Hatepe (Taupo, 186 or 232 AD)
The Hatepe eruption from the Taupo caldera system is perhaps the powerful volcanic eruption in the last 5 ka. It is submerged under Lake Taupo, the largest freshwater lake in Australia – New Zealand. It is physically a reverse volcano, where the caldera is the lowest point in a 40 km radius and the flanks slope downward into the caldera. The eruption took place before human movement into New Zealand, so there is no Maori tradition of this eruption or obvious impacts on their new settlements.
The Taupo Volcanic Zone is one of the most frequently active and productive rhyolitic systems on Earth. Hatepe was the latest of at least 27 eruptions since the 25 ka Oruanui eruption from Taupo ejected nearly 1,200 km3 in a VEI 8.1. Caldera collapse from this eruption formed the wide basin under Lake Taupo today. The caldera has poorly defined margins and is listed by Smithsonian GVP as 35 km in diameter.
As with Santorini, there is also a bit of discussion about the date of the Hatepe eruption, with the original date of 186 AD being supplanted over time by the current 232 AD eruption. The eruption itself was a complex rhyolitic eruption that generated a new caldera collapse. It is also 5x larger than any of the eruptions between Oruanui and Hatepe.
The eruption was centered on at least three vents aligned on a NE – SW fissure under eastern Lake Taupo. This was divided into at least seven phases, each of which produced diverse, pyroclastic eruption products.
The eruption sequence has five explosive phases, two of them wet (phreatomagmatic), three of them dry that deposited pyroclastic fall deposits. The sixth phase was the climactic Plinian, caldera-forming event. After some years or a decade or so, lava was extruded into the bottom of the lake building submarine domes (Horomatangi Reefs and Waitahanui Bank). Large pumice blocks broke away from the domes, floated to the surface, and were driven ashore by local winds at multiple sites on the E side of the lake.
Fall deposits deeper than 10 cm cover at least 30,000 km2. Ignimbrites cover a near circular area 20,000 km3. The Taupo ignimbrite is considered to be the most violently emplaced pyroclastic flow known. Pyroclastics from Taupo are regarded as type examples globally due to widely varying eruptive styles.
The eruption lasted somewhere between several days to several weeks, late summer to early autumn (Mar – Apr). It proceeded as follows:
Phase 1 began with minor phreatomagmatic activity. Cold lake water mixed with rising magma to generate fine-grained initial ash over a period of a few hours. Eruption column was around 10 km. Distribution of material is localized, with a volume around 0.05 km3 of loose material.
Intensity stepped up with Phase 2, as the eruption generated a Plinian column at least 30 km high. Ashfall was blown over a large area and distances. Early dilute pyroclastic flows were also erupted. Fall deposits are faintly bedded, indicating an average, moderately powerful Plinian eruption. Estimated volume of this phase is 2.5 km3.
The Phase 3 ash is fine-grained, indicating the vent widened, allowing abundant external water to access the rising and erupting magma column. Steam would have been generated in substantial amounts, condensing to produce rain, which flushed ash out of the eruption plume. This fell as wet, sticky mud draped over branches of neighboring trees and shrubs. Phase 3 was interrupted by a brief period of dry Plinian activity. Length of this phase was a few to tens of hours, erupting around 1.9 km3 of loose material.
There are at least two of what looks like breaks in activity after Phase 3. Durations are unknown. There is a more prominent break at the end of Phase 3 that lasted from some hour(s) to less than 3 weeks. A contrary suggestion is that Phase 3 and 4 were erupting simultaneously from two different vents during intense rainfall. There are eroded shallow gullies about 1 m deep in tephras E of Lake Taupo. The rain may have come from ejected lake water, condensate from Lake Taupo steam, or a meteorological storm(s).
Phase 4 renewed the eruption with phreatoplinian ash. Abundant water encountered a submarine dome of dense, degassed magma exposed in the new vent. This erupted a dense, vine grained, obsidian rich ash that landed wet. This material was also eroded by water. Phase 4 is estimated at 1.1 km3.
