THE 1951 ERUPTION OF MT. LAMINGTON, PAPUA NEW GUINEA: DIRECTED BLAST DIRECTED UPWARD

We present results of field reinvestigation of deposits of the 1951 Mt. Lamington eruption combined with analysis of the available photographs and eyewitness accounts (Taylor, 1958). The one-week-long preclimactic seismicity, ground deformation and vulcanian explosive activity accompanied intrusion of a small shallow cryptodome. This triggered gravitational collapse of the pre-historic intra-crater lava dome. The collapse triggered a powerful vertical explosion and produced a debris avalanche has a volume of 0.02-0.04 km³, a length of 8.5 km and ratio of vertical drop to length (H/L) of 0.14. The eruptive cloud was not buoyant and formed a radial pyroclastic density current (PDC) that traveled as much as 15 km from the vent and produced tree blow-down within an area of 230 km². At distances from 3 to 12 km the maximum thickness of the deposit (normally graded lapilli and coarse ash mixed with soil in the base) decreases from 55 to 5 cm, the average size of the 10 largest clasts decreases from 4.5 cm to 0.5 cm, the Md diameter decreases from -1.5 to 4.5 phi, and sorting improves from 3 to 0.7 phi. The deposit contains 80% of highly crystalline juvenile andesite clasts (vesicularity 4 – 40%) derived from the former cryptodome. The eruption sequence (cryptodome intrusion + edifice failure + blast), resulted in PDC deposits and tree blow-down features that are similar to those of the classic blasts of the Bezymianny in 1956 and the Mount St. Helens in 1980. The A, B, C strata recognized in the St. Helens and Bezymianny blast deposts are not as well developed in the Lamington blast deposit. We attribute this to the fact that, unlike the St. Helens and Bezymianny examples, the Lamington blast cloud first ascended vertically before collapsing and producing a PDC. Consequently the Lamington PDC ingested more air and was more dilute than those at St. Helens and Bezymianny. We speculate that the powerful explosion of Lamington was vertical because the rupture surface of the preceding edifice collapse unloaded the very tip of the intruding cryptodome.