The August 1883 eruption of Krakatoa was one of the deadliest volcanic explosions in modern history. The volcano, found in the middle of the Sunda Strait in between two of Indonesia’s largest islands, was on a small island which disappeared almost overnight. The eruption was so loud it could be heard in Reunion, some 3,000 miles away.
As the volcano collapsed into the sea, it generated a tsunami 37m high – tall enough to submerge a six-storey building. And as the wave raced along the shoreline of the Sunda Strait, it destroyed 300 towns and villages, and killed more than 36,000 people.
Nearly 45 years later, in 1927, a series of sporadic underwater eruptions meant part of the original volcano once again emerged above the sea, forming a new island named Anak Krakatoa, which means “Child of Krakatoa”. In December 2018, during another small eruption, one of Anak Krakatoa’s flanks collapsed into the ocean and the region’s shorelines were once again hit by a major tsunami. This time, 437 were left dead, nearly 32,000 were injured and more than 16,000 people were displaced.
Even though Anak Krakatoa had been active since June that year, local residents received no warning that a huge wave was about to hit. This is because Indonesia’s early warning system is based on ocean buoys that detect tsunamis induced by submarine earthquakes, such as those that struck on Boxing Day in 2004, in one of the most deadly natural disasters of all time.
But tsunamis caused by volcanic eruptions are rather different and, as they aren’t very common, scientists still don’t fully understand them. And Indonesia has no advanced early warning system in place for volcano-generated tsunamis.
At some point in the future, Anak Krakatoa will erupt again, generating more tsunamis. Since it is difficult to predict exactly which areas of the Sunda Strait will be affected, it is of paramount importance that residents in coastal villages are well aware of the danger.
An advanced early warning system could be installed. It would involve tide gauges to detect an increase in water levels, satellite imagery and drone mapping, and a tsunami numerical model run in real time. When this system triggered a warning, it would be fed direct to residents who live in the coastal belt. Until such a system is in place, it will be vital to get the local community involved in disaster risk management and education.
We need to tell people about the risks
But preparing for future disasters isn’t just about building breakwaters or seawalls, though these defensive structures are clearly vital for preserving beaches for tourism and local businesses like fishing. It is also about educating people so that they feel psychologically healthier, more resilient and less anxious about facing the mega tsunamis of the future.
I have previously highlighted two examples of proactive community participation in disaster-prone villages in the UK and Japan. In both cases, residents know how to act in case of a natural disaster without depending on the authorities. It is certain that the decimation of the land and deaths could be reduced if the local communities are well prepared for natural disasters like tsunamis.
Following the December 2018 Anak Krakatoa tsunami, local researchers and I conducted a detailed field survey of the coastline of Lampung province, on the north side of the strait, and some of the smaller nearby islands. We found a lack of proper tsunami defence structures or any early warning system, and houses and businesses built very close to the coast with no buffer zone. We identified high ground where residents could run to in case of a tsunami and put up signs with evacuation routes.
During this survey, I conducted a series of focus group meetings with local residents and businesses in order to make the communities more resilient and reduce their anxiety about future mega tsunamis in the area. I developed a tsunami wave propagation model to replicate the 2018 tsunami and most plausible future tsunami events, and to identify the most vulnerable coastal stretches, such as the village of Kunjir on the Lampung mainland.
I also combined field survey results, numerical model outputs and published information to make some recommendations for local communities. I suggested active collaboration between government departments and local institutions on the issue, and the formation of disaster preparedness teams for every village in Southern Lampung. The planning criteria for development of infrastructure along the coasts should also be put under review, and there should be a trauma healing programme for the victims of the 2018 Krakatoa tsunami.
We don’t know exactly when Krakatoa will next erupt, or if any future eruptions will match those of 1883 or even 2018. That’s a question for volcanologists. But we should do what we can to prepare for the worst.
