Determining the conduit, eruption and pyroclast transport dynamics of silicic submarine eruptions: Havre 2012

Abstract:

The understanding of submarine eruption dynamics significantly lags behind that of the subaerial setting due, in part, to a) capturing and characterising the eruption (magnitude, duration, eruption depth), b) characterising the dispersal style and distribution of the products and c) sampling difficulties. Studies of modern seafloor volcanic deposits are critical, especially those where real time observations and /or timing are available.

The 2012 eruption of Havre volcano in the Kermadec volcanic arc is the largest and deepest recorded submarine silicic eruption. In 2015 we conducted a subaerial-style physical volcanology investigation of the eruption deposits at Havre using a combined AUV-ROV deployment. This approach was designed to provide the critical data to answer outstanding basic and fundamental questions of magma ascent and eruption dynamics, lava and clastic transport processes in the submarine setting.

Our observations and measurements constrain for the first time the duration and eruption rate of a large submarine explosive eruption. Multiple lavas from known eruption depths permit a detailed investigation of the role of hydrostatic pressure on lava properties that modulate lava morphology.

The investigation of four pumiceous clastic units reveal multiple possible modes of fragmentation, that include: (1) passive spalling and autobrecciation of the margins of a lava, and (2) some style of explosive disruption.

One particular pumiceous clastic deposit on the volcanic edifice contains very abundant coarse m-sized clasts and lacks in lapilli-sized (<64 mm diameter) clasts. Such deposits dominated by vesicular giant pumice clasts appear to be unique to submarine eruptions. At Havre, whether this proximal deposit grain size is a characteristic imparted by secondary transport and depositional processes, or is a primary characteristic of fragmentation remains a challenge to constrain. We favour a model that includes some component of explosive fragmentation, because the inferred mass eruption rate of 10⁷ kgs^-1 is 3 – 4 orders of magnitude greater than those recorded for subaerial silicic lava eruptions. Current analytical research on these deposits will reveal the role of hydrostatic pressure on conduit dynamics and fragmentation mechanics.