On the nature & causes of volcanism in the Galápagos archipelago
Tuesday 21st May - 4.00 pm - Mineralogy seminar room
Dr Sally A Gibson, Department of Earth Sciences, University of Cambridge, UK. sally@esc.cam.ac.uk
Diversity appears to be key to understanding natural phenomena in the Galápagos archipelago. Whilst most associate this with the unusual creatures that inhabit the islands it is also true of their volcanic nature.
Historical perspective: The volcanic nature of Galápagos was based on reports of pirates, buccaneers and naval admirals until 1835, when Charles Darwin visited the archipelago during the Beagle voyage. Although widely regarded as a zoologist, Darwin was first and foremost a geologist and especially interested in the formation of volcanic islands. Whilst in Galápagos, most of his time was spent on James Island (now known as Santiago) and here he made a crucial observation regarding the occurrence of different volcanic rock types; he realised that confinement of low-density trachytes to elevated parts and higher-density basalts to lower slopes of the same volcano meant that different types of magma could form in ‘the body of a volcanic mountain’ by sinking of crystals. In this regard he was the first scientist to link the diversity of volcanic rock types to what we now refer to as crystal settling. Darwin’s theory of crystal sinking was published in 1844 but not widely accepted at the time.
21st Century importance: The Galápagos archipelago is a natural laboratory for Earth Scientists and provides a unique opportunity to test models of mantle melting. It is one of the world’s most volcanically active regions with eruptions of predominantly basaltic lavas occurring every 3 to 5 years. Galápagos is located above a mantle plume and adjacent to an oceanic spreading centre. Whilst the greatest volumes of melt occur in the west of the archipelago, close to the postulated axis of the plume, volcanism is widespread. There are no age-progressive linear relationships between activity and distance from the location of the present-day hotspot and no temporal variation in magma type as there is for example at Hawaii. The large geochemical dataset for recently erupted basalts and high-resolution seismic database allow greater constraints to be imposed on the causes of volcanism than for any other archipelago. Melt generation occurs both in the region of active mantle upwelling, which has a radius of ~100 km, and also where plume mantle is being dispersed laterally towards the adjacent spreading centre. The composition of erupted basalts is closely linked to the thickness of the underlying lithosphere: numerical modelling of geochemical and geophysical datasets has revealed that this is relatively thin (45 km) beneath the NE of the archipelago and allows the generation of tholeiitic basalts. Above the current zone of active plume upwelling the lithosphere is thicker (60 km) such that the amount of melting is lower and alkali basalts are generated. Isla Santiago is located in central Galápagos above the margin of the zone of active upwelling and also on the edge of the zone of thin lithosphere. The island is unique in that it has experienced recent eruptions of basaltic melts with extremely varied major- and trace-element and also isotopic compositions. This diversity is a manifestation of both complex physical processes and compositional variations in the underlying mantle plume.
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