Experimental Constraints on the Ejection of Ballistics During the 6 August 2012 Hydrothermal Eruption of Upper Te Maari (Tongariro), New Zealand.

Abstract:

At 6th August 2012 a hydrothermal eruption occurred at the Upper Te Maari vent within the Tongariro volcanic center (New Zealand). The eruption comprised a series of short-lived explosions, at least two laterally directed density currents and a vertical ash plume. All explosions were accompanied by ballistic ejection, some of which impacted NZ’s most popular hiking trail and a mountain lodge at 1.4 km from the vent.

Detailed mapping of the ballistic strew field (craters and where possible impactors) revealed first insights into the eruption dynamics. Ballistics could be discriminated into four main lithology groups and traced back to specific explosion-source locations in the vent region.

The main lithology types were used in rapid decompression experiments mimicking hydrothermal explosions under controlled laboratory conditions. The heterogeneity of the lithologies, including breccia with different components, cementation and compaction, necessitated an experimental setup for large samples (34 mm diameter, 70 mm length). The experiments on pre-saturated samples were conducted in a temperature range from 250°C – 300°C and applied pressure between 4 MPa – 6.5 MPa. Within this range we tested rapid decompression from both the liquid-dominated field and the vapor-dominated field. Clasts were ejected with velocities of up to 180 m/s as recorded with a high-speed camera. The resulting fragments were analyzed for their grain size distribution, shape and lithology. Besides a obvious larger clasts, a large amount of fine and very fine (<63 µm) ash was produced in all experiments. Experimental studies of this kind facilitate better constraints on the ejection characteristics of ballistics and their associated hazard. Furthermore they shed light on the energy conversion and partitioning during hydrothermal explosions.