When Is a Caldera a Caldera?

Léon Prunelle / Originally published September 30, 2014

This is a re-post of an article by Henrik, written during the Bardarbunga eruption when the caldera had started its collapse.

olcano? Erm... I don't see a volcano? Toba caldera wall seen from Samosir Island, a resurgent dome near the middle of the caldera. (Photo, Sebastian Hubarat, Tobaexplorer)

Volcano? Erm… I don’t see any volcano? Toba caldera wall seen from Samosir Island, a resurgent dome near the middle of the caldera. (Photo, Sebastian Hubarat, Tobaexplorer)

It does not take long for a newcomer to volcano-watching, if we are to call our hobby that, to come across the term “caldera”, cauldron. The term is very loosely used to describe large volcanic depressions ranging from volcanic craters a few hundred metres in diameter up to the huge depressions left behind by the very largest volcanic eruptions that may approach 100 km in their greatest extent. Consciously or subconsciously, there is always a sensationalist undertone evoked in the reader when the term caldera is used, so what is a caldera?

The archetypal caldera is Yellowstone. For many decades, scientists had realised that Yellowstone was a large volcanic feature with its geyser fields, tuffs and lava flows, but where was the volcano? There was no sign of it! It wasn’t until the advent of Space flight and high resolution orbital photography that the truth was finally realised: Yellowstone and its major explosive eruptions were so huge that the volcanic scars left were too large to leave a recognisable volcano behind. Almost wherever one is in Yellowstone National Park, one is standing on top of the volcano. Let us just add immediately that the vast majority of its volcanic or volcano-related eruptions are nowhere near huge, that there is no periodicity to the largest, caldera-forming eruptions and that Yellowstone is in no way “overdue”, whatever Michio Kaku or Steve Quayle may say.

he trail of "supervolcanic" eruptions across the state of Idaho and through the Rockies known as the Snake River Plain (ISU)

The trail of “supervolcanic” eruptions across the state of Idaho and through the Rockies known as the Snake River Plain (ISU)

With this epiphany, the scars left behind by several more such very large eruptions were identified. West of Yellowstone running through the state of Idaho and cutting a swathe through the Rockies lies the Snake River Plain, a trail of at least seven calderas left behind by several ultra-colossal (VEI 8), nowadays sometimes referred to as apocalyptic eruptions, and mega-colossal (VEI 7) eruptions dating as far back as 16 million years as the North American plate travelled over the Yellowstone hotspot. It can be traced back to the Columbia River Flood Basalts in Oregon, a large igneous province formed over a period of 10 – 15 million years and most vigorous 17 – 14 million years ago that deposited some 174,300 km3 over an area of 163,700 km2 with a maximum thickness of over 1.8 km. This is not the only area in the USA where such eruptions have occurred. Long Valley in California was home to a mega-colossal VEI 7 eruption about 760,000 BP. The Valles Caldera, New Mexico, was formed by two VEI 7 eruptions some 1.15 million years ago. What has long been regarded as the largest eruption ever identified is the massive VEI 8 eruption of 27.8 million years ago that formed the La Garita Caldera and Fish Canyon Tuff in Colorado.

Interesting though the American caldera-forming eruptions are, they are not unique, nor are they the most interesting. The Atana Ignimbrite eruption (VEI 8) that formed the Pacana Caldera in Northern Chile some 4.0 million years ago was as large as the largest of Yellowstone’s eruptions. Lake Toba in Sumatra Indonesia was even larger, occurred as recently as 74,000 BP, and is somewhat controversially linked to a bottle-neck in the human genome. If true, the Toba Catastrophe Theory claims that as many as 60% of all humans on Earth at the time could have perished. The 74,000 BP Toba Eruption was preceded by at least one other VEI 8 and two VEI 7 eruptions dating as far back as c 1.2 million years. Subsequent to the most recent eruption, a resurgent dome, Samosir Island, has appeared.

eautiful Lake Taupo, New Zealand. The only hint of its volcanic origin are the distant volcanoes on the horizon, built at the edges of the ring fault system. (Provenance unknown)

Beautiful Lake Taupo, New Zealand. The only hint of its volcanic origin are the distant volcanoes on the horizon, built at the edges of the ring fault system. (Provenance unknown)

No disrespect to the USA, but the undisputed World Champion of ultra-colossal and mega-colossal eruptions is the Taupo Volcanic Zone located on New Zealand’s North Island. Over a period from 280,000 BP to as late as 1,800 years ago, the TVZ has seen no less than two VEI 8 and five VEI 7 eruptions. The latest of those two are the Oruanui Eruption (VEI 8) 26,500 BP and the Hatepe Eruption (VEI 7) 1,800 BP of Lake Taupo.

