region. This is a semi-solid layer with a characteristic low viscosity, and is a transitional region between the crust and mantle.
Tectonic plates are gigantic parts of the crust, which "float" on the asthenosphere. In addition to a number of minor plates, there are eight major tectonic plates. Volcanic eruptions occur along their boundaries, wherein these plates move towards, against, and/or along each other, which results in the formation of transform faults.

Elements of Eruption
Most volcanoes are observed at the regions of convergence or divergence of tectonic plates. These are surface areas that are joined and connected via a slab with a single surface. It is not necessary that volcanoes form when two tectonic plates slide past each other.
In the case of a mid-oceanic ridge, they are caused by tectonic plates that diverge. Some eruptions are also caused due to plate convergence, this is mainly observed on the continental regions.
Volcanoes also form as a result 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 eruptions are also observed on other rocky planets.

Divergent Plates

At mid-oceanic ridges and nearby regions, 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 plate convergence results in extreme pressure, and leads to an expansion that is adiabatic in nature. This causes the mantle to melt partially, leading to a volcanic eruption. Most volcanic activity is observed on the ocean beds since the divergent plate boundaries are mostly present in that region.

Convergent Plates

Subduction zone is a region where the oceanic and continental plates collide with each other, and are consumed in the trenches.
When the former one gets submerged beneath the latter one, an oceanic trench is formed, leading to magma formation on account of the decreased temperature of melting. This magma is high in silica, and it mostly cools at very deep regions. However, when it does make it to the surface, there is a volcanic eruption.

Hotspots

They are usually located above the mantle plumes. In these regions, the convection surface of the Earth's mantle generates a plume of magma that rushes up through the crust.
The rising temperature results in crustal melting and a divergence of the vent. The hotspots remain dormant, and when there is renewed activity beneath the tectonic plates, the eruptions begin again.
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, and is accompanied by steam. Cryovolcanoes have been observed on the moons of Jupiter, Neptune, and Saturn.
Mud volcanoes have lower temperatures than their igneous counterparts. Shield volcanoes and fissure vents are also considered as major types of volcanic eruptions. In the latter type, the vents are linear cracks that are formed while the magma rushes towards the Earth's surface.
The former displays low-viscosity and lava movement that is at a great distance, but not accompanied by a large explosion. The other types of volcanoes include lava domes, cinder cones, stratovolcanoes, supervolcanoes, submarine, sub-glacial, and strombolian volcanoes.