Causes of Volcanic Eruptions

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Volcanoes

Volcanoes are vents in the Earth’s crust from which molten rock from below the crust erupts out onto the surface. When below the surface, this molten rock is called magma. When it is erupted or flows above the surface, it is called lava. Volcanoes also release hot gases, fragments of rock, and ash. 

The study of causes of volcanic eruptions is important because they are powerful natural forces capable of great destruction and creation. Volcanic eruptions can eradicate entire landscapes and just as easily create new landforms and reshape the land. It is estimated that 1 out of every 20 people in the world (around 350 million people) live within the vicinity of an active volcano’s ‘danger range’. To better predict the onset of volcanic eruptions and to warn nearby populations of the potential hazards,  volcanoes around the world are closely monitored and studied by volcanologists.

Volcanoes usually occur as cone-shaped mountains or hills, however, they can take many other forms. They can be steep and rise to great heights, or low, wide, dome-shaped and spread over a large area. Volcanoes can also exist as underwater ridges beneath the surface of the sea. They can be found all over our planet, on all continents, including Antarctica. There are an estimated 1,900 active volcanoes on Earth.

These volcanoes occasionally demonstrate some volcanic activity and are expected to erupt again in the future. Many are called dormant volcanoes and do not pose any risk at present but may become active again in the future. When volcanoes are no longer active and have no chance of erupting in the future, they are considered extinct.

How Volcanoes are Formed

Volcanic eruptions occur in a variety of ways and produce an equally varied set of landforms. When two plates collide, the intense heat and pressure accompanying the collision lead to violent eruptions of lava from the opening that build up to form steep, cone-shaped volcanoes, called stratovolcanoes.

Conversely, when two plates pull apart, lava seeps through the space in between. Under the ocean, this gentler flow of magma forms a crust on the seafloor. Above ground, this creates wide, rounded shield volcanoes. Volcanic eruptions may at times be so explosive and violent that the upper part of the volcano collapses, leaving behind a large pit called a caldera. 

Volcanic Eruptions

Causes of Volcanic Eruptions

In the intensely high temperatures of the Earth’s core, some rocks melt and turn into magma. Magma is less dense than the solid rock surrounding it, causing it to rise and collect in magma chambers. The magma continues to rise through the lithosphere, and after some time reaches the Earth’s surface through vents and fissures. When magma erupts onto the surface of the Earth it is called lava. 

In general, volcanic eruptions are caused by an increase in pressure in the lid of a volcano’s magma chamber. In turn, the magma is released through the volcano. 

Volcanoes are often found in the boundaries of the Earth’s tectonic plates, which either move apart or subduct and crash into one another. The separation of two tectonic plates allows magma to slowly ascend and occupy the space left behind. This results in a minor explosion of runny basaltic lava ranging between temperatures of 800–1,200 °C.

Conversely, the subduction of one plate under another features a different volcanic process. In this event, seawater, sediment, and molten rock are forced down into the magma chamber. Sediment and molten rock are incorporated into the chamber as fresh magma. Over time, this new magma accumulates until it can no longer be contained in the chamber, causing an eruption. In the process, viscous andesitic lava is released at temperatures ranging from 800–1,000 °C.

Volcanic eruptions can also be caused by factors unrelated to the Earth’s tectonic plates. A decrease in temperature can cause old magma to crystallize and sink to the bottom of a magma chamber. This forces new magma up and out of the chamber in a volcanic eruption. Another cause unrelated to plate tectonics is a decrease in the external pressure acting on the magma chamber.

Natural events such as typhoons decrease the density of rock, melting the glaciers on top of the chamber lid, which changes the composition of molten rock, reducing the ability of a magma chamber to hold back internal pressures. This process can eventually lead to an eruption. 

Types of Eruptions

Not all volcanoes erupt in the same manner. Eruptions can vary vastly in terms of how violent, how explosive, how gentle, or how frequent they occur. Much of this has to do with the composition of the magma stored in the volcano. Generally, volcanic eruptions are classified as either effusive or explosive. 

Effusive eruptions are associated with basaltic magma that is somewhat low in viscosity and gas content. When a volcano erupts thin and runny magma, the molten substance allows gas to easily escape, and it erupts in a gentle flow. Lava from this type of eruption is rarely a threat to human life, as it flows slowly enough for people to move out of the way. This type of eruption is common among Hawaiian volcanoes.

