Ash Generation During a Major Silicic Deep Marine Eruption

Abstract:

Despite water’s encompassing presence at explosive submarine eruptions, there remain many questions about the degree and nature of its interaction with magma during such events. We are analysing the microtextures and morphology of ash sampled from the 2012 rhyolitic submarine eruption at Havre seamount, which occurred at 700-1300 mbsl, producing a 400 km² sea surface pumice raft along with effusive and explosive deposits on the sea floor. The fully submarine eruption site, rhyolitic composition, known explosive and effusive phases, and extensive post-eruption ground truthing and ROV sampling in 2015 make Havre ideal for the study of processes and products of interaction between silicic magma and water.

The 2012 Havre eruption produced 4 volcaniclastic units on the surrounding seafloor, with distinct characteristics, sources and modes of formation. From base to top these are: Unit 1 (fall), Unit 2a (fine-ash settling), Unit 3 (Dome E ash), Unit 4 (Lava flow ash) and Unit 2b (fine-ash settling). Unit 1 (fall) is the oldest identified 2012 volcaniclastic unit. It is widespread, forms the base of the succession, and comprises glassy vesicular clasts 5 m to <10 µm in size. Unit 3 (near Dome E only) comprises distinct fine lapilli- to ash-sized glassy elongate tube-pumice clasts, deposited in a limited area around lava dome E. Unit 4 (only near lava flows), sampled near the caldera-floor lava flows, is composed of dark microcrystalline clasts with varied shapes. We break Unit 2 into Subunits 2a and 2b. Both subunits comprise mostly 8-60 µm ash and are quite cohesive. Unit 2 is inferred to represent the slow settling out of fine ash over a sustained period. Around Dome E and the caldera-floor lava flows are units 3 and 4, respectively, which locally split Unit 2 into the two subunits.

Morphological analyses of the ash from these units shows evidence for a range of formative processes. Brittle processes dominate, with conchoidal and stepped fractures common, and vesicular clasts preserving bubbles walls also indicate extensive brittle fracturing. Another notable component comprises fluidal ash grains. These are 10-250 µm clasts with fluidal surface features and/or fluidal shapes. They are atypical for rhyolite magmas, and indicate that surprisingly small particles were shaped by surface tension during this fully submarine eruption.