Other recent mega-colossal eruptions that formed large calderas include the 3,600 BP civilisation-destroying eruption of Thera (Santorini) from which we have derived words such as “terrible”, “terrific” and the Tera of terawatt/terabyte as well as the legend of Atlantis. It may well be mentioned in the Biblical description of the Plagues of Egypt. Two further recent such eruptions are the Kikai Caldera, Ryukyu Islands, Japan (6300BP)and Macauley Island in the Kermendec Islands, New Zealand(6,300BP) eruptions. In addition, there have been two recent VEI 7 eruptions that did not lead to a caldera collapse even if the huge summit craters, partially the result of collapse, are often referred to as “calderas”, by scientist – Mount Baekdu/Changbaishan (969 AD) North Korea-China border and Tambora, Sumbawa Island, Indonesia (1815 AD) that caused “The Year Without a Summer”, 1816.

ount Baekdu or Changbaishan on the Chinese - North Korean border, source of the 969 AD VEI 7 eruption (Wiki)

Mount Baekdu or Changbaishan on the Chinese – North Korean border, source of the 969 AD VEI 7 eruption (Wiki)

These very largest of volcanic eruptions begin with an initial eruption that removes a sufficient amount of magma that the ~5 ~ 10 km thick roof over the magma reservoir begins to buckle and drop. This forms a series of concentric ring fractures which eventually begin to erupt after which the now unsupported roof collapses into the magma reservoir leading to the major and usually tuff-forming ignimbrite eruption which partially fills the caldera. Later eruptions and erosion further fills in the caldera and intrusions thousands of years later may cause a resurgent dome to emerge near the middle. Note that neither Baekdu nor Tambora fit this pattern for caldera formation very well as the progenitor volcanoes survived!

In comparison to these giant scars, often tens and several tens of kilometres in extent, it seems ridiculous to refer to the 1½ km wide, shallow summit crater of Iceland’s Eyjafjallajökull, a volcano that gently spews out no more than some 0.1 cubic kilometres or so over a period of months with its eruptions, as a “caldera”, but unfortunately even scientists do in serious academic papers.

he 3 x 4½ km summit caldera of Mount Katmai, Alaska, that formed after the 1912 VEI 6 Novarupta eruption (Wiki)

The 3 x 4½ km summit caldera of Mount Katmai, Alaska, that formed after the 1912 VEI 6 Novarupta eruption (Wiki)

But the very large, ultra- and mega-colossal, eruptions are not the only eruptions to form collapse calderas. In addition to this, there are relatively speaking smaller eruptions that form them, mostly from the colossal VEI 6 eruptions. The most recent such example is the 1912 VEI 6 Novarupta eruption when the magma reservoir under the Katmai volcano, Alaska, emptied through a new vent well to the side of the volcano to form the “Valley of Ten Thousand Smokes”. Over the next few years, much of the edifice of the Katmai volcano collapsed into the emptied magma chamber to form a 3 x 4½ km summit caldera.

A better known and researched caldera-forming eruption is that of Mount Mazama (7,700 BP), Oregon. This followed the pattern of a ring fault forming followed by an eruption at the ring fault and the collapse of the volcanic edifice into the relatively shallowly located and emptied magma reservoir. The caldera has been filled with water to form the 8.0 x 9.7 km diameter Crater Lake and, in contrast to the non-eruptive resurgent domes of very large calderas, subsequent eruptive activity has built a central cone, Wizard Island.

opographical map of the Crater Lake caldera showing the features of subsequent intra-caldera eruptions. (USGS)

Topographical map of the Crater Lake caldera showing the features of subsequent intra-caldera eruptions. (USGS)

Calderas such as these are plentiful. Europe alone, perhaps not noted for great volcanic eruptions, has several; Laacher See in Germany, Vulsini, Colli Albani, Roccamonfina in Italy, the Rodalquilar Caldera complex in Spain to name but a few. In addition there are several ancient ones such as the caldera complex upon which Oslo, the capital of Norway, is built, Mount Snowdon & the Lake District, UK, etc. etc.

