Abstract

Ilchulbong (Sunrise Peak) tuff cone is a United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage site that owes its scientific importance to the outstanding coastal exposures that surround it. It is a type locality for the sedimentary evidence of pyroclastic transport and depositional processes that occur during phreatomagmatic basaltic eruptions. Its classically shaped cone morphology was long considered to result from a single short-lived eruption. Reanalysis of the sedimentary sequence has now revealed two subtle paraconformities in the deposits, indicating that the eruption occurred in three separate phases. The break between phases I to II was “long,” i.e., days to weeks, during which erosion and compaction of the lower cone occurred, leading to a volcanowide discontinuity surface and reworked deposits in the lower flanks. Possibly following cooling and solidification of the initial shallow conduit system, the phase II eruption resumed from a new vent location ∼600 m away, before being terminated by a very brief interval and final eruption of phase III from the same general site. The second pause led to a more subtle discontinuity, marked by differences in deposit character above and below it, and it was associated with a period of seismicity-induced slumping and slipping of deposits. Detailed geochemical study of the juvenile clasts through the sequence reveals that three separate alkali basaltic magma pulses were erupted, corresponding directly to the three eruption phases. Magmas 1 and 3 may be genetically related, with the former showing evidence for longer periods of shallow-level fractionation, whereas magma 2 originated from a slightly different mantle domain, although at a similar depth. These results demonstrate that breaks in monogenetic eruption sequences may represent deep magma system processes, and they raise fundamental questions concerning the mechanisms of triggering and eruption of multiple magma pulses in small-volume basaltic volcanoes. This study shows the importance of recognizing sedimentary evidence of time breaks in such eruption sequences as potential indications of shifts in eruption chemistry and vent location. These have important implications for the controls of vent migration at other monogenetic volcanoes and for hazard planning during future similar eruptions.

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