EMPLACEMENT MECHANISMS OF PHREATOMAGMATIC TEPHRA AT KOKO CRATER (“TUFF CONE”), O'AHU, HAWAI'I, USA

A study of phreatomagmatic basanitic tephra on the steep (30-35 degrees) flanks of Koko Crater (tuff cone) revealed interesting aspects of its eruption conditions and emplacement, that are distinct from the overlying tuff ring deposits from nearby Hanauma Bay. This distinction is difficult to interpret in terms of within-vent or wider environmental conditions at the time of their emplacement, but likely suggests that larger volumes of water were available during pre-FCI mingling at Koko. The scarcity of coral sand in the Koko tephra compared to Hanauma Bay tephra may indicate a deeper water setting for Koko. The Koko Crater tephra preserves abundant evidence of “wet” conditions during emplacement, including syn-eruptive rilling, slumps and syn-eruptive massive (debris) flows and associated side blocks, thick (>3cm) beds, large armored lapilli and a lack of traction current structures (TCS). In contrast the Hanauma tephra includes abundant TCS and thin (<1-3cm) beds and much less evidence of syn-eruptive slumping or mass flows. The Koko tephra is interpreted as having mostly been initially emplaced by wet fallout (ballistic and suspension) and syn-eruptive mass flows, and the Hanauma tephra mostly by dilute PDC with minor accompanying suspension and ballistic fallout. However, it is important to note that the interpretation of emplacement processes in such proximal steep-slope settings is complicated by high rates of deposition and syn-eruptive redeposition.