VolcanicDegassing

Krakatoa

Flooding the Santorini Caldera

Flooding the Santorini Caldera

The flooded caldera of Santorini volcano holds many secrets, buried beneath the ash and pumice of its last great eruption. In the Late Bronze Age, about 3600 years ago, an explosive eruption several times larger that of Krakatoa, 1883, wreaked devastation across this thriving island.

View, looking North, of the northern part of the Santorini caldera. The island of Thersia (left) is separated from the island of Thera (right) by a channel, that is now thought to have formed at the end of the Late Bronze Age eruption.

View, looking North, of the northern part of the Santorini caldera. The island of Therasia (left) is separated from the island of Thera (right) by a channel, that is now thought to have formed at the end of the Late Bronze Age eruption.

A great trading port, Akrotiri, was buried under metres of pumice; preserving for future generations a snapshot of Late Cycladic urban life.

Akrotiri ruins

Bronze Age street scene – Akrotiri, Santorini. Precursory earthquakes damaged the town. But there was time for the residents to clear up rubble (foreground), and rescue furniture (those are casts of bed frames in the street), before the town was buried in pumice.

Once the eruption was over and the dust had, literally, settled the geography of Santorini would have looked much like it is today: a group of three islands, forming a ring around a caldera occupied by the sea. The central ‘Kameni’ islands would not have begun to emerge for another thousand years, or more.

But when did the sea flood the caldera, and what were the consequences? It has long been suspected that the Santorini eruption triggered tsunamis, implicated in Late Bronze Age destruction along exposed coastlines around the Aegean. Observations of the Krakatoa eruption in August 1883 include harrowing accounts of the great sea waves that swept across low-lying areas of Java, Sumatra as this eruption reached a climax. These tsunami were triggered either by the convulsions of the sea-floor, as the island of Rakata collapsed into the newly-forming caldera; or, as now seems more likely, by the displacements of colossal volumes of water as the pyroclastic currents raced into the sea.

Krakatoa - before and after images.

Krakatoa – before and after: notice how much of the island of Rakata (labelled Krakatau) has disappeared in the eruption; and the large shoals of pumice that formed new islands north-west of Krakatau (Proceedings of the Royal Geographical Society).

But could tsunami have been triggered by the Santorini eruption; and how? Until now it has been thought that collapse of the caldera could have triggered a tsunami; but only if the caldera was already flooded by the sea. A growing body of geological and archaeological evidence suggests that the crater of Santorini volcano was not open to the sea, as it is today, at least at the beginning of the Late Bronze Age eruption. Large blocks of stromatolites, an algal concretion, and chicken-wire gypsum that were caught up in the eruption debris point to the presence of a muddy lake in the northern part of Santorini. Before the eruption, this would probably have been an area of hot, bubbling mud-pools, fed by volcanic heat and fluids from depth. Among the wall-paintings from Akrotiri is one, the flotilla fresco, that shows a city on a hill, with a river running by. Are these features that might have been found inside the caldera of the volcano?

Metre-wide block of stromatolite, thrown out during the LBA eruption

Metre-wide block of stromatolite, thrown out during the LBA eruption, now in the entrance to the Heliotopos hotel.

In a new paper in Nature Communications, we present some new evidence that may help to resolve some of these questions. High-resolution mapping of the seafloor shows the presence of a remarkable scour at the northern end of the deep channel that separates the islands of Therasia and Thera, below the town of Oia. This feature can be seen on older sea-floor maps; but not in such detail. Using sound-waves to look at the structures that this channel cuts through shows that the channel didn’t form until most of the material thrown out in the eruption had already been deposited. The shape of the scour will be familiar to anyone who has built dams of sand on a wet beach, and then let it fail. This shape shows that the channel formed quickly, and that the sea broke through and poured down a great waterfall into the caldera, but only after the eruption was over. Any tsunami waves that formed during the eruption must have been triggered by the entry of the pyroclastic currents into the sea.

Image of the northern 'breach' in the Santorini caldera, and the scour and channel that cut through it.

Image of the northern ‘breach’ in the Santorini caldera, and the scour and channel that cut through it. From Nomikou et al., 2016.

This new evidence, along with new ‘cosmic ray exposure’ age measurements showing that the cliffs of the northern part of Santorini are ancient and formed before the Late Bronze Age, may strengthen arguments that the pre-eruption interior of Santorini island was not filled with seawater as it is today. It doesn’t, however, have any direct connection to the events that led to the collapse of the major Late Cycladic culture on Crete, the Minoan, a few decades later. Disruption caused by the loss of a major trading hub and short-term damage caused by fallout of ash across the islands of the eastern Aegean might well have contributed to the eventual decline and fall of Minoan culture; but the later destruction of the Cretan palaces must have another cause. Earthquake, anyone?

Selected references 

Heiken G, McCoy F, 1984, Caldera development during the Minoan eruption, Thira, Cyclades, Greece. Journal of Geophysical Research 89, 8441–8462

Manning SW et al., 2006, Chronology for the Aegean Late Bronze Age 1700-1400 B.C.,  Science 312, 565-569

Nomikou P et al., 2016, Post-eruptive flooding of Santorini caldera and implications for tsunami generation, Nature Communications 7: 13332, 8 November [open access]

Novikova T et al., 2011, Modelling of tsunami generated by the giant Late Bronze Age eruption of Thera, South Aegean Sea, Greece. Geophys J Int 186, 665-680.

