No place like home
Signpost on Tristan da Cunha
14 November 2011 by Anna Hicks
Being part of the most remote population on Earth should protect you from the chaos and disruption some light volcanic ashing brings to our crowded European countries. Far from it. Anna Hicks explains how her research is bringing us closer to understanding the risks that go with living in the shadow of one of nature's deadliest forces.
Risk comes in many forms, and for the 264-strong community of Tristan da Cunha - a remote volcanic island in the South Atlantic - an eruption could mean the stark choice between camping in a small field surrounded by a large ocean, and making a week-long boat journey to the nearest temporary refuge, leaving everything you own and your whole way of life behind.
The evacuation of people from Tristan, near or far, would be governed by the style and location of the next eruption. Knowing where and when that will be is a major challenge. Like many other ocean island volcanoes, Tristan does not erupt only from the top; magma can spew from vents on the slopes, near the shore or even under the ocean.
Fifty years ago this month, the volcano re-activated and lava erupted just a few hundred metres from the island's only village, Edinburgh of the Seven Seas. The whole population was forced out by the encroaching lava and endured several uncomfortable nights in the vegetable patches to the west and on nearby Nightingale Island - normally inhabited only by penguins, seals and albatross. Eventually the islanders were picked up by a Dutch liner and evacuated to the UK.
The main settlement of Edinburgh of the Seven Seas
Islander Lars Repetto clearly remembers the moment that they left Tristan. "Everybody was taken off at the beach," he says. "It was so frightening because the volcano was coming down; red hot lava flowing down right where we had to get off and we tried to get off as quick as possible. Everybody got off and nobody got hurt."
The islanders lived and worked in England for two years, but in 1963 they voted to return. For them, life in the UK could never match the freedom, independence and security that Tristan offered. "We decided there's no place like home, sweet home so we came back to this island. We love this island," Harold Green told me in February 2011.
In recent years Tristan has been quiet, but a submarine eruption that washed up pumice onto its shores in 2004 showed that the islanders remain at risk. Complacency is their worst enemy. One of the most useful contributions that scientists can make is to reduce the uncertainty about when, where and how the island might erupt. To help do this, we aimed to determine ages of the most recent eruptions to identify any patterns in activity.
We applied a method known as argon-argon dating. This approach uses the known natural radioactive decay of an isotope of potassium (40K) to an isotope of argon (40Ar), with a known half-life of 1,250 million years, to determine the age of rocks and minerals. Despite the robustness of the technique, dating the Tristan rocks was going to be tricky as the island is very young. If too little time had elapsed for the rocks to accumulate measurable amounts of 40Ar, then the derived ages for the volcanic eruptions could be riddled with uncertainty. But this was only one of many difficulties...
Getting to Tristan was the first hurdle. The island, a British Overseas Territory, is more than 2,800km from the nearest mainland city, Cape Town in South Africa. This is also the point of embarkation for ships that transport visitors and islanders to Tristan about ten times a year. Entry by air is impossible. Rising abruptly out of the ocean to a height of 2060m, the island's precipitous flanks are furrowed with deep ravines, many of which are virtually inaccessible.
Although Tristan is only 12km (seven miles) across at its widest, the undulating topography made traversing it incredibly arduous. After weeks of hiking and sampling rocks, stopping occasionally to enjoy the fearsome beauty of the wild volcanic terrain, I left Tristan with half the island in my pockets, and set about preparing the rock samples for argon-argon dating.
The volcanic eruption of 1961
Within six months, the rocks were individually prepared by crushing, sieving and separating identical sand-sized particles by hand. We then delivered the prepared samples to the NERC Argon Isotope Facility for dating. The conventional way of extracting the argon from the rocks and minerals is to heat the samples in a small vacuum furnace. However, the furnace itself contributes a background amount of argon; when working with young samples, this can mask the argon we are looking for.
To combat this, we commissioned a brand-new scanning laser system designed to heat samples uniformly. This meant we could precisely control the temperature at which the argon was extracted to assess whether there was any potential for contamination from atmospheric and mantle sources. Without the ability to remove the effect of these contaminants from the radiogenic 40Ar signal, our hard work would have been worthless.
The results were astonishing. All 16 samples produced robust data (statistically reproducible measurements of key argon isotope ratios). What about the ages? Not only did we manage to date precisely some of the youngest lavas on the island, but we also broke a laboratory record for the youngest basalt ever dated! This particular rock was sampled just a kilometre from Edinburgh. The lava flow is less than 2,700 years old. Had the Romans decided to invade Tristan, it would still have been hot!
What about the last eruption from the summit? Previous researchers assumed activity here had ceased more than 15,000 years ago. An eruption from the summit would pose very different risks from eruptions from the flanks lower down, with Tristan's steep slopes and channelling canyons directing lava, hot gases and ash down towards the village.
Dating the summit was wise - it had been active as recently as 5,000 years ago. In fact, contrary to popular belief, all three 'zones' on the island - summit, flanks and low-lying coastal areas - have been volcanically active during the last 16,000 years. Although the last two eruptions have occurred near sea level, an eruption higher up cannot be discounted.
Tristan da Cunha, as photographed from space by NASA
So when is the next one due? It is almost impossible to say with any certainty - the next strategy should involve monitoring the volcano closely for signs of reactivation, and scientists need to support the community in their efforts to prepare. I reported our study's findings back to the islanders in real time. The data prompted immediate review, revision and improvement of their disaster management plan, and the islanders also conducted their first ever evacuation drill!
During this period of repose, it is critical that scientists, decision- and policy-makers work with the islanders to continue to learn about the volcano and its behaviour. The argon-argon dating results have justified our concern, and the Tristan community has to prepare for an inevitable eruption. When, where and how is uncertain. Given the frequency of eruptions since Tristan emerged from the sea over 100,000 years ago, we need to watch closely for unusual activity and focus our future research efforts accordingly.
Improved knowledge of the system and basic monitoring is essential, but scientists and decision-makers also need to confront the volcano's inherent uncertainty and create an open dialogue about what we do and don't know. Only then can the islanders continue to plan for different eruption scenarios and reduce their risk from future volcanic activity.
Anna Hicks is a PhD candidate, with supervisors at the University of East Anglia and the British Geological Survey. She is carrying out the research described in this article in collaboration with Dr Darren Mark at the Scottish Universities Environmental Research Centre.