The total grain size distribution of tephra from Strombolian explosions: in the air
Abstract
Grain size distribution is one of the fundamental physical parameters of pyroclastic deposits, key to understanding both magma fragmentation and tephra emplacement processes. Most commonly, the grain size distribution of pyroclasts from one eruption is obtained by field sampling and analysis of the deposit, integration of multiple sampling sites being required to obtain a total grain size distribution. This process requires assumptions on the distribution of the deposits and on the lack of in-flight changes to its grain size distribution. To release these assumptions we use high-speed imaging to measure the in-flight grain size distribution of pyroclasts for the first tens of meters above an eruptive vent. We focus on lapilli and bombs from Strombolian activity at Stromboli and Etna (Italy) and Kilauea (Hawaii) volcanoes. The activity was recorded by using a range of high-speed and high-definition cameras filming at a range of temporal and spatial resolutions. The high-speed videos were pre-processed for background subtraction, then thresholded for gray level intensity, and finally the individual particles measured for their 2-D size and shape by using image processing. The proposed analysis allows for a quantification of the grain size distribution of the flying pyroclasts in one frame. Integration over multiple frames provides changes in the grain size distribution over time and the total grain size distribution at the end of the explosion, as well as an estimate of the total erupted mass. Irrespectively of the source volcano, the total grain size distribution from Strombolian activity is always unimodal, well-sorted, mostly symmetric, and with a mode around 0.1 m.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V23I0315S
- Keywords:
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- 3275 Uncertainty quantification;
- MATHEMATICAL GEOPHYSICS;
- 8419 Volcano monitoring;
- VOLCANOLOGY;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY;
- 8494 Instruments and techniques;
- VOLCANOLOGY