I would like to take a closer look at a caldera in Vanuatu. This one is Kuwae, hotly debated as the source of a massive eruption around 1450. This eruption was accompanied with dry fog in Turkey, massive snowfall in China, and significant tsunami damage in northern New Zealand and Australia. As the caldera is for the most part submerged, the normal study of volcanic materials left over from a Crater Lake (Mazama) or Santorini caliber eruption are not present. It is this lack of eruption debris that makes tying the eruption and associated global climate impact to this particular caldera so difficult.
The best popular article about Kuwae I have come across was written by Dr. Erik Klemetti in his Eruptions blog in 2012. It is well worth the time to read as it represents a professional analysis of the eruption. http://www.wired.com/2012/05/kuwae-eruption-of-the-1450s-missing-or-mythical-caldera/
Kuwae is a submarine caldera measuring some 6 x 12 km. It occupies the area between Epi and Tongoa islands. The caldera has a pair of basins. Karua is an active submarine volcano near the northern rim of the caldera that poked its head above the waves several times since first observed in 1897.
The caldera sits some 76 km SSE of Ambrym in the nation of Vanuatu. I wrote about Ambrym last March. https://volcanohotspot.wordpress.com/2015/03/24/recent-eruption-at-ambrym-winding-down/
The active cone built short-lived islands measuring a kilometer or two long and a few meters above the waves. These islands typically last months to as long as a year.
Local tradition has the three islands surrounding Kuwae all part of the original island named Kuwae that was destroyed in a cataclysmic eruption around 1452 AD. These stories are backed up by some tephra and pyroclastic deposits on Epi and Tongoa. But as we will see later, local tradition may not tell the entire story.
Vanuatu is roughly 1,500 km equidistant from Western Australia, Northern New Zealand and Eastern New Guinea. There are nearly 49,000 people living within 100 km of the caldera.
The single active volcano in Kuwae is Karua. It is basaltic and has breached the surface of the Pacific several times since first observed in 1879. The majority of the eruptions have been explosive in nature likely due to the interaction of basalt and salt water.
Some 17 eruptions, a third of them unconfirmed have been reported from Karua since 1897. The 1897 – 1901 eruption built a small island. Three eruptions in 1949 were a pair of VEI 3s with a VEI2. These were the largest and closest spaced of historic eruptions.
The most recent eruption was unconfirmed in 1980.
Eruptive products are typically basalt to dacite with the occasional excursion to andesite.
In the worst case, if Kuwae was the source of both the dust, volcanic gasses and created the caldera in 1452, it would have taken a cataclysmic eruption. Estimates of the eruption have some 32 – 39 km3 of magma erupted. Oregon State’s Volcano World suggests that the caldera forming eruption proceeded in three phases. The first was moderate phreatomagmatic and magmatic activity from a central vent over a period of months to years. The second was phreatomagmatic eruptions of dacite gradually shifting to magmatic eruption which produced at least two major pyroclastic flows and the collapse of the caldera. Final phase were additional eruptions of dacitic pyroclastic flows as the caldera collapse proceeded to the north.
Total vertical collapse of the cone is estimated to be over 1,100 m, from around 600 meters above the waves to over 400 meters below the ocean surface. Note that there is no geologic record that I was able to find of the actual ancestral cone although it is mentioned in local tradition.
But there is a problem with the characterization of the eruption as this large and productive, as pyroclastic flow deposits on the surrounding islands are not consistent with a recent, large, highly vigorous eruption. A 2007 paper by Nemeth, Cronin and White describe deposits on the surrounding islands as small-volume dacitic pyroclastic flow deposits lacking associated widespread fall deposits. They propose these eruptive deposits as similar to those of large maar-forming phreatomagmatic eruptions of the Laacher See.
Analysis of pyroclastic flow deposits next to the caldera itself show three layers measuring some 10 meters thick. Depth of these flows quickly tail off, with only scattered ash layers a few tens of centimeters thick 3 – 5 km from the caldera.
Analysis of near vent deposits at Santorin and Krakatau show the Kuwae deposits are not similar and therefore not consistent with a large recent eruption from Kuwae.
The oral histories do support a major eruption from the Caldera around 1452. But there is no evidence so far of widespread human catastrophe at the time with locals being engulfed by massive pyroclastic flows. Indeed, the areas of Tongoa with the thickest flow emplacement were resettled starting some six years following the eruption. The oral histories of large destructive eruption appear to be confined only to the vicinity of the caldera on Tongoa.
