When the ground starts to rise beneath your feet, it is time to sit up. Fishermen would be the first to notice, being unable to leave their harbours due to lack of sea. Governments would discuss the risk of reduction in tax income from fishing, and would commission research. The scientists report evidence of widespread withdrawal of the sea but their findings leave some uncertainty and they need more funding. An intergovernmental Panel of Ground Change (IPGC) is set up, and their first report, issued after 20 years, proposes that ground changes should be limited to two meters; it leads to animated and emotional discussions. A strong rebuttal is published in the Daily Mail and Fox News which report that no change in sea level is seen in Paris, Texas, an unchanging 300 miles from the sea, and can we now stop this nonsense? The next day the Daily Mail in a lapse of memory reports that scientists have found that rising ground cures cancer. The Financial Times writes that the inflation is creating free land which can be sold for profit and save the hapless banks. The Washington Post reports that the President seems out of touch and is unaware of the danger of the ground getting too close to the Sun and burning up. The Sun runs the headlines ‘Sun saves our bacon’ and ‘The Sun has won’. The Department of Homeland Security claims that there is no evidence that the land is rising but that the sea may be going down; it demands funding to build a wall to keep out the falling sea. The Canadian government orders all scientists to cease talking about ground level change, or, preferably, stop talking altogether. The Dutch are delighted; they also renew their historical claim on England. The Solid Earth Society puts out a press release which no one reads. And in the mean time, no one wonders what is pushing up the ground.
In the real world, some towns do have to cope with changing ground levels. Naples is best known for this: the submerged Roman city of Baiae, once the Panama City Beach of its days, is now a diving destination and underwater archaeological park as much as 12 meter below the sea; but nearby is evidence of clams having grown on pillars which are now 6 meter above the sea. Not many modern seaside towns could cope with those kind of ups and downs!
Inflating ground is a well-known precursor of volcanic activity. It may be caused by a growing magma chamber, or by circulating water; one may expect the former to lead to an eruption, and the latter to a phreatic explosion. Often such expectations are correct. Basaltic volcanoes especially inflate before erupting. But volcanoes often show inflation without a subsequent eruption. The magma may fail to reach some critical pressure, or a dyke is stopped in its tracks. And sometimes a volcano erupts without any inflationary sign. Turning inflation into a sure-sign prediction of a specific eruption date (or even eruption decade) is still beyond us.
This post is about inflation and the causes of inflation, and at the end describes some volcanoes which are inflation hot spots. It is a bit longer than the average VC post – you may want to get yourself a coffee first.
Uplift and rebound
That shifting ground underneath you may not be volcanic. The most dramatic long-term uplift is a memory from the ice age. Rock is strong – but 2 kilometer of ice is heavy, and the rock, and especially the more fluid mantle underneath, had difficulty accommodating the weight of those enormous ice sheets that covered the frozen north. The mantle, essentially hot sand, is not as stiff as the lithosphere above it, and gives away first. The ice pushes down, and the mantle slowly moves sideways (not down: sand doesn’t compress much). As if on treacle, the ground slowly sinks, until a new equilibrium is achieved when it has gone down by a third of the thickness of the oppressive ice sheet. Indeed, parts of Antartica but also Greenland have been pushed below sea level by the immense weight of the ice.
Once the ice sheets melt, the mantle and the ground rebound. But anything that involves the mantle is slow; it takes many thousands of years before the memory of the ice age has been completely erased. North Scandinavia is still rebounding from the last ice age, even after 10,000 year. There are ancient beaches -and harbours- now well above the water line. In Gotland, Sweden, the uplift is still about 2 mm per year. It was much faster immediately after the ice age, of course. The ground here became depressed by 300 meter, and the uplift has so far recovered 270 meter. 30 meter is still to come.
On the other side of the Atlantic, the Hudson Bay is the remnant of the depression left by the Canadian ice cap. The ice cap was 2 km thick. At the centre of the ice cap, the ground was depressed by 700 meters. Much of this has been recovered, but not all. The Hudson Bay is still 270 meter deep at its deepest point; the average is around 100 meter. But getting less.
This weight effect can also affect volcanoes. A kilometers-tall mountain depresses the ground on which it rests. A volcano is a lot less heavy than an ice sheet and therefore the weight can be carried by the lithosphere: it doesn’t reach the mantle. The lithosphere is stiff and can carry a bit of extra weight, so at first the volcanoes don’t depress the crust, even as they grow. But once they get huge, the lithosphere does give way. Mauna Loa, the world’s largest volcano, is sinking by 2cm per year as the underlying lithosphere adjusts to the weight of the mountain. (But the lava flows on top make Mauna Loa grow at the same rate, so the net effect is no change.)
