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Why do volcanoes erupt? – Nicholas, age 3 years and 11 months, Northmead, NSW.
The rock inside the planet we live on can melt to form molten rock called magma. This magma is lighter than the rocks around it and so it rises upwards. Where the magma eventually reaches the surface we get an eruption and volcanoes form.
The top part of the Earth is made up of a number of hard pieces called tectonic plates. Magma and volcanoes often form where the plates are pulled apart or pushed together but we also find some volcanoes in the middle of tectonic plates.
Volcanoes have many different shapes and sizes, some look like steep mountains (stratovolcanoes), others look like bumps (shield volcanoes) and some are flat with a hole (a crater or caldera) in the centre that is often filled with water.
The shape of the volcano and how explosively it erupts depend largely on how “sticky” and how “fizzy” (how much gas) the magma is that is erupted.
For example, if you try to blow bubbles in cooking oil though a straw, the bubbles can escape quite easily because the cooking oil is runny.
If you try to blow bubbles in jam or peanut butter you would find it very difficult because the jam and peanut butter are very sticky, they wouldn’t move much at all if you tried to pour them out of the jar.
It is the same with volcanoes. When magma rises towards the surface gas bubbles start to form. Whether or not they can escape as the magma is rising affects how explosive the eruption will be.
Where the magma is runny like cooking oil and doesn’t have much bubbly gas mixed in it, such as places like Hawaii, then we see lots of slow-moving lava flows and shield volcanoes. Lava is what we call magma when it reaches the surface.
Here are some pictures of a recent Hawaiian eruption:
However, where the magma is very sticky, like jam or peanut butter, and if it contains a lot of bubbly gas then the gas can get stuck and eruptions can be very powerful and explosive, like the recent eruptions at Fuego volcano in Guatemala.
Damage caused by eruptions
In explosive eruptions the frothy, bubbly magma can be ripped apart into tiny bits called volcanic ash. This is not ash like you get after a barbecue or fire, it does not crumble away in your fingers. It is very sharp and is dangerous to breathe in.
Some explosive volcanoes can send ash high up into the sky and it can travel around the world over different countries. If aeroplanes travel through an ash cloud from a volcano it can cause a lot of damage to the engine.
Other explosive eruptions create fast-moving, hot clouds of volcanic ash, gas and rocks that travel down the sides of the volcanoes and destroy pretty much everything in their path.
The benefits of volcanoes
Despite the great damage they can cause, volcanoes also help us to live. Volcanic ash provides food for the soil around volcanoes which helps us grow plants to eat. The heat from some volcanoes is used to make energy to power lights, fridges, televisions and computers in people’s houses.
You can find some more information about different types of volcanoes here and here.
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Recent photographs and video from the devastating eruption of Fuego volcano in Guatemala show people stood watching and filming hot, cloud-like flows of gas, ash and volcanic material (known as pyroclastic flows) travelling towards them down the slopes of the volcano.
From this it is clear that some people do not fully understand the risks of the volcanoes they live near.
Although each volcano is different, and each presents different risks to the people near to them, there are some generalisations that help us understand what these risks are likely to be.
Three points are clear: location matters, explosiveness can be predicted to an extent, and fast-moving pyroclastic flows of volcanic material are deadly.
The outer layer of the Earth, called the lithosphere (crust and upper mantle), is broken up into a number of rigid tectonic plates. Volcanoes typically occur where the plates move apart from one another, for example at underwater mid-ocean ridges, or collide together at subduction zones.
We also find volcanoes in the middle of tectonic plates – these are called “intraplate” volcanoes, such as the Hawaiian and Galápagos oceanic islands.
The magma (molten rock) that feeds volcanoes is generated in different ways in these settings, and different volcanic landforms result.
Hawaii is in the middle of a tectonic plate and volcanic activity there forms relatively low-profile, shield volcanoes. Typically, these volcanoes are built up by many fluid lava flows into broad, gently sloping domes, which resemble a warrior’s shield.
In contrast, Fuego is situated in a subduction zone environment (one plate going under another) where steep-sided, stratovolcanoes, or composite volcanoes are most common. These often symmetrical, conical volcanoes form from the build up of layers of lava and pyroclastic (fragmented volcanic) rocks.
