How are volcanoes formed?
Volcanoes are remarkable and fascinating geological wonders that arise from the shifting of tectonic plates beneath the Earth’s surface. Essentially, they are openings in the Earth’s crust that allow hot, molten rock (magma), ash, and gases to escape into the atmosphere. The mechanism of volcano formation starts with the movement of tectonic plates, which can either collide, separate, or slide past each other: this movement creates the required conditions for magma from the mantle to rise toward the surface. As the magma erupts, it cools and solidifies, gradually forming layers of volcanic material. Over time, these layers build up, creating the mountains we know as volcanoes.
There are five types of volcanoes, the main two being stratovolcanoes and shield volcanoes:
– Stratovolcanoes produce very viscous, high-silica lava, and typically have a conical shape. The lavas tend to erupt from the summit rather than flow, and a huge quantity of ashes is produced. They can also erupt in the form of a pyroclastic flow which is an avalanche of red-hot, thick syrup-like, fast-travelling hot gas and volcanic material cascading down the slopes. Pyroclastic flows will destroy virtually everything in their paths. An example is the eruption of Mount Vesuvius on August 24, 79 CE that destroyed the Italian city Pompeii.
– Shield volcanoes produce low-viscosity basaltic lava which flows readily, a bit like oil would. Dissolved gasses escape easily as the lava emerges and little ash is produced. They are composed mainly of lava which can pour out of cracks (fissure eruptions). Their gentle slopes are the result of the low-viscosity lava which travels slowly over long distances, before cooling and solidifying, visible on the Hawaiian Islands.
The other types of volcanoes are cinder cone volcanoes which are small, steeped-sided volcanoes, formed of volcanic debris such as Paricutín in Mexico, one of the youngest volcanoes on Earth, which formed over 9 years in the middle of a corn field from 1943 to 1952. Cinder cone volcanoes often form on the flanks of larger volcanoes, within volcanic fields, or along volcanic rift zones; lava domes (volcanic domes) which are formed by the slow extrusion of lava and grow gradually as lava continues to ooze out of the vent and piles up around it. Since its massive eruption in 1980, Mt St Helens in the USA has formed several domes within its crater; lastly submarine volcanoes such as Laki Fissure in Iceland, which are found on the ocean floor and are often ‘pillow shaped’ due to the rapid cooling of lava in water.
Different types of tectonic activity lead to the birth of volcanoes and although plate tectonic movements are the primary drivers of volcanic activity, there are three main processes leading to their formation: subduction, rift zones and hot spots. Ugandan calderas, Mount Kilimanjaro and the East African Rift (EAR) Valley are excellent examples.
The outer shell of the Earth, called the lithosphere, is divided into several large and small tectonic plates which float on the semi-fluid asthenosphere below them. When two tectonic plates converge, one plate will be forced to slide beneath the other into the asthenosphere: this is called subduction. The subducted plate gets heated and gives off volatile substances which promote the melting of the surrounding mantle. The magma or molten rock is less dense that the surrounding solid rock and will therefore rise to the surface. These subduction zone volcanoes mostly produce rhyolite (felsic rock rich in silica) and andesite (intermediate rock with a silica content between that of rhyolite and basalt). Subduction takes place mostly around the Pacific Ocean rim or ‘Ring of fire’, where volcanic activity is intense. Some of the most famous stratovolcanoes of the Ring of Fire volcanoes are Mount St Helens in the US, which erupted catastrophically in 1980, Mount Fuji in Japan (active but dormant) or Mount Pinatubo in the Philippines (its June 1991 eruption being one of the largest of the 20th century).
Another way volcanoes can form is through rift zones (or divergent boundaries). These are areas where tectonic plates are moving apart. As the plates separate, basalt-rich magma from the mantle rises to fill the gap, creating new crust as it cools and solidifies. This process can lead to the formation of volcanic islands and mid-ocean ridges. A good example is Heimay, a volcanic island in Iceland which lies in the rift zone where the Eurasian plate and the North American plates are moving apart. The East African Rift (EAR) or East African Rift System (EARS) as mentioned, is an example of an active continental rift zone.
Some volcanoes form over hotspots, which are areas where basalt-rich magma from deep within the mantle melts through the crust. Hotspots remain stationary while tectonic plates move over them, creating chains of volcanoes. A well-known example is the Hawaiian Islands. By dating the formation and movement of the islands, scientists have been able to plot their path of movement.
Volcanoes are formed through dynamic geological processes driven by the movement of tectonic plates know as Subduction Zones where one tectonic plate is forced beneath another, and Rift Zones or Divergent Boundaries where at mid-ocean ridges and rift zones, tectonic plates move apart. This latter process forms submarine volcanoes and volcanic islands. The third process is Hotspots where magma plumes from deep within the Earth’s mantle rise to the surface, forming volcanoes. These processes result in various types of volcanoes, including shield volcanoes, stratovolcanoes, cinder cone volcanoes, and lava domes, each with unique characteristics and eruption styles. Although volcanoes are mostly associated with destruction, they also provide many benefits such as fertile soil, geothermal energy, new land formation, unique ecosystems, and they offer an insight into the Earth’s processes.
References/Sources
How on Earth? Terrence McCarthy