Prehistoric Surges of the Black Rapids Glacier, Alaska: Using the Past to Assess Future Hazards

The Black Rapids Glacier is a surging glacier in the Eastern Alaska Range that undergoes rapid advances and retreats alternating with periods of stagnation. Its most recent surge occurred in the winter of 1936/7 when the glacier raced forward 6.4 kilometers in four months. Little is known about how often the Black Rapids Glacier surges, despite the fact that both the Richardson Highway and the Trans-Alaska Pipeline (TAPS) lie within 1.5 km of the 1936/7 surge limit. Glaciologists have speculated this glacier surges every 60-70 years, but this was not based on long-term records. Our goal is to assess the hazards this glacier poses for the TAPS and Richardson Highway by establishing a secure chronology for its prehistoric surges. Building on a reconnaissance study by Richard D. Reger in the 1990s, we are using standard stratigraphic methods, lake coring, 14C dating, and lichenometric dating to constrain the ages of three terminal moraines that lie outboard of the 1936/7 moraine. The innermost of these moraines was deposited prior to AD 1370. The next moraine in the sequence was deposited prior to AD 280. The oldest, outermost moraine was deposited between 2400 and 3000 years before present and was deposited by ice that overrode the area now occupied by the TAPS and the Richardson Highway. This oldest advance also dammed a lake some 60 m deep on its up-valley flank. The causes of the Black Rapids Glacier's surges remain unknown, but may relate to landslides along the Denali Fault that periodically blanket its ablation zone with a thick layer of rock debris, insulating the ice below from melt.