Distinguishing between primary and secondary volcaniclastic deposits

Abstract

The distinction between primary and secondary volcaniclastic deposits, which are currently defined as the “direct” products of volcanic eruptions and the “reworked” products of the former, respectively, is the first step to interpreting volcaniclastic deposits, particularly the genetic connection with active volcanism. The distinction appears straightforward, but is not always applicable to natural deposits. During the 3.7 ka BP eruption of the Songaksan tuff ring, Jeju Island, Korea, there was an invasion of typhoon. The tuff ring was partly submerged underwater and affected by wave activity for over a day, resulting in a peculiar volcaniclastic deposit composed of both vent-derived (primary) and substrate-derived (reworked or secondary) volcaniclastic particles. We propose a new term “reprocessed” for a category of volcaniclastic deposits or particles, which originated directly from volcanic eruption but was deposited finally by nonvolcanic processes. Here we show that both reprocessed and reworked particles can coexist in the same volcaniclastic deposit, making it impossible to differentiate it into either a primary or a secondary deposit according to the current definition of volcaniclastic deposits. We thus define the secondary volcaniclastic deposits as comprising either or both of reprocessed and reworked volcaniclastic particles.