Eisele, Steffen (2015) Quaternary explosive volcanism of the Cape Verde Archipelago : on- and offshore tephrostratigraphy. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 123 pp.

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Abstract

The Cape Verde Archipelago is a volcanic intraplate system that comprises one of the most active ocean island volcanoes (Fogo) in the world. Like many other ocean islands the Cape Verde Archipelago has produced highly explosive eruptions but despite their importance in assessing potential future hazards only few previous volcanological studies have focused on these eruptions. This study has been conducted to investigate the Pleistocene to Holocene highly explosive volcanic activity at the Cape Verde Archipelago. In this context eruption frequencies of the various eruptive centers, the compositional evolution of the erupted magmas, and their pre-eruptive storage conditions and eruption triggers are studied. To address these topics I combined results from marine tephrostratigraphic investigations, using sediment gravity cores recovered during the RV METEOR cruise M80/3, with stratigraphic field work on Santo Antão, Fogo and Brava. These islands are hosting the youngest volcanism in the archipelago. Further I used various chronological approaches to establish the first detailed tephrostratigraphic framework for the Cape Verde Archipelago covering the last ~220 kyr. It comprises 54 highly explosive eruptions from four volcanic centers at the southwestern and northwestern part of the Archipelago. Forty-three highly explosive eruptions of sub-Plinian to Plinian dimensions from Fogo are identified in the marine record within the last 155 kyr. These are entirely of mafic magma compositions documenting impressively that such very hazardous volcanic activity is by no means limited to evolved magma compositions. The longterm average frequency of one eruption every 3,000 years has increased to one eruption every 2,000 years over the last 30 kyr. The occurrence of such large eruptions needs to be considered for future hazard assessment. Another very hazardous event, the huge flank collapse of Fogo island of hitherto unknown age, can be dated to 117 ka by the marine tephra record. The largest known phonolitic eruptions of the Cape Verdes produced the tephras of the Cão Grande Formation on Santo Antão. Two previously recorded tephras originated from a basanitic and a nephelinitic magmatic suite, respectively. My new fieldwork identified two more tephra units associated to the Cão Grande Formation. I showed that the four large eruptions, in total, alternatingly originated from the basanitic and a nephelinitic system. Mineral thermobarometric calculations constrain the coexistence of the magma systems very close to each other below the island. Both magmatic systems contemporaneously evolved to highly differentiated phonolitic compositions but remained isolated from each other; only one of the tephras contains evidence for magma from the nephelinitic suite having been admixed to the basanite-suite reservoir. This mixing probably triggered the eruption of this tephra since the resident magma was still water-undersaturated. Two other tephras, however, derive from evolved magmas that had reached water saturation in their reservoirs making gas-overpressure the likely eruption trigger. Plenty phonolitic explosive eruptions took place on Brava as evidenced by numerous tephra deposits on the island, however, within the last 155 kyr only one eruption succeeded to create a widespread ash deposit covering the surrounding ocean floor. As no traces of the caldera forming eruption and the caldera filling ignimbrites were identified in the marine cores, these events most likely took place before 155 ka. Two young widespread tephra layers from the Cadamosto seamount were identified in the marine record. Contrary to the lava rocks typically recovered by dredging, these tephras show that highly explosive phonolitic submarine eruptions generated at >1400 m water depths are an important part of the seamount evolution. In conclusion the presented tephrostratigraphic record provides the first evidence of frequent highly explosive volcanic eruptions from varying volcanic complexes at the Cape Verde Archipelago during the past 220 ka. The detailed study of the Cão Grande Formation provides insights of magma chamber and eruption evolution at one of these complexes that may be typical also for future evolved explosive eruptions on Santo Antão. The manuscripts presented in this study provide essential results to assess potential hazards from future eruptions.

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