Calving Glaciers
With Emphasis on Columbia
Glacier, Prince William Sound*
Megan Kennedy, College of Wooster
Department
of Geology
*This
material is based upon work supported by the National Science
Foundation under Grant No. 9910805. Any opinions, findings, and
conclusions or recommendations expressed in this material are
those of the author and do not necessarily reflect the views
of the National Science Foundation.
Calving Glaciers
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Intro to Calving Glaciers
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.
Figure 2: The ELA (equilibrium line altitude) and AAR (accumulation
area ratio) are the two primary climatic variables that drive
glacial advance and retreat (Mercer, 1961).
While non-calving glacial cycles are
driven primarily by changes in climate, tidewater glaciers are
also forced by a number of other factors in the system. In addition
to climatic controls on calving, they must also respond to non-climatic
conditions in and around the fjord in which they terminate. Some
tidewater calving glaciers might respond equally to these controls
as they do to climatic controls (Mann, 1986).
The first of these is the water depth
in the fjord (Figure 3). As a glacier advances, it creates a
moraine shoal below sea level, which is full of sediment and
debris at the front of its margin. This pile of sediment acts
as a support for the front of the ice as long as the glacier
continues to advance. However, when the glacier begins its retreat
(often dictated by climatic controls explained above), the moraine
shoal stays grounded in place, and the terminus is over deeper
water. Tidewater glaciers are known to experience rapid retreat
and linear calving rates as the fjord water deepens because of
a loss of stability at the margin (Figure 4). One key result
of this loss of stability is the formation and detachment of
icebergs due to crevassing throughout the ice; hence, calving
begins and drives the system back further.
Figure 3: Calving glacier advance/retreat cycle (Post,
1975). Columbia Glacier is now in the retreating phase. However,
it did advance during the Holocene to create the terminal moraine
shoal at the end of its fjord before beginning its retreat in
1982, as seen in figure 6.
Another non-climatic influence on calving
rates is ice thickness at the terminus. Thicker ice is also a
sign of lesser stability and greater stresses at the calving
front (Warren, 1992). A linear relationship has been found between
calving rate and both ice thickness and water depth, as seen
in figure 4 (Van der Veen, 1995).
Figure 4: These graphs display the data collected by Brown
et al. (1982) on 12 Alaskan tidewater glaciers, where Uc is the
best fit calving rate for each parameter (Van der Veen, 1995).
The linear trends are apparent.
Finally, fjord geometry is an equally
influential non-climatic factor in controlling tidewater glacier
calving (Figure 5). As fjord width increases, the calving margin
also widens, causing an increase in the surface area of the ice
(Mercer, 1961). Again, this leads to lesser stability and greater
stresses at the terminus, and retreat rate due to calving increases.
Fjord geometry has been documented to quicken calving rates of
tidewater glaciers so much that once the glacier reaches a certain
point in its rapid retreat phase, it is no longer affected by
climatic controls until a more narrow, stabilizing section of
the fjord is reached (Mercer, 1961).
Figure 5: The affects of fjord geometry on tidewater
calving rates (Mercer, 1961). Retreat rates increase as the fjord
widens (A), whereas advance rates increase as the fjord narrows
(B). Columbia Glacier has retreated during recent years in part
due to its widening fjord shape.
Do all calving glaciers
behave the same?
Several attempts have been made to
quantitatively determine a "universal calving law"
that can be applied to any calving tidewater glacier in the world
(i.e. Brown et al., 1982, Venteris, 1999). While numerical models
have been formulated for individual glaciers and regions of glaciers,
this hypothetical universal law is unlikely to exist because
of the different climatic and non-climatic mechanisms that control
calving rates.
One question that remains to be answered
is whether calving depends on glacial retreat, or vice versa
(Venteris, 1999). Some researchers agree that the linear relationship
between calving rate and water depth (Figure 4) leads to the
assumption that calving rates drive glacial retreat (Brown et
al., 1986, Meier, 1994). However, others have stated that retreat
drives calving rate because of the observation that calving occurs
faster when the terminus position retreats into deeper water
(Van der Veen, 1996). This relationship is unclear to date, and
further study is needed to fully understand this aspect of tidewater
glaciers.
What is a calving glacier?
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.
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Columbia Glacier
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.
The retreat of Columbia Glacier has
not only lead to our better understanding of the way tidewater
glaciers experience calving, but it has also allowed glaciologists
to determine when it was advancing. As the ice calves off at
the margin, it exposes areas on either side of the fjord that
are covered with sub-fossil trees that were run over by the glacier
during its initial advance. Using dendrochronology and cross-dating, the results of recent calving by Columbia Glacier
have been used as indicators of fluctuations in the past climate
of Prince William Sound.
What is the importance of studying 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. |
Works Cited
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.
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. |
