Lacuna band (surface depressions) occurrence and conditions of formation, Bering Glacier, Alaska

Abstract

Bering Glacier lacunas are steep-sided flat-floored depressions ranging from 40 to 60 m wide, 80 to 120m long and 35 to 50m deep. They are confined within a band of debris-free ice (1.5km wide, 5km long) parallel to the eastern margin of the Bering piedmont lobe. After the 1993-95 surge displacedthe lacuna bandseveral kilometersonto theforeland, a new bandof lacunas began to form 5-6 years later in the same location as occupied by the displaced band. Conditions essential to lacuna formation were initiated during the surge, as overriding ice was thrust into position across the trend of a subglacial trough, leading to stagnation deep within the trough. It is proposed that stagnation combined with englacial water movement altered ice crystal fabric and resistance to ablation. Exposure of this ice through normal ablation led to differential ablation and the formation of lacunas.