2. Magma and gas affect explosiveness
The volcanic landforms and eruptive styles we see in different settings are largely a result of the differences in the composition of the magma (molten rock) erupted, its temperature and its gas content in these contrasting tectonic settings.
Large shield volcanoes in the middle of tectonic plates, such as Kilauea volcano in Hawaii, erupts high temperature, low silica lava. This is runnier (less viscous) than magma typically erupted at subduction zone volcanoes (like Fuego).
This means that any volatiles (dissolved gases such as water, carbon dioxide and sulphur dioxide) in the Kilauea magma are able to escape more easily compared to in a stickier, higher silica, magma that characterises subduction zone volcanoes.
And so “Hawaiian-style” eruptions are characterised by lava fountaining and flows of hot fluid lava that normally travel slow enough for people to walk away from and evacuate. This is exactly what we have been seeing over the last month in Kilauea’s East Rift Zone.
In contrast, at subduction zone volcanoes – such as Fuego – the higher water content of the magma and the typically more silica-rich, sticky magmas erupt more explosively. It is harder for gas bubbles formed to escape as magma rises to the surface, which then take up more space and over pressure the system.
Subduction zone volcanoes can produce high columns of gas and ash reaching tens of kilometres into the atmosphere, and scalding hot, fast-moving, cloud-like currents of gas, ash and volcanic material. These pyroclastic flows, or “pyroclastic density currents”, race down the volcano at speeds over 80 km/hr.
Some news reports of eruptions at Fuego have incorrectly termed these pyroclastic flows “rivers of lava”. They are very different to lava flows and much more hazardous.
Clear and accurate communication of volcanic eruptions is crucial if those near the volcano are to understand the real risks.
3. Pyroclastic flows are deadly
Pyroclastic flows are extremely hazardous and deadly. They were responsible for deaths in Pompeii and Herculaneum from the AD79 eruption of Vesuvius in Italy.
Even the famous volcanologists Katia and Maurice Kraft underestimated the reach of a pyroclastic flow during an eruption at Unzen volcano on June 3, 1991, which killed them along with many others.
Historic subduction zone volcanic eruptions producing devastating pyroclastic flows include:
Vesuvius, Italy AD 79
Tambora, Indonesia (1815)
Krakatau (Krakatoa), Indonesia (1883)
Mt Pelée, Caribbean (1902)
Mt St Helens, USA (1980)
Mt Pinatubo, Philippines (1991)
Unzen, Japan (1991).
At Fuego, the loose, fragmented volcanic material (known as tephra) lying on the slopes after eruptions may be remobilised by rain to form volcanic mudflows known as lahars. These pose a significant current and future risk for the people surrounding Fuego compared to those living in Hawaii.
Pyroclastic density currents were the main cause of death from volcanic activity in the 20th Century, killing around 45,000 people, almost 50% of all volcanic deaths in that time period (total deaths from volcanic activity is estimated to be 91,724).
While eyes are diverted toward eruptions in Central America and the Pacific Ocean, Indonesia has raised the alert level on some of its volcanoes this week. It now has 21 volcanoes on alert levels 2-4 (yellow, orange and red) on a scale of 1-4.
Local authorities will be vital in managing and communicating the risks of these volcanoes, as well as around Fuego and Kilauea.
Yet Kilauea is one of the world’s most active volcanoes, and has been erupting continually since 1983. So what has triggered this sudden change in activity, threatening homes and livelihoods? The answer relates to what is happening beneath the volcano.
Activity at Kilauea is driven by the buoyant upwelling of a plume of hot mantle, which provides the heat to generate magma beneath the volcano. This magma has the potential to erupt from several different locations, or vents, on the volcano.
Typically, the crater at the summit of the volcano is where eruptions are expected to occur, but the geology of Kilauea is complex and a rift on the eastern side of the volcano also allows magma to erupt from its flanks.
Over the past decade both the summit crater and a vent on the eastern rift, called Pu’u O’o, have been continually active. The summit crater has hosted a lava lake since March 2008.
Lava lakes are relatively rare features seen at only a handful of volcanoes around the world. The fact that they do not cool and solidify tells us that lava lakes are regularly replenished by fresh magma from below.