Tourists visiting Whakaari/White Island on December 9 last year had no warning of its imminent violent eruption. The explosion of acidic steam and gases killed 21 people, and most survivors suffered critical injuries and severe burns.
The tragedy prompted us to develop an early alert system. Our research shows patterns of seismic activity before an eruption that make advance warning possible. Had our system been in place, it would have raised the alert 16 hours before the volcano’s deadly eruption.
We were also motivated by the fact that several other New Zealand volcanoes pose similar threats. Explosions and surges at the popular visitor destination Waimangu geothermal area killed three people in 1903, an eruption at Raoul Island in 2006 killed one person, ballistics at Mt Ruapehu in 2007 caused serious injuries and tourists narrowly escaped two eruptions on a popular day walk in the Tongariro national park in 2012.
Our automated warning system provides real-time hazard information and a much greater level of safety to protect tourists and help operators determine when it is safe to visit volcanoes.
A history of eruptions
New Zealand has a network of monitoring instruments that measure even the smallest earth movements continuously. This GeoNet network delivers high-rate data from volcanoes, including Whakaari, but it is not currently used as a real-time warning system for volcanic eruptions.
Although aligned with international best practice, GeoNet’s current Volcano Alert Level (VAL) system is updated too slowly, because it relies mainly on expert judgement and consensus. Nor does it estimate the probability of a future eruption — instead, it gives a backward view of the state of the volcano. All past eruptions at Whakaari occurred at alert levels 1 or 2 (unrest), and the level was then raised only after the event.
Our study uses machine learning algorithms and the past decade of continuous monitoring data. During this time there were five recorded eruptions at Whakaari, many similar to the 2019 event. Since 1826, there have been more than 30 eruptions at Whakaari. Not all were as violent as 2019, but because there is hot water and steam trapped in a hydrothermal area above a shallow layer of magma, we can expect destructive explosions every one to three years.
Last year’s eruption was preceded by 17 hours of seismic warning. This began with a strong four-hour burst of seismic activity, which we think was fresh magmatic fluid rising up to add pressure to the gas and water trapped in the rock above.
This led to its eventual bursting, like a pressure cooker lid being blasted off. A similar signal was recorded 30 hours before an eruption in August 2013, and it was present (although less obvious) in two other eruptions in 2012.
Building an early warning system
We used sophisticated machine-learning algorithms to analyse the seismic data for undiscovered patterns in the lead-up to eruptions. The four-hour energy burst proved a signal that often heralded an imminent eruption.
We then used these pre-eruption patterns to teach a computer model to raise an alert and tested whether it could anticipate other eruptions it had not learned from. This model will continue to “learn by experience”. Each successive event we use to teach it improves its ability to forecast the future.
We have also studied how best to optimise when alerts are issued to make the most effective warning system. The main trade-off is between a system that is highly sensitive and raises lots of alerts versus one that sets the bar quite high, but also misses some eruptions.
We settled on a threshold that generates an alert each time the likelihood of an eruption exceeds 8.5%. This means that when an alert is raised – each lasting about five days – there is about a 1-in-12 chance an eruption will happen.
This system would have raised an alert for four of the last five major eruptions at Whakaari. It would have provided a 16-hour warning for the 2019 eruption. But these evaluations have been made with the benefit of hindsight: forecasting systems can only prove their worth on future data.
We think there is a good chance eruptions like the 2019 event or larger will be detected. The trade-off is that the alerts, if acted upon, would keep the island off-limits to visitors for about one month each year.
Where to from here
We have been operating the system for five months now, on a 24/7 basis, and are working with GNS Science on how best to integrate this to strengthen their existing protocols and provide more timely warnings at New Zealand volcanoes.
We plan to develop the system for New Zealand’s other active volcanoes, including Mt Tongariro and Mt Ruapehu, which receive tens of thousands of visitors each year. Eventually, this could be valuable for other volcanoes around the world, such as Mt Ontake in Japan, where a 2014 eruption killed 63 people.
