fli) Crary
er ice at equivalent depth than on the flanks. A possible disadvantage is that the dome summit may not be representative. This research was supported by National Science Foundation grant DPP 87-16243 and the British Antarctic Survey.
References Raymond, Charles F. 1983. Deformation in the vicinity of ice divides. Journal of Glaciology, 29(103):356-373. Raymond, C. F. and B. R. Weertman. 1990. Glaciological observations
on Dyer Plateau, Antarctic Peninsula. Antarctic Journal of the U.S., 25(5):90-92. Reeh, N. 1988. A flow-line model for calculating the surface profile and the velocity, strain-rate, and stress in an ice sheet. Journal of Glaciology, 34(116):46-54. Rogers, J. C. and E. R. LaChapelle. 1974. The measurement of vertical strain in glacier bore holes. Journal of Glaciology, 13(68):315-319. Thompson, L. G., R. Mulvaney, and D. A. Peel. 1989. A cooperative climatological-glaciological program in the Antarctic Peninsula. Antarctic Journal of the U.S., 25(5):69-70. Thompson, L. C. Personal communication, 5 April 1990. Field report: 1989-1990. Cooperative Climatological-Glaciological Program in the Antarctic Peninsula (unpublished field report).
Possible effect of subglacial volcanism on changes in the west antarctic ice sheet
,
90°W
South ' F 'I 200 300 400 500 km Pole 0 100
JOHN C. BEHRENDT
U.S. Geological Survey Denver, Colorado 80225
\*I/ fli)
Station
0
Rapid changes in the west antarctic ice sheet (WAIS) may effect future global sea level changes. Hypothesis (Alley and Whillans 1991) to account for observed changes in the ice streams of the WAIS (figure la) include variations in the generation and transport of water and basal debris, and in ice strength, but not greenhouse warming. Alley and Whillans (1991) note that "the water responsible for separating the glacier from its bed is produced by frictional dissipation and geothermal heat," but assume that changes in geothermal flux would ordinarily be expected to have slower effects than glaciological parameters. I suggest episodic subglacial volcanism and geothermal heating may have significantly greater effects on the WAIS than generally appreciated. The WAIS flows through the active, largely aseismic West Antarctic rift system (WS) (figure 2) that defines the subsea level bed (Bryd Subglacial Basin) of the glacier (LeMasurier 1990; Behrendt et al. 1991). Various lines of evidence summarized in Behrendt et al. (1991) indicate high heat flow and shallow asthenosphere beneath the extended, weak lithosphere underlying the West Antarctic rift system (figure 2) and the WAIS. Behrendt and Cooper (1991) suggest a possible synergistic relation between Cenozoic tectonism, episodic mountain uplift, and volcanismin the West Antarctic rift system and the waxing and waning of the antarctic ice sheet beginning about earliest Oligocene time. A few active volcanoes and late Cenozoic volcanic rocks (figure 2) are exposed throughout the West Antarctic rift system 40
( /?
\\
(7
Crary Ice ice Rise
NS
\\ \ \ \
low km
Ross
Shelf 8Oo
Antarctica
Map Area
Calving t
Figure 1. Portion of WAIS flowing into the Ross Sea with Ice and Whliians 1991). along both flanks. No part of the rift system can be considerec inactive (LeMasurier 1990; Behrendt et al. 1991). Short-wavelength, high-amplitude anomalies (figure ib) observed on widely spaced aeromagnetic profiles collected in the 1960s (Behrendt 1964) occur over exposed volcanoes, and over the ice streams and their catchment areas. These latter anomalies, whose sources were calculated to be at or near the base of the ice sheet, have also ANTARCTIC JOURNAL
900W
850S
SOUTH POLE
850S
105°W
120°W Boos
135°W
750S
1500W
700S
165°W
1800
165°E
Figure 2. Magnetic anomalies (Behrendt 1964) along aeromagnetic track lines overlapping area of a. Contour interval for elevation, 200 meters.
