What is a calving glacier?
When a glacier experiences calving, large pieces of the ice break off at the margin, usually during the retreating stage in the glacial life cycle. Specifically, tidewater calving glaciers are those whose margin, or terminus, ends into a body of water (Figure 1). When calving occurs, these broken pieces of ice fall into the water and become icebergs. Calving is the most efficient way for tidewater glaciers to experience a loss of their ice mass, as well as for the world's oceans to gain more water through the melting of icebergs (Van der Veen, 1997).
Figure 1: Photo of the calving margin at Columbia Glacier, Alaska, a tidewater calving glacier. Extensive research has been conducted here on various topics, including calving speeds, glacial advance rates, and the affects of climatic versus non-climatic controls on calving, as discussed in the following sections.
When do tidewater glaciers experience calving?
There are two types of controls on tidewater glacier advance and retreat and are therefore important in governing when and to what extent a glacier calves. These are climatic and non-climatic controls. Climatic controls drive the glacial system on a global scale and most often serve as the basis for wide-spread glacial advance or retreat. Of these, temperature and precipitation are the most predominant. Mercer (1961) described two climatic variables that are crucial to a glacier's lifecycle (Figure 2). First, the equilibrium line altitude (ELA) is what separates the zone of accumulation (where snow falls and is compacted into ice) from the zone of ablation (where melting and calving occur). Glaciers begin to calve and retreat when ablation below the ELA is greater than accumulation above the ELA and is therefore dependent upon climate. Second, the accumulation area ratio (AAR) is the point above or below which a glacier will advance or retreat, respectively. It is a ratio of the glacier's accumulation area to the glacier's total area. When the AAR drops below 0.6, retreat and calving occur (Powell et al., 1991). These two variables maintain a complex relationship that serves as the basis for the climatic controls on glacial advance and retreat.
What is the importance of studying Columbia Glacier?
Among the world's most studied tidewater glaciers is Columbia Glacier, located in Prince William Sound just west of Valdez, Alaska (Figure 6). It is well documented to have been experiencing calving and rapid retreat up its fjord since 1982. The water depth and fjord geometry at Columbia Glacier has allowed for its continued retreat to date, even though its AAR is above the equilibrium level of 0.6 required for it to begin advancing again (Lapham, 2001). Thus, it is apparent that non-climatic variables currently make more of an impact on the calving behavior of this glacier than do climatic variables, and it will most likely continue this process of retreat until the water in the fjord becomes more shallow (Post, 1975). Click here to read more about the advance and retreat of Columbia Glacier.
Figure 6: Topographic image of the Columbia glacier. Notable features include water depth, ice thickness, and shape of the fjord (Post, 1998; modified from Post pers. comm.). This image shows the ice calving margin as it was in July, 2000, although it has retreated further since this time.
Brown, C.S., M.F. Meier, and A. Post. 1982. Calving speed of Alaska tidewater glaciers, with application to Columbia Glacier. U.S. Geological Survey Professional Paper 1044-9612, p. C1-C13.
Lapham, K.A. 2001. The past thousand years of glacial change from Columbia Glacier, Prince William Sound, Alaska. Unpublished Undergraduate Thesis - The College of Wooster.
Mann, D.H. 1986. Reliability of a fjord glacier's fluctuations for paleoclimatic reconstructions. Quaternary Research 25:10-24.
Mercer, J.H. 1961. The response of fjord glaciers to changes in the firn limit. Journal of Glaciology 3(29): 850-858.
Post, A., B. Hallet, and L.A. Rasmussen. 1998. Preliminary bathymetry of the forebay, Columbia Glacier, Alaska. Open File Report No. 4-B, sheet 2.
Post, A. 1975. Preliminary hydrography and historic terminal changes of Columbia Glacier, Alaska. U.S. Geological Survey Hydrographic Investigations Atlas 559, 3 sheets.
Powell, R.D. 1991. Grounding-line systems as second-order controls on fluctuations of tidewater termini of temperate glaciers, p. 75-93. In: Anderson, J.B. & Ashley, G.M. (eds.), Glacial marine sedimentation; Paleoclimatic significance. The Geological Society of America, Special Paper 261.
Van der Veen, C.J. (ed.). 1997. Calving Glaciers: Report of a Workshop, February 28 - March 2, 1997. BPRC Report No. 15, Byrd Polar Research Center, The Ohio State University, Columbus, Ohio, 194 pages.
Van der Veen, C.J. 1996. Tidewater calving. Journal of Glaciology 42(141): 375-385.
Van der Veen, C.J. 1995. Controls on calving rate and basal sliding: observations from Columbia Glacier, Alaska, prior to and during its rapid retreat, 1976-1993. BPRC Report No. 11, Byrd Polar Research Center, The Ohio State University, Columbus, Ohio, 72 pages.
Venteris, E.R. 1999. Rapid tidewater glacier retreat: a comparison between Columbia Glacier, Alaska and Patagonian calving glaciers. Global and Planetary Change 22:131-138.
Warren, C.R. 1992. Iceberg calving and the glacioclimatic record. Processes in Physical Geography 16(3): 253-282.