Because of the immense public value of these kinds of early warning systems, we have made all our data and software available open-source.
Although most eruptions at Whakaari appear to be predictable, there are likely to be future events that defy warning. In 2016 there was an eruption that had no obvious seismic precursor and this would not have been anticipated by our warning system.
Eruptions at other volcanoes may be predictable using similar methods if there is enough data to train models. In any case, human operators, whether assisted or not by early warning systems, will continue to play an important role in safeguarding those living near or visiting volcanoes.
Spectacular images of recent volcanic eruptions in Hawaii are a little disheartening – especially given news reports suggesting there is a sleeping volcano under Melbourne that could awaken and erupt at any moment.
Understanding the geological differences between Melbourne and Hawaii is really helpful in working out how we can keep an eye on future risks in Australia.
The Newer Volcanics Province
Victoria and South Australia do host an active volcanic field, called the Newer Volcanics Province (NVP). This is not a single volcano with a large single chamber of molten rock (magma) — the common image of a volcano — but a widespread field of multiple small volcanoes, each with a small volume of magma.
Melbourne lies at the eastern end of the NVP, and the most recent eruptions in this area occurred over a million years ago.
Mt Gambier in southeastern South Australia represents the western margin of the volcanic field and the most recent eruption — only 5,000 years ago.
Between Melbourne and Mt Gambier there are more than 400 small volcanoes that erupted over a period of 6 million years.
The NVP was most active between 4.5 million to 5,000 years ago and volcanologists consider the field to still be “active” with the potential for future eruptions.
We do not know when the next eruption will take place.
The NVP is located within a tectonic plate – and not along a plate edge like the Ring of Fire volcanoes (for example, Mt Agung on Bali).
Tectonic plates are large slabs of rock made up of the Earth’s crust and uppermost part of the mantle (the lithosphere) which form the outer shell of the Earth, and move around slowly relative to each other.
Curious Kids: Why do volcanoes erupt?
Volcanoes act in different ways
While Kilauea volcano in Hawaii is also located within a tectonic plate, it has several key differences with the NVP in Southeastern Australia.
Magma source and volume
While Hawaii sources large volumes of magma from deep within the Earth, the NVP only receives small amounts of magma from just below the Earth’s crust.
It’s worth noting here that the makeup of the magma is similar in both locations, with both erupting runny basalt – a type of rock low in silica, and high in iron and magnesium.
We suspect that in Australia’s NVP, magma can move very fast from its source to the surface (on a time scale of days). This can bring rock fragments of the mantle (xenoliths) to the surface as the magma moves too fast for them to melt.
Hawaiian volcanoes can erupt numerous times, but NVP volcanoes are largely monogenetic — that is, each only erupt once or over a restricted period of time.
Hawaii is located on the oceanic crust of the Pacific Tectonic Plate, which is a thin (around 7 km) layer of material that is dense and rich in iron. The magma can rise through this crust quite easily.
In contrast, the NVP is located on continental crust which is much thicker (about 30km), richer in silica and much less dense. Magma finds it much harder to travel through this kind of material.
Is there a new volcano on Hawaii?
Water adds danger
The explosivity of a volcanic eruption can depend on availability of water.
“Dry” eruptions – where magma has little-to-no interaction with ground water or water on the Earth’s surface – typically produces mildly explosive eruptions such as lava fire fountains, showers of lava fragments and lava flows.
The most explosive, hazardous eruptions form where rising magma interacts with ground water, surface water or sea water. These “wet”, (phreatomagmatic) eruptions can produce deadly, fast moving, ground-hugging currents of gas and volcanic material – called pyroclastic surges, and send abundant fine volcanic ash into the atmosphere.
The Australian Mt Gambier eruption 5,000 years ago was a “wet” eruption, and had a volcanic explosivity index of 4 on a scale of 0-8 (where 0 represents a lava eruption, 1 a spectacular lava “fire” fountain as recently witnessed in Hawaii, and 8 represents a catastrophic explosive super-eruption).