1992 REVIEW
41
900J
/100
Figure 3. Generalized Isostatically adjusted bedrock elevation map after ice removal, assuming sufficient time, In area of the west antarctic rift system (Behrendt and Cooper 1991). Contour interval is 500 meters. The approximate limits of the grounded ice are indicated by a dotted pattern. The 3,000-kilometer-long, 750-kilometer-wide west antarctic rift system, which comprises areas having below-sealevel elevations bounded by the rift shoulder, Is filled by the WAIS as indicated. The rift shoulder escarpment has present elevations ranging from 3 to 5 kilometers. The volcanic centers on the continental shelf are determined from seismic reflection data (Behrendt et al. 1991). Additional (active?) volcanic centers are probable beneath the WAIS. been interpreted as due to late Cenozioc volcanic rocks (Behrendt 1964,1983; Behrendt et al. 1991). Volcanoes exposed above the ice in the WAIS show evidence (LeMasurier 1990) of having been erupted from beneath the ice. Several of the subglacial fixed points in the ice streams interpreted from satellite images (Bindshalder and Scambos 1991) suggest buried volcanoes. There is no teleseismic evidence for explosive eruptions beneath the ice, this is most likely due to few seismographs and generally nonexplosive volcanism (LeMasurier 1990; Behrendt et al. 1991). I propose that subglacial volcanic eruptions and ice flow in volcanically active areas should be considered to possibly have a forcing effect on the thermal regime resulting in increased melting at the base of the ice streams. The modest eruption of Mount St. Helens (in a different type
42
of tectonic setting) on 18 May 1980, released about 1.7 x 1018 joules (Decker and Decker 1981). An equivalent volcanic eruption beneath the WAIS could result in melting a 1-centimeter thick layer beneath a 700 by 700 kilometer area (the order of the ice streams and a significant part of their catchment area). If an eruption of this magnitude occurred once every 1 to 10 years in an area this size it probably would have a significant effect on the glacial regime; if once every 1,000 to 10,000 years, the results would be negligible. Of course energy released by volcanic eruptions varies by orders of magnitude, particularly when duration (only one day of the Mount St. Helens eruption is considered above) of particular eruptions is considered. The only point of this overly simplistic calculation is that active volcanism in the vst antarctic rift system, which might be reasonably expected (LeMasurier 1990; Behrendt et al. 1991), must be considered when studying changes in the WAIS. Programs such as the southeast Ross transect zone (CASERTZ) program (Blankenship et al. 1991) presently surveying a 330kilometer-wide swath of aeromagnetic, gravity, and radar icesounding data across the West Antarctic rift system give evidence of subglacial volcanism and concentrations of meltwater beneath the WAIS. This work was supported in part by National Science Foundation grant DPP 92-03170. References
Alley, R. B. and I. M. Whillans. 1991. Changes in the west antartic ice sheet. Science, 254:950-963. Behrendt, J. C. 1964. Distribution of narrow-width magnetic anomalies in Antarctica. Science, 144, 3641,995-999. Behrendt, J . C. 1983. Petroleum and mineral resources of Antarctic. Geological Survey Circular, 909:75. Behrendt, J. C. and A. K. Cooper. 1991. Evidence of rapid Cenozoic uplift of the shoulder of the sest antarctic rift system and a speculation on possible climate forcing. Geology, 19:315-319. Behrendt, J. C., W. E. LeMasurier, A. K. Cooper, F. Tessensohn, A. Trehu, and D. Damaske. 1991b. Geophysical studies of the sest antarctic rift system. Tectonics, 67-112. Bindshadler, R. A. and T. A. Scambos. 1991. Satellite-image-derived velocity field of an antarctic ice stream. Science, 252:242-246. Blankenship, D. D., S. M. Hodge, R. E. Bell, K. A. Najmulski, D. L. Wright, and J. C. Behrendt. 1991. Corridor aerogeophysics of the southeastern Ross transect zone (CASERTZ) 1990-1991. In Abstracts Sixth International Symposium on Antarctic Earth Science, National Institute of Polar Research, Tokyo, 64-65. Decker, R. and B. Decker. 1981. The eruptions of Mount St. Helens. Scientific American, 244(3):68-80. LeMasurier, W. E. 1990. Late Cenozioc volcanism on the antarctic plat: An overview. In W. E. LeMasurier and J . W. Thompson (Eds.), Volcanoes of the antarctic plate and southern oceans. (Antarctic Research Series, 48.) Washington, D.C.: American Geophysical Union, 1-19.