But there is one type of caldera that defies the traditional description and which is impossible to tie down to a single, specific event and that is best exemplified by the large Icelandic calderas of subglacial origin, i.e. Katla and Askja. While it is thought that the calderas of both Torfájökull and Tindfjallajökull are the result of single large, possibly VEI 6 eruptions, some 50-70,000 years ago, those of Katla, Askja and the volcanoes below both Hofsjökull and Langjökull are much harder to explain in those terms. The calderas of both Askja and Katla are far larger than can be explained by eruptions known to have occurred from analysis of ash strata. Both volcanoes seem to be limited to no more than medium-sized VEI 5 eruptions whereas in both cases, at least a low-end VEI 7 – which we know never has happened – would be required to produce their large calderas. It is here that what we are now seeing at Bardarbunga might have a bearing.

erial photograph of the sub-glacially formed Askja volcano with its large caldera containing the nested caldera of Lake Öskjuvötn, which resulted from post-eruption collapse following the volcano's 1875 VEI 5 eruption (RUV)

Aerial photograph of the sub-glacially formed Askja volcano with its large caldera containing the nested caldera of Lake Öskjuvötn, which resulted from post-eruption collapse following the volcano’s 1875 VEI 5 eruption (RUV)

That Bardarbunga is now undergoing a collapse event is beyond question. Furthermore, to judge by previous eruptions in the area emanating from or involving this central volcano, this is not the first such time in its history. Until this eruption started, the caldera as shown on the IMO maps was significantly smaller but was enlarged to its current extent by human reinterpretation. The collation of earthquake data strongly suggests that a collapse event is happening at the N to NNW margin of the newly defined caldera where the data indicates that the deep magma reservoir extends beyond the limits of the “rescheduled” caldera. If this continues, the caldera will eventually be enlarged in this direction.

If this is indeed what is currently happening and has happened before, we may have obtained a vital clue as to how and why the Icelandic subglacial calderas defy the correlation between the size of the caldera and the eruptions that formed them.

/ Pyrite

33 thoughts on “When Is a Caldera a Caldera?

  1. Thanks for the repost! This one stuck with me till today so it was a great pleasure to re-read 🙂

  2. It occurs to me that there might be a mechanism by which subduction could be initiating off Portugal despite the relative youth of the oceanic crust in that region.

    Specifically, because Africa is ramming its way north into Eurasia, the western part of the latter plate is being crumpled and chunks of it shoved aside. That will deform the plate by pushing the western parts (north)west, the central parts north, and the eastern parts (north)east (relative to the midline of the collision region).

    This could push the buoyant felsic crust of the Iberian peninsula westish into the Atlantic, perhaps enough to force the denser mafic Atlantic seabottom beneath it.

    But that seabottom is still fairly buoyant compared to the asthenosphere, so the subduction, if subduction there is, will be very shallow-angle. The Atlantic plate will grind along the undersurface of Europe until geometrically forced down by the roots of the Alps and/or African and subducted Mediterranean material.

    That, in turn, could contribute to volcanism and other phenomena, much as the shallow Farallon plate underlying western NA causes occasional burps in the southwestern US and in Mexico. Perhaps Laacher See is part of this. The extension someone mentioned in central Europe also likely relates to the African impact: north of the compression zone forming the Alps, if parts of Eurasia are being forced to radiate out away from that zone there will be radial zones of extension north of the Alps. Such extension was also put forth as a possible explanation for Laacher See.

    • Very elegant reasoning and it makes a very nice hypothesis to put to test.
      It could well be that a subduction zone is already developing across western Europe due to the impact of the African plate.

      Alongside with it, subduction already happens in the west side of the Atlantic, in the Caribbean sea.

  3. Nice and interesting to read this again. The Holohraun event was an excellent pedagogic lesson!
    Now perhaps new lessons on way. Strong, relatively shallow swarm at Arnes, Iceland. What is this about?

    • It is within the south Iceland seismic zone, near is eastern end point. Lots of short faults crossing the transform zone, and it is on one of those. This region is mainly tectonic. Irpsit mentioned some lava flows in the area but they must be rare. A nice history of this zone (which must have taken a lot of time to put together!) is in http://www.jonfr.com/volcano/?p=745 There were two large quakes here in June 2000, and today’s quake is just to the east of those. Interesting, there may be some influence of stress from Hekla on this area. The 2000 quakes followed after a Hekla eruption.

