INVOLVEMENT OF A HYDROTHERMAL SYSTEM IN THE CATASTROPHIC 5600 BP ERUPTION OF MOUNT RAINIER

About 5600 years ago, an eruption of Mount Rainier triggered an edifice (sector) collapse that caused the 4-km³ Osceola Mudflow and coeval phreatic and pumiceous magmatic tephras. The phreatic tephras, like the Osceola Mudflow, are clay-rich and contain hydrothermal minerals typical of acid-sulfate systems. Our study demonstrates, through mineralogy and stratigraphy, the genetic link between the Osceola Mudflow and its tephra and suggests how the hydrothermal system might have influenced the eruption.

The Osceola Mudflow and its tephra are heavily laden with alteration minerals indicating that they are cogenetic. Both contain pyrite, quartz, smectite, mixed-layer illite-smectite, and kaolinite. These minerals do not all occur together in other lahars or tephra layers. Crystal form, lack of framboids, and size distinguish pyrite in the Osceola tephra from modern low-temperature pyrite. Their shape and intergrowth with other hydrothermal minerals show that the pyrite formed prior to eruption and was abraded during transport. Abundant mixed layer illite-smectite (60% illite) and quartz in the tephra and matrix of the Osceola Mudflow suggest hypogene alteration at temperatures greater than about 150-200° C. From proportions of these minerals in deposits, we infer that the pre-eruption volume of intensely altered rock within the edifice could have been 0.5 km³ or more.

Weak, hydrothermally altered rock provided the eruption a path of least resistance through the edifice and potential edifice-collapse surfaces. Clay-rich lobes of the tephra have traits that suggest laterally directed, phreatic explosions. Each of the lobes has an axis that roughly matches the medial axis of the scarp left by the Osceola sector collapse. Distribution by high wind to the NE is inconsistent with the broad ESE arc of the coeval pumiceous tephra. Explosive decompression of the hydrothermal system’s super-heated water during edifice collapse may have blasted the unusual, pyrite-bearing clay-rich tephra to the NE.