How Do Volcanoes
Erupt?

Volcanic eruptions are extremely fast and noisy geological phenomena that remind us that even though our planet appears calm and static most of the time, it is constantly active beneath the surface. Often, eruptions are the end result of lengthy processes that have been unfolding for millions of years, deep beneath the Earth's surface. These complex phenomena are governed by various physical and chemical principles. 

A Balance of Temperature and Pressure

In order to understand how a volcanic eruption develops, we must delve deep into the Earth's interior and examine the long-term processes occuring there. The Earth is composed of layers upon layers: at its center lies the solid inner core, which is primarily composed of iron. Surrounding the inner core is the molten outer core, which is also predominantly composed of iron, but in a liquid state. Both are encompassed by the mantle, the thickest layer of the Earth, which is composed of solid, hot rock. Enveloping the mantle is the crust - a thin layer of cool rock composed of separate plates known as tectonic plates.

The source of the molten lava that is ejected during volcanic eruptions originates from the rocks of the mantle that melt and are pressurized towards the Earth’s surface through the crust. Molten rocks that are still located beneath the surface are referred to as magma. The mantle, which constitutes about 84% of the Earth's volume, is always at a higher temperature than the normal melting temperature of the types of rocks it is composed of, but the high pressure exerted on it prevents them from melting, as their melting temperature increases with pressure.

The source of the immensely high temperatures deep within the Earth originates from the decay of radioactive elements found in the core, as well as residual heat left over from the formation of the solar system. Over time, the Earth gradually cools down, and volcanic eruptions represent one of the ways for releasing this heat into space.

Several geological processes can cause local changes in pressure, temperature, or composition of the mantle, leading to the melting of the mantle rocks into magma, which may eventually erupt onto the Earth’s surface. During each of these processes, magma of different compositions is formed, which directly affects its viscosity (i.e. its degree of fluidity) and the amount of gases dissolved in it. These gases are released from the magma as the pressure drops, similar to how carbon dioxide is released from a carbonated beverage. Both properties have a significant impact on the nature of the volcanic eruption that will occur, and the type of volcanic mountain that will form as a result of the eruption.

When magma forms in the mantle, it tends to rise towards the crust and accumulate in a subterranean reservoir, known as a magma chamber. The magma is stored in this chamber at high pressure, until a point of weakness forms at the surface, which allows it to erupt upwards. In many cases, an eruption occurs when new magma, coming from a deeper origin, is pushed upwards into a separate magma chamber, thereby further increasing the pressure within the chamber.

Emitting gases for over 250 years. The Isle of Wight in New Zealand, the site of the Whakaari volcano eruption in December 2019. Photo: Shutterstock

A Song of Water, Gas and Fire

Volcanic eruptions can be categorized into several types, distinguished by the viscosity of the magma, the accumulation of volcanic gases emitted from it, and the involvement of water in these processes. When the cause of a volcanic eruption is the pressure of liquid magma or volcanic gases, it is referred to as a "magmatic" eruption.

One example of this is the Hawaiian eruption, named after the Hawaiian Islands where it frequently occurs. In this type of eruption, the magma involved is thin and low in gaseous content, and flows continuously from cracks that have opened in the ground, taking a long time to solidify into rock. Initially, lava flows out of multiple nearby cracks, and the flow stops once the subterranean streams merge, creating several wide basins. A lava lake typically forms within these basins, and lava fountains of tens of meters or more in height may erupt from it. These eruptions are the calmest among volcanic events and can persist for a very long time without subsiding.

In a Strombolian eruption, named after the Stromboli volcano in Italy, moderately viscous magma rich in gases is involved. As the magma rises through a crack in the Earth's crust, the pressure on it decreases, causing the gases trapped inside to form bubbles that merge together. As these bubbles grow to a certain volume, they begin to rise through the magma up the slope of the volcano. However, when the bubbles reach the surface, the pressure they were under drops dramatically. This causes them to rapidly expand and explode like massive soap bubbles, spewing lava and rocks in all directions.

Strombolian eruptions are usually short-lived and recurrent. The non-destructive nature of the eruption, which results from the fact that the gas bubbles explode outside the volcano rather than inside it, preserves the volcano's structure. As a result, the volcano may continue to erupt intermittently for hundreds of years.

In a Vulcanian volcanic eruption, which is named after the Vulcano volcano in Italy, viscous magma that is rich in gases is involved. This occurs when the slow-moving magma in the crater does not allow the gases trapped within it to escape. The pressure of the gases gradually builds up until it overcomes the layer of rock that makes up the mountain and the magma surrounding it. The result is a large explosion that could potentially destroy the surrounding area of the crater.

Finally, a Plinian eruption, named after the Roman author Pliny the Elder who documented the eruption of Mount Vesuvius in 79 AD, is also characterized by a dense and gas-rich magma. However, during this process, gas bubbles released begin to connect within the magma chamber itself. They rise up in the volcanic conduit alongside the magma and continue to grow. When they reach a critical size relative to the surrounding magma, they explode. The narrow space inside the conduit pushes the gases and magma upwards, creating an eruption column (a plume of condensed magma) and hot air that can rise up to tens of kilometers in the atmosphere.

The eruption of Mount Vesuvius in 79, which covered the Roman cities Pompeii and Herculaneum with ash, is one of the most famous Plinian eruptions. In addition to the abundant ash that fell from the sky, the eruption caused other destructive effects that contributed to the great disaster.

Another type of volcanic eruption occurs when magma comes into contact with water. This can happen, for example, when groundwater seeps into a magma chamber or to hot rocks adjacent to it, creating hot steam that may be trapped in the depths of the earth.

This trapped subterranean steam is subjected to extremely high pressure. Occasionally it is released to the surface in a controlled manner, in the form of geysers and hot springs. However, when such a release does not occur, a violent explosion can occur, no less severe than a magmatic eruption. These stream-driven eruptions are called phreatic eruptions (derived from the Greek term for "water well"), and they can occur without any liquid magma discharge. When magma does erupt along with the steam, it is referred to as a phreatomagmatic eruption.

On December 9th, 2019, the surprising eruption of the volcanic peak Whakaari on the Isle of Wight, New Zealand, occurred. Tourists were present in the area during the eruption, likely on a ship near the island, and there were reports of fatalities and injuries. Based on initial information, it was difficult to determine what type of eruption occurred, as this volcano can experience various types of eruptions. Experts later determined that the event was a phreatic eruption, which is characterized by the sudden release of steam and volcanic gasses under high pressure.

In March 2021, a volcanic eruption occurred in Iceland's Geldingadalur valley, attracting both tourists and scientists. The eruption was unique as it occurred in a fissure that had been dormant for 800 years, providing scientists with an opportunity to study eruptions and their environmental impact. Although the exact cause of the eruption is not fully understood, it is believed that a small earthquake triggered the event, causing the fissure to open and magma to escape. The volcanic activity in the region is thought to be linked to the movement of tectonic plates, since Iceland is situated on the Mid-Atlantic Ridge, where the North American and Eurasian plates meet. The slow lava flow from the eruption lasted for six months and attracted hundreds of thousands of tourists, making it the country’s longest volcanic eruption in 50 years.

The volcanic eruption in Geldingadalur valley, Iceland. Photograph: Shutterstock