All you need to know about volcanoes

What is a Volcano?

Volcano is a rupture in the crust of the earth which allows hot lava, ash and gases to escape from the magma chamber below the surface. The word ‘volcano’ is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy, whose name in turn originates from Vulcan, the name of the god of fire in Roman mythology

How do volcanoes form?

The earth has three layers- crust, mantle and core. The crust is the upper layer, while mantle is the middle one and the core is the inner most layer. Each layer has its own temperature; the temperature of the core is greater than 6000 Kelvin, which makes the iron and other metals in the core melt. The molten form of the metals is called magma. When a large body of magma is formed, it rises through the denser rock layers towards the earth’s surface. The magma that has reached the surface is called Lava.

Volcanoes are generally found where tectonic plates diverge or converge. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates coming together.

Volcanoes can also form where there is stretching and thinning of the crust's interior plates, e.g., in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of plate hypothesis volcanism. Volcanism away from the plate boundaries has also been explained as mantle plumes.

These so-called hotspots, for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth. Volcanoes are usually not created where two tectonic plates slide past one another.

Divergent plate boundaries

At the mid-oceanic ridges, two tectonic plates diverge from one another as a new oceanic crust is formed by the cooling and solidifying hot molten rock. Because the crust is very thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion and the partial melting of the mantle, causing volcanism and the creation of the new oceanic crust.

Most divergent plate boundaries are at the bottom of the oceans; therefore, most volcanic activity is submarine, forming a new seafloor. Black smokers (also known as deep sea vents) are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.

Convergent plate boundaries

Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. In a process called flux melting, 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 often it does not reach the surface but cools at the depth. When it reaches the surface, a volcano is formed. Typical examples of this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.

Hotspot

Hotspots is the name given to volcanic areas believed to be formed by mantle plumes, which are hypothesised to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting. Because tectonic plates move across them, each volcano becomes dormant and is eventually reformed as the plate advances over the postulated plume.

Types of Volcanoes

Scientists have broadly classified volcanoes into 10 types based on their morphology.

Fissure vents

Volcanic fissure vents are flat, linear cracks through which lava emerges.

Shield volcanoes

Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings.

Lava domes

Lava domes are built by slow eruptions of highly viscous lava. They are sometimes formed within the crater of a previous volcanic eruption, they can produce violent, explosive eruptions, but their lava generally does not flow far from the originating vent.

Cryptodomes/ Cinder Cones

Cryptodomes are formed when viscous lava is forced upward causing the surface to bulge. Cinder cones result from eruptions of mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 meters high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own.

Strato volcano

Strato volcanoes or composite volcanoes are tall conical mountains composed of lava flows and other eject in alternate layers, the strata that gives rise to the name. Strato volcanoes are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash, and lava.

Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process repeats, they are hazardous. Not only do strato volcanoes have greater pressure build up from the underlying lava flow than shield volcanoes, but their fissure vents and monogenetic volcanic fields (volcanic cones) have more powerful eruptions, as they are many times under extension

Supervolcanoes

A supervolcano usually has a large caldera and can produce devastation on an enormous, sometimes continental, scale. Such volcanoes are able to severely cool global temperatures for many years after the eruption due to the huge volumes of sulfur and ash released into the atmosphere. They are the most dangerous type of volcano. Examples include: Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States.

Because of the enormous area they may cover, supervolcanoes are hard to identify centuries after an eruption. Similarly, large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted (even though the lava flow is non-explosive).

Submarine volcanoes

Submarine volcanoes are common features of the ocean floor. In shallow water, active volcanoes disclose their presence by blasting steam and rocky debris high above the ocean's surface. In the ocean's deep, the tremendous weight of the water above prevents the explosive release of steam and gases; however, they can be detected by hydrophones and discoloration of water because of volcanic gases.

Pillow lava is a common eruptive product of submarine volcanoes and is characterised by thick sequences of discontinuous pillow-shaped masses which form under water. Even large submarine eruptions may not disturb the ocean surface due to the rapid cooling effect and increased buoyancy of water (as compared to air) which often causes volcanic vents to form steep pillars on the ocean floor.

Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals. Over time, the formations created by submarine volcanoes may become so large that they break the ocean surface as new islands or floating pumice rafts.

Subglacial volcanoes

Subglacial volcanoes develop underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the ice-cap melts the lava on top collapses leaving a flat-topped mountain. These volcanoes are also called table mountains.

Mud volcanoes

Mud volcanoes or mud domes are formations created by geo-excreted liquids and gases, although there are several processes which may cause such activity. The largest structures are 10 kilometers in diameter and reach 700 meters high.