NEAR-SURFACE ERUPTIVE STATE OF WET VERSUS DRY MAGMA

The strong effect of dissolved water on the low-pressure (i.e., near surface) phase relations of basalt is well known. Magma essentially free of dissolved water (i.e., Dry Magma) cools during ascent and solidifies only by adiabatic expansion and conductive heat losses to the wall rock (see Fig.). The slope of the liquidus of Dry Magma relative to the steeper adiabatic cooling gradient (~0.5 °C/km) stalls and reverses solidification. Indigenous crystals are melted and as soon as the magma becomes superheated vigorous thermal convection ensues (e.g., Marsh, 1989; Hort et al., 1999). Thermal convection rapidly reduces the temperature to the liquidus where convection ceases and conductive losses may further drop the temperature promoting crystallization. The inevitable result is that the eruption temperature of Dry Magma will generally be near the liquidus, which is commonly observed for Hawaiian magmas and inferred for ocean ridge magmas.

For magma containing significant amounts (>~1 wt.%) of dissolved water (Wet Magma), the low-P liquidus and solidus have negative slopes. Magma near its liquidus temperature and saturated with water at 200 MPa, for example, will be at a temperature near or below the 1-atm solidus temperature. Isentropic ascent from a near liquidus temperature here also causes cooling, promoting solidification (e.g., Mastin and Ghiorso, 2001). Exsolving water with approach to the surface promotes rapid vesiculation leading to fragmentation and tephra production. With continued ascent the still water-saturated magma traverses the phase field and undergoes a combination of rapid crystallization and quenching, becoming a glassy highly viscous (~109 p) mass of greatly reduced mobility. This is reflected in the high effective viscosity regulating flows from cinder cones associated with wet basalt, which matches well with the rheology of dry basalt glass of Webb and Dingwell (1990). Wet basalt is explosive, but relatively immobile as lava. Dry Magma is not explosive, but highly mobile as lava. [The views expressed herein are the authors'. They do not reflect an NRC staff position, or any judgment or determination by the Advisory Committee on Nuclear Waste or the NRC, regarding the matters addressed or the acceptability of a license application for a geologic repository at YM.]