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Volcanic hazards
Volcanic hazards

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1.4 Lahars, jökulhlaups and gasses

The eruption that claimed most lives in the 100 years before this course was written was the 1985 eruption of Nevado del Ruiz in Colombia, which caused a lahar that annihilated the town of Armero

A jökulhlaup (if you pronounce it 'yer-kul-h-loip' you won't be far wrong) is in some ways a dilute equivalent of a lahar. It consists of a violent flood of water transporting rocks, gravel and often blocks of ice, but the nature of the material available at its source means that it lacks the high load of ash and/or mud that distinguishes a lahar. You will have studied a jökulhlaup in Activity 1.

Gas emitted from volcanoes poses many problems. It can blight land downwind from a persistently active volcano (Figure 7), poison the grazing after a large eruption, and accumulate in depressions or flow downhill to asphyxiate people. When you visit the Global Volcanism Program website you will probably see gases identified by their chemical formula rather than name, so they are listed for you here in Table 1. With the exception of water vapour and hydrogen fluoride, these gases are all denser than air so, if emitted passively (rather than being expelled in a hot and therefore buoyant eruption column), they will tend to flow downhill.

Figure 7 Contrasting conditions (a) away from and (b) beneath the persistent sulfur dioxide plume from the Masaya volcano in Nicaragua (colour versions of TYVET Figure 5.23).
Figure 7 Contrasting conditions (a) away from and (b) beneath the persistent sulfur dioxide plume from the Masaya volcano in Nicaragua (colour versions of TYVET Figure 5.23).
Table 1 Chemical formula of common volcanic gases
carbon dioxideCO2
hydrogen fluorideHF
water vapourH2O
hydrogen sulfideH2S
sulfur dioxideSO2

Activity 2 The Grimsvötn jökulhlaups

Timing: 15 minutes

Go to the Global Volcanism Program website, and the Icelandic volcano, Grimsvötn page [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . Select 'Reports', then 'Available Bulletin Reports'. At the time of writing, there are reports of jökulhlaups in 1972, 1982, 1983, 1996, 1998, 2004 and 2011 but by the time you look there may have been another one. For this activity, I simply want you to look at the photographs of the 2004 event, which are Figures 7-10. If you click on any of these, you will see an enlarged image which lets you see more detail (use your browser's 'Back' button to return to the previous view).

In particular, I would like you to note the gravelly coastal plain in Figure 10 flooded by the jökulhlaup that you can see emerging from the distant glacier but (in this case) not washing away the adequately engineered road that had been rebuilt after the 1996 jökulhlaup.

Look at Figure 8. Write a description (two or three sentences) describing what you can see below the angled eruption column. Which way was the wind blowing (relative to this view)?


I hope you wrote something like this:

Below the eruption column curtains of ash can be made out, falling towards the ground. The ground beneath the column (which is actually the top of a glacier) is blackened by the airfall that has accumulated there. The column is deflected to the left, showing that the wind was blowing from right to left when this picture was taken.

(If you were particularly insightful, you might also have noted that the pattern of airfall on the ground shows that the wind direction has remained constant throughout the eruption.)

Incidentally, if you tried to find a description of the 1996 jökulhlaup on this site, you will have been disappointed because the 1996 reports only cover the eruption leading up to the jökulhlaup.

Activity 3 Long Valley skier fatalities in 2006

Timing: 10 minutes

Go to the Global Volcanism Program website, and the western USA volcano, Long Valley and select the report for 06/2006.

Read the section of the report for that date headed Ski-area accident, and then write a description (one or two sentences) stating which volcanic gases are mentioned, and which one caused the deaths.


The report mentions H2S (which Table 1 identifies as hydrogen sulfide), of which one sometimes 'gets a whiff' [rotten eggs] while riding in the nearby chairlift, but what appears to have killed the ski-patrol officers was CO2 (carbon dioxide) that probably made up 90% of the air in the snow-hole into which they had fallen. With that in mind, I hope you wrote something like:

The two volcanic gases mentioned in the report are hydrogen sulfide and carbon dioxide. It was the carbon dioxide that appears to have caused the deaths.

As a point of interest, this incident was at Mammoth Mountain, where tree deaths because of carbon dioxide in the soil are described near the foot of TYVET p. 158. You may have noticed the map showing 'tree kill' immediately above the Ski-area accident report.