The Year Leading to a Supereruption

Abstract:

Supereruptions – which catastrophically eject 100s-1000s of km³ of magma to the surface in a matter of days to a few months – have been described as the ultimate geologic hazard; yet, we lack direct knowledge of the processes and signals leading to them. We use zoning in quartz to assess the timescales over which the transition from quiescent pre-eruptive crystallization to syn-eruptive decompression and crystallization takes place, focusing on the Bishop Tuff (CA). Quartz crystals in the Bishop Tuff have distinctive rims (<200 μm thick), which are Ti-rich and bright in cathodoluminescence (CL). We use high-resolution CL imaging to obtain estimates of these timescales with unprecedented precision.

We performed CL imaging using an SEM, employing a 5 kV electron beam, which yields much improved spatial resolution but reduced CL contrast. We imaged rim-interior contacts using a pixel size of ~0.2 μm, with dwell times per pixel of 1-3.2 ms. Contacts were placed vertically to maximize their sharpness. Maximum timescales of crystallization were determined using a 1D model, assuming initial step functions. Minimum quartz growth rates were calculated using measured rim thicknesses.

Maximum rim growth times span from ~1 min to 35 years, with a median of ~4 days. More than 70% of rim growth times are less than 1 year, showing that quartz rims have mostly grown in the days to months prior to eruption. Minimum growth rates show distinct modes between 10^-8 and 10^-10 m/s (depending on sample), revealing very fast crystal growth rates (100s of nm to 10s of μm per day).

Crystallization times for quartz crystal interiors based on diffusion chronometry, melt inclusion faceting, and crystal size distributions show that establishment of a melt-rich magma body (or bodies) took place within a few millennia of eruption. Gas-saturated crystallization led to accumulation of bubbles, which eventually drove destabilization and eruption. The onset of decompression is recorded as a microlite population observed in crystal size distributions. Concomitant rim growth reveals that decompression initiated within a year of eruption, with most of the growth happening in the weeks or days preceding eruption. These are the timescales over which signs of an impending eruption can be felt at the surface, providing much needed warning for hazard mitigation.