Answer:
by igneous activity is very low, but its potential consequences are nevertheless important to
performance assessments. An ongoing critical area of concern is the nature and magnitude of the
thermal interaction of magma with tunnel walls, high-level nuclear waste packages, and waste
forms. Previous assessments consider a variety of dynamic scenarios, but large uncertainties
remain in understanding the rheological nature of the magma likely to be involved and its
behavior within a repository drift. Here we specifically address the issue of magma rheology
during degassing, cooling, and solidification as basaltic magma approaches Earth’s surface and
enters a drift. Magma containing significant amounts (>~2 wt.%) of dissolved water (Wet
Magma), as is anticipated for this region, at or near its liquidus temperature and saturated with
water at 200 MPa is at a temperature near or below the 1-atm solidus temperature. Isentropic
ascent from this near liquidus temperature promotes extensive solidification and/or
glassification. 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 solidification phase field and undergoes a combination of rapid crystallization and
quenching, becoming a glassy highly viscous (~108
Pa s) mass of greatly reduced mobility. This
immobility is reflected in the high effective viscosity regulating flows from nearby cinder cones
associated with wet basalt. This also matches well with the experimentally determined rheology
of dry basalt glass. This rheology greatly restricts the mobility of basalt within repository drifts,
amounting to < 10 m per day. Magma in this state quenches rapidly (~10 cm/ min.) on waste
packages. Wet basalt is explosive, but relatively immobile as lava. Dry Magma is not explosive,
but highly mobile as lava. Previous studies have tended to use an inconsistent set of mixed
magma properties involving both extremes. The net effect of our results is that the portion of a
repository hypothetically affected by invading magma is likely to be minimal and the number of
waste packages affected may be very small. Moreover, the waste packages and/or waste
materials affected will most likely be encased in quenched magma.
