VolcanicDegassing

St Vincent

Why does it always rain (ash) on me?

Why does it always rain (ash) on me?

On May 1st, 1812, a remarkable weather system reached Barbados. ‘At half-past twelve AM a heavy dark cloud obscured the heavens completely. [..]  at half past one a sandy grit began to fall in small quantities‘. Through the night there was the sound of explosions and thunder, and by late afternoon, Barbados had been blanketed in several centimetres depth of ash. The origin of the ‘May dust’, as it came to be known, was soon found to be due to a violent eruption of the Soufriere volcano, on the nearby island of St Vincent.

The German explorer and botanist Robert Schomburgk later realised that this ‘rain of ashes’ showed that there must be winds in the upper reaches of the atmosphere in the Caribbean that blow in the opposite direction to the Trade Winds, near the surface. When he was living in Barbados, in 1846, Schomburgk could still find patches of the May dust across the island, which he collected and shared with Christian Ehrenberg – a scientist who had recently discovered traces of aquatic organisms in wind-blown Atlantic dusts. Later work on archived samples of this May dust showed that they were rich in bacteria and fungal spores, perhaps carried in place by high-level winds. Barbados, it turns out, is no stranger to dust. Mostly, this is carried in from Africa, and is usually only detected by careful air-sampling using carefully cleaned filters.  But, as we saw in 1812; and then again in 1902, 1903 and 1979, everytime there is an explosive eruption of the Soufriere of St Vincent, Barbados is coated in another layer of mineral dust and volcanic glass.

Two hundred years on, and we are still learning how the interaction of the atmospheric winds with the topography of the planetary surface controls where ash is carried, and then dropped, from erupting volcanic plumes. In a new paper, we use sensitive models of the wind-field, that account for the reversal of wind directions and the fine-scale topography of the islands of St Vincent and Barbados, to simulate ash fallout after the 1902 and 1979 eruptions. The computer simulations show how these two factors influence the distribution of ash after an eruption, but also that we can reproduce the observations made on the ground in 1902, and 1979. This approach means that we now have a way to improve forecasts of where volcanic ash will go, and when and how much will land on the ground during an eruption. Given how sensitive human lifelines – transport, water, food and power supplies – are to disruption by volcanic ash, this could be an important step forwards.

Selected references.

Darwin, C (1846) An account of the fine dust which often falls on vessels in the Atlantic Ocean, Quarterly Journal of the Geological Society, London, 2, 26-30.

Delaney, AC et al (1967) Airborne dust collected at Barbados, Geochimica et Cosmochimica Acta, 31, 885-909.

Poulidis, AP et al (2018) Meteorological controls on local and regional volcanic ash dispersal. Scientific Reports, 8, 6873.

Pyle, DM et al (2018) The 1902-3 eruptions of the Soufriere, St Vincent: impacts, relief and response. Journal of Volcanology and Geothermal Research.

Schomburgk, RH (1848) The History of Barbados. Longman, Brown, Green and Longmans, London. pp. 69-72.

Wilson, T (2012) Volcanic ash impacts on critical infrastructure, Physics and Chemistry of the Earth A/B/C, 45-46, 5-23.

This work was inspired by the STREVA project, funded by the UK Natural Environment Research Council (NERC), and is the product of a collaboration between scientists based in the UK, Caribbean, Japan and Singapore.

There’s (volcanic) dust in the archives

There’s (volcanic) dust in the archives

There’s not much that beats the thrill of discovery.. particularly when it turns up in your own backyard.  This summer, I have been on the hunt for records and reports of the 1902 eruptions of St Vincent, a lush volcanic island in the Eastern Caribbean. There are indeed many reports from this eruption, carefully documented in official records from the time. But, more surprisingly, there are samples – and many of them in the UK: packets, vials and boxes of ash; chunks of rock and more, in museum collections and archives in both the Natural History Museum, and at the British Geological Survey. Here is just a snapshot of some of the incredible samples from the British Geological Survey Archives.

From the BGS archives

Four vials of volcanic ash – all collected on Barbados. The smallest vial is of ash from the 1812 eruption of St Vincent. The three other vials are samples of ash that fell on Barbados during eruptions between May 1902 and March 1903.

Along with the samples are the original envelopes in which they were sent, and handwritten notes documenting the sample: priceless tools, when you want to look back at an eruption that took place over 100 years ago.

1902 Barbados ash

1902 Barbados ash sample sent to Horace Woodward, who was in charge of the Geological Survey’s office in Jermyn Street, London, at that time – which included the Museum of Practical Geology. Memo reads ‘Dust from Mt Soufriere St Vincent, collected on the deck of the SS ‘Statia’ lying at Barbados, 90 miles distant, travelling against a strong S E wind and covering everything to the depth of 5 inches. 1903′.

