Spatial and Temporal Evolution of Particle Size and Shape during Plinian Eruptions: The 7.7 ka Cleetwood Eruption of Mt. Mazama (Oregon, USA)

Abstract:

The transport and fall of tephra produced by explosive eruptions pose widespread and disruptive hazards. With potential for ash to disrupt air traffic and impact agriculture, water and air quality, the ability to model tephra dispersion and how it evolves throughout an eruption is key. Primary controls on tephra dispersal and sedimentation include the size, shape and density distributions of ejected particles, which are challenging to constrain. Here we show a novel combination of techniques that can reveal these parameters on a markedly shortened time scale, while simultaneously increasing the number of individual particles and parameters analyzed. We focus on the size, shape and componentry distributions of tephra from the Plinian 7.7 ka Cleetwood eruption of Mount Mazama that preceded the caldera-forming eruption of Crater Lake (Oregon, USA). We quantify in detail how these characteristics evolve in space (outcrop to outcrop) and time (vertical location within the stratigraphy). We analyze tephra from four 22 cm to 1.35 m thick sections, located 6 to 67 km away from the proposed vent location and situated along the main dispersal axis. In the field, changes in the physical characteristics of the deposit and tephra were used to further subdivide each section into distinct layers representing different phases of the eruption; these were then cross-corelated between outcrops. High precision particle size distributions (PSD) and shape analyses of tephra ranging in size from 0.1 to 40 mm were conducted using a high speed, high resolution particle size and shape analyzer. We evaluate tephra componentry using a novel deep learning neural network approach. Preliminary results show that the phase of maximum eruptive intensity exhibits a systematic decrease in fractal dimension in both space and time. Shape analysis reveals pumice elongation with decreasing clast size, with the most elongation occurring at 200-300 μm due to a higher abundance of elongated vesicles. Similar observations from Medicine Lake Volcano were recently interpreted as lateral variations in the size and shape of the products from primary fragmentation, with smaller, elongated particles being formed towards the edges of the conduit where shearing is more prominent.