      Earthquakes here tend to come in pairs..

      • Two areas in SISZ await a major earthquake. One is west of the 2008 earthquake pairs (this is slightly east of Hengill), up to M5, but risk is minor there.

        The second area is west of Hekla, near Arnes, and where the swarm happened today, and this area, yes, is “due” to a M6-7.5 quake sometime in the future. Its where usually quakes are at its most intense along the SISZ, often quite destructive by Icelandic standards.

        Hekla and SISZ. From what I gather in its long list of (Hekla) eruptions and SISZ major quakes, these often happen in the same year or within a period of 1-2 years. Sometimes Hekla erupts before a major SISZ episode, sometimes its the other way around. This happened several times in historical times. Definitively there is some sort of (indirect) connection!

    • From the comment of the IMO facebook page. Giggle translation Icelandic – English.

      “Heavy tremor activity was in the country today, May 6th.
      Kl. 12:08 a magnitude 4.4 earthquake occurred about 2 km south east of Árnesi. A minute later (12:09), another measured in similar areas of size 3.3. The earthquakes were found in a well-known sprawling area and found widely in southern Iceland, among others. In Árnes, in Selfoss and in Reykjavík. No reports of injury or injury to people have been received. Small earthquake activity has subsequently been measured in the area, but the next few days will be monitored.

      Later today, around 20:50, a tremor of 3.2 was observed in Bárðarbunga.
      For the most part, it is worth mentioning that the South American tigers were today known as the most likely crook last year in 1630, but probably the tremor was 7.0 in size. The location of the earthquake can be seen from now on the attached picture showing the main cracks in South Iceland and the years in which the biggest (known) earthquakes occurred. The tremor from today is marked with a red star.”

        • A improved translation of the sentence could be “It’s interesting to mention that the quakes today were on a known fault that probably snapped last in the year 1630, likely resulting in a quake of about 7,0 in size.”

  4. Do we know why the relatively-small Askja 1875 eruption formed a caldera?

    • Because it wasn’t relatively small, it was pretty big. With that said, the caldera is actually a pretty accurate representation of the tephra emitted during the eruption.

      the listed size is approximately 3×4 in area, with a depth of 267 meters (.267 km). If we were to assume a square shape for estimation purposes, we would get a volume of around 3.2 kilometers of displaced magma. This is likely an over-representation however, since the shape of the hole is not a perfect square, and is likely more narrow towards the bottom, and rounded on the sides.


      Going off what the abstract here says, the caldera formation actually started prior to the big blast, and the onset of the caldera dropping along with magma mixing is likely what led to the big VEI-5 eruption in 1875. From what we know, .3 km^3 was erupted from a rift in 1874, which led to the initial drop in the smaller caldera (similar to Bardarbunga). From that rifting eruption alone, we can assume there is a lot more magma that was implaced into that rift that did not erupt above ground, but still escaped from the caldera itself. When you combine the volume of magma erupted during the fissure eruption portion and then add that to the VEI-5 blast, the size seems roughly on par with what the volume of the caldera itself.

      • In 1874 a rift eruption erupted around 0.3km3 (probably underestimated; could have easily been 1km3, a figure more similar to Holuhraun similar size).
        2.5km3 erupted in 1875 caldera VEI5 blast.

  5. As Dutchman I have a caldera in my backyard of the neighbours. It is called Laacher See in the south-eastern part of the VulkanEifel in southwest Germany. Are there currently any signs of increasing volcanic activity there?

      • Although Laacher See is still emitting CO2 gases from mofettes on the northeastern shore of the lake. These mofettes produce a little bit smell like rotten egg.

        At http://www.q-mag.org/germany-a-danger-of-volcanic-eruptions.html I read that a new volcanic eruption in the VulkanEifel can happen within months from now. There is no volcano monitoring system there.

        “Locally limited eruptions, like those at the Ulmener Maar, appear to scientists as the most likely scenario. For this to happen, only a little more magma would need to accumulate underground. This could happen in a matter of months, explains geophysicist Joachim Ritter of the University of Karlsruhe, who has plumbed the underground with sound-waves in the frame of the “Eifel-Plume-project.” Should the gas pressure increase, the 1000°C stone mush could shoot up, declares Ritter.

        The subterranean Eifel is moving. Especially between the Laacher See and Coblence, small earthquakes are regular reminders of the lurking danger. Possibly, rising groundwater, which is being heated up by the magma reservoir 50 km down, is triggering the vibrations. The underground around the Laacher See reaches a temperature of 60 to 70 degrees at a depth of one kilometer – an unusual reading.

