Abstract
After the Hunga Volcano eruption of 15 January 2022, the ash volume estimated by on-land fall thickness appeared anomalously small for the explosive magnitude. The eruptive column from this event reached at least 55 km high. Ash dusting (<1 mm) was reported on islands as far as 200 km NE from the volcano in the Vava'u group. Ashfall in the Ha'apai group and Tongatapu Islands, approximately 70 km NE and SE from the volcano, respectively, did not exceed 40 mm in thicknesses. Deep-water sampling campaigns, carried out by both the Korean Polar Research Institute and the University of Washington with the Woods Hole Oceanographic Institution, shed light on deposits located up to 100 km W and S of the volcano. Gravity- and box-cores were collected at approximately 2000-3200 m depth, returning deposits rich in juvenile volcanic ash. At 100 km NW of Hunga, deposits reached a thickness of approximately 1 cm, while 40 km from the eruptive centre (location of a deep basin), deposits were a minimum of 22 cm thick. Subaerial (sa) and submarine (sm) samples were studied, and preliminary results show that both are fine-grained and poorly sorted, with 36.1+ or -4.8 wt.% of fine ash (<63 mu m), for sa samples, and 37.7+ or -16.6 wt.% of fine ash for sm. Unusually, both sa and sm samples contain few particles of very fine range (<0.01 wt.% finer than 1 mu m). In general sm samples are unimodal, and those to the W exhibit a mean size decreasing with increasing distance to the source. However, an anomalously fine-grained sm sample 32 km SSW from the volcano, points to a different emplacement mechanism compared to those to the west. Work is in progress to unravel the deposition mechanism of the sm deposits, which could be via fall and/or turbidity currents generated by PDCs entering the ocean.