Explosive eruptions involve magma that is much more viscous and with a higher gas content. When a volcano erupts thick and viscous magma, gases can easily be trapped and struggle to be released. This builds up immense pressure inside the volcano until the gases are finally released in a violent explosion. This type of eruption is accompanied by the expulsion of magma into the air, which breaks apart into pieces called tephra. The size of tephra can range between micrometres to boulders as big as a house. 

Hydrothermal

Hydrothermal eruptions are typically very small eruptions driven by the heat present in hydrothermal systems. This type of eruption pulverises the surrounding rocks and does not include magma, but it can be accompanied by volcanic ash. 

Phreatic

Phreatic eruptions are driven by the interaction of water and the heat produced by magma. In this type of eruption, the water may come from a variety of sources including lakes, seas, groundwater, surface run-off, and hydrothermal systems. Much like hydrothermal eruptions, phreatic eruptions pulverise the surrounding rocks. They can also produce ash but do not produce any new magma.   

Phreatomagmatic

Phreatomagmatic eruptions are the result of new magma or lava interacting with water. The water may come from lakes, seas, surface run-off, groundwater, and hydrothermal systems. These eruptions have the potential to be very explosive.  

Lava

Lava is magma that is erupted at the ground surface. Lava can erupt in many ways such as lava flows, lava fountains, and lava domes. Lava flows are the non-explosive effusions of lava. In general, these flow slower than a walking pace. As the name suggests, lava fountains are fragments of runny lava erupted from a single vent or a line of vents.

Lava fountains may form spatter piles. If the fragments form fast enough, they can also form lava flows. Lava domes are rounded dome-shaped mounds formed by the gradual eruption of viscous lava that accumulates over the vent instead of flowing out. The lava from this type of eruption is viscous and has lost most of its gas content. The volume of lava domes may range from some cubic metres to as large as cubic kilometres.

Icelandic

The effusion of molten basaltic lava flowing from long, parallel fissures is characteristic of Icelandic eruptions. They usually build lava plateaus. 

Hawaiian

Similar to Icelandic eruptions, Hawaiian eruptions feature fluid lava flows that come from a volcano’s summit and radial fissures. However, this type of volcanic eruption forms large, gentle-sloped shield volcanoes. Hawaiian eruptions can also form fire fountains and lava flows. They are considered the least violent type among volcanic eruptions. 

Strombolian

Strombolian eruptions, named after the Stromboli volcano, are characterised by small frequent outbursts and nearly perpetual minute eruptions. These involve bursts of expanding gases that spew out clouts of glowing lava. Strombolian eruptions are also regarded as one of the least violent types of volcanic eruptions. 

Vulcanian

Named after Vulcano Island found near Stromboli, Vulcanian eruptions feature moderate explosions of gas mixed with volcanic ash. This combination results in turbulent eruption clouds, dark in colour, that quickly rise into the atmosphere and expand into odd shapes. 

Pelean

Pelean eruptions, named after the violent eruption of Mount Pelée on the island Martinique in 1902, are known to have explosive outbursts capable of great destruction. Like its namesake, this kind of volcanic eruption produces a dense mixture of gas and hot volcanic material called pyroclastic flows. The pyroclastic flows are heavier than air, have low viscosity, and rush down valleys and slopes at high velocity. 

Plinian

Plinian eruptions are highly violent and explosive. This type of eruption has immense and almost continuous jetting blasts produced by boiling magma containing high concentrations of gas. These powerful blasts rip apart the magma conduit, with gases and volcanic material rushing upwards like a rocket. Plinian eruptions can produce clouds for hours without stopping. The eruption clouds can reach as far as the stratosphere. Plinian ash clouds generate large amounts of static electricity, hence lightning strikes are a common in the vicinity. 

Notable Volcanic Eruptions

20th Century

Mauna Loa, 1984

The study of causes of volcanic eruptions is important because they are powerful natural forces capable of great destruction and creation. Keep reading to learn more.

Found on the island of Hawaii, Mauna Loa is a massive shield volcano that erupts showers and streams of incandescent lava roughly every three and a half years.  After a year of increased seismic activity, Mauna Loa began to erupt in the early morning of March 25, 1984. Lava fountains erupted from a fissure along the summit’s caldera, lighting up the volcano’s clouds and fumes with a red glow.

Lava erupted from the fissure in the summit pooled in Mauna Loa’s caldera, forming a lake of molten rock that quickly cooled and solidified. Throughout the day, the summit fissure extended and created new lines of lava fountains along the volcano. The last vents created had the most vigorous output of lava, amounting to roughly 500,000 cubic metres per hour. The great amounts of outpouring lava from this volcanic event made Mauna Loa the world’s largest volcano to date. 

