Two successive ash-flow cooling units of the Black Mountain volcanic center, the Pahute Mesa and Trail Ridge members of the late Miocene Thirsty Canyon Tuff, Possess nearly indistinguishable reverse polarity remanent magnetization of about 178¿ declination and -10¿ inclination. These are interpreted to reflect thermoremanent magnetization either during a single geomagnetic field excursion or polarity transition, or possibly from a field oriented near the limit of secular variation. Regardless of the type of geomagnetic field behavior, the Pahute Mesa Member was apparently overalin by the Trail Ridge Member before appreciable shift of the anomalous geomagnetic field. This strongly suggests that the time between eruption of the two ash-flow sheets was very short, perhaps less than 100 years, a conclusion that is independently supported by the local absence of a complete cooling break between the two units. The interval between eruption of the Pahute Mesa and Trail Ridge members was too brief for reestablishment of chemical stratification in the Black Mountain magma system. Nevertheless, a small volume of highly evolved comendite magma developed and was erupted as lava eithin the Black Mountain caldera and and as a component of the Trail Ridge Member. The short repose interval and rapid development of the comendite, together with evidence for mixing of magmas of silicic to near-mafic composition (Vogel et al., this issue), and the presence of air-fall tuff at the base of the Trail Ridge, lead us to conclude that eruption of the Trail Ridge Member was probably triggered by injection of hot, relatively unevolved magma into the Black Mountain magma body. ¿ American Geophysical Union 1989