An abrupt switch to a dry and very powerful magmatically driven eruption started Phase 5. The Plinian plume is estimated at 35 – 40 km. This deposited pumice over a wide area. Eruption intensity reached near extreme levels with magma being discharged at very high rates. There are at least 11 smallish pyroclastic flow deposits erupted. These were either deposited by a partial plume collapse or a small overflow from the vent. Most of this material went E, well beyond the coastline and into the Pacific. This plume is described as ultra-Plinian, the first known example of this class of eruption at the time. Estimates of plume height range 30 – 55 km, near the theoretical upper limit of plume height. Best estimate of erupted material is around 7.7 km3.
Phase 6 was the climatic phase of the eruption, an extremely violent emplacement of the Taupo ignimbrite due to plume collapse from Phase 5. The vent(s) widened, and ground subsided forming the new caldera within the older Oruanui collapse structure. 30 km3 of pyroclastic material was ejected in as little as 15 minutes. It was hot, highly fluid, and flowed at high speeds (600 – 900 km/hr), fast enough to easily crest neighboring peaks nearly 2,000 m high. Only Ruapehu was high enough at 2,797 m to block the flow. The base layer of this flow was nearly frictionless. It covered low points in the terrain it covered, smoothing it a bit. It also scoured underlying soil in places and created flow front jets along the leading edge of the wave front.
The violence of the energy release by the quick collapse of the eruptive column should have been large enough to create what is called a volcano-meteorological tsunami, essentially detonation shock wave due to the collapsing column displacing a large volume of air laterally causing pressure waves that may have reached coastal areas worldwide.
Sometime after the end of Phase 7, a few years to decades, rhyolite lava domes were erupted of the newly reformed Lake Taupo. The domes are topped with glassy, pumaceous rhyolite debris, some of which detached, floated to the surface, beached and cooled.
Santorini (1600 BC)
Santorini (Thera, Thira) is the emergent remnant of a volcanic caldera. It is the southernmost member of the Cyclades, about 200 km SE of the Greek mainland. There are several islands around the rim of the caldera, with an area of 73 km2. Highest point on the island is 367 m. Population as of 2011 was 15,500. Thera is the classical Greek name for the islands. The islands were named Santorini by the Latin Empire of the 13th Century from the name of the Saint Irene cathedral in the village of Perissa.
The volcanic system has been quite vigorous, with 15 listed eruptions in the VOGRIPA database in the last 360 ka. These ranged in power from a relatively recent VEI 4.1 in 726 AD to the massive Minoan eruption 1600 BC, a VEI 7.3 that ejected over 120 km3 of material, creating a 12 x 7 km caldera, wiping out the neighboring Minoan civilization on Crete, 141 km S. There have been attempts to tie the effects of the Minoan eruption to some of the Biblical Plagues visited on Pharoh during the time of Moses. That discussion is both loud and ongoing.
The Minoan eruption is well up on the list of Largest Volcanic Eruptions in the last XXXX years. It had demonstrably global impact. Worse, it took place in the eastern Mediterranean, where multiple Bronze Age civilizations were concentrated, impacting the Greeks, Minoans, Hittites, Egyptians and multiple civilizations in the greater Middle East, all downwind from the eruption.
One of the oddities of the eruption is a 100-year discrepancy in dating the eruption. Dating things in this period is typically done by two techniques, archeological methods and scientific methods (Carbon14, tree rings, ice cores, stalagmites). The scientific numbers are generally 100 years or so earlier than the dates determined by the archeologists. Part of this may be due to limitations in Carbon14 dating techniques. Carbon14 dating generates what is called a calibration curve based on variations in Carbon14 levels in the atmosphere over time. This curve has flat spots with little to no variation, making matching the two dates difficult.
Volcanism at Santorini began about 2 Ma, with initial eruptions some 20 km SW of Santorini. Center of eruptions shifted to the current location of the island and caldera some 400 ka and began the current cycle of construction and destruction of shield volcanoes. The archipelago was close to its current shape, a caldera ringed with islands, and an active stratovolcano island in the center by the time of the Cape Riva eruption 21 ka.