The accompanying ash column is estimated to have reached 5km to 10km into the atmosphere.
On Hawaii explosive eruptions are rarer because the magma has a low gas content and groundwater aquifers are not as large as in the NVP. However, when lava flows into the sea there are often phreatic or steam explosions which can be hazardous to nearby spectators.
There’s a lot we don’t know
Another important factor relates to how we keep an eye on volcano risk at the two sites. Kilauea on Hawaii is extremely well monitored, and tracking magma moving underground has helped predict eruptions.
In contrast, the NVP is less well monitored, likely because there is no present volcanic activity, and it’s a huge region.
However, warning signs of an eruption are likely to be similar in the NVP to those on Hawaii – small earthquakes, minor uplift and/or subsidence of the ground, changes in ground temperature and gas or steam rising out of the ground.
Also, based on present knowledge of the NVP, there is no clear eruption pattern we can use to try to predict when or where the next eruption will be.
If the NVP were to erupt, significant impacts on our lives would likely occur. These may include:
- the closure of surrounding roads by lava flows and ash fallout
- volcanic ash and rocks loading roofs of local buildings
- contamination of water reservoirs by ash
- damage to machinery and electricity infrastructure by infiltrating ash
- respiratory problems for people prone to asthma, and
- disruption to air traffic across southeastern Australia due to drifting ash clouds driven by prevailing south-westerly winds.
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Further scientific research is required on active volcanic fields such as the NVP to know how fast magma travels from its source to the surface, how much warning we might have before an eruption, and how long an eruption and its impacts might last.
Heather Handley, Associate Professor in Volcanology and Geochemistry, Macquarie University; Jozua van Otterloo, Assistant Lecturer in Volcanology, Monash University, and Ray Cas, Professor emeritus, Monash University
Over the past few weeks we’ve seen increasingly spectacular images reported in the news of the ongoing eruption at Kilauea volcano, on the Pacific island of Hawai’i.
These have been tempered by reports of growing destruction, with houses and infrastructure bulldozed, buried or burned by lava flows.
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.
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.
This is an article from I’ve Always Wondered, a new series where readers send in questions they’d like an expert to answer. Send your question to email@example.com
Do underground nuclear tests affect Earth’s tectonic plates, and cause earthquakes or volcanic eruptions? – Anne Carroll, Victoria
Apart from escalating global fears about conflict, North Korea’s recent nuclear tests have raised questions about geological events caused by underground explosions.
So can an underground test cause an earthquake? The short answer is yes: a nuclear explosion can cause small earthquakes. But it is unlikely to affect the earth’s tectonic plates or cause a volcanic eruption.
Although a nuclear explosion releases a lot of energy in the immediate region, the amount of energy is small compared to other stresses on tectonic plates.
What are tectonic plates?
Tectonic plates are slabs of the earth’s crust which move very slowly over the surface of the earth. Mountain ranges form at the edges of the plates when they collide, and ocean basins form when they move apart.
Volcanoes occur mostly where plates are colliding. One plate overrides another, pushing it down to where it may partly melt. The partially melted rock – also known as lava – then rises to the surface, causing a volcano.
The movement of tectonic plates also causes earthquakes, which is why 90% of them occur at the plate boundaries. All but the deepest earthquakes occur along faults, which are breaks in the crust where rocks can move past each other in response to stress. This stress can be from both natural events and human activities.
Human induced earthquakes
“Induced seismicity” is the term used to describe earthquakes caused by human activities.
Human induced earthquakes can be caused by anything that changes the stresses on rocks beneath the surface. These include processes that add or remove great loads from the surface, such as mining, building dams or tall buildings.
Other processes that change the amount of pressure on rocks can include fluid injection from drilling, or extraction of water from aquifers.