Contrast the depth of pyroclastic flow materials with that of the Valley of Ten Thousand Smokes inundated by the Novarupta – Katmai eruption of 1912. It was first visited some four years after the eruption. Visitors found tens of thousands of vents in a 100 km2 valley filled in places with 200 meters of pyroclastic flow. And this from an eruption slightly smaller than Kuwae is supposed to be at just under 30 km3 of material ejected.
From all this, it does not appear that there was enough material produced to support a caldera-forming eruption in 1452 with over 30 km3 of ejected material.
An alternate explanation would have the caldera formation via a largely submerged eruption. This would start to explain the rather thin coverage of pyroclastic and tephra deposits on surrounding islands. It would explain the associated Pacific paleotsunamis. It would explain the presence of the caldera itself.
We do not yet have enough supporting data for a large eruption at the suggested date. Klemetti points out that a close analysis of ice core samples show perhaps two large eruptions around 1452, with the second one being 5 – 6 years later. The second source of volcanic dust and gasses is as yet unidentified. It is entirely possible that three eruptions are involved in this short period of time, two of which injected massive amounts of volcanic dust and gas into the atmosphere and a third out of Kuwae itself associated with the caldera formation and tsunamis.
At this point, whether or not there was a massive volcanic eruption from Kuwae is still hotly debated.
If there is an eruption that ends up creating an underwater caldera, sudden movement of that much rock (molten or not) will in turn move a lot of water in a short period of time. The most famous example of that would be the caldera formation of Krakatoa in 1883 that created tsunamis over 30 meters high inundating the shorelines of Sumatra and Java, killing some 36,000.
There should be evidence of tsunamis on surrounding islands near and far. Not unsurprisingly, such evidence does exist on the northern shore lines of New Zealand and Western Australia. The problem with such evidence is that there are a lot of things in that part of the world that can cause tsunamis, as New Zealand is very active from the standpoint of tectonics, earthquakes and volcanic activity. While it is possible to date tsunami events, it is problematic to identify their source.
The 1450 AD tsunami in New Zealand was large enough to force Maori abandonment of coastal regions. The referenced New Zealand analysis ties the tsunami event to near simultaneous rupture of 3 – 4 major faults on and around the North Island.
One of the other things about Vanuatu is the not uncommon appearance and disappearance of islands between the islands. Islands that appear are usually accompanied with some sort of Surtsey-like volcanic eruption. Sometimes the disappearance is accompanied by large waves and earthquakes. Sometimes they are not as the waves quietly reclaim a low new volcanic island over the course of months.
A 2006 paper discusses the disappearance of several islands in the vicinity of Ambrym and Kuwae over the last 800 years. Most of these sudden disappearances were accompanied with earthquakes and waves indicating possible flank collapses. Indeed, the long-time locals do not tend to inhabit small offshore islands indicating perhaps an ancestral knowledge of the danger posed by small offshore islands. http://repository.usp.ac.fj/4021/
The tectonics of Vanuatu are driven by the subduction of the Pacific Plate and the Australian Plate. Previously the Pacific Plate was subducting westward under the Australian Plate. The location of that subduction was the Vityaz Trench located in the northern part of Vanuatu. This subduction drove activity 25 – 14 My ago.
But that subduction changed and the Australian Plate is now subducting under the Pacific Plate via the New Hebrides Trench. It is this subduction that is now feeding the volcanic islands of Vanuatu. The volcanism shifted south as subduction switched from Pacific to Australian some 7 – 4 My ago.
Convergence of an oceanic ridge started some 3 – 1.5 My ago. This wrenched existing fault lines parallel to the line of the New Hebrides Trench, roughly NW – SE. It is through those fault lines that melted plate finds its way to the surface to fuel the volcanic islands of Vanuatu.
The tectonics of the region are complex with a spreading zone in the Pacific Plate to the east of Vanuatu. Plate convergence is 9 cm/year. The subduction slab sits some 150 km below the surface. The trench is relatively close at some 125 km. Crust under the caldera is 15 – 25 km thick. http://www.geohazards.gov.vu/index.php/general-volcano-information
Whether the eruption of 1452 AD drove the tsunami events and the volcanic ash and gas signals in the ice cores remains to be seen. Clearly something, perhaps several something’s happened in that part of the world. While there are some very enticing connections between the caldera, the tsunamis and change in global weather that argue in support of such a conclusion, there is also the lack of massive pyroclastic flows and tephra deposits on neighboring islands that argue against it.
Unfortunately, at this time, we simply do not have enough information to pin this cause on that effect. Given the number of subsea calderas in that part of the world, the convoluted tectonic impact mess between the eastern end of the Australian Plate and the Pacific Plate, earthquakes and island flank collapses, I believe we are going to need a lot more information before precisely determining what happened.
Post this as a yet unsolved and most interesting mystery.