Progress of the main eruption is entirely tied to the shape of the island. Before the main eruption, the best guess about the island is that it had a small caldera, open to the NW, with a large intracaldera island. Phases 1 – 3 built the central island, increased its height, changed its shape to conical. Debris from these phases covered the original island and filled the caldera shape with debris. Phase 4 was the caldera collapse.
The Minoan eruption proceeded in four major phases. There were multiple earthquakes and some unknown amount of volcanic activity that chased inhabitants off the island. They had sufficient time to evacuate and took jewelry and complex tools with them. No corpses were found in an excavation of Akrotiri, which was inhabited at the time of the eruption. Shortly after the series of earthquakes, Akrotiri was visited again, in what appears to have been an effort to salvage valuable items not destroyed by the earthquakes. This was abruptly halted by what appears to be the first pyroclastic eruption. Small amounts of ash and lapilli were ejected from a vent in the center of the island. A dormant period of several months may have taken place.
The first phase of the main eruption ejected a Plinian plume that deposited up to 6 m of pumice and ash. Energy of this phase was low. Material was ejected by volcanic gas expansion. Water had not yet entered the vent. This phase lasted 1 – 8 hours. Pyroclastics mixed into loose deposits only on the top layers and first came in contact with sea water. The vent for the precursory and first main phase was located in the northern part of the caldera. At the time, the caldera had at least one intracaldera island similar to what we see today with the Kameni Islands.
The next phase was phreatic, as seawater entered the vent, triggering a phreatomagmatic explosion much more powerful than the initial eruption. The eruption ejected much heavier material, unevenly mixed. Lapilli about a centimeter in diameter was ejected, mixed with ash and a few larger chunks. This put a nearly 6 m layer on the western part of the island. Black lava blocks as large as 5 m in diameter were ejected. Pyroclastic surge deposits up to 10 m thick are interbedded with pumice fall layers from the ongoing Plinian phase. The second phase lasted about an hour. The vent migrated into the flooded caldera bay and ruptured in a southerly direction.
The third phase was the largest and most powerful of the eruption. Pyroclastics flowed in a continuous stream, sweeping away huge blocks, up to 20 m. These were typically dacites. The blocks were embedded in ash flows, lapilli flows, and in the end mudflows. Deposits of this phase are up to 55 m thick. The vent shifted northward during this phase. The pre-existing intracaldera island started to be destroyed by explosions. The largest blocks came from the destroyed island.
It is at this point where the narrative diverges. One description has seawater entering the vent, mixing with volcanic material creating a massive lahar that was ejected, flowing over the 400 m high walls of the caldera. So much of this was ejected that the chamber collapsed, dragging the overlying island down with it. On the outside, the flows went into the sea, widening it with shallow coastal plains. A second description has the vent still in the northern part of the caldera and accumulating eruptive products from all three earlier phases filling the caldera, perhaps completely, building a tuff cone, blocking seawater access to the central vent.
The final phase erupted several different types of ejecta: ignimbrite layers, lahar flows, ash flows and huge amounts of debris. These layers alternated between each other. There was ash fall likely during this phase. Most of the material flowed off the edge of the island. While they are only 1 m thick at the caldera, they are as much as 40 m thick outside. Rocks ejected during this phase are smaller, not more than 2 m. Lahars flowed back into the caldera in at least two places. Juvenile material was ejected from multiple vents during this phase, likely due to a gradual collapse forming the new (present day) caldera.
Tehpras from this eruption covered most of the eastern Mediterranean and is used as an important date marker in archeology. This is also why dating the eruption became such a hot topic in archeology. Pumice deposits 10 – 80 m thick extend 20 – 30 km out from the island in all directions.