In contrast, Pu’u O’o, 18km east of the summit crater, has been pouring out lava flows since 1983. In the first 20 years of this eruption, 2.1km³ of lava flows were produced, equivalent in volume to 840,000 Olympic swimming pools. All of this tells us that Kilauea volcano regularly receives lots of magma to erupt.
Over the past three weeks activity at Pu’u O’o has stopped, while a series of fissures has opened roughly 20km further east in a subdivision known as Leilani Estates.
This area was previously affected by lava flows in 1955.
To date, 23 fissures have opened, starting off simply as cracks in the ground, with some developing into highly active vents from which significant lava flows are forming.
At the moment, the longest flows are about 6km long, having reached the ocean. This is a further cause for concern, as the lava reacts with seawater to form a corrosive mist.
Meanwhile, at the summit of the volcano, the lava lake has drained from the crater, sparking fears of more explosive eruptions, as draining magma interacts with groundwater.
Satellite instruments and high-resolution GPS are being used to monitor changes in the shape of the volcano and have found that the summit region is deflating, while the lower east rift zone, where new fissures have opened in recent days, is inflating.
The magma reservoirs that feed eruptions on Kilauea can be imagined as balloons, which grow when they are filled and shrink when they are emptied. Deflation at the summit, combined with observations that the lava lake has drained (at a rate of up to 100m over two days!), suggest that the magma reservoir feeding the summit is emptying.
Where is the magma going? Observations of ground inflation around the newly opened fissures to the east indicate that the magma is being diverted down the east rift and accumulating and erupting there instead.
Exactly what has caused this rerouting of the magma is still not clear. A magnitude 6.9 earthquake occurred in the area on May 4 and this may have opened a new pathway for magma to erupt, influencing the geometry of the lower east rift zone.
Lessons for the future
By combining measurements from Kilauea of ground deformation, earthquake patterns and gas emissions during the current eruption, with observations of the lava that is erupted, volcanologists will be able to piece together a much clearer picture of what triggered this significant change in eruption over the past few weeks.
This knowledge will be crucial in planning for future eruptions, both at Kilauea and at other volcanoes.
Eruptions from the flanks of a volcano can pose a much more significant hazard for the local population than those from a volcano’s summit, as many more people live in the areas that are directly affected.
This has been amply displayed over the past few weeks on Kilauea by the fissures opening in people’s gardens and lava flows destroying homes and infrastructure.
But Kilauea is not the only volcano to have flank eruptions. For example, lava flows famously emerged from the lower slopes of Mt Etna in 1669, destroying villages and partially surrounding the regional centre of Catania, on the east coast of Sicily, Italy.
Lessons learned from the current eruption of Kilauea can equally be applied to other volcanoes, like Etna, where more densely populated surroundings mean that the hazards posed by such an eruption would be even greater.
In Indonesia, more than 197 million people live within 100km of a volcano, including more than 8.6 million inside a 10km radius.
The country has a record of some of the most deadly volcanic eruptions in history, and right now there are ongoing eruptions at the Agung, Sinabung and Dukono volcanoes. But other volcanoes in the region are active too, including Kadovar in Papua New Guinea, Mayon in the Philippines, and Kusatsu-Shiranesan in Japan.
Although it all seems to be happening at once, it’s normal for the Asia-Pacific region to have frequent earthquake and volcanic activity.
But we still need to keep a close eye on things, and local volcanic authorities are monitoring activity to manage risks and evacuations adequately.
These volcanoes are part of the Pacific “Ring of Fire”, a horseshoe-shaped belt of earthquakes and volcanoes that runs for some 40,000km, roughly around the edge of the Pacific Ocean. The Ring stretches from South America, up to North America and across the Bering straight, and down through Japan, the Philippines, Papua New Guinea, Vanuatu and New Zealand. It generates around 90% of the world’s earthquakes and contains 75% of its active volcanoes.
Here are the volcanoes on my Asia-Pacific watch list this week.
Agung, Bali, Indonesia
Mount Agung in Bali has been highly scrutinised for the past few months, largely because of Bali’s popularity as a tourist destination.
After a series of volcanic earthquakes (more than 1,000 per day at its peak), eruptions began on November 21, 2017.
In the evening of January 19 an explosion of fire (known as a “strombolian” eruption) ejected glowing rocks up to 1km from the crater. The alert level remains at the highest level, with an exclusion zone in place.
