Rock strength as a metric of welding intensity in pyroclastic deposits

Welding of pyroclastic deposits describes the flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature. Traditionally, this process is mapped using metrics such as density, porosity or fabric. Here we develop rock strength as an ancillary tool for mapping variations in welding intensity. Rock strength can be measured as point load strength or as uniaxial compressive strength (UCS). The point load strength test (PLST) is an efficient, portable means of measuring relative rock strength and is easily used in field studies. Our measurements on a variety of rock types, including welded ignimbrite, are used to develop an empirical relationship between the point load measurements and the more standardized rock strength rating based on UCS. Strong materials (PLST>4 MPa) show a linear relationship described by UCS = 24.4.PLST. Weaker materials, such as pyroclastic rocks, (PLST<5 MPa) require a nonlinear model: UCS = 3.86.PLST (super 2) +5.65.PLST. The potential for using rock strength to map variations in welding intensity within pyroclastic deposits is demonstrated using data collected from a stratigraphic section through the Bandelier Tuff, New Mexico. Four discrete zones of welding intensity based on rock strength ratings are identified. This classification scheme provides an objective means of quantitatively tracking variations in welding intensity in the field.