Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Final Report Summary - ISLAND FREEBOARD (What can island isostasy tell us about hotspot dynamics?)

The Cape Verde Archipelago is a group of 10 volcanic islands located in the tropical North Atlantic, 600 km offshore western Africa. The archipelago is the type-example of a hotspot in a stationary plate environment with respect to its melting source, and exhibits ubiquitous evidence for quaternary uplift. This uplift is possibly driven by a regional growth pulse of the Cape Verde Rise, the largest bathymetric anomaly in the Earth's oceans, and whose origins are still very enigmatic. This project was designed to date quaternary uplift tracers across the Cape Verde Islands and test a regional vs local mechanism for the uplift in order to gain insight on the origins of the Cape Verde Rise and processes of island growth. If considerable uplift trends occur at a regional scale, a mechanism associated with hotspot swell development and dynamics (i.e. dynamic topography) must be invoked as the source of isostatic imbalance. Conversely, if highly differential uplift trends occur at a more local scale, affecting nearby islands differently, local intrusion-related mechanisms are the most likely source of uplift.

Quaternary uplift tracers in the Cape Verdes consist essentially of relative sea-level markers such as marine terraces (wave-cut surfaces and raised beach deposits) and lava deltas, which are potentially dateable. In this project we employed cutting-edge surface exposure dating techniques, as well as U-Th (on corals) and Ar-Ar (on lava deltas), to date uplift features. Detailed fieldwork allowed the discrimination of each island’s quaternary uplift features. The northwestern islands of Santo Antão and São Vicente are devoid of quaternary uplift features above +20m, and so is the young Fogo volcano. The other islands exhibit variable evidence for quaternary uplift, up to +130 m. The islands’ uplift features were studied and screened for dateable material, such as preserved wave-cut surfaces, young lava deltas and pristine corals. Samples were collected at selected sites, and subsequently prepared in the lab, for its respective geochronology method. Surface exposure dating on samples from seemingly preserved wave-cut surfaces was performed. For the purpose, a method for extracting olivine concentrates from glassy, almost-aphyric submarine volcanic rocks was successfully optimised. Surface exposure dating yielded results with mixed success - most samples exhibit signs of complex exposure ages, and ages that are inconsistently younger than expected, most possibly denoting, respectively, cosmic ray shielding by now-eroded aeolian sands and significant denudation not evident in the field. Using U-Th geochronology on corals we were able to date some of the younger/lower (<20 m) terraces. However, given the complex exposure ages of the higher terraces, long-term uplift curves were still subject to very large uncertainties affecting a full discrimination between the competing uplift models. In order to overcome this scientific obstacle, additional Ar-Ar geochronology on lava deltas - particularly from the key islands of Santiago and São Nicolau - was planed and implemented. This additional geochronology was designed to complement the existing age dataset and increase the resolution of the improved uplift reconstructions, helping in the discrimination between uplift models. With an increased resolution on the uplift curves, we expect a fuller, more complete picture on the processes driving both the regional and local uplift; however, a major conclusion arising from this work is that competing volcanic and intrusive processes govern island growth (with clear implications to other settings), and that islands can emerge above sea level by uplift rather than summit eruptions, as we were able to demonstrate.

Crucially, the field and laboratorial work carried out during the project allowed the fellow to provide a significant breakthrough in another, rather unexpected aspect of volcanic island research: the generation of megatsunamis by island flank collapses. During the fieldwork associated with the project, the fellow discovered compelling field evidence for a megatsunami (with over 270 m of wave run-up) that impacted the Cape Verde islands with devastating near-source effects. The deposits were successfully dated to 73,000 years, using the same cosmogenic nuclide geochronology techniques used for uplift studies. As the fellow was able to demonstrate, this giant tsunami was triggered by a flank collapse at Fogo, the youngest of the Cape Verde Islands and one of the most active oceanic volcanoes on Earth. This study constitutes the first time that megatsunami clasts were successfully dated using 3He geochronology, pioneering this technique in tsunami research and allowing a much finer time-constrain on the Fogo collapse event. Additionally, the deposits found in the Cape Verdes constitute the first megatsunami cliff-top boulder deposits known to date, and document one of the largest megatsunamis known to be preserved in the geological record. The scientific implications of this discovery are tremendous and far reaching, as it confirms that flank collapses can happen catastrophically and trigger giant tsunamis, solving a decades-long debate on the tsunamigenic potential of volcanic island flank collapses. This new evidence therefore reinforces the hazard potential of volcanic island collapses and stands as a warning that such hazard should not be underestimated, particularly in areas where volcanic island edifices are close to other islands or to highly populated continental margins, such as in the NE Atlantic. The hazard/societal implications exposed by this study - to Cape Verde, Europe, Africa, and other parts of the Atlantic basin - are therefore clear. The main beneficiaries of knowledge arising from this research include: (a) the scientific community and in particular the geohazards community; (b) hazard monitoring & risk management agencies (especially in the Atlantic basin); (c) civil protection decision-making bodies; (d) insurance companies; and (d) the general public.

The study of island successions on both the Cape Verdes and the Azores also helped shed light on insular coastal sediment dynamics, e.g. it showed that storms are the main agent of sediment transport and deposition on narrow insular shelves. More importantly the wide knowledge amassed during the course of this project allowed the fellow to spearheaded a comprehensive review on processes of coastal evolution at volcanic islands, the first to describe in fine detail coastal evolution from island emergence to island disappearance.

Also, during the course of this project, Fogo volcano started to erupt on the morning of 23rd November 2014, after almost 20 years of quiescence. The eruption provided a unique opportunity to study in loco the active processes of island growth, and so R. Ramalho rapidly integrated an international mission to work closely with the local authorities on monitoring the eruption and assuming an advisory role on volcanic crisis management. Fogo’s 2014/2015 eruption eventually proved to be one of the most destructive within local recorded history, and the observations/data gathered during the eruption will potentially help comprehending this hazardous volcanic system.

Researcher’s URLs:

Project URL:

Video and photo essay on megatsunami study:

Fogo 2014/2015 Eruption report/photo essay:

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