Mount Pinatubo, 1991

In 1991, many geologists were taken by surprise when areas in the vicinity of Mount Pinatubo in the Philippines began experiencing earthquakes and steam explosions. At the time, the mountain did not have the typical cone shape of a volcano, its summit cut by erosion into jagged edges with steep slopes. Mount Pinatubo was not even included in the world’s catalogues of volcanoes, and there was no recorded history of past eruptions.

Despite the initial shock, scientists at the Philippine Institute of Volcanology (PHIVOLCS) took these warning signs seriously and were quick to start monitoring the reawakened volcano. They knew that a volcano grows deadlier the more time there is between its eruptions.

The eruption of Mount Pinatubo happened in several stages. In March 1991, small local earthquakes began on Pinatubo’s northwest face. In April, steam explosions opened three large fumaroles along a three-kilometre fissure high on the north flank of the volcano. Nearby residents were advised to evacuate. Throughout April and May, PHIVOLCS recorded multiple small earthquakes per day.

The month of June witnessed an increase in seismic activity, along with ash eruptions and magma rising to the volcano’s surface. On the morning of June 12, Mount Pinatubo’s first major explosive eruption happened, sending pillars of gas and ash 20 kilometres into the atmosphere. Further large explosions occurred in the following days. Finally, on June 15, Mount Pinatubo discharged a powerful Plinian-type eruption that lasted around nine hours. At the same time, a typhoon struck the area. Many nearby areas were devastated by earthquakes, ashfall, great pyroclastic flows, along with strong winds, rain, flooding, and the collapse of buildings caused by wet ash. 

Frequently Asked Questions

What causes volcanic eruptions?

Volcanic eruptions are primarily caused by the movement of tectonic plates, where magma rises to the surface through cracks or weaknesses in the Earth’s crust, resulting in the release of gases, ash, and lava.

How do subduction zones contribute to volcanic eruptions?

Subduction zones occur where one tectonic plate is forced beneath another. As the subducting plate sinks into the mantle, it can generate magma that rises to the surface, leading to volcanic activity.

Can volcanic eruptions be triggered by other factors besides plate tectonics?

Yes, volcanic eruptions can also be triggered by other factors such as mantle plumes, where hot material rises from deep within the Earth, or by volcanic activity associated with mid-ocean ridges and hotspots.

Are all volcanic eruptions explosive?

No, volcanic eruptions can vary in their explosiveness. Some eruptions are relatively calm, characterized by the gentle flow of lava, while others can be highly explosive, producing pyroclastic flows, ash clouds, and volcanic projectiles.

Can volcanic eruptions be predicted?

While volcanic activity can be monitored for signs of potential eruptions, accurately predicting the timing and size of eruptions is challenging. Monitoring techniques include seismic activity, gas emissions, ground deformation, and thermal imaging.

Resources

How Do Volcanoes Erupt?. (n.d.). Retrieved from United States Geological Survey:                                https://www.usgs.gov/faqs/how-do-volcanoes-erupt?qt-news_science_products=0#qt-news_science_products

The Big Question: Why do volcanoes erupt?. (n.d.). Retrieved from BBC Newsround:                                https://www.bbc.co.uk/newsround/44100737

Types of Volcanoes & Eruptions. (n.d.). Retrieved from GNS Science:                                https://www.gns.cri.nz/Home/Learning/Science-Topics/Volcanoes/Types-of-Volcanoes-Eruptions

Volcano. (n.d.). Retrieved from Britannica:
https://www.britannica.com/science/volcano

Volcanoes. (n.d.). Retrieved from National Geographic:   https://www.nationalgeographic.org/media/volcano-satellite-images/

Volcanoes, explained. (n.d.). Retrieved from National Geographic:          https://www.nationalgeographic.com/environment/article/volcanoes

Why do volcanic eruptions occur?. (n.d.). Retrieved from Cosmos:                                https://cosmosmagazine.com/geoscience/why-do-volcanoes-erupt/

Cite/Link to This Article

  • "Causes of Volcanic Eruptions". Geography Revision. Accessed on March 29, 2024. https://geography-revision.co.uk/gcse/causes-of-volcanic-eruptions/.

  • "Causes of Volcanic Eruptions". Geography Revision, https://geography-revision.co.uk/gcse/causes-of-volcanic-eruptions/. Accessed 29 March, 2024.

  • Causes of Volcanic Eruptions. Geography Revision. Retrieved from https://geography-revision.co.uk/gcse/causes-of-volcanic-eruptions/.