2 posts ago I started talking about the pinnacle of basaltic explosivity, 2 posts later there is no mention of anything bigger than the Tarawera 1886 eruption. The eruption of Tarawera was a relatively common scenario of a dyke intruding below a lake, sure, the one responsible was a giant volcanic system of the Taupo Rift, but you are still in all your right if you are thinking I was being overly exaggerated!
If that is the case, do not worry, we have just looked into 3 cases of the last 120 years when we can actually look much further back, centuries and millennia back. So where could we find those big basalt blasts? The 3 volcanoes so far studied are Fernandina, Miyakejima and Taal, each one of them is recurrent, and has performed depressurization eruptions at least twice.
We need first a way to drain the volcano laterally, large enough flank eruptions, or rifts which can swallow a vast amount of magma. Next what we need is water to run into the magma reservoir when pressure drops low enough. Miyakejima taught us that the danger lies in water reaching near the surface. No matter how much water is dancing around the reservoir if it can’t be released into steam at the surface the big basalt blast is inhibited. When released into steam, a shallow explosion can depressurize the deeper superheated water, that’s when it uncorks and all hell breaks loose. But water wasn’t initially at the surface in Miyakejima. As its caldera deepened, water started to run into it and it was then that conditions turned from dry to wet.
There are many volcanoes that can erupt in this way, in fact I will end with a list of all volcanoes I believe have potential to produce them, but some deserve special mention. One stands above them all, as the most dangerous and the best studied. I feel some people may have been expecting Masaya in this article, here it comes.
This year Nik Wallenda had the unique experience of walking above a lava lake, walking on a tightrope! And he wouldn’t have worn any harness hadn’t he been insisted on wearing it! I wonder if he was aware about the history of Masaya, it probably wouldn’t have mattered to him, for someone determined to defy the forces of gravity and volcanoes some past explosive eruptions are the smallest issue.
Masaya looks like a small shield volcano. A bubbling fiery cauldron inside a pit crater complex opens at the top of a gently sloping cone covered in lava flows, might give the wrong impression of being gentle. But if we look at its eruptive history, what we find are three gigantic phreatoplinian/plinian events. These are the San Antonio Tephra with a volume of 14 km3 6000 years ago, the Masaya Triple Layer with 3.4 km3 2100 years ago and the brutal Masaya Tuff, 10 km3, at 1900 years.
Now, I of course think these eruptions were triggered by depressurisation of the magma reservoir (a BBB), otherwise I would not be talking about them… But I should first give some evidence. To begin with, Masaya is located in a rift, the Managua Graben, with its enormous caldera close to the water table. The proposed model predicts this is the ideal location for them to happen. Second, the Masaya of the last few centuries is a gentle small-erupting volcano that wouldn’t seem able to produce such massive eruptions unless there was a really good and extraordinary reason, which the BBB model gives. Third, the eruptions show strong water involvement. As an example, people escaping the Masaya Triple Layer left deep footprints into wet soft ash. Last, the Masaya Tuff was an eruption very similar to that of Taal in 1911, therefore with the same style of mechanism?
Taal obliterated an area of about 150 km2 with a base surge in 1911 resulting in the death of 1300 people. So what if I told you that the Masaya Tuff was the same kind of eruption but 50 times bigger in volume? The entire deposit shows the characteristics of an exaggerated base surge (cross-bedding, accretionary lapilli, indurated…). It extended 20-30 km in all directions. The base of the tuff is abundant in plant molds, this is probably all the vegetation in the area being flattened to the ground. And how long did it take? It looks like one big explosion, which must may have been over is less than an hour. This is frightening when you consider that about 2 million people live in this area, including entire Managua, the capital of Nicaragua. The city would be blasted by a wave of mud, rocks and maybe acid, reducing it to mud-plastered ruins. Now you know what I was thinking when 2 posts ago I mentioned the pinnacle of basaltic explosivity. Is there a more destructive eruption that a basaltic volcano can come up with?
The Masaya Tuff was followed by a second eruption phase, the Ticuantepe Lapilli. In terms of volume it was a small compared to what had come just earlier. But it was an energetic plinian eruption that sent a buoyant plume 25 km high into the atmosphere (contrasting with the horizontal flow of the Masaya Tuff), it threw dense blocks of old rock 30 cm across, 15 km away. Most of the volume are lapilli fragments of fresh lava. The eruption still has signs of water interaction, like layers of tuff, but the style is drastically different, much more “magmatic”. What caused this sudden change in style?
The 3 eruption cases that I have described in previous posts turned out to be once again the key to solve the enigma. Taal and Fernandina had more in common with the Masaya Tuff though at a much smaller scale. They are hydrothermal blasts. However the 2000 August 18th eruption of Miyakejima had more in common with the Ticuantepe lapilli. It was a more magmatic eruption, while still phreatomagmatic, there was a lot of fresh lava towards the end. The difference between them is that Taal and Fernandina blew long before or without a caldera collapse, while Miyakejima blew during a collapse event. And this suddenly made perfect sense. Let me explain.
