Chilled margin fragmentation as a trigger for transition from Strombolian to phreatomagmatic explosive activity at Cova de Paul Crater, Santo Antao, Cape Verde Islands.

Sudden transitions from effusive or mildly explosive eruptions to violent phreatomagmatic explosive activity can occur at high-elevation vents on oceanic island volcanoes. Here, we describe near-vent deposits around the large Cova de Paul crater on Santo Antao, Cape Verde Islands, which provide insights into the transition from Strombolian magmatic to violently explosive phreatomagmatic activity during one such eruption. The Strombolian deposits change from welded spatter to bedded clinkery scoria to angular blocky scoria, and contain a distinct population of strongly banded, low-vesicularity angular sub-glassy clasts with strongly prolate vesicles. These sub-glassy clasts also form a discrete layer of larger blocks at the transition to the overlying phreatomagmatic deposits. They are compositionally identical to the Strombolian scoria and are interpreted to be fragments of flow-banded chilled margins from the walls of the eruptive conduit. The layer of large sub-glassy blocks records disintegration and expulsion of the chilled margins at the transition to phreatomagmatic activity. We interpret the sequence from spatter through clinkery scoria to blocky scoria as recording an increase in groundwater inflow into the conduit from pressurised aquifers, but at coolant input ratios below those required for explosions. The shattering of the chilled margins to produce the bed of large sub-glassy blocks appears to have allowed a much greater water flow into the conduit, leading to the phreatomagmatic explosive phase of the eruption. [ABSTRACT FROM AUTHOR]