Danger 3: The Soufrière Hills “cat and mouse” eruption

 

 

This section will focus on the Belham Valley and the communities around its mouth because they remain a part of Montserrat at considerable risk from the eruption, if this continues. Anyone who has watched any member of the cat family hunting can see what this heading means. Domestic cats are infinitely patient when they are hunting mice. They just sit and wait – edging ever closer – until the mice make a mistake and get within range (Fig 5).

 

 

Fig. 5  Cat closing in on a mouse (Bob Thompson).

 

 

In memoriam to Pansy, who was an excellent mouser and whose remains are now buried in the north of England, while her ageing human companions in life have fled to the warmer southern end of the country.

 

 

 

 

The Soufrière Hills volcano has been playing cat-and-mouse with its surrounding population for 13 years so far. During this period most of the ghauts and river valleys radiating from it have been attacked by eruptions and filled by pyroclastic flows and subsequent lahars. At first the Belham Valley seemed to be almost immune from this creeping volcanic chaos (Fig. 6). Then something entirely unexpected happened on 25 June 1997. At one side of the surge cloud that swept through Streatham village a small very fluid pyroclastic flow formed and flowed down Belham Valley to the Cork Hill area (Fig. 8). Topic 7 of this website is focused on this phenomenon.

 

 

 

Fig. 6  Montserrat in the 1970s. Not a speck of volcanic ash to spoil paradise (Princesa Daley).

Fig. 7  Belham Valley laid out below the NW slopes of the volcano and its summit dome. Summit of Gages Peak in the foreground (V L Hards, MVO).

Fig. 8  First incursion of the Soufriere Hills pyroclastic flows into the Belham Valley. On 25 June 1997 a type of concentrated secondary PF, never seen before, formed by draining down-slope of ash that had just been deposited from the surges above, and which then continued down Belham Valley to Cork Hill (Copyright NERC).

 

Next the volcano did a typical “patient cat” tactic: nothing at all (except lahars) to Belham Valley for a decade! Next the summit lava dome shifted subtly in December 2006 (not at all obvious to distant onlookers) and began to grow in such a position that it might be able to collapse on a large scale down Tyer’s Ghaut and hence Belham Valley. Finally on 8 January 2007 it collapsed on a large enough scale (Fig. 9) to send a pyroclastic flow down the valley as far as Cork Hill (Fig. 10). Although this PF reached to within a kilometre of the beginning of the densely populated area around Happy Hill, Fleming, Frith, Salem, Old Towne and Olveston (Figs 11-12), its products are not very obvious from the down-valley locations (see photos from Old Towne below). Nevertheless, this PF both partially filled the upper and middle parts of the river gorge and smoothed it, so that a future PF could more easily reach the lower ground. This progression of events is the same as happened more rapidly in ghauts all around the volcano during the early years of the eruption. Click on these links to see YouTube videos of both the 8 January eruption and PF, and the high level of volcanic activity on 7 January.

 

 

Fig. 9  Pyroclastic flow of 8 Jan 2007 infilling Tyer’s Ghaut and the section of Belham Valley passing St George’s Hill (Vicky Hards, MVO). A decade later in the life of this patient and dangerous volcano.

Fig. 10  PF deposits (block and ash) of 8 Jan 2007 partially filling the Belham Valley down to beside Cork Hill (Vicky  Hards, MVO). This makes it easier for a big future surge (if this ever happens) to “escape” the valley and reach Old Towne, Salem and surrounding area.

Fig. 11  Termination of the 8 January 2007 PF; about 1 km  from the Salem/Frith/Old Towne area (Vicky Hards, MVO).

 

 

 This large PF was accompanied by a huge surge that devastated most of the upper Belham Valley.

 

Greg Scott

Same view uphill; the river valley was completely filled by PF debris.

 

Greg Scott

The best angle for seeing how the PF remained confined to the valley floor, whilst the surge spread out widely on both sides.

 

Greg Scott

 

 

S marks Salem

The modelled surge overwhelms the MVO area......

 

Paddy Smith

   ......and then devastates Salem and Olveston

 

Paddy Smith

 

Since then the SHV feline has steadily approached the Salem area in the manner used by all stalking cats -- short creeps forward, punctuated by still periods. First, things started moving again abruptly on the night of 28 July 2008, when the new (since 2007) Gages Vent, high on Gages Wall, erupted violently and sent pyroclastic flows in several directions. Subsequent short periods of lava extrusion on the dome in August and December 2008 increased its volume and height. Further explosions during this period generated pyroclastic flows and threatened to destabilise the entire dome. But after 3 January 2009 the volcano returned again to being as quiet as a mouse. Following its summer break, the lava dome returned to serious activity On 4 October 2009 and did some spectacular tricks before returning to sleep after 11 February 2010.

