VolcanicDegassing

volcanology

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.

Lahar: Lost in translation?

Lahar: Lost in translation?

Since late September, the eyes of the volcano world have turned to Gunung Agung. This prominent volcano in Bali last erupted in 1963, when it released enough sulphur dioxide to form a global stratospheric sulphate aerosol layer that led to vivid sunsets, and short-term global cooling. The 1963 eruption was one of the largest and deadliest in Indonesia in the 20th century; and many of the casualties were caught up in the violent pyroclastic flows and mudflows, or lahars, that swept down the flanks of the volcano in March and May 1963.

Even though the eruptive activity at Agung is currently at a low level, the immediate hazard is once again due to the lahars – slurries of ash and water, that may form during heavy rain, and run rapidly down the volcanic flanks. Lahars have already featured prominently in English language news reports; with some described as ‘cold lava‘. Cold lava, it turns out, is the ‘Google translate’ rendering of the phrase lahar dingin, from some reports of the activity. The problem is not one of Google’s making – but goes back to the way the word ended up in English usage.

Lahar is one of the few words from Javanese that has entered the English language. The Oxford English Dictionary entry for lahar describes it as a ‘destructive mudflow on the slopes of a volcano’; originating in the 1920’s. A contemporary Indonesian – English dictionary offers two translations for the Indonesian word lahar – 1, lava; 2 mudflow. Older dictionaries have only the first rendering of lahar, meaning lava.  Although this isn’t reflected in English dictionaries, lahar was a geographical term familiar to Dutch colonialists in Java by the mid-19th Century.

The botanist and geologist Franz Junghuhn used the term lahar to describe the ravines on the slopes of Kelud, Java (Junghuhn, 1854). In reports of the January 1864 eruption of Kelud, Colonel Versteeg, of the engineering corps of the Dutch colonial army, describes how his workmen shouted “lahar is coming“, shortly before a hot stream of ash and water swept by (Versteeg, 1864). John Hageman experienced the same eruption ‘We heard a thunderous roar approaching .. and a moment later the sizzling fluid rolled by‘.  At Kelud, these destructive flows formed when the  eruption emptied the summit crater lake ‘in a single shock; and the water mixed with sand and stones flowed .. to the south-west and north-west along the so-called Lahars or ravines.‘ So, while lahar is only ever used in English as a technical term to describe a particular sort of flow; its non-technical meaning in Indonesian is clearly rather broader.

The use of lahar to mean a ‘flow of volcaniclastic debris and water’ first came into use in volcanology in the early 1920’s, following another eruption of Kelud in 1919. This eruption caused a terrible loss of life as the crater lake failed, sending cascades of lahars down the ravines that drained the flanks of the volcano. These flows and their deposits were described by the geologist Georg Kemmerling in 1921. Kemmerling distinguished hot (eruptive) and cold (non eruptive) lahars; and recognised that their deposits were dominated by sand-sized material, rather than mud. The next year Berend Escher (brother of the artist, M C Escher) compared the lahars of Kelud to the newly-described deposits of the Valley of Ten Thousand Smokes, emplaced during the 1912 eruption of Novarupta (Alaska). British Geologist John Brooke Scrivenor wrote a short account of the Kelud lahars in 1929; and the term later gained widespread use in volcanology after the 1940’s.

Globally, lahars are one of the most significant hazards posed by volcanoes, due to their capacity to rapidly inundate areas many tens of kilometres from the erupting volcano; while the threat from lahars may continue long after an eruption has ceased. We can only hope that the preventative measures taken around Agung are sufficient to reduce the threats from lahars to those living nearby.

Sources

Brown SK et al., 2017, Volcanic fatalities database: analysis of volcanic threat with distance and victim classification, Journal of Applied Volcanology 6:15

Escher, BG, 1922, On the hot ‘lahar’ (mud flow) of the Valley of Ten Thousand Smokes, Alaska, Koninklijke Nederlandse Akademie van Wetenschappen, 24, 282-293.

Hageman, J, 1865, Bijdragen tot de kennis der uitbarsting van den Keloed op 3 Januarij 1864, Natuurkundig tijdschrift voor Nederlandsch Indië, 28, 475. [in Dutch].

Kemmerling, GLL, 1921, De uitbarsting van den G. Keloet in den nacht van den 19den op den 20sten Mei 1919: Dienst van het Mijnwezen in Nederlandsch Oost Indie, Vulkanol, Mededeel., 2, 120 p. [in Dutch].

Meinel, AB & MP Meinel, 1967, Volcanic Sunset-Glow Stratum: Origin, Science 155, 189

Pierson, TC, NJ Wood, CL Driedger, 2014, Reducing risk from lahar hazards: concepts, case studies, and roles for scientists, Journal of Applied Volcanology 3:16

Rigg, J, 1862, A dictionary of the Sunda language of Java, Verhandelingen van het Bataviaasch Genootschap van Kunsten en Wetenschappen, Deel XXIX.

Versteeg, WF, 1864, Vervolg op de Aanteekeningen omtrent aardbevingen in den indischen archipel, 27, 129. [in Dutch].

Zen, MT & D Hadikusumo, 1964, Preliminary report on the 1963 eruption of Mt Agung in Bali (Indonesia), Bull Volcanique 27, 269-299