From Dormant to Deadly – What Causes a Volcano to Erupt? Uncovering the Secrets of These Explosive Natural Phenomena

As we know, volcanoes are the center attraction of many tourist attractions. Some volcanoes are less active than others, and some are even extinct. But what causes a volcano to erupt in the first place?

Volcanoes are defined as an opening in the Earth’s surface, or a vent, that allows hot magma to travel up to the surface. Magma can remain stagnant in storage areas called magma chambers.

If enough pressure builds up, the magma emerges from vents.

A volcano erupts when the rising magma reaches and emerges above the Earth’s surface. Once the magma exits the vents of a volcano, it becomes lava. There are a lot of complex processes that magma goes through to produce a volcanic eruption.

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What is a Volcano?

When you think of a volcano, a steep mountain-shaped geological feature may come to mind.

However, volcanoes can come in different shapes and sizes. The openings or vents present in areas where magma can be produced is what defines a volcano.

The two main features present in every type of volcano include vents and conduits. Vents need to be present to allow magma to escape above the Earth’s surface. Conduits are the reservoirs that magma travels through to reach the vents.

Magma is molten rock. Magma isn’t permanently present in the Earth’s mantle or crust. It’s created by partial melting of the upper mantle. The upper mantle is located below the Earth’s crust.

The movement of the Earth’s tectonic plates can create volcanoes. This is why areas located at the plate boundaries typically have volcanoes.

Three Main Types of Volcanoes

There are different types of volcanoes, which experience different processes of magma production and eruption. There are technically eight types of volcanoes, but the three major and most common types include:

• Cinder cone – Small volcanoes typically less than 1,000 ft (305 m) tall. Cinder cones usually only experience one eruption that can last up to a few months.
• Composite volcanoes – Mid-sized volcanoes that can reach up to 20,000 ft (6,096 m) above sea level or more. These volcanoes can have numerous eruptions that take place over the course of hundreds of thousands of years.
• Shield volcanoes – Massive volcanoes that can be miles wide and up to 33,000 ft (10,058 m) tall with gentle slopes. These volcanoes are mostly made up of fluid lava flows.

Volcanoes can differ in shape, size, types of magma, and eruption style. Cinder cones are made of volcanic ash, cinders, and rocks called tephra. Composite volcanoes are made of pyroclastic deposits, lava flows, and volcanic domes.

Pyroclastic deposits refer to various fragments that spew out during volcanic eruptions. Volcanic pyroclastic materials can come in the form of crystals, volcanic glass, ash, and pyroclastic rocks.

The other types of volcanoes that are less common include volcanic domes, maars and tuff rings, fissure volcanoes, monogenetic volcanic fields, and calderas.

What Causes a Volcano to Erupt?

Volcanic eruptions occur after magma goes through a lengthy process within the Earth’s layers. A volcano needs magma to erupt. 

However, fresh magma doesn’t need to escape a volcano to have eruptive activity. Magma is produced deep within the Earth in the upper part of the mantle.

Rocks in the upper mantle slowly melt into magma. This process can be triggered by moving tectonic plates or hot spots.

Movement of Tectonic Plates

There are three types of plate boundaries, but only two are responsible for creating volcanoes.

When plates move away from each other and new crust is formed between the plates it’s called constructive boundaries.

Destructive boundaries are plates that move toward each other. The old crust between the two plates is pushed together and can either be pushed up or dragged down. Mountain ranges are created by destructive plate boundaries.

Hot spots are unique because they don’t occur at plate boundaries. Hot spots are areas with super hot magma called mantle plume. The superheated magma heats up the Earth’s thin crust and causes melting.

The Hawaiian Islands are a good example of volcanoes that have formed as a result of a hot spot. The volcanic island chain formed away from plate boundaries in the middle of the Pacific Plate.

Rising of Magma

Once magma is produced by partial melting of the upper mantle, it can rise. The cause of rising magma can depend on how the tectonic plates are moving.

If the plates are moving away, the magma can rise to fill the space between the plates. Magma is capable of rising because of buoyancy. It’s lighter than the rock around it. 

If the plates are converging, the pressure this creates can cause the magma to rise.

Magma doesn’t always immediately rise to the Earth’s surface. Sometimes it can become stagnant. Magma can sit in magma chambers for long periods of time before rising to the Earth’s surface. It can crystallize in the magma chambers if it remains stagnant long enough.

Magma is made of several components, including molten rock, crystals, and gas. Molten rock contains chemical elements, such as iron, magnesium, and potassium.

Once magma reaches the Earth’s surface, it can break through a weak area of the crust. It escapes through openings or vents. This is what causes a volcano to erupt.

Once the magma exits the volcano, it becomes lava.

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How Do Volcanoes Erupt?

Volcanoes can erupt in different ways depending on the composition of the magma. Magma can be thick or runny.

If the magma is thin, or has a low viscosity, it typically results in lava flows. This refers to a volcanic eruption where magma oozes out of the vents and flows down as lava.

If the magma is thick, or has a high viscosity, it shoots out violently.

Silica is a chemical compound that can greatly influence magma viscosity. Magma with a high silica content is thicker. Thin magma has a low silica content. Other elements present can also impact the thickness of magma.