Paris R, 2015, Source mechanisms of volcanic tsunamis, Phil Trans Roy Soc London A 373: 20140380

Pichler H, Schiering W, 1977, The Thera eruption and Late Minoan-IB destructions on Crete, Nature 267, 819 – 822

Acknowledgements

Thanks to NERC for funding for my work on Santorini; the organisers of NEMO 2016 and Heliotopos Conferences for a recent visit to Santorini; to Christos Doumas, Lefteris Zorzos, Clairy Palyvou and Maya Efstathiou for introductions to Akrotiri and the Late Bronze Age, and Floyd McCoy for discussions.

August Anniversaries: the eruption of Krakatoa

August 27th marks the anniversary of the culmination of the great eruption of Krakatoa (or Krakatau) in Indonesia in 1883. This devastating eruption has become the archetype of a volcanic catastrophe, even though it was a geologically modest example of a ‘caldera forming’ event. The eruption of Krakatoa quickly made the headlines around the world, in part because newly installed undersea cables allowed the news of the event to be wired rapidly across the globe.

Precursory eruption of Krakatoa in May 1883. From Symonds (1888).

Precursory eruption of Krakatoa in May 1883, several months before the climactic events of August 1883. From Symons (1888).

The Krakatoa eruption was one of the first major eruptions to be intensively studied by scientists. The journal Nature published an editorial explaining the ‘Scientific Basis of the Java Catastrophe’ shortly after news of the eruption broke, and over the next few weeks published reports with the first descriptions and explanations of some of the many widespread effects of the eruption – from the appearance of great floating rafts of pumice, to the many oceanic, atmospheric and other phenomena that accompanied the event. In February 1884, the Royal Society set up the Krakatoa committee, chaired by a meteorologist George Symons, to collect information on ‘the various accounts of the volcanic eruption.. and its attendant phenomena’, including ‘authenticated facts respecting the fall of pumice and dust .. unusual disturbances of barometric pressure and sea level.. and exceptional effects of light and colour in the atmosphere‘. These results were published in 1888 in a wonderfully illustrated monograph, which still stands as one of the most complete accounts of a major volcanic eruption and its widespread effects.

 Drawings of views of Krakatoa rock samples in thin section, under a microscope.  This set of images are of lavas from Krakatoa. These are quite rich in crystals (white feldspar; green pyroxene), set in a fine matrix of glass (colourless to brown).  Images on the left: about 1 cm across; on the right – about 1 mm across.

Drawings of microscope views of Krakatoa rock samples in thin section. This set of images are of lavas from Krakatoa, from Symons (1888). These are quite rich in crystals (white feldspar; green pyroxene), set in a fine matrix of glass (colourless to brown). Images on the left: about 1 cm across; on the right – about 1 mm across.

 Drawings of views of 1883 Krakatoa pumice and ash samples in thin section, under a microscope.  The pumice samples are mainly made of glass (very pale colour) with gas bubbles.  The top 4 images are each about 1 cm across, and show the texture of lumps of pumice.  The bottom two images are each about 1 mm across, and show the ‘ash’ that fell over a thousand miles away from Krakatoa, onto the ship Arabella (left), and the ‘ash’ formed by grinding up a sample of pumice.

Drawings of microscopic images of 1883 Krakatoa pumice and ash samples . The pumice samples are mainly made of glass (very pale colour) with gas bubbles. The top 4 images are each about 1 cm across, and show the texture of lumps of pumice. The bottom two images are each about 1 mm across, and show the ‘ash’ that fell over a thousand miles away from Krakatoa on the ship Arabella (left), and the ‘ash’ formed by grinding up a sample of pumice. From Symons (1888).

The most celebrated impact of the Krakatoa eruption in terms of science was the recognition that the many optical effects that followed the eruption, including both spectacular sunsets (an example from London, in November 1883, below) and the discovery of ‘Bishops’ Rings‘, must be  the consequences of the global spread of volcanic pollutants, high in the atmosphere. We now know that the major constituents of this haze were tiny droplets of sulphate, forming a thin aerosol layer in the stratosphere which scattered and absorbed incoming solar radiation.

Sunset at Chelsea, 4.40 pm,  November 26th, 1883. From Symonds (1888).

Sunset at Chelsea, 4.40 pm, November 26th, 1883. From Symons (1888).

Although the Krakatoa eruption was one of the largest eruptions of the past 200 years, it is not exceptional in comparison to other eruptions from the geological record. In terms of eruption ‘size’ it rates as a ‘6’ on the Volcanic Explosivity Index, having erupted an estimated 12 cubic kilometres of magma. Eruptions of this sort of size occur once every 100 – 200 years around the globe; while the largest known explosive volcanic eruptions erupt many thousands of cubic kilometres of magma over a very short period of time. Krakatoa remains the best documented example of a ‘caldera-forming’ eruption, during which an entire volcanic edifice collapses. In this case, the eruption and the formation of the caldera had catastrophic consequences for tens of thousands of people along the shores of Java and Sumatra, as a series of major tsunamis triggered by these events swept ashore.

Reference

G.J. Symons (Editor, 1888), The eruption of Krakatoa and subsequent phenomena.  Report of the Krakatoa Committee of the Royal Society.  London : Trübner & Co.

Links to some other posts on Krakatoa

David Bressan’s excellent post for the Scientific American Blog

Bill McGuire’s post on Krakatoa for the OUP Blog

Jeremy Plester’s Weatherwatch piece for The Guardian

Cynthia Wood’s post for Damn Interesting

A page on the Krakatoa eruption from the United States Geological Survey