Experimental Welding of the Bishop Tuff

Pyroclastic flows emplaced and subsequently welded into what is now the Bishop Tuff record the eruptive history of the Long Valley Caldera. Material density and competence vary throughout the Bishop Tuff, indicating variable welding conditions within the ignimbrites. Inferences about welding conditions in different regions within the Bishop Tuff have been made through modeling and observational studies relating time, temperature, stress, thickness, and density. The relationships among stress, temperature, and strain that lead to welding have not yet been experimentally verified, which makes it difficult to quantitatively characterize the history of welding in this deposit. To determine these relationships, we deformed unwelded tuff at atmospheric pressure and temperatures ranging from 790-950°C under uniaxial loads of 1, 2, 4, and 8 MPa. For each temperature, an unloaded sample was sintered beside each loaded sample. Samples were held under load for half an hour and then quenched below 700°C before unloading. Before and after deformation, density was measured by two different methods, which gave comparable results. After the high-temperature deformation experiments, these samples were placed into a uniaxial press and loaded until failure at room temperature. The same failure tests were performed on a sample of naturally welded tuff as a reference for material strengthening as a result of a high degree of welding. We use density and room-temperature failure strength as proxies for the degree of welding. Measurements of density, strain, and failure strength reveal that the degree of welding primarily correlates with strain at high temperature, rather than with stress or temperature. Extrapolation of our experimental data suggests that 100-240% strain is required to achieve welding similar to the naturally welded tuff. This calibration provides a method for estimating strain during welding of Bishop Tuff. Specimens were collected and tested as part of the RORD REU program.