There is something special and awe-inspiring about watching new land form. This is what is now happening in Hawai’i as its Kīlauea volcano erupts. Lava is reaching the ocean and building land while producing spectacular plumes of steam. These eruptions are hugely important for the creation of new land. But they are also dangerous. Where the lava meets the ocean, corrosive acid mist is produced and glass particles are shattered and flung into the air. Volcanic explosions can also hurl lava blocks hundreds of metres and produce waves of scalding hot water.
At Kīlauea, lava is erupting from a line of vents on the volcano’s flanks, and is moving downslope to the edge of the island, where it enters the ocean. This is a process that has been witnessed many times at Hawai’i and other volcanic islands. And it is through many thousands of such eruptions that volcanic islands like Hawai’i form.
The new lava being added to Hawai’i by this latest Kīlauea eruption replaces older land that is being lost by erosion, and so prolongs the island’s lifespan. In contrast, older islands to the north-west have no active volcanoes, so they are being eroded by the ocean and will eventually disappear beneath the waves. The opposite is happening to the south-east of Hawai’i, where an underwater volcano (Lōʻihi Seamount) is building the foundations of what will eventually become the next volcanic island in this area.
How lava gets to the ocean at Hawai’i
The lava erupting from the current Kīlauea vents has a temperature of roughly 1150 degrees °C, and has a journey of between 4.5km and 5.5km to reach the ocean. As this lava moves swiftly in channels, it loses little heat and so it can enter the ocean at a temperature of over 1000 degrees°C.
What happens when lava meets the ocean?
We are witnessing one of the most spectacular sights in nature – billowing white plumes of steam (technically water droplets) as hot lava boils seawater. Although these billowing steam clouds appear harmless, they are dangerous because they contain small glass shards (fragmented lava) and acid mist (from seawater). This acid mist known as “laze” (lava haze) can be hot and corrosive. If anyone goes to near it, they can experience breathing difficulties and irritation of their eyes and skin.
Apart from the laze, the entry of lava into the ocean is usually a gentle process, and when steam is free to expand and move away, there are no violent steam-driven explosions.
But a hidden danger lurks beneath the ocean. The lava entering the sea breaks up into blobs (known as pillows), angular blocks, and smaller fragments of glass that form a steep slope beneath the water. This is called a lava delta.
A newly formed lava delta is an unstable beast, and it can collapse without warning. This can trap water within the hot rock, leading to violent steam-driven explosions that can hurl metre-sized blocks up to 250 metres. Explosions occur because when the water turns to steam it suddenly expands to around 1,700 times its original volume. Waves of scalding water can also injure people who are too close. People have died and been seriously injured during lava delta collapses
So, the ocean entry points where lava and seawater meet are doubly dangerous, and anyone in the area should pay careful attention to official advice on staying away from them.
What more can we learn from these eruptions?
Once lava deltas have cooled and become stable they represent new land. Studies have revealed that lava deltas have distinctive features, and this has enabled volcanologists to recognise lava deltas in older rocks.
Remarkable examples of lava deltas have been discovered near the top of extinct volcanoes (called tuyas) in Iceland and Antarctica. These deltas can only form in water and the only plausible source of this water in this case is melted ice. This means that these volcanoes had melted water-filled holes up to 1.5km deep in ice sheets, which is an astonishing feat. In fact, these lava deltas are the only remaining evidence of long-vanished ice sheets.
It is a privilege to see these incredible scenes of lava meeting the ocean. The ongoing eruptions form part of the natural process that enables beautiful volcano islands like Hawai’i to exist. But the creation of new land here can also bring danger to those who get too close, whether it be collapsing lava deltas or acid mist.
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.
Antarctica is a vast icy wasteland covered by the world’s largest ice sheet. This ice sheet contains about 90% of fresh water on the planet. It acts as a massive heat sink and its meltwater drives the world’s oceanic circulation. Its existence is therefore a fundamental part of Earth’s climate.
Less well known is that Antarctica is also host to several active volcanoes, part of a huge “volcanic province” which extends for thousands of kilometres along the western edge of the continent. Although the volcanic province has been known and studied for decades, about 100 “new” volcanoes were recently discovered beneath the ice by scientists who used satellite data and ice-penetrating radar to search for hidden peaks.