We know that Krakatau, which generated a 30 m tsunami and pyroclastic flows that traveled 40 km across the ocean ejected only a third of the material that Santorini did. Volcanic effects from the Minoan eruption must have been dramatic and deadly to neighboring islands and shorelines in the eastern Mediterranean.
Macauley (6.3 ka)
Macauley Island is located midway between New Zealand and Tonga. It is the emergent portion of a large submarine volcano topped with a 10.5 x 7 km caldera. There are a pair of islets to the east of the main island. It is surrounded by high cliffs. The main island is uninhabited, just over 3 km2 in area, topping out at 238 m above sea level.
The underlying volcano rises from sea floor 1,700 m below the waves. It is 23 x 30 km, elongated to the E – NE. The caldera floor is 1,100 m deep, with the rim 600 m. There are N – NE trending lineaments, with jumbled blocks covering portions of the caldera floor next to Macauley Island. These are thought to be flank collapses of the western caldera rim. The caldera floor is covered with thick pumice layer.
There is a 9 km long dome or ridge NNW of the caldera. It is topped by a line of cones rising to 80 m below sea level. Macauley cone is located on the SE caldera rim. It rises within 250 m of the surface. It is topped with a small crater. The crater floor is covered with ash and sulfur. There are multiple vents around the caldera rim and on the flanks of the main volcano. Some of these source lava flows into the caldera and down the flanks. There is evidence of sector collapse or pyroclastic flow “sediment waves” on the flanks of the main volcano. These are typically 100 m wide by a kilometer long.
Total volume of the volcano is 269 km3.
Most of the rocks on Macauley Island are basaltic. The island has a complex volcanic history. Cliffs on the N side of the island have five different geologic formations. Tephra layers dating 130 ka, 40 ka, and 30 ka. The island gently dips away from the NW side of the island. All of them appear to be emplaced above sea level. Some of the early eruptions took place below sea level. A much larger portion of the island was exposed above sea level at the height of the last ice age.
The largest eruption is called the Sandy Bay eruption, producing a massive tephra. Activity before this eruption created a shield volcano with at least one crater. There was little erosion before phreatomagmatic eruptions produced tephras and lavas. Water may have entered the main vent. Lava flows on the shield were due to Hawaiian style eruptions.
The Sandy Bay Tephra was erupted 6.3 ka during the formation of the Macauley caldera in a VEI 7 eruption. The vent was a shallow submarine vent close to the island. Total volume is thought to be in the neighborhood of 100 km3 based on volume of the caldera afterwards. An initial submarine eruption jet breached the surface, producing at least 30 pyroclastic flows, surges, and tephra fallout layers on Macauley Island. The flows were cold, likely due to interaction with water and buried vegetation on the island leaving wood casts.
The Sandy Bay Tephra is a white dacite, in contrast with dark colors of the rest of Macauley eruptive products. It is layered lapilli, pumice, sand and fine ash. Total thickness ranges 100 – 15 m. Older rocks were integrated into the erupting magma. The tephra layers have been eroded.
Tephras from this eruption have been identified around the island. The eruption produced large amounts of pumice, which would have formed pumice rafts which were transported widely in the South Pacific. There may have also been tsunamis. This eruption is the only known felsic eruption from Macauley.
Subsequent eruptions built the Haszard Formation, including scorias and lava flows. There was a submarine phreatomagmatic phase of these eruptions that produced the Parakeet Tuff, depositing lapilli and volcanic ash. These eruptions were thought to come from the SE sector of the newly formed caldera and may have begun decades or centuries after Sandy Bay. These deposits filled in erosive canyons carved into the Sandy Bay Tephra. There was at least one lake formed on the E end of the island with volcanic deposits damming a valley.
There are anecdotal reports of earthquakes and sulfur smells at the northern cliffs, next to the oldest rocks of Macauley Island. There is extensive hydrothermal activity at submarine Macauley cone. This includes white fluids, occasional bubbles, black smokers. Elemental sulfur is located at some of these vents. Water from hydrothermal activity can be found as far as 7 km from the cone. There are a couple additional vents suspected in the caldera itself. A lake of molten sulfur likely filled the crater of Macauley cone, leaving sulfur deposits over a meter thick.
