Evolution of the Phreatomagmatic Cova de Paul Eruption, Santo Antao, Cape Verde Islands: Links Between Eruption Development and Crater Growth

Episodes of phreatomagmatic explosive activity that excavate large deep craters occur at high-elevation vents on many oceanic island volcanoes. The water driving these explosions is sourced from freshwater aquifers within the volcanic edifices, whose location and other characteristics will influence crater growth and final geometry. Here we describe phreatomagmatic deposits from the single eruption that formed the large Cova de Paul crater on the island of Santo Antao, Cape Verde Islands. These deposits, emplaced after an initial Strombolian phase of activity, record wide fluctuations in the intensity of phreatomagmatic explosivity. The largest explosions produced low-temperature, indurated lithic-rich phreatomagmatic ignimbrites and surge deposits; these occur as isolated and often erosive-based units in more distal areas but are interbedded in proximal outcrops with airfall to mixed fall and flow breccia and ash beds containing varying proportions of lithic and juvenile clasts, pointing to a series of climactic explosions within an extended period of milder phreatomagmatic explosive activity. We find that whereas the lithic clasts in the mixed units can be matched with the varied rock units in the exposed crater walls, the ignimbrite and surge units also contain hydrothermally altered clasts that appear to have originated from deeper in the volcanic edifice. They imply that during the climactic explosions the crater was excavated to several hundred meters depth below the surface. The phreatomagmatic explosive phase of the CDPC eruption was initiated by shattering of the chilled margins of the eruptive conduit, which had formed during the early Strombolian activity. The remains of these chilled margins form a distinctive bed of large flow-banded angular sub-glassy juvenile clasts that separates the Strombolian deposits from the later phreatomagmatic deposits. At this point water inflow from pressurized aquifers into the conduit attained a critical coolant input ratio prompting the onset of the phreatomagmatic explosions (Tarff and Day 2013). Only rare small angular sub-glassy clasts are present in the rest of the eruptive sequence, suggesting that the chilled margins never re-established. Instead, we propose that subsequent variations in the intensity of phreatomagmatic activity were controlled by fluctuations in water flow to the conduit as the crater was excavated to deeper levels and different aquifers were tapped and depleted. The CDPC deposits can be compared with older phreatomagmatic ignimbrites in other ocean islands, where the source vents have been eroded. Tarff, R. W. and Day, S. J. (2013). "Chilled margin fragmentation as a trigger for transition from Strombolian to phreatomagmatic explosive activity at Cova de Paul Crater, Santo Antao, Cape Verde Islands." Bulletin of Volcanology 75(7): 1-14.