Capelinhos 1957-1958, Faial, Azores: deposits formed by an emergent surtseyan eruption

The 1957-1958 eruption of Capelinhos, Faial island, Azores. involved three periods of surtseyan, hydromagmatic activity: two in 1957 and one in 1958. Deposits from this eruption are exposed both in sea cliffs cut into the fla nks of the tuff cone and more distally >1 km from the vent. Five lithofacie s are identified: lithofacies I is composed of even thickness beds with lat erally continuous internal stratigraphy and is interpreted to have been for med by fallout. Lithofacies II consists of beds with internally discontinuo us lenses. and has sand-wave structures that increase in abundance toward t he outer margins of the tuff cone. This lithofacies is interpreted as havin g been deposited from pyroclastic surges. Lithofacies III is composed of ma ntle-bedded deposits with laterally discontinuous internal stratigraphy. Th is lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges. In the outer flanks of the tuff cone. lithofacies III grades laterally into fallou t beds of lithofacies I. Lithofacies IV consists of alternating beds of coa rse ash aggregates and non-aggregated fine ash, and is particularly well de veloped in distal regions. Some of this facies was formed by fallout. Alter nating beds also occur plastered against obstacles up to 2 km from the vent , indicating an origin from wet pyroclastic surges. The orientation of plas tered tephra indicates that the surges were deflected by topography as they decelerated. The distinction between surge and fallout in distal regions i s uncertain because wind-drifted fallout and decelerating surge clouds can generate similar deposits. Lithofacies V consists of scoria lapilli beds in terpreted to be fallout from hawaiian-style fire-fountaining in the later s tages of the eruption. Juvenile pyroclasts within hydromagmatic deposits ar e predominantly poorly vesicular (25-60% of clasts < 30% vesicles). However , on both micro- and macroscopic scales, there is a wide range in clast ves icularity (up to 70% vesicles) indicating that, although fragmentation was predominantly hydromagmatic. vesiculation and magmatic-volatile-driven frag mentation operated simultaneously.