Types of Volcanic Eruptions

There are three main types of volcanic eruptions, including:

• Magmatic eruption – Magma that has traveled through conduits causes lava and/or tephra to eject from the volcano.
• Phreatomagmatic eruption – Hot magma interacts with groundwater or surface water to create a steam-driven eruption.
• Phreatic eruption – Magma superheats groundwater, causing a steam-driven eruption.

Phreatomagmatic and phreatic eruption styles cause volcanoes to release large, billowing steam clouds. Although both are steam-driven, magma in phreatic eruptions doesn’t interact directly with water.

Tephra is a general term used to describe all fragments of rock and material that are ejected into the air during a volcanic eruption. 

You may be most familiar with magmatic eruptions. Lava is most apparent and visible during this type of eruption.

Lava, tephra, and volcanic gasses can be released when a volcano erupts. A volcano can ooze lava out slowly or violently spew lava and tephra into the air.

Measurement of Volcano Eruptions

Volcanoes have their own scale that measures the severity of an eruption. It’s referred to as the Volcanic Explosivity Index (VEI). It resembles the Richter scale that measures the magnitude of earthquakes.

The VEI consists of eight levels of severity. A volcano with a VEI 0 equals non-explosive. A volcano with a VEI 8 is described as an “apocalyptic” eruption.

An example of a VEI 8 eruption is the Yellowstone eruption that occurred about 640,000 years ago. At least 240 cubic miles (1,000 cubic km) of explosive materials was released. The eruption resulted in the formation of the Yellowstone Caldera located in Yellowstone National Park.

The most intense volcanic eruption that occurred in 2022 was a VEI 5. The eruption occurred from the Hunga Tonga-Hunga Ha’apai volcano located in Tonga, Oceania. It began in December 2021 and ended in January 2022.

Volcanic eruptions can last for mere days, while others can go on for months or years.

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Classifications of Volcanoes

Outside of different types of volcanoes, there are also other classifications used to describe them. Classifications can be determined by the type of volcano, when it was active, how long it’s been active, and the number of eruptions.

Holocene vs Pleistocene Volcanoes

Volcanoes can be classified as Holocene or Pleistocene volcanoes. Holocene volcanoes are those that have been active during the Holocene period between 11,700 years ago to present-day.

Pleistocene volcanoes are those that haven’t been active since the Pleistocene epoch, which occurred between about 2.5 million years ago to 11,700 years ago.

Monogenetic vs Polygenetic Volcanoes

Volcanoes are also classified as monogenetic or polygenetic. Monogenetic volcanoes are volcanoes that typically only have one eruption in their lifespan. These types of volcanoes include cinder cones and maars and tuff rings.

Maars are shallow, broad volcanic craters that are typically filled with water and formed by phreatomagmatic eruptions. 

Tuff rings are circular volcanic landforms that are formed by explosive eruptions of gas-charged magma that deposit layers of volcanic ash and tuff around a central vent.

Polygenetic volcanoes experience numerous eruptions throughout their lifespan. This category includes composite, shield, and caldera volcanoes.

Active vs Dormant vs Extinct Volcanoes

Volcanoes can be described as active, dormant, or extinct.

Holocene volcanoes are considered active. Volcanoes can be potentially active even if they haven’t had an eruption in thousands of years.

Active volcanoes can also be considered dormant. Volcanoes that haven’t erupted recently, but may erupt in the future are considered dormant.

Volcanoes that aren’t expected to have any eruption activity in the future are considered extinct.

There are at least 1,350 potentially active volcanoes around the world as of March 2023. This doesn’t include potentially active volcanoes on the ocean floor.

Notable Volcanoes and Eruptions

There are several record-breaking volcanoes and eruptions worth noting. These amazing geological structures can reach soaring heights, erupt very frequently, and have colossal eruptions.

Mauna Loa

Mauna Loa is a volcano located on Hawai’i Island. It’s considered the largest, most active volcano in the world. The highest point of Mauna Loa stands at an elevation of 13,679 ft (4,169 m). Mauna Loa is a shield volcano that oozes basaltic lava.

Mauna Loa last erupted on November 27, 2022. The eruption ended on December 10, 2022. The volcano has experienced at least 110 confirmed eruptions throughout the Holocene period.

Mount Tambora

Mount Tambora is a stratovolcano located on Sumbawa in the Lesser Sunda Islands of Indonesia. 

The last eruption activity of Mount Tambora occurred in 1967. However, it experienced increased seismic activity and steam venting in April 2011.

According to the US Geological Survey, Mount Tambora holds the record for the deadliest volcano eruption. 

An 1815 eruption caused 92,000 fatalities. The deaths were largely due to crop failures and food shortages, leading to starvation over the years that followed. The 1815 eruption was measured a 7 on the VEI.

Santa María

Santa María is an active volcano located in Guatemala. It holds the record for the longest eruption.

The long-lasting eruption of Santa María began in June 1922 and is still continuing as of February 2023. The maximum level of the eruption was recorded as a VEI 3. Between February and July 2022, Santa María continuously released gas and steam emissions.

Nevados Ojos del Salado

Nevados Ojos del Salado is a volcano located within the Andes Mountain Range along the border of Chile and Argentina.

It’s considered dormant because it hasn’t experienced a confirmed eruption in hundreds of years. However, an uncertain eruption was recorded in November 1993.

Nevados Ojos del Salado is the tallest volcano in the world. It has a summit elevation of 22,569 ft (6,879 m).

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What Causes a Volcano to Erupt FAQ