Geological evolution and construction of a glacierized active intra-oceanic arc volcano: Visokoi Island (South Sandwich Islands)

Visokoi is a small volcanic island in the remote South Sandwich Islands and is unique in being dominated by the basaltic andesite products of highly explosive eruptions. Here, its geology is described in detail for the first time and can be used to characterize the construction of an active glacierized volcano in an intra-oceanic volcanic arc setting. More than 90% of the volcano is submarine and is composed of (1) a ~ 2.5 km-high mound formed of pillow lava and tuff breccia flanked by a low apron of mass flow deposits, together with (2) an overlying unit ~ 200 m thick composed of Surtseyan volcanic products representing a shoaling (and ultimately emergent) volcanic stage. The succeeding island commenced as a small volcanic shield composed of subaerial ‘a ‘ā lavas whose construction terminated in a caldera collapse that repressurized the magma chamber, presaging a major transition to highly explosive pyroclastic eruptions. They were mainly of sub-Plinian and Plinian type and their recognition on the island provides the first viable explanation for the presence of compositionally similar marine tephras sampled by drilling > 500 km from source, previously considered enigmatic. Eruptions probably took place under ice-poor conditions but evidence for quenching of juvenile clasts suggests that the magmas interacted with water high in the conduit sourced from melting of a small ice cap. The major period of high-discharge sub-Plinian and Plinian eruptions appears to have ended and any future events shall probably comprise small-volume eruptions forming Strombolian scoria cones or glaciovolcanic tuff cones.