 

The NASA image below of the Belham Valley area (Fig. 12) tells you the whole story to date of the steady march by increasingly large PFs towards the Salem/Old Towne occupied houses. The numbers along the valley floor mark approximately where consecutive major PFs have reached. Notice the label for the Bishops View Road, where two photos below were taken. Remember that a secondary surge-derived PF reached to below Cork Hill school on 25 June 1997.

 

 

Fig. 12  NASA space image of Montserrat, with the volcano and Belham area enlarged and labelled with as many relevant things as I can fit. The numbers in puce pink along the Belham Valley floor refer to items detailed below.

 

 

Rather than more text, the numbered events on the Belham "map" are  illustrated below.

 

 

EVENT 1.  The termination of the 8 January 2007 PF (Vicky Hards, MVO)

EVENT 2.  Infilling and smoothing of Tyers Ghaut by many small PFs during early 2009 (Rod Stewart, MVO, and Greg Scott)

EVENT 3. The 10 December 2009 PF creeps far enough down the valley to trigger an upgraded hazard alert and some evacuation downstream.

 

The next 3 photos are just spectacular explosion clouds. Go to Topic 11 to see photos by Melody Schroer of the 11 February 2010 PFs cruising down the Belham River.

 

 

8 January 2007 eruption cloud over Plymouth area (Vicky Hards, MVO)

3 January 2009 explosion (Kathleen O'Donnell)

EVENT 5. 11 February 2010 PFs entering sea at Trants (Bennette Roach, editor Montserrat Reporter)

 

 

In order to update the stalking cat  idea, we can say that on 11 February 2010 the cat pounced .............. but missed! Actually is was quite a near miss. If the dome collapse had been larger or centred just a bit further to the west along the northern side of the dome, a large part of the collapsed material could have gone down the Belham Valley, rather than towards Trants. Let's hope that future collapses, if they happen, are equally thoughtful in their directions.

 

 

Belham Valley floor from Bishops View Road, Old Towne. Big PFs are now routinely reaching to just before the corner of the valley in the middle distance. Will it be total panic and evacuation, or "business as usual" if/when future PFs begin to appear in this view?

 

The report of SAC meeting 17 repeats "when", rather than "if".

The wheel tracks show that adults with 4x4 vehicles already drive up this past of the Belham Valley river bed, across thick lahar deposits. What is going to happen if/when hot PFs begin to reach down this stretch of the valley. Adults will hopefully obey orders to stay away but CAN WE BE SURE THAT ALL YOUNG MONTSERRATIANS WILL RESIST BEING THEIR USUAL DAREDEVIL SELVES AND SNEAKING DOWN TO TAKE  A CLOSER LOOK?

During the BBC radio programme "Saturday Live" on 27 November 2010 a former island school head told how, on the morning of 25 June 1997, one lad from the school refused all her efforts to dissuade him and set off into the danger zone to collect some school books he had abandoned when evacuated. Miraculously, the helicopter rescued him next day. That's typical of boys!

 

 

JUNE 2010 UPDATE

 

The passage in the Science Advisory Committee Technical Report 14 about the threat of PF events in the Lower Belham Valley (pages 8-11) should be compulsory reading for anyone interested in the SHV eruption and its interface with residents around the valley. The SAC spell out that the risk of dome collapse and associated PFs reaching down Belham to the currently inhabited areas around Salem and Old Towne have been hugely greater since late 2006 than they were during previous years. Slow but steady raising of the upper valley floor by small PFs and lahars has systematically increased that risk and continues to do so. Greg Scott (Caribbean Helicopters, Antigua) was flying the laser surveying instrument to re-map the valley topography accurately during mid-June. These data will then be used by the Penn State group to improve their computer models that predict the PF deposits resulting from potential future dome collapses. As the two illustrations below show, these predictions are already astonishingly accurately and have been so since the SAC8 report in 2007. The SAC group comment in their June 2010 report that, using the pre-new-survey topography, their computer model needs only about 2 million cubic metres of dome to collapse towards the Belham Valley for the PFs to reach to below Happy Hill. This would send PFs down the entire stretch of the valley in the two views above. There have been many events of such a volume since 1995.

 

 

Satellite view of the 11 Feb 2010 partial dome collapse deposits (both this illustration and the adjacent one are copied from the SAC 14 technical Report in June 2010)

2007 computer model by the Penn State research group (see below) for dome collapse about half or less in volume that the 11 Feb 2010 one (the original SAC14 captions are below the illustrations.

 

On a strictly personal note I recall that there were furious debates during 2007, via blogs on the Montserrat Reporter website, about the decision by MVO Director Vicky Hards (fully supported by the SAC of course) to recommend evacuation of the residents close to the Lower Belham Valley after the 8 January 2007 eruption and to maintain that evacuation until it was partially reversed by Governor Barnes Jones and her other advisors. Some of Vicky Hards' critics operated openly via letters to the Reporter and suchlike but many others hid behind anonymous blogs and some even resorted to personal attacks by means of anonymous telephone calls to her during the night. Such behaviour is a criminal offence in many countries and could potentially have caused catastrophe on Montserrat, by diverting her concentration from a developing massive dome collapse or explosion. To give a specific example of this disgraceful behaviour, I quote an anonymous Reporter blog on 21 August 2007 which included the comment, "Please also be aware that Director Hards has changed definitions of danger levels to suit her desires and apparently is now changing the definitions of pyroclastic flows, rockfalls etc". If that person has any integrity whatsoever he/she will apologise to to Vicky for this outrageous slur on her professionalism.