These sub-ice volcanoes may be dormant. But what would happen if Antarctica’s volcanoes awoke?
We can get some idea by looking to the past. One of Antarctica’s volcanoes, Mount Takahe, is found close to the remote centre of the West Antarctic Ice Sheet. In a new study, scientists implicate Takahe in a series of eruptions rich in ozone-consuming halogens that occurred about 18,000 years ago. These eruptions, they claim, triggered an ancient ozone hole, warmed the southern hemisphere which caused glaciers to melt, and helped bring the last ice age to a close.
This sort of environmental impact is unusual. For it to happen again would require a series of eruptions, similarly enriched in halogens, from one or more volcanoes that are currently exposed above the ice. Such a scenario is unlikely although, as the Takahe study shows, not impossible. More likely is that one or more of the many subglacial volcanoes, some of which are known to be active, will erupt at some unknown time in the future.
Eruptions below the ice
Because of the enormous thickness of overlying ice, it is unlikely that volcanic gases would make it into the atmosphere. So an eruption wouldn’t have an impact like that postulated for Takahe. However, the volcanoes would melt huge caverns in the base of the ice and create enormous quantities of meltwater. Because the West Antarctic Ice Sheet is wet rather than frozen to its bed – imagine an ice cube on a kitchen work top – the meltwater would act as a lubricant and could cause the overlying ice to slip and move more rapidly. These volcanoes can also stabilise the ice, however, as they give it something to grip onto – imagine that same ice cube snagging onto a lump-shaped object.
In any case, the volume of water that would be generated by even a large volcano is a pinprick compared with the volume of overlying ice. So a single eruption won’t have much effect on the ice flow. What would make a big difference, is if several volcanoes erupt close to or beneath any of West Antarctica’s prominent “ice streams”.
Ice streams are rivers of ice that flow much faster than their surroundings. They are the zones along which most of the ice in Antarctica is delivered to the ocean, and therefore fluctuations in their speed can affect the sea level. If the additional “lubricant” provided by multiple volcanic eruptions was channelled beneath ice streams, the subsequent rapid flow may dump unusual amounts of West Antarctica’s thick interior ice into the ocean, causing sea levels to rise.
Under-ice volcanoes are probably what triggered rapid flow of ancient ice streams into the vast Ross Ice Shelf, Antarctica’s largest ice shelf. Something similar might have occurred about 2,000 years ago with a small volcano in the Hudson Mountains that lie underneath the West Antarctica Ice Sheet – if it erupted again today it could cause the nearby Pine Island Glacier to speed up.
The volcano–ice melt feedback loop
Most dramatically of all, a large series of eruptions could destabilise many more subglacial volcanoes. As volcanoes cool and crystallise, their magma chambers become pressurised and all that prevents the volcanic gases from escaping violently in an eruption is the weight of overlying rock or, in this case, several kilometres of ice. As that ice becomes much thinner, the pressure reduction may trigger eruptions. More eruptions and ice melting would mean even more meltwater being channelled under the ice streams.
Potentially a runaway effect may take place, with the thinning ice triggering more and more eruptions. Something similar occurred in Iceland, which saw an increase in volcanic eruptions when glaciers began to recede at the end of the last ice age.
So it seems the greatest threat from Antarctica’s many volcanoes will be if several erupt within a few decades of each other. If those volcanoes have already grown above the ice and their gases were rich in halogens then enhanced warming and rapid deglaciation may result. But eruptions probably need to take place repeatedly over many tens to hundreds of years to have a climatic impact.
More likely is the generation of large quantities of meltwater during subglacial eruptions that might lubricate West Antarctica’s ice streams. The eruption of even a single volcano situated strategically close to any of Antarctica’s ice streams can cause significant amounts of ice to be swept into the sea. However, the resulting thinning of the inland ice is also likely to trigger further subglacial eruptions generating meltwater over a wider area and potentially causing a runaway effect on ice flow.
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.
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.
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.
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.