There have been very few issues for tourists visiting Bali so far, apart from a temporary closure of Denpasar airport in late November 2017. However, thousands of Agung’s local residents are still displaced from their homes, with many still stationed in evacuation centres. It remains uncertain when those living closest will be able to return home.
Sinabung volcano awoke in 2010 after a 400-year sleep, and is currently one of the most active volcanoes in Indonesia. It has been pretty much in constant eruption since September 2013, and there are still frequent volcanic earthquakes.
Eruptions have produced ash plumes reaching as high as 11km into the atmosphere, as well as ash fall and lava flows. There have also been volcanic mudflows (“lahars”) and fast-moving, hot flows of gas, ash and rock fragments (“pyroclastic flows”), which have killed 25 people.
The initial activity in 2010 saw around 30,000 people evacuated. In August last year the Indonesian National Disaster Management Authority (BNPB) reported that there were 7,214 people displaced, and a further 2,863 living in refugee camps. For the locals, life seemingly goes on in the midst of eruptions.
The alert level currently remains at 4 (on a scale of 1-4), with exclusion zones of 3-7km around the volcano.
Mayon, around 330km southeast of Manila, is a picture-perfect volcano with its steep-sided conical cone, typical of stratovolcanoes. It is one of the most active volcanoes in the Philippines, with 24 confirmed eruptive periods in the past 100 years. Mayon’s most violent eruption in 1814 killed more than 1,200 people and destroyed several towns.
The recent eruption began on January 13, 2018, and is continuing, with several episodes of dramatic lava fountaining, one lasting 74 minutes.
Eruptions during January 23-29 generated 3-5km-high ash plumes and multiple pyroclastic flows, which travelled more than 5km down drainage channels. The alert is at level 4 (on a scale of 1 to 5) and an 8km danger zone is in place.
Lava flows have currently made their way up to 4.5km down river valleys from the summit crater.
The Philippine Institute of Volcanology and Seismology (PHIVOLCS) estimated on January 27 that the total volume of material deposited from ash fall and pyroclastic flows amounted to 10.5 million cubic metres. Remobilisation of this loose volcanic material by rainfall to form volcanic mudflows is a major concern.
According to news articles, more than 75,000 people have been evacuated, along with the temporary closure of Legazpi airport around 15km away.
Kadovar, Papua New Guinea
Until January 2018, when it began erupting, I hadn’t heard of Kadovar. It’s a 2km-wide, 365m-high emergent summit of a stratovolcano off the coast of Papua New Guinea.
The volcano had no confirmed historic eruptions before 2018. However, it is possible that William Dampier, a 17th-century pirate and later maritime adventurer, witnessed an eruption at Kadovar during a voyage in search of Terra Australis.
Activity began on January 5, 2018, with rising plumes of ash and steam from the volcano. The island’s inhabitants, some literally living on the crater rim, began evacuating at that time. People were initially taken by boat to neighbouring Blup Blup island but then to the mainland along with other nearby islanders, due to the close proximity of the eruption and logistics of providing people with supplies.
The Rabaul Volcano Observatory reported that activity significantly escalated on January 12, with a large explosive eruption and volcanic rocks ejected to the south. Large amounts of sulfur dioxide have been detected since January 8, and continue to be released along with ash and steam plumes. A lava “dome” has been observed glowing at night.
The impact from the eruption is not just confined to those on Kadovar and nearby islands, with satellite imagery tracking an ash plume from Kadovar travelling over tens of kilometres.
On January 23, 2018, an eruption occurred at Kusatsu-Shirane volcano without any prior warning, catching Japan’s Meteorological Agency and volcanic experts, not to mention the skiers on the volcano, by surprise.
The ejected volcanic rocks, which landed up to 1km away from the vent, injured several people. A member of the Ground Self-Defence Force who was skiing in a training exercise was killed.
The Japan Meteorological Agency has since analysed the deposits of the eruption and state that there was no new magma erupted on January 23.
Japan has more than 100 active volcanoes, with many monitored 24/7 by Japan’s Meteorological Agency.
Living near volcanoes
Indonesia, the Philippines and Japan have the greatest numbers of people living within 100km of their volcanoes. The populations of small volcanic island nations, such as Tonga and Samoa, almost all live within 100km.