ANTARCTIC JOURNAL
References Raymond, Charles F. 1983. Deformation in the vicinity of ice divides. Journal of Glaciology, 29(103):356-373. Raymond, C. F. and B. R. Weertman. 1990. Glaciological observations
on Dyer Plateau, Antarctic Peninsula. Antarctic Journal of the U.S., 25(5):90-92. Reeh, N. 1988. A flow-line model for calculating the surface profile and the velocity, strain-rate, and stress in an ice sheet. Journal of Glaciology, 34(116):46-54. Rogers, J. C. and E. R. LaChapelle. 1974. The measurement of vertical strain in glacier bore holes. Journal of Glaciology, 13(68):315-319. Thompson, L. G., R. Mulvaney, and D. A. Peel. 1989. A cooperative climatological-glaciological program in the Antarctic Peninsula. Antarctic Journal of the U.S., 25(5):69-70. Thompson, L. C. Personal communication, 5 April 1990. Field report: 1989-1990. Cooperative Climatological-Glaciological Program in the Antarctic Peninsula (unpublished field report).
Possible effect of subglacial volcanism on changes in the west antarctic ice sheet
,
90°W
South ' F 'I 200 300 400 500 km Pole 0 100
JOHN C. BEHRENDT
U.S. Geological Survey Denver, Colorado 80225
\*I/ fli)
Station
0
Rapid changes in the west antarctic ice sheet (WAIS) may effect future global sea level changes. Hypothesis (Alley and Whillans 1991) to account for observed changes in the ice streams of the WAIS (figure la) include variations in the generation and transport of water and basal debris, and in ice strength, but not greenhouse warming. Alley and Whillans (1991) note that "the water responsible for separating the glacier from its bed is produced by frictional dissipation and geothermal heat," but assume that changes in geothermal flux would ordinarily be expected to have slower effects than glaciological parameters. I suggest episodic subglacial volcanism and geothermal heating may have significantly greater effects on the WAIS than generally appreciated. The WAIS flows through the active, largely aseismic West Antarctic rift system (WS) (figure 2) that defines the subsea level bed (Bryd Subglacial Basin) of the glacier (LeMasurier 1990; Behrendt et al. 1991). Various lines of evidence summarized in Behrendt et al. (1991) indicate high heat flow and shallow asthenosphere beneath the extended, weak lithosphere underlying the West Antarctic rift system (figure 2) and the WAIS. Behrendt and Cooper (1991) suggest a possible synergistic relation between Cenozoic tectonism, episodic mountain uplift, and volcanismin the West Antarctic rift system and the waxing and waning of the antarctic ice sheet beginning about earliest Oligocene time. A few active volcanoes and late Cenozoic volcanic rocks (figure 2) are exposed throughout the West Antarctic rift system 40
( /?
\\
(7
Crary Ice ice Rise
NS
\\ \ \ \
low km
Ross
Shelf 8Oo
Antarctica
Map Area
Calving t
Figure 1. Portion of WAIS flowing into the Ross Sea with Ice and Whliians 1991). along both flanks. No part of the rift system can be considerec inactive (LeMasurier 1990; Behrendt et al. 1991). Short-wavelength, high-amplitude anomalies (figure ib) observed on widely spaced aeromagnetic profiles collected in the 1960s (Behrendt 1964) occur over exposed volcanoes, and over the ice streams and their catchment areas. These latter anomalies, whose sources were calculated to be at or near the base of the ice sheet, have also ANTARCTIC JOURNAL
900W
850S
SOUTH POLE
850S
105°W
120°W Boos
135°W
750S
1500W
700S
165°W
1800
165°E
Figure 2. Magnetic anomalies (Behrendt 1964) along aeromagnetic track lines overlapping area of a. Contour interval for elevation, 200 meters.