Now assume that Mauna Loa has just exploded, and the explosion has removed tens of cubic kilometers of rock. (Don’t worry: this is an experiment in imagination. Mauna Loa is completely safe.) What happens to the ground below? Suddenly the weight is gone. And the lithosphere rises in response. But in practice, this is exceedingly slow. The lithosphere is stiffer than the mantle, and adjusts even slower. 10,000 year is nothing to it. If Mauna Loa were to lose 1 kilometer of its height, the recovery from the depressed ground would be no faster than 1 mm per year. So even after extreme eruptions, this can be ignored. Sudden weight loss does not change your height – at first. Still, in the end gravity always wins. Mauna Loa is destined to sink below the sea, even if it will take a few million years. It is like a slow Hollywood remake of the Sinking of the Lusitania.
But in reality, large volcanoes can show quite a bit of reflation after a caldera-forming eruption. So if this is not the lithosphere re-adjusting, what is it? As always with volcanoes, it is the magma. (A good line to win any debate: “It is the magma, stupid”.) After the explosion/eruption, there is less weight pressing down on the magma reservoir. Less pressure means that the melting temperature drops a bit, and this allows more melt. The newly melted magma rises, and creates a new magma chamber underneath the caldera. It may slowly re-solidify there. The final result is a large bulge in the centre of the caldera, pushed up by this risen decompression magma; the bulge is called a resurgent dome.
A good example is the huge Toba caldera. It is filled with an enormous lake, and the lake is filled with a large central island. This island is the resurgent dome. There are three resurgent calderas in the US, namely Yellowstone, Long Valley, and Valles Caldera. The resurgence can be very large. Toba resurged by well over a kilometer. But it is also rather slow (although not as slow as gravity): the resurgence may take 10,000 year or longer. After its stupendous eruption (the largest for the past million year), Toba resurged by 10 cm per year; this lasted many millennia.
Although the resurgent dome may erupt, often it doesn’t: in itself, the resurgence is an afterthought from the original explosion, and not a sign of an impending new eruption. Resurgence is comparatively safe, as volcanoes go; it takes so long that the magma has time to solidify in its new place. So when large, long-term inflation inside an old caldera is seen, it tends to be identified as a resurgent dome: this model works, can explain the observed changes, and there is no need to panic. What more could you want?
Inflation in volcanoes
But non-resurgent volcanoes also show uplift. This happens when magma is accumulating, and it is a warning sign for a potential eruption. The basic model for the uplift was devised by a Japanese seismologist called Mogi, and with all the creativity of scientific language, is called the Mogi model. It assumes a spherical magma chamber, with a certain diameter and pressure, located at a depth below the surface which is much larger than its diameter. This round blob pushes up the surface over an area beyond its own diameter.
The size and shape of the uplifted bulge depends on the volume of the magma chamber, the pressure, and the strength of the rock, all of which are approximately known. There are equations where you plug in the measured surface motion, in three directions, and out comes the depth and size of the magma chamber. It involves a few questionable assumptions: magma chambers are probably not round, and they may be closer to the surface than the equation wants, but in the right circumstances the model has worked well enough. (Rumour has it that the mathematics of horse racing, developed for the gambling industry, began with the sentence ‘Let’s assume a spherical horse..’) (Mogi himself wrote It is the writer’s opinion that the assumed spherical origin seems to harmonize well with the idea of the magma reservoir under the earth’s surface.) There is however an obvious problem. It begs the question how did the new magma get there? Magic? Did Scotty beam it down (or up)? Or did it achieve reality as an ‘alternative fact’? None of this seems quite compatible with physics, geology, or even volcanology. Either the magma migrated from elsewhere, or it formed in situ from newly melted rock. If it migrated, the observed inflation will be compensated by deflation somewhere else.