Human-induced earthquakes have been reported from every continent except Antarctica. Induced earthquakes only occur where there is already some stress on the rocks. The human activity adds enough stress to the rocks to reach the “tipping point” and trigger the earthquake.
Nuclear explosions can induce small earthquakes along existing faults near a test site. Some underground nuclear tests have fractured the ground surface above the explosions, causing movement on faults adjacent to explosion sites.
Earthquakes from nuclear testing
The 3 September 2017 North Korean nuclear test generated shock waves equivalent to a magnitude 6.3 earthquake. Eight minutes later, a magnitude 4.1 event was detected at the same site. This may have been linked to a collapse of a tunnel related to the blast.
Several small earthquakes measured since the event may have been induced by the nuclear test, but the largest is only a magnitude 3.6. An earthquake of this size would not be felt outside of the immediate area.
The largest induced earthquake ever measured from nuclear testing was a magnitude 4.9 in the Soviet Union. An earthquake of this size can cause damage locally but does not affect the full thickness of the earth’s crust. This means it would not have any effect on the movement of tectonic plates.
Historical data from nuclear testing (mostly in the USA) shows that earthquakes associated with nuclear testing typically occur when the explosion itself measures greater than magnitude 5, 10–70 days after the tests, at depths of less than 5km, and closer than around 15km to the explosion site. More recent studies have concluded that nuclear tests are unlikely to induce earthquakes more than about 50km from the test site.
Concerns have also been raised about the risk of volcanic eruptions induced by the nuclear tests in North Korea. Paektu Mountain is about 100km from the test site and last erupted in 1903.
In the 1970s, the USA conducted a number of nuclear tests in the Aleutian Islands, a volcanic island arc chain containing 62 active volcanoes.
One of the blasts, named Cannikin, was the largest underground nuclear test ever conducted by the USA. There were fears that the blast would cause a huge earthquake and tsunami. The blast did result in some induced earthquakes, but the largest was a magnitude 4.0 and there was no increase in volcanic activity.
Based on this evidence, it seems unlikely a nuclear test by North Korea will trigger an eruption of Paektu Mountain. If the volcano was on the verge of erupting, then an induced earthquake from a nuclear blast could influence the timing of the eruption. However, given the distance from the test site then even this is not likely.
Monitoring nuclear tests
The Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) has a global monitoring system to detect nuclear tests, including seismometers to measure the shock waves from the blast and other technologies.
Seismologists can analyse the seismic data to determine if the shock waves were from a naturally occurring earthquake or a nuclear blast. Shock waves from nuclear blasts have different properties to those from naturally occurring earthquakes.
Testing was much more common before the CTBTO was formed: between 1945 and 1996 more than 2,000 nuclear tests were conducted worldwide, including 1,032 by the USA and 715 by the Soviet Union.
Since 1996 only three countries have tested nuclear devices: India, Pakistan and North Korea. North Korea has conducted six underground nuclear tests at the same site between 2006 and 2017.
It’s more than three weeks since the alert level on Bali’s Mount Agung was raised to its highest level. An eruption was expected imminently and thousands of people were evacuated, but the volcano has still not erupted.
I keep getting emails from people asking me whether they should travel to Bali. I tell them to check the Australian’s government’s Smartraveller website, or contact their airline or tour operator.
They should also keep an eye on the media and any updates from the Indonesian Centre for Volcanology and Geological Hazard Mitigation.
Reports this week from the Indonesian National Disaster Management Authority show a decline in seismic energy recorded near the volcano.
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.
On the neighbouring island of Java, the explosive 2010 eruption of Mount Merapi saw more than 400,000 people evacuated and 367 killed. This was preceded by increased earthquake activity over a period of about two months. It was the volcano’s largest eruption since 1872.
The monitoring data and studies of the volcanic rocks produced by the Merapi eruption suggest the relatively fast movement of a large volume of gas-rich magma was the reason for the unusually large 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?