 

During my brief stay on Montserrat in February 2010, several people commented to me that almost the entire island population now regretted their unsupportive attitude to Vicky Hards during 2007 because it is now crystal clear that she was totally correct in her decision to call for and maintain the January 2007 evacuation. Perhaps individual apologies and letters of support to her from individuals, and also from entire groups who opposed her judgements from the depths of their ignorance about erupting volcanoes, would go some way to redressing the harm that Montserrat residents did to her in 2007.

 

Since that year, the two subsequent brief eruption periods of SHV have steadily increased the potential threat to residents around Lower Belham Valley, as detailed above. The laser survey by Greg Scott has now appeared (November 2010) and shows dramatically how every PF entering the Belham Valley contributes to slowly filling it and thus making it progressively easier for a PF to reach further down that valley than before.

 

 

 

 

Newly-published computer modelling by the SAC (Feb 2011).

 

The area where people hope to drill for geothermal energy, and then install a geothermal power station, is in the vicinity of Delvins (marked). The unexpected new feature of the latest (post-lidar-survey) modelling is that it appears that the surges accompanying an extremely big PF might leak southwards through the gap between Garibaldi and St George's Hills, thus reaching the proposed drilling area. For such a surge to be big enough to be a major problem for the drillers and their rigs, the modelling suggests a PF with much the same volume the huge one that covered Trants etc on 11 Feb 2010. This would need another ~20% collapse of the entire dome; i.e. the entire sector overlooking Tyer's Ghaut and Gages Mt. You can gaze yourselves from the observatory to decide whether such an event is an appreciable risk -- my instinct is no (at least for a while).

 

If such a sector collapse of the dome did happen, complete with surges tearing through the Delvins area, could drillers and their rigs be protected? Go to page 3 of my Merapi topic and see that the answer is most definitely "Yes"! A concrete shelter, designed much as a local stone sugar mill, would provide comfortable safety for the drillers -- even if some of the actual block-and-ash flow arrived there. A wall about 300 metres long and 10 metres high would block any such PF before the drilling sites (and subsequent power station). Reinforced concrete about one metre thick should be ample;  maybe this should be both V-shaped and tilted away from Belham, for maximum stability. Alternatively, simple use thinner concrete and pile loads of earth, rocks etc in front of it. The Berliners and subsequently the Israelis are experts in erecting such walls and might even donate spare materials from theirs!

 

 

 

Here is a helicopter pilot's view of the Belham Valley in November 2010. Note how recent PFs and lahars are rapidly filling the entire valley floor.

 

Paddy Smith

 

 

Two Greg Scott 2009 views of the eastern side of SHV show how this side of the summit dome appears to have hardened enough to begin to show us what this peak may look like after the eruption ends. The silver lining to Montserrat's volcanic cloud may be a spectacular crag that rock climbers flock to do their gymnastic thing on.

 

 

 

To end this update, here are two recent (2010-11) views of the volcano, showing how crucial Gages Mountain is to the protection of the Belham Valley area from pyroclastic flows. I suspect that if we could go back a millennium and visit the island when its only inhabitants were Carib tribespeople, we would find that they worshipped the goddess of Gages Mountain as their protector from the ravages of the volcano.

 

From St George's Hill the dome looks HUGE now and only Gages Mt (left) prevents it from collapsing down the Belham Valley.

(Dee Forlife, Feb 2011)

From Broderick's. Chances Peak is between the camera and the new dome but it is hard to tell them apart in this view. Gages Mt is now on the left. The new dome can clearly grow larger before it overtops Gages. This is unlikely to happen because a very large dome normally collapses in other directions, as in February 2010.

 

 

The two images below are from the late 2010 eruption of Merapi, where about 350 people were killed -- mostly by pyroclastic surges. The NASA "false colour" image from space (left) shows deposits from a big PF (pale grey) extending for many km down a river valley. The dark grey-coloured thin zones along both sides of the PF are surge deposits. Note how these extend very widely on both sides of the PF for the first few km from Merapi. This looks suspiciously like evidence for a laterally-directed blast when the PF formed. The right-hand image shows details of the wide tongue of surge deposits to the left of the river. It's clear that these did terrible damage before they stopped on a golf course.

 

The point I'm trying to make is that, if a dome collapse and blast from SHV happens in the future, there remains a small but non-zero chance that Salem could suddenly look like the right-hand image. Think about this next time you grumble when told to evacuate yet again.

 

 

 

 

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