If the Masaya Tuff was a hydrothermal blast then a jet of steam would blow open a path from the surface to the magma reservoir. What would happen if a collapse event took place as the jet was still active? Remember collapse events? They mean that the ground suddenly drops on top of the magma reservoir and pressurizes it, magma would be squeezed out into the steam jet, you can imagine what a mess this would be. Volcanic gasses being violently released, the magma meeting whatever water remains in the conduit, magma shooting upwards at amazing speeds, a plinian/phreatoplinian eruption is the reasonable result. This is an unproven model of course, but the only one as far as I know than can succesfully explain the formation of the caldera, the extraordinary explosivity of the often gentle basaltic magma and the sharp changes in eruption style at the same time.
The hydrothermal blast can happen before, at the same time, after a collapse event, or with no collapse whatsoever. There can be multiple blasts and collapse events too. This gives a wide variety of eruptions a BBBs can take. Here I have illustrated, one of them, the Masaya Tuff model:
The terrible past of the Managua Graben is written in rock. The fiery cauldron of Masaya is a reminder that this area is geologically alive, but there is no surviving memory among the people. Can we blame them? This region has dormant for so long, and it could be centuries or thousands of years before the next disaster is ready. Hopefully we could say the same for all volcanoes of the world, but some just don’t know how to stay put. Enter the hyperactive volcano who is an angry goddess and a giant volcanic laboratory at the same time:
A volcano of legend, abode of Pele, the terrible goddess of volcanoes, fire, lightning, and so on, feared by native hawaiians. Then as the western world arrived to Hawaii, Kilauea went for a whole century of highly touristic lava lake activity. No surprise it started to be considered as one the safest volcanoes of the world. But now it stands as the number 1 most dangerous volcano of the U.S. in the ranking of the USGS. Yes, it ranks above volcanoes such as Mount Saint Helens or Rainier. And believe me, there is good reason, and I have additional reason to believe it poses a threat in the near-present.
Back when Pele was feared, not like now that Aila’au is the one getting blamed when the volcano does something destructive, the explosivity of Kilauea was well known through oral history. The last story passed down and thus best recorded is that of Keonehelelei, means “the falling sands”.
In 1790 a civil war was taking place on the island, after the death of the previous king, ambitious Kamehameha had tried to seize the rule, sparking a conflict. It was finally down to the cousins Kamehameha and Keoua. Learning that his homeland was under attack, Keoua decided to race there through the quickest route, which led through Kilauea, was his fate sealed at that moment? The victory of Kamehameha solidified? Or was the ambition of Kamehameha, who would later unify the islands, too strong to oppose from the beginning? Who knows.
While encamped at the summit an eruption started, explosions continued for 3 days, which they used in trying to appease Pele. Eventually Keoua decided to advance, they walked over a recent layer of wet accretionary lapilli in which their footprints became imprinted, much like at Masaya. I think about the people of Masaya, were they able to escape or would their fate be as sad as that of the people of Keoua? Just as they marched, the paroxysmal phase of the 1790 eruption started. The army had been divided into three groups, the first party felt as if they were suffocating and some “were burned to death by the sand”, this could mean either acid or heat. The last party was caught in the ash fall but was fine. When the darkness of the eruption cleared they resumed the advance to find everyone in the second party dead, they had been caught in a base or pyroclastic surge (hard to tell the difference and their definitions are not clearly set). The bodies were lying on the ground or sitting, some were touching noses, a gesture of affection. In Hawaii warriors travelled with their families.
There are many estimates on the number of casualties, but they are usually cited to be 400-800. Kilauea is a shield volcano, it is covered mostly in lava flows but it can do this. Fernandina is also a shield volcano… You see where I’m going. Don’t underestimate any shield/basalt volcano that has a caldera or lake at its summit, or better don’t underestimate any volcano at all.
The 1790 eruption fits the characteristics: It is mostly lithic, very violent, wet, there is also evidence for a rift eruption and a caldera collapse at the time, so I consider it to be an example of a big basalt blast.
Some of you may be wondering about the fate of Keoua. He survived, but later lost the war. Even more of you are probably wondering about the new lake on top of Kilauea. Yes, it is bad news. Where the collapse of 2018 was dry, a new collapse in the caldera as it currently is will turn phreatoplinian in a way akin to 1790. There are many ways the lake can blow (dyke, open conduit, phreatic) but the most worrisome is probably if it is depressurization-triggered, the 1790 scenario. Kilauea collapses frequently, sometimes sizable collapses have taken place just 10 years apart from each other. With its south flank moving and its east rift inflating, another collapse in the next few decades is a very real possibility, or even a likely possibility. Some small towns are located dangerously close to the caldera of Kilauea. If someone from HVO asked me, I would tell them to prepare for the worst.