New melt will often start with an impulse of heat from below, perhaps carried up by magma rising from the deep. The heat seeps into the existing magma chamber, and causes more rock to melt, enlarging the chamber or increasing the melt fraction. Magma has a lower density than the rock it forms from: the magma expands. It is the pressure from this expansion that causes the rock above to rise, to accommodate the increased volume. At the same time the growing magma flows upward through any cracks in the rock it finds, carrying the heat with it. The speed of the rise depends on the size of the cracks. For a typical size of 2 mm, the ‘speed’ is somewhere around 1 meter per year. It beats continental drift, but not much else. Enlarge the cracks or break the rock with a well-timed earthquake, and it can go much faster. As the magma rises, the inflation becomes more focussed on the area directly above the chamber. Whilst it was deeper, it pushed up a larger area. This gives the funny effect that as the magma rises, some distance away the inflation turns to deflation, while in the centre it increases. Think sombrero.
The new heat can also affect any water inside the volcano. Hot water can circulate much faster than magma, and this can therefore cause fast inflation when the water enters new areas. Large, intermittent variations inside a caldera are often due to circulating hot water. Gases should also not be ignored: underground volatiles can add tremendous pressure. An earthquake may rupture an underground layer or rock, allowing gas underneath to escape, and cause sudden deflation. Or shake the magma, allow volatiles to come out, and up goes the mountain. In volcanoes, nothing is simple.
So there are at least three ways a volcano can inflate: resurgence, a magma/heat pulse (with or without volatiles), or hydrothermal circulation. How can you tell which one is happening in your favourite volcano? That is not so easy! The only reliable way to see underground is to wait for an eruption and analyse what comes out: new magma, recycled and reheated magma, gas, and/or water. Obviously, that only works if there is an eruption. Non-erupting volcanoes are the bane of volcanology.
Volcanoes in inflation
Let’s look at some real-world examples inflation, a round-up of the usual suspects.
Nowadays, measuring inflation is fairly easy, using GPS or radar. But those only go back a few decades. How do you determine inflation which predates GPS? Actually, just as in glacial rebound: it is best seen at the coast, from historic water lines, like the dirt rims in an empty bath tub. The known cases of prehistoric inflation are indeed all at sea (or at least at water). The following are the best known, and largest, cases.
Siwi caldera, Vanuatu
At the southern end of the 1200 km long Vanuatu volcanic arc lies Tanna Island, a land of rain forests, beautiful beaches, and coffee plantations – my kind of paradise. Lonely Planet calls it ‘an extraordinary place‘ with ‘the world’s most accessible active volcano‘. The volcano in question is Yasur, an 800-year old dome which is indeed the most active volcano in Vanuatu – and that says something. It is located inside a 2-km wide caldera. An eruption some 20,000 year ago (not well dated) formed the much larger Siwi caldera; Yasur lies on the rim of this caldera. Within the ancient caldera is the Yenkahe block, 6 by 3 kilometer, and 250 meter high. The block contains a series of coral terraces, all well above the sea level. Coral does not form on dry land, or even in the wetness of a rain forest: clearly much of this block was once below the sea, and it came up step-wise. The dates of the terraces show that the area has been rising for a period of 1000 year or more, at a rate of 16 cm per year, by more than 150 meter overall. The inflation rate is the second largest known in the world over such a long period.
Naturally, this being inside a caldera, it is called a resurgent dome. But it wasn’t a massive caldera eruption (1-2 km3), and it is an area of very active volcanism – there is plenty of new magma sloshing around. It has been proposed that the magma chamber underneath Yasur may be migrating east, pushing up Yenkahe in the process.
There are no obvious signs of an impending eruption. In fact, nearby Yasur provides a safety valve, and this would limit the pressure which can build up. But there is another danger. The area is prone to landslides, and the continuing uplift can destabilize the slope. There is a risk that a large landslide would enter the sea, and the resulting tsunami, modeled as up to 4-5 meter high, would put some of the tourist areas at risk. The dense rain forest makes it difficult to assess the risk of this.
Iwo Jima (Iwo To)
This is the most extreme, but also least understood, currently on-going long-term uplift. The island, 4 by 8 km long, is famous as a war shrine. In the US, it will always be known as Iwo Jima, but in Japan, an older name (with the same spelling) has been re-instated: Iwo To. The different names call out a common heritage, a memory to be kept alive in order to prevent a future madness. Painful memories have turned into a common purpose, and made friends out of sworn enemies of the past. But there is also a volcanic heritage here: there is much more island now than there used to be. Iwo Jima is coming up at an average rate of 25 cm per year, and appears to have done this for at least 1000 year. The island is covered in old beaches, now far above the water line. Some of the old beach lines are bent, indicating that inflation was not always uniform across the coast, but every part of the island has seen some uplift. One of the beach lines dates from the time when Iwo Jima was discovered, in 1779. This is now 50 meter above the sea. A beach 100 meter high is carbon-dated to 750 year BP. The average long-term uplift is about 15 cm per year. But the rate does fluctuate. Between 1779 and 1887 the uplift was significantly less than 10 meter and possibly zero, and a carbon-dated beach 1350 year old seems not much higher than the one at 750 meter.
