Lesser Antilles island arc


Montserrat’s volcano is different. Montserrat is one of a slightly curved row of Caribbean islands, known as the Lesser Antilles “island arc”. If you look at the plate tectonic map (Fig. 7), you can find the relatively small Caribbean Plate (bang in the centre of the world of course, as everyone from the Caribbean knows!) and see that its eastern (right-hand) end is curved, marking the site of the Lesser Antilles. This is one of the places where an old tectonic plate sinks back into the convecting mantle beneath. Geologists call this process “subduction”. If you compare the view from space below (Fig. 12) with the cartoon (Fig. 13), you should be able to get a good idea of what’s going on beneath Montserrat. The magic ingredient that makes the mantle above the sinking slab melt is water. The sinking slab used to form the sea bed and therefore contains water that escapes from the slab as it is heated by the surrounding mantle. This lowers the melting temperature of mantle rocks enough to produce the volcanoes of island arcs.


Fig. 12  The Lesser Antilles islands from space (NASA).

Fig. 13  Diagram showing what is going on deep in the earth beneath the Lesser Antilles region.


The north end of the arc of active volcanoes above the Lesser Antilles subduction zone is Saba (Fig 14). North of this island the boundary between the Atlantic and Caribbean tectonic plates turns abruptly from roughly NE-SW to E-W. Therefore the convergence between two large plates is instead taken up on the Puerto Rico and Hispaniola region by a zone where the two plate slide past each other. The sliding is distributed along the margins of several small tectonic fragments ("microplates"), rather than occurring along a single fault, such as the famous San Andreas fault in California.

Fig. 14  Volcanic northern Antillean islands (Smith and Roobol). Island names in black; active/dormant volcanoes in red.

Fig. 15  Approximate boundaries of the small tectonic plates to the north of the Lesser Antilles island arc (Eric Calais). Sideways (lateral) slip on the Enriquillo fault (yellow) caused the devastating Haiti earthquake in January 2010.


White Island, New Zealand is a good candidate for the "ideal" explosive subduction-related volcano ( 2001, Greg Vaughan USGS).


If you visit NZ and wish to do something expensive but truly unforgettable, fly into the crater by helicopter and explore this amazing place. Yes it's safe, PROVIDED you are very careful where you step!




Partial crystallisation of “wet” basic magma


The problem with this extra dissolved water is that it also affects the way in which the basalt behaves as it rises beneath the volcanoes. Much of the basic magma from the mantle that reaches the roots of volcanoes does not erupt at once but hangs around at various depths in the lithosphere. Some cools and crystallises completely but other batches only partially crystallise and are then erupted later.


This idea that magma does not crystallise at a fixed temperature but does so over a range of hundreds of degrees Centigrade sounds odd to non-scientists. The substance that most of us experience, both as a liquid and frozen solid, is water/ice. Pure water freezes entirely to ice at a single temperature – zero Centigrade. Pure liquid water and ice have exactly the same chemical compositions and therefore, if you extract some of the remaining liquid at the moment when water is freezing, that liquid also has exactly the same chemical composition. For magmas (complicated natural mixtures of liquid molten silicates) the crystallisation rules are totally different. The crystals that form during its long crystallisation process are never the same in composition as the original silicate liquid. Therefore, if natural processes under a volcano can allow the remaining liquid to separate from its early-formed crystals (for instance if the crystals become glued to the walls of the magma reservoir), the composition of such a liquid is different from the original one. In the case of Montserrat and similar magmas, this new liquid is enriched in the chemical element silicon Si – always referred to as its oxide “silica” (SiO2) in earth science writings.


 All that Montserrat residents need to know is that this partial crystallisation of basic magma and separation of residual liquids (enriched in water by the removal of water-free minerals during partial crystallisation) is going on all the time under the Soufrière Hills volcano and is a key process in producing the magmas that erupt so violently. It is this magma that causes trouble for surrounding residents because silica-rich magma is characteristically very viscous and tends to come out of the volcano like toffee or half-dried cement Figs 16 and 17). Geologists have several names for variants of silica-rich magmas and the rocks they form when cooled. The specific Montserrat magma and lava is “andesite”.


Fig. 16. Two fine examples of viscous lava domes on volcano summits.

Dome of extremely viscous (sticky) lava inside the summit crater of Colima volcano, Mexico (Juan Carlos Gavilanes Ruiz).

A terrifying sight! An incandescent lava dome in the summit crater of Kelud volcano, Indonesia (Tom Pfeiffer ).


Fig. 17  Viscous lava squeezing up (like toothpaste) to the surface of the Mt St Helens lava dome, USA (Don Swanson, USGS).

Fig. 18  Typical volcanic fumarole depositing sulphur as the gas escapes; White Island crater, Bay of Plenty, New Zealand (Bob Thompson).



This is why each new lava flow on Montserrat volcano is just a huge steep lump (“lava dome”), looking like rocky toothpaste (long past its sell-by date).  Small holes and cracks on the dome, called “fumaroles” by geologists, emit jets of steam and other gases, and also deposit yellow sulphur on the surrounding rocks Fig. 18). When silica-rich lava is squeezed out peacefully, there’s no hazard for anyone.  The problem is the vast amount of water contained in the melt. Below the surface this is under huge pressure and, if anything happens to release that pressure suddenly (such as a dome collapsing or a big fissure reaching the surface), the water in the underground magma flashes into steam and the whole thing explodes. This is a bit like the messy experiment of shaking a bottle of soda and then taking the lid off: the soda water represents the magma, the gas (carbon dioxide) represents the dissolved water and taking off the lid represents a dome collapse.  This takes us straight to the thoroughly antisocial habits of the Montserrat’s Soufrière Hills volcano (SHV)!