Indonesia has the greatest total population located within 10km (more than 8.6 million), 30km (more than 68 million) and 100km (more than 179 million), and a record of some of the most deadly volcanic eruptions in history.
The eruption of Tambora in 1812-15, was the largest eruption in the last 10,000 years and killed around 100,000 Indonesians (due to the eruption and the ensuing famine). The infamous eruption of Krakatau (Krakatoa) killed an estimated 35,000 people, almost all due to volcanic-generated tsunamis. Volcanic mudflows (lahars) generated by the eruptions of 1586 and 1919 at Kelut (Kelud) in Java took the lives of 10,000 and 5,000 people, respectively.
Keeping watch on the world’s volcanoes is a big job for the local volcanic agencies. This is particularly true when volcanoes erupt for the first time in history (Kadovar is a good example) or there were no warning signals before eruption, as at Kusatsu-Shirane.
But does that mean the threat of any eruption is over?
A few false starts
The last major eruption of Mount Agung was in 1963. Since then, there have been two known periods of activity at the volcano site without an ensuing eruption.
In 1989, a few volcanic earthquakes occurred and hot, sulphur-rich gas emissions were observed with no eruption.
Between 2007 to 2009, satellite data showed inflation (swelling) of the volcano at a rate of about 8cm per year, probably caused by the inflow of new magma (molten rock) into the shallow plumbing system. This was followed by deflation for the next two years, again without an eruption.
The current volcanic activity – mainly the number of earthquakes – has not subsided since the alert level was raised to level 4. It continues to fluctuate at high levels, with more than 600 earthquakes a day. This indicates that the threat of an eruption is still high, despite a general decline in overall seismic energy.
This past weekend saw the highest number of daily earthquakes, with more than 1,100 recorded on Saturday October 14.
The latest statement from the Indonesian Centre for Volcanology and Geological Hazard Mitigation was released on October 5. It said earthquake data indicates that pressure is continuing to build up under the volcano due to the increasing magma volume and as magma moves towards the surface.
It’s all about the gas
Magma contains dissolved gases (volatiles) such as water, carbon dioxide and sulphur dioxide. As magma moves towards the surface, the pressure becomes less and so gas bubbles form, akin to taking the top off a fizzy drink bottle. These gas bubbles take up additional space in the magma and increase the overall pressure of the system.
The amount of gas, and whether or not gas is able to escape from the magma prior to eruption, are major factors that determine how explosive (or not) any volcanic eruption will be.
If the gas bubbles forming in the magma stay within as it ascends beneath Mount Agung, then it could lead to a more explosive eruption. If the gas formed is able to escape, it might depressurise the system enough to erupt less violently or not at all.
White gas plumes, composed mainly of water vapour, have been observed. They have typically reached 50-200m above the crater rim at Mont Agung, and up to 1,500m on October 7. This water vapour is likely due to the hydrologic system heating up in response to the intruding magma at depth.
During the 1963 eruption, Mount Agung produced a significant amount of sulphur-rich gas that caused an estimated global cooling of 0.1-0.4℃. In this current phase of activity, we are yet to see any significant release of sulphur dioxide from the intruding magma.
How big would an eruption be?
It’s not easy to predict how big any eruption at Mount Agung would be. Analysis of volcanic material deposited during previous eruptions over the past 5,000 years suggests that about 25% of them have been of similar or larger size than the 1963 eruption.
In 2010, the Indonesian Center of Volcanology and Geological Hazard Mitigation issued timely forecasts of the size of the eruption phases at Merapi, saving an estimated 10,000–20,000 lives.
The waiting game
The Indonesians are keeping a close eye on seismic activity at Mount Agung and the public can watch a live seismogram.
The last two eruptions of Mount Agung in 1843 and 1963 had a Volcanic Explosivity Index (VEI) of 5, on a scale of 0-8. A 0 would be something like a lava flow on Hawaii that you could generally walk or run from, and 8 would be a supervolcanic eruption like Yellowstone (640,000 years ago and 2.1 million years ago) in the United States or Toba (74,000 years ago) in North Sumatra, Indonesia.
Based on a history of explosive activity at the volcano, the Indonesian authorities are maintaining the current hazard zone of up to 12km from the summit of Mount Agung.
It’s still considered more likely than not that it will erupt, but the question remains: when?