Over time the uplift seems to have accelerated, based on the rate of 25 cm per year since 1952. Between 2006 and 2012 there was an episode with an uplift of 3 meter; we do not know whether such an episode is unusual.
The rate and duration have lead to the suggestion that Iwo Jima is the perfect example of a resurgent dome. However, this is not universally accepted, and Newhall & Dzurisin in a US Geological Bulletin of 1988 write An important and unanswered question is whether the historical uplift is rebound since the last caldera-forming eruption, or rather a precursory signal of another caldera-forming eruption to come. The literature mentions that Iwo Jima sits inside a 9-km caldera. This comes from a bathymetric map published in 1983. This map and two profiles are reproduced here. It shows that Iwo Jima is part of a flat-topped area, 9 kilometer wide, beyond which the mountain drops steeply down, but there is not much evidence of a clear caldera ring. One profile shows two symmetric bumps, 9 kilometer apart, but profiles in other directions do not. If there is no clear caldera, the case for post-caldera resurgence is not strong. The flat top can also have formed through wave erosion. Even if there is a caldera, we still don’t know what is going on.
The uplift affects more than just the main island. The small island just to the northwest is also growing. We don’t know what happens under the sea but the entire top of the undersea mountain may be coming up. You’re gonna need a bigger boat.
Is Iwo Jima the most dangerous volcano in the world? Perhaps it is.
Laguna del Maule
This Chilean lake is a relief in a thirsty semi-desert. The landscape is a geological treasure, but not a touristic highlight. It still made world news, when the southern shore was found to have been uplifted by 1.5 meter in 8 years, from 2007 to 2014. This was attributed to 0.02km
There have been 36 eruptions here during the past 20,000 year – once every 600 year. The largest of these eruptions, 23,000 year ago, ejected 20 km3 of ash, amounting to a few km3 of rock. This puts the magma supply rate in context: it has by now recovered the loss from that eruption. Laguna del Maule has not erupted since the Spanish arrived, 450 year ago. It seems a strong candidate for an eruption, even if fairly minor, in the next few centuries. You read it here first.
Socorro, New Mexico
Socorro is perhaps not the liveliest place in the world. The main excitement is floating down the Rio Grande on a tube, looking at the big ‘M’ painted on the local hill, or watching the sandhill cranes and roadrunners in the Bosque del Apache. To the east is the site of the first nuclear explosion in the world, an attraction perhaps best avoided (it is open to the public for two days each year), and a town to the south is called Truth or Consequences, reportedly the only town in the world named after a TV show. With the American penchant for shortening names, it quickly became ‘T or C’. Still, Socorro grows on you. And the locals appreciate visitors, as I found out during a hike, when sitting down for lunch. Immediately three vultures started circling above us, visibly thinking: “They stopped moving! Lunch!”.
Socorro also grows underneath you. The excitement is all underground. The area shows slow inflation, at 1.5 mm per year, which is caused by a magma sill 19 km deep and 150 meter thick. The sill may be the largest magma chamber in the US, with a length of 60 km stretching along the Rio Grande rift valley. The volume is between 100 and 1000 km3. The enlargement of the magma body has caused earthquakes, with a swarm between 1999 and 2004, but also a M5.8 in 1906. The inflation has been going on since at least 1911, but the magma intrusion itself has an estimated age of 1400 year. (Albuquerque, to the north, is subsiding, but this is attributed to over-extraction of ground water.) The area is littered with volcanic cones, a sign of things to come. But in Socorro, nothing happens fast. It takes its time. Still, we’re not in Kansas anymore.
The Taupo Volcanic Zone
New Zealand is volcanically hyperactive. The source of its problem is the Taupo Volcanic Zone on North Island, terminating in what is appropriately called the Bay of Plenty. Based on its history, this is the area (relative to its size) most likely to host the Earth’s next problematic eruption. But detailed monitoring has shown that the entire area is deflating, due to cooling of magma. The entire area? No. One region, away from the known volcanoes, and discovered only last year, shows inflation. It is just on the coast or perhaps centred in the Bay of Plenty. The uplift is at a rate of 0.5-1 cm per year, over an area of close to 300 km2. The required magma supply rate is around 0.01 km3 per year. Geological evidence indicates that this has been going on for at least 1700 years! Just when you think New Zealand is safe, it isn’t. You can’t trust sheep.
