The next step in merging multiple perspectives on silicic magma systems should be to test them by drilling

Eichelberger, John

Abstract:

Penetration of silicic magma by geothermal boreholes has been reported to have occurred more than once at each of three locations, Krafla Caldera in Iceland, Menengai Caldera in Kenya, and Kilauea, in Hawaii, USA. Well control was maintained through normal geothermal practice. Some striking similarities emerge:

1. Depths of the encounters were 2.1 -2.7 km, though magma had not interpreted to be so shallow by geophysical techniques.

2. The transition from hydrothermal to magmatic regimes is short, <30 m at Krafla, so >15^oC/m.

3. The first sign of magma was a “soft” formation and fragments of clear glass with sparse crystals, near-liquidus magma.

4. Fragments of melt-bearing intrusive were also recovered, interpreted at Krafla to reflect partial melting at the roof of the magma body.

Drilling into magma bodies, large or small, opens a completely new window on the conditions and processes of silicic magma storage in the crust. Coring molten rock in lava lakes is routine, and it should be possible to core magma and obtain the entire transition intact from deep subsolidus to liquidus. To answer questions posed in the workshop description:

1. Although 3-D resolution of magmatic regimes through integration of geophysical techniques will undoubtedly improve with time, the question will always remain – resolution of what? These methodologies cannot be significantly improved until they are tested against ground truth. Volcanology now is like studying hydrocarbon reservoirs before any had been drilled.

2. By maintaining boreholes as observation ports and drilling new ones, we should over decadal periods observe processes and changes that will enlighten our hypotheses about longer time scales and which are orders of magnitude longer than possible in the laboratory.

3. We can stimulate the magma through fluid injection (this has already been done accidently without ill effects) and watch for characteristic signals of unrest.

Standard logs can be run immediately after drilling and coring because the wells will be cold. Thermocouple strings can be left in wells that will withstand magmatic temperatures. Dewared or temperature-hardened instruments can be run as the well recovers. Tethered sondes can be developed to be deployed in the magma.