Some of these samples are timed and dated, and can be linked to particular phases of the eruption. Here is one example – of the ash that fell during the opening stages of the eruption on Barbados.

First hour

‘Volcanic dust collected at Barbados during the night May 7-8 1902. This spec. fell during the first hour’. Collected by WG Freeman, a botanist at the Imperial Department of Agriculture for the West Indies in Barbados.

Other samples can be used to map the distribution of ash and coarser samples that fell across St Vincent – here’s an example of a ‘gravel’ grade sample from Rosebank on St Vincent.

1902 ash

1902 tephra sample collected on St Vincent by  Henry Powell, Curator of the Botanic Station on St Vincent. ‘Sample of volcanic sand which fell at Rosebank (Leeward) on night of 3rd and morning of 4th Sept. 1902’

Among the most amazing discoveries, are examples of damage to economically valuable plants – this one, a sample of Breadfruit leaf that was damaged during the latter stages of the eruption in March 1903.

image

‘A leaf of the Bread Fruit Tree (Artocarpus incisa) gathered in St Vincent about 12 miles from the Soufriere and showing perforation caused by volcanic stones etc.’ Collected by WG Freeman in 1903.

Together, these sorts of samples will allow us to go back and investigate what was actually happening during the eruption, in a way that is rarely possible, even for modern events.

Links – read more about the eruptions of St Vincent on the London Volcano blog.

Friday Field Photo – St Vincent, 1902

Roseau Dry River, St Vincent, 1902. Photo by Tempest Anderson, from Volcanic Studies.

Roseau Dry River, St Vincent, 1902. Photo by Tempest Anderson, published in his book  Volcanic Studies in Many Lands (1903).

Today’s field photo is by Tempest Anderson, of the ‘Roseau Dry River flowing with Boiling Mud’, a picture taken in the aftermath of the May 1902 eruptions of the Soufrière of St Vincent.  The full published caption explains the origins of this boiling mud – a phenomenon we now call a lahar:

This is a small stream in the Wallibu Basin. When the water undermines the banks and the hot ashes fall into the river, the stream is often dammed up, and the giving way of the obstruction is associated with a great discharge of boiling mud. In one of our ascents of the Soufriere, we had crossed the Rozeau Dry River without difficulty in the morning when the weather was fine, but heavy rain had fallen before our return in the afternoon, and the river was full of boiling mud, coming in gushes, as shown in the picture. After some trouble our men cut down two trees which had been killed by the eruptions, and made a bridge by which we crossed. The banks show the characteristic erosion by the rain rills.’

 Lahars are a very common feature at volcanoes, and they may often continue to be triggered by strong rainfall events for months or years after the eruptive activity has ceased.

 Tempest Anderson was an opthalmologist by profession, and also an inveterate traveller and photographer. He accompanied  John Flett to the Caribbean to document the eruptions on St Vincent and Martinique, in 1902, and a selection of his photographs can be found on the website of the Yorkshire Museum. The Soufrière of St Vincent is the centrepiece of the London Volcano project, which runs from 9-13 June.

Further Reading

Anderson and Flett’s Report on the Eruptions of the Soufriere, in St Vincent, in 1902..  Philosophical Transactions of the Royal Society, London, A 200, 353-553 (1903)

Anderson, T., Volcanic Studies in Many Lands, John Murray, 1903.

The destruction of St Pierre, Martinique: 8 May, 1902

May 8th marks the anniversary of one of the worst volcanic disasters on record: the destruction of St Pierre, Martinique, in 1902, at the climax of the eruption of Mont Pelée. Below are a snapshot of images from one of the contemporary accounts of the disaster, ‘The volcano’s deadly work‘, written by Charles Morris in 1902. This eruption followed just one day after a similarly destructive eruption of the nearby Soufrière of St Vincent.

photo 2

Original caption ‘The only photograph taken of the volcanic outbreak of Mt Pelee, May 8, 1902, during the height of the eruption, a scene as grand as it was appalling’. This shows ash clouds rising off the flanks of the volcano, presumably associated with the emplacement of the dense, hot pyroclastic ‘nuee ardentes’ that accompanied the eruption to devastating effect.

photo 4

Original caption ‘Interior of a steamship at St Pierre, after the whirlwind of fire’. There was considerable damage incurred to boats that were afloat offshore from St Pierre, as the hot ash clouds travelled a short distance across the water, and engulfed them.

photo 1

Original caption ‘The clock that told the story. The ruins of the hospital of St Pierre and the clock with the hands pointing to 7.50, which indicated the time at which the city was overwhelmed’.