Recently inflating volcanoes
The ones above are long-lasting uplifts, but where eruptions do not seem particularly imminent. Ground level measurements have revealed other cases, not as long lasting, but increasing the chance of an eruption. Here are some. This list is certainly not complete!
South Sister, Oregon
The area of inflation is just west of South Sister, which is one of the three sisters in Central Oregon. The inflation began in 1996; it is caused by a growing magma chamber 5-7 km underground. Measurements from the European Space Agency (don’t tell the new administration – they might ban these immigrant data) shows that it started at a restrained 1 cm per year in 1996, and quadrupled after 1998. From 2004 the uplift has slowed but not ceased; the total uplift has been 25 cm. The magma volume responsible for the uplift amounts to 0.06 km3. Eruptions here are relatively infrequent, with the last one 1200 year ago. The inflation is centered several kilometers from South Sister – there is no volcano directly above it. Either the magma will find a sideways outlet, or a new cone will form once the magma decides to come up. A younger Sister to join the family.
There are (too) many volcanoes at the Altiplano, the unlivably high plane in the Chilean/Bolivian Andes. A few birds live here, drawing whatever oxygen they can from the thin, bone-dry and frigid air. Be careful, and acclimatise, or the altitude sickness can kill you. Monitoring these volcanoes is a Herculean job, assuming Hercules could live on this little oxygen. (Excessive muscles put a lot of oxygen demand on the body, whilst turning off the brain only saves 25 per cent of your body’s oxygen needs, so the net effect is that Hercules would do better at sea level.)
Monitoring in the Altiplano began in 1980. It quickly revealed a lurking danger. One of the volcanoes, double-peaked Uturuncu, was growing at a rate of 1-1.5 cm per year, starting in the early 1990’s. That is not that much for what is now an (almost exactly) 6km tall volcano (at that rate it would have taken half a million year to get to its current size, which is not unreasonable), and it is a well-behaved volcano which has not erupted for 300,000 year and shows no sign of impending doom apart from a sickening sulphur smell. (Until recently there was an active sulphur mine on the mountain.) Still, something is going on.
The inflation is remarkably stable, and although there is some microseismicity, there are no major earthquakes. Perhaps a ductile layer is allowing the magma chamber to quietly expand without building up large stress. The magma chamber is 15 km deep, and may be as much as 100 km wide which would make it the largest magma body in the world. The uplift is centred just southeast of the peak. The region of uplift is surrounded by a ring of deflation: this means that the magma is rising – slowly, but notably. When it reaches the surface, perhaps in another 1,000 year, things could get very interesting. Watch this space.
Uturuncu tends not to do explosive eruptions. It seems that magma usually stalls at shallow depths, and has time to exhale its volatiles, before deciding to erupt. A big friendly giant.
To VC readers, this will be a familiar name: Henrik wrote his magnificent post on this, one of the most dangerous volcanoes in the world. Campi Flegrei is where the phenomenon of the ground moving up and down was discovered. The entrance to the in-land harbour, Portus Julios, is now 12 meter below the sea, which shows the local deflation over 2000 years, although this deflation did not happen the same everywhere. But what goes down can come up. All eruptions here are preceded by rapid uplift. Before the 3700 BP eruption phase (which lasted 900 year), there was 40 meter of uplift. We don’t precisely know when the post-Roman deflation began. It was long lasting but not uninterrupted. In 1502, the astonished locals noticed that the sea was receding. This motion continued for over 30 years, and in 1534 the first earthquakes began. In 1538, during one week the ground came up by 6 meters, a rate reminiscent of magic beans. The rise was uncontainable and an eruption followed, building the Monte Nuovo (new mountain). One week later, when the eruption ended, this new mountain was over 100 meter tall. Somewhat ironic, the mountain grew in the middle of what had been the main lake of Portus Julios’s inland harbour.
