Volcano goes BOOM, and then…
I’m sure we’re all familiar with the notorious things about volcanoes: that big ash plume, planes grounded, lava spewing all through a town and burning houses, and so on. These are famous for a reason: they are striking and so out of the ordinary, at least compared to life in the UK. But volcanic eruptions don’t necessarily very quickly go pop and then go back to sleep when the media stop reporting them. Some volcanoes can erupt for many years during a single ‘eruption’ and the impacts of these eruptions can be much more long term than you might expect because of the way they change the landscape.
A clear day looking at Soufriere Hills Volcano.
I am a geomorphologist, which means that I study the way the Earth’s surface is shaped, and how earth surface processes (i.e. flowing water and debris) mediate the shaping of the landscape by erosion and deposition (contrary to Hollywood’s woefully uninformed belief, no, I do not study the Earth’s magnetic field… did anyone else see Annihilation? Unbelievable… ). I am particularly interested in how volcanic eruptions alter their surrounding landscape and how it then behaves in response. The geomorphic impact of eruptions can be extremely variable, depending on the type of eruption and the magnitude of the eruption. For example, on one hand in Hawaii, runny mafic lavas spew out of fissures, flow like rivers over the landscape, until they reach the sea where they dribble incandescent molten rock to the depths of the Pacific. This spewage leads to very shallow slopes or seemingly rather flat landscapes of bare black rock, stripped of life. In contrast to this, some other volcanoes may unleash cataclysmic explosions, flinging fragmented lava up into the atmosphere and over their surrounding environment. This sort of behaviour fills up valleys with volcanic fragments, smoothes out the landscape, strips hill slopes of vegetation and can deposit ash many 100s of kilometres away.
The latter explosive eruptions are what I focus on, particularly the eruption of Soufriere Hills Volcano, Montserrat. Here, in 1995, after 300 years of quiescence, the volcano started squeezing out an andesitic lava dome from an old crater left by the last eruption. Since then the volcanic activity, consisting of a great deal of lava dome growth and collapses, explosions, pyroclastic flows and ash fall, has claimed 19 lives, destroyed the main town of Plymouth, and caused the permanent evacuation/resettlement of around 10,000 Montserratians. Volcanic activity continued in a start-stop fashion until 2010 and since then there has been no eruption of new lava.
Montserrat is covered in deeply incised valleys, locally known as ghauts (pronounced like ‘guts’ ). These are carved out by the intense tropical rains that fall over the island, particularly over the hills. The eruption of around 1 cubic kilometre of lava in fifteen years dramatically altered the form of the volcano, with the majority of the ghauts draining the volcano being filled up with loose and poorly consolidated pyroclastic material; in some valleys there has been around 300 metres of infilling (I can’t get my head around that).
‘Yeah, yeah, but why do we care now? What are you being paid for? People are safe, right?’ Well, no, not completely. For one thing the volcano isn’t ‘asleep’ yet, and so the possibility for more eruption still exists. But the other thing is that the landscape is still responding to the volcanic perturbation! So this is where I come in with my geomorph magic…
Rain can fall heavily in tropical Montserrat and when it does, just like anywhere else, some water will soak into the soil, but in some cases it might not soak in fast enough and flow over the surface instead, i.e. the state of the substrate determines the hydrologic response to rainfall. In Montserrat, the deposition of all this volcanic material has drastically altered the surface properties which has in turn changed the behaviour of rain water when it reaches the ground. After eruption, vegetation is damaged or destroyed, which limits interception, increasing the rate of water delivery to the surface. Further, ash fall leaves a layer of fine ash on the surface which can be hydrophobic, preventing the water from infiltrating. Both of these examples increase the likelihood of more water being available to flow over the surface. When there is sufficient overland flow, the volcanic material can be eroded and picked up by the water, forming what is known as a lahar. These lahars are dangerous phenomena that occur on volcanic slopes at many volcanoes, and in Montserrat they are responsible in large part for the destruction of Plymouth and still occur to this day.
A view of Soufriere Hills Volcano from the buried town of Plymouth. Credit: J. Christie.
Over time, after or between eruptions, the state of the land surface changes: lahars erode the material, removing it from the deposit (preferentially removing the finest material), reducing the amount of material available to be eroded;, channels form which makes the flow of water more efficient, which reduces the amount of material that gets eroded; and vegetation regrows, which reduces rate of water delivery to the surface and subsequent runoff magnitudes. The two figures below show the removal of vegetation after a large eruption in 2010, and the subsequent recovery. So, as these factors evolve, so too does the probability of a lahar for a given rainfall magnitude. THIS is what I am interested in.
Satellite images of Montserrat from a) 2010, demonstrating the impact of eruption on the landscape, and b) 2018, demonstrating the extent of vegetation recovery since 2010. Credit: ASTER, NASA
By looking at the geomorphic changes that have/are occurring around Soufriere Hills Volcano (i.e. input/removal of volcanic material, changes in vegetation cover), and the rainfall patterns that have been recorded, I hope to develop a better understanding of the key factors that control the probability of lahar triggering around the Soufriere Hills Volcano. This is important because, for example, in the Belham River Valley (one of the valleys draining the volcano), people are mining the volcanic material for export from the middle of the lahar channels, so are potentially in harms way at work. Furthermore, there is a small community that lives across the valley from the rest of the island and the only way across is over the dry channel. When a lahar occurs, these miners are potentially at risk if they have failed to be warned and evacuated, and this community can be cut off for several days because the flows erode through the make-shift crossings which then need to be rebuilt. The hope is that improved understanding of lahar triggering conditions might help authorities better manage land use and risk into the future.