Mass and style of eruptions in experimental geysers

Abstract

In the present study, we conducted laboratory experiments of geysers to reproduce the time predictability of natural geysers in Yellowstone and other geothermal areas. We measured pressure and temperature in a hot water chamber, flux from a cold water reservoir, and mass erupted by each eruption (total number of eruptions are up to 100), varying experimental conditions such as the heating rate, water quality, and system geometry. We observed two styles of eruptions, “jet” and “flow” depending on the maximum height reached. Under some conditions, only jet events occurred, while under other conditions, jet and flow events co-occurred. Based on the statistical analysis of the erupted mass, an experiment setup that produces only jet events exhibits a narrower frequency distribution with a relatively large average mass. As the proportion of flow events increases, the frequency distribution of the erupted mass widens with relatively small average mass. The temperature measurements indicated that jet-dominated experimental setups had smaller temperature fluctuations than flow-dominated setup. We proposed a triggering condition involving boiling of water that defined the onset of an eruption. We assumed two thresholds of the efficiency of decompression boiling that defined explosivity and eruption development on the basis of hydrodynamic energetics. Using the triggering condition and the two thresholds, to explain experimental correlations between erupted mass, eruption style, and the magnitude of thermal fluctuation, we conducted a Monte Carlo simulation in a square consisting of 256×256 parcels with the superheating temperature as a stochastic variable by a Gaussian probability density function (PDF). The results showed that when the PDF has a larger average and smaller standard deviation, the event tends to be explosive and large fraction of water is evacuated, as in jet events. Decreasing the average temperature or increasing the standard deviation of the PDF shifts the events to an explosive style followed by an effusive event and to an event that produces only effusive flow. This transition of eruption styles from explosive to effusive and the relationship with the erupted mass is consistent with results of the laboratory experiments, suggesting that the spatial distribution pattern of supersaturated portions just prior to an eruption is a factor controlling the style and transition of the eruption.