Volcanic activity. It’s the reason why the town of El Rodeo in Guatemala is currently uninhabitable, why the Big Island of Hawaii gained 1.5 kilometres of new coastline in June, and why Denpasar airport in Bali has closed twice this year.
But these eruptions should not be seen as destructive attacks on certain places or the people that live in them. They have nothing to do with even the country that hosts them. They occur in specific regions because of much larger-scale processes originating deep within the Earth.
According to the United States Geological Survey (USGS), approximately 1,500 potentially active volcanoes exist on land around the globe. Here’s a look at four of the world’s most volcanically active spots, and the different processes responsible for their eruptions. As you’ll see, there is no one-size-fits-all volcano.
Most volcanic eruptions go unnoticed. That’s because they happen continuously on the ocean floor where cracks in the Earth’s outer layer, the lithosphere (comprising the crust and solid upper mantle), form at so-called divergent plate boundaries. These margins form due to convection in the underlying mantle, which causes hot, less dense molten material, called magma, to rise to the surface. As it forces its way through the lithospheric plate, magma breaks the outer shell. Lava, the surface-equivalent of magma, fills the crack and pushes the broken pieces in opposite directions.
Volcanism from this activity created Iceland. The country is located on the Mid-Atlantic Ridge, which forms the seam between the Eurasian and North American plates. Iceland is one of the few places where this type of spreading centre pops above sea level.
However, volcanism on Iceland also happens because of its location over a hot spot. These spots develop above abnormally hot, deep regions of the mantle known as plumes.
Each plume melts the overlying material and buoyant magma rises through the lithosphere – picture a lava lamp – to erupt at the surface.
This volcanic double whammy produces both gentle fissure eruptions of basaltic lava as well as stratovolcanoes that are characterised by periodic non-explosive lava flows and explosive, pyroclastic eruptions, which produce clouds of ash, gas and debris.
In 2010, the two-month eruption of the ice-capped Eyjafjallajökull stratovolcano – the one that no one outside Iceland can pronounce – attracted a lot of media attention because the resulting ash cloud grounded thousands of flights across Europe.
In fact, it was a relatively small eruption. It is believed that a major eruption in Iceland is long overdue. Four other volcanoes are all showing signs of increased activity, including the country’s most feared one, called Katla.
More than 197 million Indonesians live within 100 km of a volcano, with nearly nine million of those within 10 km. Indonesia has more volcanoes than any other country in the world. The 1815 eruption of its Mount Tambora still holds the record for the largest in recent history.
Indonesia is one of many places located within the world’s most volcanically, and seismically, active zone, known as the Pacific Ring of Fire. This 40,000 km horseshoe-shaped region, bordering the Pacific Ocean, is where many tectonic plates bang into each other.
In this so-called convergent plate boundary setting, the process of subduction generates volcanism. Subduction occurs because when two plates collide, the higher density plate containing oceanic crust sinks beneath another less dense plate, which contains either continental crust or younger, hotter and therefore less dense oceanic crust. As the plate descends into the mantle, it releases fluids that trigger melting of the overriding plate, thus producing magma. This then rises and erupts at the surface to form an arc-shaped chain of volcanoes, inward of, but parallel to, the subducting plate margin.
Indonesia marks the junction between many converging plates and, thus, the subduction processes and volcanism are complex. Most of Indonesia’s volcanoes, however, are part of the Sundra Arc, an island volcanic range caused by the subduction of the Indo-Australian Plate beneath the Eurasian Plate. Volcanism in eastern Indonesia is mainly caused by the subduction of the Pacific Plate under the Eurasian Plate.
The stratovolcanoes that form in convergent plate boundary settings are the most dangerous because they are characterised by incredibly fast, highly explosive pyroclastic flows. One of Indonesia’s stratovolcanoes, Mount Agung, erupted on 29 June for the second time in a year, spewing ash more than two km into the air and grounding hundreds of flights to the popular tourist destination, Bali.
The June 3 eruption of the Guatemalan stratovolcano, Volcan de Fuego (Volcano of Fire), devastated Guatemalans, and the rest of the world, as horrifying images and videos of people trying to escape the quick-moving pyroclastic flow filled the news.
Like Indonesia, Guatemala’s location within the Ring of Fire and the subduction-related processes that go along with its location are responsible for the volcanoes found here. Located on the other side of the Pacific Ocean, volcanism is caused by the subduction of the much smaller Cocos Plate beneath the North American-Caribbean Plate.
Unlike Indonesia, however, the convergent boundary between these two plates occurs on land instead of within the ocean. Therefore, the Guatemalan arc does not form islands but a northwest-southeast trending chain of onshore volcanoes.
The same process is responsible for the formation of the Andes – the world’s longest continental mountain range – further south along the western coast of South America. In this case, subduction of the Nazca-Antarctic Plate beneath the South American Plate causes volcanism in countries such as Chile and Peru.
When someone mentions Hawaii, it’s hard not to picture a volcano. But Hawaii’s volcanoes are actually not typical. That’s because they are not found on a plate boundary. In fact, Hawaii is slap-bang in the middle of the Pacific Plate – the world’s largest.
Like Iceland, Hawaii is also underlain by a hot spot. However, because the Pacific Plate is moving to the northwest over this relatively fixed mantle anomaly, the resulting volcanism creates a linear chain of islands within the Pacific Ocean. A volcano forming over the hot spot will be carried away, over millions of years, by the moving tectonic plate. As a new volcano begins to form, the older one becomes extinct, cools and sinks to form a submarine mountain. Through this process, the islands of Hawaii have been forming for the past 70 million years.
The typical shield volcanoes that form in this geological setting are produced from gentle eruptions of basaltic lava and are rarely explosive. The youngest Hawaiian shield volcano, Kilauea, erupted intensely on 3 May of this year, and 1,170 degree Celsius lava has been flowing over the island and into the ocean ever since. Kilauea, which has been continuously oozing since 1983, is regarded as one of the world’s most active volcanoes, if not the most.
AND THE WORLD’S LEAST VOLCANIC PLACE?
It may be surprising to hear that despite the Himalayas, like the Andes, being located on a very active convergent plate boundary, they are not volcanically active. In fact, there are barely any volcanoes at all within the mountain range.
This is because the two colliding plates that are responsible for the formation of the Himalayas contain continental crust at the convergent plate boundary, distinct from the oceanic-continental or oceanic-oceanic crustal boundaries in the Guatemalan and Indonesian cases, respectively.
As the two colliding plates have similar compositions, and therefore densities, and both their densities are much lower than the underlying mantle, neither plate is subducted. It’s a bit like wood floating on water. As subduction causes the lithospheric partial melting that generates the magma in convergent plate boundary settings, volcanism is not common in continent-continent collisions.
Unfortunately, Himalayan people don’t get off that easily though, because devastating earthquakes go hand-in-hand with this sort of setting.
Related reading: Each volcano has unique warning signs that eruption is imminent
Vhairi Mackintosh is a scientific educator, writer and researcher based in Melbourne who holds a PhD in earth sciences.
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