1992 REVIEW
41
900J
/100
Figure 3. Generalized Isostatically adjusted bedrock elevation map after ice removal, assuming sufficient time, In area of the west antarctic rift system (Behrendt and Cooper 1991). Contour interval is 500 meters. The approximate limits of the grounded ice are indicated by a dotted pattern. The 3,000-kilometer-long, 750-kilometer-wide west antarctic rift system, which comprises areas having below-sealevel elevations bounded by the rift shoulder, Is filled by the WAIS as indicated. The rift shoulder escarpment has present elevations ranging from 3 to 5 kilometers. The volcanic centers on the continental shelf are determined from seismic reflection data (Behrendt et al. 1991). Additional (active?) volcanic centers are probable beneath the WAIS. been interpreted as due to late Cenozioc volcanic rocks (Behrendt 1964,1983; Behrendt et al. 1991). Volcanoes exposed above the ice in the WAIS show evidence (LeMasurier 1990) of having been erupted from beneath the ice. Several of the subglacial fixed points in the ice streams interpreted from satellite images (Bindshalder and Scambos 1991) suggest buried volcanoes. There is no teleseismic evidence for explosive eruptions beneath the ice, this is most likely due to few seismographs and generally nonexplosive volcanism (LeMasurier 1990; Behrendt et al. 1991). I propose that subglacial volcanic eruptions and ice flow in volcanically active areas should be considered to possibly have a forcing effect on the thermal regime resulting in increased melting at the base of the ice streams. The modest eruption of Mount St. Helens (in a different type
42
of tectonic setting) on 18 May 1980, released about 1.7 x 1018 joules (Decker and Decker 1981). An equivalent volcanic eruption beneath the WAIS could result in melting a 1-centimeter thick layer beneath a 700 by 700 kilometer area (the order of the ice streams and a significant part of their catchment area). If an eruption of this magnitude occurred once every 1 to 10 years in an area this size it probably would have a significant effect on the glacial regime; if once every 1,000 to 10,000 years, the results would be negligible. Of course energy released by volcanic eruptions varies by orders of magnitude, particularly when duration (only one day of the Mount St. Helens eruption is considered above) of particular eruptions is considered. The only point of this overly simplistic calculation is that active volcanism in the vst antarctic rift system, which might be reasonably expected (LeMasurier 1990; Behrendt et al. 1991), must be considered when studying changes in the WAIS. Programs such as the southeast Ross transect zone (CASERTZ) program (Blankenship et al. 1991) presently surveying a 330kilometer-wide swath of aeromagnetic, gravity, and radar icesounding data across the West Antarctic rift system give evidence of subglacial volcanism and concentrations of meltwater beneath the WAIS. This work was supported in part by National Science Foundation grant DPP 92-03170. References
Alley, R. B. and I. M. Whillans. 1991. Changes in the west antartic ice sheet. Science, 254:950-963. Behrendt, J. C. 1964. Distribution of narrow-width magnetic anomalies in Antarctica. Science, 144, 3641,995-999. Behrendt, J . C. 1983. Petroleum and mineral resources of Antarctic. Geological Survey Circular, 909:75. Behrendt, J. C. and A. K. Cooper. 1991. Evidence of rapid Cenozoic uplift of the shoulder of the sest antarctic rift system and a speculation on possible climate forcing. Geology, 19:315-319. Behrendt, J. C., W. E. LeMasurier, A. K. Cooper, F. Tessensohn, A. Trehu, and D. Damaske. 1991b. Geophysical studies of the sest antarctic rift system. Tectonics, 67-112. Bindshadler, R. A. and T. A. Scambos. 1991. Satellite-image-derived velocity field of an antarctic ice stream. Science, 252:242-246. Blankenship, D. D., S. M. Hodge, R. E. Bell, K. A. Najmulski, D. L. Wright, and J. C. Behrendt. 1991. Corridor aerogeophysics of the southeastern Ross transect zone (CASERTZ) 1990-1991. In Abstracts Sixth International Symposium on Antarctic Earth Science, National Institute of Polar Research, Tokyo, 64-65. Decker, R. and B. Decker. 1981. The eruptions of Mount St. Helens. Scientific American, 244(3):68-80. LeMasurier, W. E. 1990. Late Cenozioc volcanism on the antarctic plat: An overview. In W. E. LeMasurier and J . W. Thompson (Eds.), Volcanoes of the antarctic plate and southern oceans. (Antarctic Research Series, 48.) Washington, D.C.: American Geophysical Union, 1-19.
ANTARCTIC JOURNAL
