Dr. Terence J. Hughes
Department of Earth Sciences/Climate Change Institute/University of Maine
Orono, ME 04469-5790, USA
1 April 2006

1968-1972
Upon the recommendation of Hans Weertman, a glaciologist at Northwestern who was on my M.S. and Ph.D. committees, I made glacier ice the material holding my primary scientific interest. On 1 January 1968, Colin Bull began my glaciological career in the Institute of Polar Studies (now the Byrd Polar Research Center) at The Ohio State University (OSU). That spring I fell under the spell of J. Tuzo Wilson, a pioneering geophysicist who made global plate tectonics possible by discovering transform faults. He taught a course, World Geology, that led directly to my first paper in glaciology. It was a theoretical study that predicted ice may rise in plumes by thermal convection in the lower part of the Antarctic Ice Sheet and, under the right conditions, allow a massive conversion of gravitational potential energy into kinetic energy of motion that could make the whole ice sheet surge into the ocean, resulting in global cooling that would begin a new Ice Age glaciation cycle. It was published in Science in 1970. Although I published many more papers on thermal convection in ice sheets, skeptics cited a lack of convincing observational support. In 1985, the Journal of Glaciology published my arguments that downwarped radio-echo reflecting horizons in the Antarctic Ice Sheet could be interpreted as manifestations of thermal convection, as well as the echo-free zone in ice near the bed where convection may have scrambled radio-echo horizons.

In 1971, the Journal of Applied Physics published my paper pointing out that thermal convection in polar ice sheets would be a model for thermal convection in Earth’s mantle. If ice-sheet convection were confirmed, the Antarctic Ice Sheet could be treated as a simplified (one crystal structure) miniature mantle accessible by deep drilling that could be used to study thermal convection in Earth's inaccessible mantle that moves Wilson's crustal tectonic plates and causes continental drift. That would make it my most important idea by far, and it would be a great boon to glaciology. It's my only idea that has merited Invited Paper status, in 1975, at a Symposium on the Thermal Regime of Glaciers and Ice Sheets sponsored by the International Glaciological Society. Nobody has asked for a reprint of my paper. Convection moves ice upward, which is topsy-turvy.

In 1972, the Journal of Applied Physics published my derivation of the Rayleigh number needed to initiate thermal convection in crystalline materials like ice sheets. The middle third of the Antarctic Ice Sheet is heavier than the bottom third because it is colder. That makes it an enormous reservoir of gravitational potential energy poised for release as gravitational motion not considered in conventional glaciology. The Rayleigh criterion for initiating thermal convection in viscous fluids is satisfied, when modified for convection in viscoplastic crystalline solids, ice specifically. Viscoplastic deformation begins as transient creep, which starts with an infinite strain rate that gives a Rayleigh number infinitely above the critical Rayleigh number for initiating thermal convection in the Antarctic Ice Sheet. Only as slow steady-state creep sets in can the Rayleigh number drop below the critical value in some parts of the ice sheet. It other parts it stays higher and recrystallization of ice in the bottom third allows fast steady-state creep, so the Rayleigh number can climb once again. But intermittent transient thermal convection should always be possible. So why doesn’t the cold ice ceiling collapse into the warm ice basement? If it did, would collapse be catastrophic? Everywhere at once? What then would be the implications for thermal convection in Earth’s crystalline mantle, and for plate tectonics in Earth’s lithosphere? These questions are important.

For me, the question is not whether ice-sheet convection occurs, but how? I see it in ice streams, perhaps even causing ice streams, as reported in the Journal of Glaciology. In 1976 I wondered if ice streams were warmer than adjacent ice, so they were “buoyed up” enough for basal water to flow in and uncouple ice from the bed, allowing stream flow. By 1992 I argued that stream flow begins when the cold ice ceiling collapses into the warm arm ice basement in directions of ice-sheet spreading, creating the concave longitudinal and transverse profiles that characterize ice streams. Basal water driven toward lowered ice by the hydrostatic pressure gradient might uncouple ice from the bed enough to initiate stream flow. Some displaced basement ice might rise in the lateral shear zones and join ice flowing toward the lowered ice from the sides. Warm basement ice displaced downstream carries the cold sinking ice in extending flow, producing transverse surface crevasses. Ice advected downstream never completes a convection “circuit” but ninety percent of Antarctic ice is discharged by ice streams, so most of the gravitational potential energy stored in the cold ice ceiling is released as kinetic energy by the motion of ice streams. Alternatively, ice streams could form by other processes and stream flow then allows this kind of thermal convection. This chicken-or-egg question has two answers, topsy and turvy. Either way, this release of gravitational potential energy is not considered in theories of ice streams. I’m working on it.