SLOW AND FAST MODES OF TIDEWATER GLACIER ADVANCE

Reconstructed fiord histories suggest that Alaskan tidewater glaciers have two distinct modes of advance. The slow mode, with advance rates of a few tens of meters per year, occurs in deep water and is typical of most termini today. The contrasting fast mode occurs in shallow water and can cause large and rapid ice margin expansions; for example, Guyot Glacier in Icy Bay advanced at an average rate of 295 m/yr between 1791 and 1886. Here, data from the literature are used to examine the likely physical controls on these two modes of advance.

Advance rate in the slow mode is controlled by the rate at which a morainal bank beneath the iceberg-calving margin is eroded and moved down-fiord. Simple geometry shows that X = e / tan Θ, where X is morainal bank advance rate, e is effective erosion rate of the ice-contact slope, and Θ is the angle of the ice-contact slope; for typical erosion rates and slope angles, advance rates are less than 50 m/yr. Hydraulic geometry analysis shows that ice flow and channel geometry are in equilibrium at slowly advancing termini that are underlain and fringed by sedimentary deposits. This equilibrium, which is comparable to that of river channels with easily eroded boundaries, implies that the slow mode of advance is relatively stable and that the terminus ice-sediment system as a whole can adjust to small or gradual changes in ice discharge or fiord geometry.

The fast mode of advance will occur if the terminus enters shallow water and iceberg calving becomes inhibited. Advance rate will be controlled by the ice volume flux at the terminus; this is very high for many tidewater glaciers today and so advance rates of hundreds of meters per year or more are quite possible. This mode of rapid advance is distinctly different from a Variegated-type surge because it is initiated by conditions at the terminus and occurs under steady ice flow. This mechanism may have been responsible for the rapid closure of Russell Fiord by Hubbard Glacier in 1986 and rapid advances of iceberg-calving ice sheet lobes during the Late Wisconsinan, both of which have previously been attributed to Variegated-type surges.