Before I list all the shield volcanoes not to be underestimated, there is one last place that deserves special mention, we keep cutting across the Pacific Ocean and reach the land of exotic volcanoes.
Central Vanuatu is home to 3 adjacent and unusual volcanoes, Ambrym, Ambae and Gaua. They are a puzzle of mysteries and contradictions. They are basaltic but their surfaces are covered in thick ash deposits. They are in a geologic setting that should be highly compressive, where an ancient volcanic arc is colliding with the active one, but they have very efficient rift zones. The volume of their edifices and the supply they enjoy seems exaggerated for normal arc volcanoes, Ambae for example has a volume of 2500 km3, more than 4 times the largest volcano of the Cascade Arc! With their large calderas and 2500 to 3500 mm of annual rainfall, these 3 should be quite the hotspot for big basalt blasts, could this be the case?
Ambrym stands out for its high activity. It has been erupting almost every year of its short recorded history and in a myriad of ways, from submarine, to vulcanian, or subplinian, hawaiian and the impressive feat of sustaining 4 lava lakes at the same time. The island has been described as a giant tuff cone. There seems to be more ash than lava, and it is topped by a large 12 km wide caldera. Among the many eruptions that have shaped the island there is likely to be a contribution from some big basalt blasts to have accumulated such an impressive and widespread thickness of ash.
Does a large caldera like that of Ambrym mean potential for a larger explosive eruption? Intuitively it would seem so. Logically a broad caldera means that the hydrothermal system can be more voluminous and that there is a greater area of contact between water and the magma reservoir which may allow a higher heat flux if pressure falls. If the roof drops and magma is squeezed out there is more magma available for this too. It seems therefore that Ambrym may be particularly suited for large magnitude events, Gaua too. The tens of meters thickness of pyroclastic material at Gaua and the few hundreds at Ambrym do look suspicious, so far study is poor and there are no good dates available.
Ambrym poses a near-future threat, like Kilauea. The pit craters at the summit have kept deepening since 1943. It should be about time they hit the water table. A corner of the 12km caldera started rupturing during an intrusion in 2018, the next collapse might well rupture all around.
To the north of Ambrym we find Ambae, here they had to deal with damaging eruptions in 2017-2018, but these are really child’s play compared to what this volcano should be able to do. The potential to generate lahars has been remarked before, this is certainly a hazard but I am much more worried about a base surge. The summit lakes of Ambae are perched 1400 m above sea level, there is no obstacle to contain a base surge, the steep slopes would add even more speed to it and it would probably be highly acidic (Lake Voui is an acid lake). An eruption similar to the one of Taal in 1911 would be amplified by topography and lay waste to most of the island faster than people can realize what is going on.
Eruptions of Ambae in 1870 and 1914 resulted in casualties but nothing else is known about them, so it is possible that they may represent phreatoplinian events related to the formation of Lake Voui. No cone existed inside the caldera of Ambae before the eruptions this century, this suggests the caldera may have been pristine (from 1914?), freshly formed. So it is an interesting possibility for any researcher, and it could help figure out exactly what are the volcanic hazards this island faces.
Once thought to be safe, basalt volcanoes can actually be quite deadly. Their greatest weapon? Probably the base surge. We have seen Taal deal a deadly devastating blow with a shockingly small volume, likewise Kilauea is to blame for the deadliest eruption in US history. Falling blocks, lahar, ashfall, tsunamis are among the other possible threats of big basalt blasts. I have evaluated the potential of many mafic/basaltic systems to produce these events and compiled the list below you may find volcanoes that you wouldn’t expect to see, but if you start to look at them with different eyes then I am relieved for this series has served its purpose.
At some volcanoes conditions to produce BBBs do not seem to be met, but it may not be completely impossible either. Unlikely:
Sierra Negra, Darwin, Mauna Loa, San Carlos, Tolbachik, Bardarbunga.
The following could meet the conditions but I found this uncertain. Possible:
Nyamuragira, San Joaquin, Villarrica, Wolf, Ecuador, Pinta, San Salvador, Yasur, Hachijojima, Isla Tortuga, Fuji.
This group shows the adequate conditions or there is evidence for past phreatomagmatic eruptions or both. Probable:
Nyiragongo, Etna, Cerro Azul (Galapagos), Marchena, Genovesa, Piton de la Fournaise, Grimsvotn, Karthala, Niuafo’ou, Ambae, Aogashima, Izu-Oshima, Kilauea.
The following are probably capable and the size of their calderas suggests large magnitude events could happen. High VEI 5 to 6. Probable:
Masaya, Gaua, Ambrym, Taal.
The last ones have been confirmed to produce BBBs, as there is clear evidence for lateral draining as the trigger:
Miyakejima, Fernandina, Taal.
On the explosive eruptions of Masaya, a very complete thesis on all 3 events: