Pyroclastic flows generated by volcanic explosions

Lava domes would not be so troublesome if block-and-ash PFs generated by dome collapse were the only hazard they inflicted on their surroundings. The night views of the Montserrat domes show clearly that their interiors are made of molten but very stiff and viscous lava. This usually contains abundant dissolved water at high pressures – ready to flash into steam and explode, if the containing pressure is lowered. A larger-than-usual dome collapse is a common trigger for such explosions, as lava in the volcano below the dome decompresses explosively. In addition, if the underground water pressure is very high, the source of the explosion may be deeper beneath the surface. Although such explosions usually flush out some shattered cold rock from the volcano, most of the ejected material is a foam made of lava swelled by countless gas bubbles. This material is known as “pumice” and PFs largely composed of pumice are characteristic of these explosions. The erupted material flies up into the air and then much of it falls back to earth again and cascades down the slopes of the volcano, concentrating in ghauts/gullies/valleys, if there are any for the flow to use.

Although most of the explosions are roughly vertical, some can occur sideways and form extremely dangerous lateral blasts (see below).

The photo chosen to illustrate a vertical explosion and PFs cascading from the erupted material is a typical “vulcanian” eruption of the Soufrière Hills volcano on 5 Feb 2010 (Fig. 13)

Fig. 14 shows an extraordinarily "lucky" photo from space of a much more powerful similar volcanic explosion from a Russian island volcano, rather like Montserrat, caught by a passing astronaut. Some of the SHV explosions have also reached tens of km high. The cloud cap on top of the Sarychev Peak eruption column is formed by air pushed upwards so fast that it chilled and expanded, forcing its water vapour to crystallise into ice. Of course the Sarychev explosion would only be called a "vulcanian" one, if it ceased immediately after this photo was taken. If the eruption continued for longer, it would be classified as "sub-plinian" or "plinian", depending on its violence and duration.

Lateral volcanic blasts

The huge explosion that began the St Helens eruption on 18 May 1980 was a lateral blast. The cause was a lava dome that grew underneath the north slopes of the volcano, making them bulge (Fig. 17). When an earthquake caused the covering rock to fall away in a landslide, the high-pressure steam inside exploded instantly and sent a massive pyroclastic surge and flow sideways for many miles. Although amazing photos of this blast exist, they are all strictly copyrighted and so I 've chosen a US Geological Survey (USGS) computer graphic from Wikipedia instead (Fig. 18). You can sometimes find the “forbidden” photos via “Google Images”. View the crucial pseudomovie -- constructed from a series of still photos because nobody had a movie camera trained on the mountain at that moment -- at [http://youtube.com/watch?v=bgRnVhbfIKQ]. Of course the copyright owner, Gary Rosenquist, may have had it removed by the time you read this. If you spend more time reading all the messages below the pseudomovie, please don’t blame me if you find some obscene and stupid ones there. There are also some eyewitness comments on that eruption.

More remarkable 2002 photos by Marco Fulle of Stromboli Online have recorded a unique glimpse of what takes place at the moment a lava dome begins to collapse. In Fig. 19 a strong earthquake is shaking the Montserrat SHV dome violently and generating rockfalls all over its surface; precisely the same happened at St Helens. In Fig. 20, taken only a few seconds later, the dome is beginning to collapse and generate pyroclastic flows. For the next few hours there is no way of telling the eventual size of the collapse and whether or not it will trigger an explosive eruption, which might be vertical, sideways (lateral) or both. On Mt St Helens in 1980 a massive plinian eruption started immediately and lasted for 10 hours.

Fig. 19	Fig. 20

Those of you who know the Montserrat eruption history well will of course remember that something very similar to the St Helens lateral blast happened during the night of 26 December 1997, when Galway’s Wall finally collapsed and vast amounts of debris avalanche/PF/surge material swept down White River valley and the adjacent slopes Figs 21-22). The mess is still impressive 13 years on and a reminder of what could still possible happen to the Belham Valley and surrounding area, if a future underground lava dome was to swell beneath the NW slopes of the volcano. Fortunately the 11 February 2010 dome collapse and associated explosions involved only about 20% of the visible dome and was directed towards the long-evacuated NE slopes of the volcano.