Type of Submission
Poster
Keywords
Welded Tuff, Mount Mazama, Crater Lake, Welding Intensity, Fiamme, Glass Transition Temperature, X-Ray Computed Tomography, Ash-Fall Deposit, Volcanic Eruption, Volcanic Rock
Proposal
Welded tuff is a common volcanic rock type. When hot ash and other pyroclastic material builds up around a volcano, it will weld together if the debris is hotter than the glass transition temperature (Tg). Tg varies based on the composition of the source magma but is generally at least 600 °C. At this temperature, ash-sized glass shards begin to cohere, tuff deposit volume and porosity decrease, and deposit interiors become progressively deformed. Any pumice clasts become flattened and are then known as fiamme. Quane and Russell (2005) proposed a 6-tier semi-quantitative classification of welding intensity based on physical characteristics of tuff including petrographic texture, density, porosity, point load strength, uniaxial compressive strength, and fiamme oblateness. Rank I, the lowest, consists of undeformed pumice lapilli in a loosely-packed, unconsolidated matrix, while Rank VI is obsidian-like vitrophyre.
The Wineglass Welded Tuff (WWT) is an orange, rhyodacitic (70% SiO2) welded pyroclastic density current (PDC, also known as ash flow) deposit from the single-vent phase of the climactic eruption of Mount Mazama. It is thickest and most intensely welded in paleotopographic lows near the caldera (crater) rim. In many places, the WWT was eroded down to rather densely welded tuff by PDCs from the subsequent ring-vent phase. The more intensely welded it was at that time, the more energy it would have taken for the later PDCs to erode it. Therefore, understanding how intensely welded the WWT was will aid our understanding of the erosive power of PDCs, facilitating better emergency response and planning with regards to modern volcanic hazards.
In this project we use Avizo™ software to identify fiamme and measure their oblateness in X-ray computed tomography (XCT) scans of WWT samples. Preliminary oblateness values correspond to a welding intensity of II in the scheme of Quane and Russell (2005). Their definition of Rank II states that deposits are rendered coherent by some intraclast adhesion but pumice lapilli and ash remain undeformed. We interpret this as a minimum rank; examination of hand samples leads us to believe the true rank may be higher.
References:
Quane, S.L., and Russell, J.K., 2005, Ranking welding intensity in pyroclastic deposits: Bulletin of Volcanology, v. 67, p. 129–143, doi:10.1007/s00445-004-0367-5.
Copyright
© 2025 Micah Beachy. All rights reserved.
Publication Date
2025
Intensity of Welding in the Wineglass Welded Tuff, Mount Mazama, Oregon, Determined by XCT Analysis
Welded tuff is a common volcanic rock type. When hot ash and other pyroclastic material builds up around a volcano, it will weld together if the debris is hotter than the glass transition temperature (Tg). Tg varies based on the composition of the source magma but is generally at least 600 °C. At this temperature, ash-sized glass shards begin to cohere, tuff deposit volume and porosity decrease, and deposit interiors become progressively deformed. Any pumice clasts become flattened and are then known as fiamme. Quane and Russell (2005) proposed a 6-tier semi-quantitative classification of welding intensity based on physical characteristics of tuff including petrographic texture, density, porosity, point load strength, uniaxial compressive strength, and fiamme oblateness. Rank I, the lowest, consists of undeformed pumice lapilli in a loosely-packed, unconsolidated matrix, while Rank VI is obsidian-like vitrophyre.
The Wineglass Welded Tuff (WWT) is an orange, rhyodacitic (70% SiO2) welded pyroclastic density current (PDC, also known as ash flow) deposit from the single-vent phase of the climactic eruption of Mount Mazama. It is thickest and most intensely welded in paleotopographic lows near the caldera (crater) rim. In many places, the WWT was eroded down to rather densely welded tuff by PDCs from the subsequent ring-vent phase. The more intensely welded it was at that time, the more energy it would have taken for the later PDCs to erode it. Therefore, understanding how intensely welded the WWT was will aid our understanding of the erosive power of PDCs, facilitating better emergency response and planning with regards to modern volcanic hazards.
In this project we use Avizo™ software to identify fiamme and measure their oblateness in X-ray computed tomography (XCT) scans of WWT samples. Preliminary oblateness values correspond to a welding intensity of II in the scheme of Quane and Russell (2005). Their definition of Rank II states that deposits are rendered coherent by some intraclast adhesion but pumice lapilli and ash remain undeformed. We interpret this as a minimum rank; examination of hand samples leads us to believe the true rank may be higher.
References:
Quane, S.L., and Russell, J.K., 2005, Ranking welding intensity in pyroclastic deposits: Bulletin of Volcanology, v. 67, p. 129–143, doi:10.1007/s00445-004-0367-5.