        In the 1990s, scientists saw in these phenomena the tailing-out of Eifel volcanism. Today, they reinterpret the signals as a sign of ongoing activity. Another sign of life from the Eifel volcanoes bubbles up from the Laacher See: bubbles in the water signal carbone dioxide originating from the magma. As it is rising, it is thought that magma releases an increased amount of CO2.

        Geologist Schreiber is of the opinion that ants would be first to have warning of an impending eruption. Just as chimney-fires would chase away storks from chimneys, carbon dioxide drives out insects from their nests, which they tend to install preferably on tectonic fissures.

        At first, Schreiber’s theory elicited loud protests from ant experts. But since he has presented his proofs to his colleagues in papers and in seminars, the opposition has abatted. Schreiber and his colleagues have offered their observations to scientific publications.

        Should the Eifel-ants leave their heaps en masse, it would be an alarm signal, Schreiber says. Otherwise, there might not be any other signs of an increase in the sub-terranean magma. For there are but few measuring instruments installed in the Eifel. “A systematic monitoring of the volcanoes is not possible,” deplores Schreibe”

        See also http://www.swr.de/spuren-im-stein/eifel/warum-bebt-die-eifel/-/id=16535154/did=17335306/nid=16535154/cvcit9/index.html about molten magma below the Laacher See area.

        • Carbon Dioxide emissions is not really a sign of a potential volcanic eruption. Just about every volcano emits Co2, even long extinct ones such as what we see in Arkansas.

          Now, if we were to see a dramatic uptick in co2 emissions, that could signal magma rising, but even that wouldn’t necessarily be indicative of a future eruption.

          And one clarification, I should correct myself in saying that an eruption from the Rhine Graben is definitely possible, but something on the magnitude to the Laacher See eruption is extremely unlikely. That eruption was the ONLY time in the entire history of the Rhine graben that something of that size ocurred. It was a black swan as we like to call it. Not saying it can’t happen again, but the likelihood of it occurring in our lifetime is extremely small. Now, smaller maar-forming eruptions are definitely a risk, especially if they form in a populated region, but eruptions of that type are notoriously hard to predict.

        • Reference to cbus05’s comment about long extinct volcanoes.
          A prime example is Jackson Volcano, last active about 81 myr ago. It’s still a prodigious source of CO2, producing at commercially viable levels. (aka Jackson Dome)

        • This regions is certainly not extinct. There are many regions around the world with the occasional eruption, separated by 10,000 years of solitude. I think it is mainly phreatic eruptions, so no magma reaches the surface. It is also monogenetic, meaning each eruption is in a different place. That might be the scary bit. So chances are low but the risk is not non-zero. If it is phreatic, it cold come quite unexpected. Guess of chance of an eruption in the next 100 year: make it 0.1-1%. Very unlikely, but high enough that is should feature on some risk analyses.

          • The VulkanEifel volcanic district comprises of two distinct volcanic regions, the Western one around Daun and Gerolstein (Westeifel). And the Eastern one with Laacher See (Osteifel). I think that the Eastern one has to do with the Lower Rhine Rift, the Neuwieder Becken south of Laacher See is an extension of the Lower Rhine Rift.

  6. As usual Madame Bardy is jealous Saturday
    06.05.2017 20:50:02 64.658 -17.508 4.3 km 3.2 99.0 2.1 km NNE of Bárðarbunga (IMO)

  7. Seems like a large crater has been found at the Falklands

    “Following analysis on the basin in Falkland islands, a team of scientists concluded the basin exhibited traits of a large ancient meteorite impact. The impact crater is among one of the world’s largest impact crater.

    The basin is located in the Falkland Plateau in the northwest of West Falkland islands. The basin is a huge a 250 kilometers (150 miles) diameter, and scientists from U.S., Argentina, and Paraguay found convincing geophysical evidence of the large ancient meteorite impact on the basin.”


    • Wonder how many big impacts that has struck since life became more advanced (cambrium). Most of them must have landed in oceans and the scars are lost….

      • This was the area where we saw precursor activity a few days ago. Nn M6.4 is certainly tectonic, and a lot of what you will be aftershocks. But the volcano may also be a bit shaken. There may be a bit higher chance of an eruption over the next few days.

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