The pyroclastic deposits and eruption history of Ascension Island

Thesis



In this study, palaeomagnetic methods have been combined with field and volcanological techniques to identify, classify and correlate the pyroclastic deposits found on Ascension Island, South Atlantic, allowing them to be placed into a temporal and geographic framework. Pyroclastic material is abundant on the island and, in general, wellpreserved, making Ascension an ideal site to study the nature and distribution of the pyroclastic products of this type of composite volcano or stratovolcano. A better understanding of the nature and distribution of the products of past pyroclastic eruptions on Ascension should enhance our ability to assess volcanic hazard around stratovolcanoes world-wide.

Field mapping and stratigraphic logging have revealed the presence of several major pyroclastic sequences on Ascension. These comprise extensive felsic (pumice) and mafic (scoria) lapilli deposits, two major and several minor exposures of welded material and numerous breccia deposits that exhibit great variation in juvenile/lithic content, matrix type and content and internal structure. Preliminary interpretations of the deposits were made in the field, based on features such as welding, grain shape and internal structures. However many of the deposits - particularly the breccia deposits - display ambiguous field characteristics that could be attributed to pyroclastic or epiclastic processes and their origins could not therefore be determined from field characteristics alone.

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Authors: Hobson, K; Hobson, Kate Elizabeth

• Alternative title: a paleomagnetic and volcanalogical study
• Publication date: 2001
• DOI: 10.5287/ora-pv4nb8gmj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: South Pacific Ocean; Paleomagnetism