Similarities – Speculation Begins
What do these eruptions have in common? All of them either erupt through the sea or a large lake. All of them left a moderate sized caldera. Three of them (Hunga Tonga Hunga Ha’apai, Macauley, Hatepe) are located in the same tectonic system, the Kermadec volcanic arc. Hatepe (Taupo) is located where the Kermadec volcanic arc penetrates into northern New Zealand and erupted through a lake. Three of the five (Hunga Tonga, Krakatau, Santorini) created known tsunamis. Macauley should have.
With that in mind, can we build useful (the most important term) criteria to even know where to look for possible future extremely violent eruptions? While we can build criteria, its utility remains to be seen. Here are a few possible criteria:
- Look for moderate sized calderas (at least 5 km diameter).
- Those calderas need to have access to large quantities of water. Undersea, or submerged calderas are likely a good starting point. Large lakes would also work, though they typical crater lake may not supply enough water to ramp up the violence and is often ejected by new eruptions.
- Robust magma supply.
Where would I look for these sorts of systems? The following is a notional list, most certainly not comprehensive or particularly well-informed. Think of it as head scratching by an amateur.
The Kermadec – Tonga volcanic arc seems a good place to start. There are multiple underwater calderas with a history of violent eruptions. The spreading center of the Taupo volcanic zone in New Zealand is part of this. I would look at southern Japan, southern Kyushu (Aira, Ata and Kikai). The Izu – Bonin – Mariana volcanic arc / back arc system has multiple submarine calderas and is quite active. The resurgent Iwo Jima (Ioto) is located along this back arc. The Kuril Islands should also have multiple candidates. In Central America, Ilopango is a candidate, as are any of the stratovolcanoes surrounding Lake Nicaragua. Last, but certainly not least, would be Campi Flegrei in Italy. One of the popular articles on Hunga Tonga noted that there at least 10 other known submarine volcanoes similar to Hunga Tonga that may be candidates for similar eruptions. There are more, but you get the idea.
Things that argue against this list
In this discussion, we are on the horns of a dilemma. On the one hand, we observed two extremely violent volcanic eruptions in the last 140 years, the most recent in 2022. How many more of these took place throughout history? Until we can get some sort of handle on the length of eruption, it will be extremely difficult to characterize a particular historic eruption as particularly violent. Today, even the temporal lengths of most violent portions of massive eruptions is open for vigorous discussion among the experts, though I get the impression that the more we know, the shorter the vigorous parts of eruptions tend to be. There appear to be violent pulses embedded in a longer, less violent eruption, not unlike subvortices or suction vortices embedded in the main part of a tornado vortex .
Conclusions
As with everything else in this business, the more we figure out, the more we find we actually don’t know. It is not that the data is bad. Rather it might be that data does not yet exist, as it is generated by the new instrumentation. Though I think it is more likely that we don’t yet know how to interpret what we think we are seeing. Given that the two most recent of these sorts of eruptions were the smallest, but the best observed, I would speculate that Bruce Stout who asked this question may very well be correct, that these things are much more common than anyone suspects. Whatever happens, and whatever the destination, I expect it will be a fascinating journey.
Additional information – Hatepe
A supervolcano’s colossal eruption has been lying about its age, RG Andrews, Gizmodo, Oct 2018
The Taupo eruption, New Zealand, I. General Aspects, Wilson & Walker, May 1984
Additional information – Santorini
The size of the Minoan eruption, Volcano Discovery
Minoan eruption, gigatos, Nov 2021, Trenfo
Caldera development during the Minoan eruption, Thira, Cyclades, Greece, Heiken & McCoy, Sept 1984
The Minoan eruption of Santorini, Greece, Bond & Sparks, 1976
Additional information – Macauley Island
Volcanic history of Macauley Island, Kermadec Ridge, New Zealand, Lloyd, et al, 1996
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