But what comes up must go down and renewed subsidence followed the eruption. Between 1820 and 1968 the sea shore subsided at 1.5 cm per year. There was a brief reversal in the 1950’s. In 1968, the reversal came back with a vengeance; 1 meter of uplift had happened by 1970. Between 1982 and 1984 a further 2 meter of uplift occurred. Slow inflation returned in 2005, accelerating in 2012 to 8 cm per year. This is not dramatic by Campi Flegrei standards, enough to be noticed but nothing to raise alarm bells. Up-and-down movements by several meters are common in large calderas; it can caused by underground water moving around. Yellowstone in particular is known for this.
But now, there may be more going on. Fumarole temperatures are rising, and new gas vents are opening in the burning fields. There is more heat underground. A paper a few months ago, by Chiodini and collaborators, reported that the pressure is increasing, on a trajectory that would make an eruption possible by 2020. They point out strong similarities to the pre-eruption phases of Rabaul and Sierra Negra (Galapagos), with similar accelerating inflation over years to a decade prior to the eruption. This prediction won’t worry the locals who have too much in common with Californians. Here, tomorrow is another day. Vedi Napoli e poi muori. A city to die for.
There are too many volcanoes to list them all! In 2012, a radar study by Estelle Chausard and Falk Amelung measured ground elevation across the entire West Sunda Arc, Indonesia, between 2006 and 2012, an heroic effort. In this region (Java and Sumatra) six volcanoes showed significant inflation, with magma accumulating at 1-3 km depth. Three of these subsequently erupted.
The six were Sinabung, Kerinci, Slamet (these three erupted within 2 years after), Lawu, Lamongan, Agung. Anak Krakatau showed deflation, related to a number of eruptions. In all cases, the inflation was around 10 cm. The inflation was centred on the summits, apart from Lamongan where it arose north of the summit: this is also the only case where the uplift happened in one fast episode, rather than continuously.
Interestingly, one could say sneakingly, Merapi showed no inflation, but still erupted afterwards.
This overactive Japanese volcano on the southern rim of Aira caldera is a strong case study of a link between inflation and eruption. There were significant eruptions in 1914 (1.5 km3) and 1779. The 1914 event was preceded by strong inflation of the caldera. The eruption caused subsidence, but inflation restarted and the volcano is again at roughly the level it was before the earlier eruption. The inflation was at a rate of 1.5cm per year from 1996 to 2007. The magma inflow rate is estimated at 0.01 km3 per year, filling up a 10-km deep chamber. Modeling shows that the current activity of Sakurajima is not keeping up with the inflow: the volcano is charging up. The same model predicts 130 years between eruptions, which does fit the previous events well (scientists generally prefer to predict the past: Niels Bohr warned that “Prediction is very difficult, especially about the future.” Einstein had similar doubts: “I never think of the future. It comes soon enough.”) The inflating reservoir is not located underneath Sakurajima, but within the northeast part of the caldera. But there is clearly a good connection between this reservoir and Sakurajima.
Predicting the future (un-Bohr-like and un-wise), a significant eruption can be expected by 2044. And that’s all I have to say about that.
Longonot and Paka
InSAR data from the European Space Agency has shown inflation of 10 cm in Longonot between 2004 and 2006, and 20 cm in Paka in 2006 and 2007, both in Kenya. Paka, in the Kenyan rift valley, is a shield volcano with several small craters and cones. The last major eruption (VEI4) was over 7000 year ago but some of the cones may only be a few hundred years old. The recent uplift was preceded by similar uplift nearby, which subsequently reversed: magma flowed into the volcano sideways.
Longonot, 60 kilometer from Nairobi, is more impressive. It has a 10-kilometer wide caldera, 21,000 year old, in which a new summit has grown with its own 2-kilometer crater. It may have been active as recent as the 1860’s.
In both cases the uplift ceased and has so far not restarted. The 2-5 kilometer-deep magma chambers are clearly still active, and an eruption may be possible but we do not know what the chances are. African volcanoes are not well-monitored: we will need much more such data just to know which ones need a warning system. They are volcanoes in the mist.
Volcanoes inflate for all kind of reasons, and inflation does not necessarily equate to danger. Large calderas commonly show ups and downs, largely driven by water circulation, and not related to an eruption. There is little need to be worried about Yellowstone (neither does it show anything which would warrant its inclusion in this post.) But inflation can be an eruption precursor, a warning sign that should not be ignored, hoping that if you do nothing for long enough, it will go away. It is part of the puzzle of volcanoes: if the inflationary piece fits with other pieces that indicate increasing activity, it may be time to update the tourist accommodation and dust off the emergency plans. Up – and away.