Geology topics
S. M. Hodge
Publications and source records attributed to S. M. Hodge.
Influence of subglacial geology on the onset of a West Antarctic ice stream from aerogeophysical observations
Marine ice-sheet collapse can contribute to rapid sea-level rise . Today, the West Antarctic Ice Sheet contains an amount of ice equivalent to approximately six metres of sea-level rise, but most of the ice is in the slowly moving interior reservoir. A relatively small fraction of the ice sheet comprises several rapidly flowing ice streams which drain the ice to the sea. The evolution of this drainage system almost certainly governs the process of ice-sheet collapse . The thick and slow-moving interior ice reservoir is generally fixed to the underlying bedrock while the ice streams glide over lubricated beds at velocities of up to several hundred metres per year. The source of the basal lubricant — a water-saturated till overlain by a water system — may be linked to the underlying geology. The West Antarctic Ice Sheet rests over a geologically complex region characterized by thin crust, high heat flows, active volcanism and sedimentary basins . Here we use aerogeophysical measurements to constrain the geological setting of the onset of an active West Antarctic ice stream. The onset coincides with a sediment-filled basin incised by a steep-sided valley. This observation supports the suggestion that ice-stream dynamics — and therefore the response of the West Antarctice Ice Sheet to changes in climate — are strongly modulated by the underlying geology.
Climate variations and changes in mass of three glaciers in western North America
Time series of net and seasonal mass balances for three glaciers in western North America, one in the Pacific Northwest and two in Alaska, show various relationships to Pacific hemisphere climate indexes. During the winter season the two coastal, maritime-regime glaciers, over 2000 km apart, are affected almost identically, albeit inversely, by atmospheric and oceanic conditions in both the tropical and North Pacific. The two Alaska glaciers, only 350 km apart, have almost no coherence. Lag correlations show that in winter the maritime glaciers are influenced by concurrent conditions in the North Pacific, but by conditions in the tropical Pacific in August-September of the prior northern summer. The winter balance variations contain interannual El Nino-Southern Oscillation variability superimposed on North Pacific interdecadal variability; the interdecadal 1976-77 climate regime shift is clearly evident. The summer balances and the continental-regime glacier have a general lack of correlations, with no clear, strong, consistent patterns, probably a result of being influenced more by local processes or by circulation patterns outside the Pacific Ocean basin. The results show the Pacific Northwest is strongly influenced by conditions in the tropical Pacific, but that this teleconnection has broken down in recent years, starting in 1989. During the seven years since then (1989-95), all three glaciers have shown, for the first time, coherent signals, which were net mass loss at the highest rate in the entire record. The authors' results agree with those of other recent studies that suggest these recent years are unusual and may be a signature of climate warming.
Satellite imagery of the onset of streaming flow of ice streams C and D, West Antarctica
Five overlapping Landsat multispectral scanner satellite images of the interior of the West Antarctic ice sheet were enhanced with principal component analysis, high-pass filtering, and linear contrast stretching and merged into a mosaic by aligning surface features in the overlap areas. The mosaic was registered to geodetic coordinates, to an accuracy of about 1 km, using the five scene centers as control points. The onset of streaming flow of two tributaries of ice stream C and one tributary of ice stream D is visible in the mosaic. The onset appears to occur within a relatively short distance, less than the width of the ice stream, typically at a subglacial topographic feature, such as a step or ridge. The ice streams extend farther up into the interior than previously mapped. Ice stream D starts about 150 km from the ice divide, at an altitude of about 1500 m, approximately halfway up the convex-upward dome shape of the interior ice sheet. Ice stream D is relatively much longer than ice stream C, possibly because ice stream D is currently active, whereas ice stream C is currently inactive. The grounded portion of the West Antarctic ice sheet is perhaps best conceptualized as an ice sheet in which ice streams are embedded over most of its area, with slow-moving ice converging into fast-moving ice streams in a widely distributed pattern, much like that of streams and rivers in a hydrologic basin. A relic margin appears to parallel most of the south margin of the tributary of ice stream D, separated from the active shear margin by about 10 km or less for a distance of over 200 km. This means there is now evidence for recent changes having occurred in three of the five major ice streams which drain most of West Antarctica (B, C, and D), two of which (B and D) are currently active.
Aeromagnetic map of the eastern Ross Transect Zone, West Antarctica, Folio A
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Active volcanism beneath the West Antarctic ice sheet and implications for ice-sheet stability
IT is widely understood that the collapse of the West Antarctic ice sheet (WAIS) would cause a global sea level rise of 6 m, yet there continues to be considerable debate about the detailed response of this ice sheet to climate change1-3. Because its bed is grounded well below sea level, the stability of the WAIS may depend on geologically controlled conditions at the base which are independent of climate. In particular, heat supplied to the base of the ice sheet could increase basal melting and thereby trigger ice streaming, by providing the water for a lubricating basal layer of till on which ice streams are thought to slide4,5. Ice streams act to protect the reservoir of slowly moving inland ice from exposure to oceanic degradation, thus enhancing ice-sheet stability. Here we present aerogeophysical evidence for active volcanism and associated elevated heat flow beneath the WAIS near the critical region where ice streaming begins. If this heat flow is indeed controlling ice-stream formation, then penetration of ocean waters inland of the thin hot crust of the active portion of the West Antarctic rift system could lead to the disappearance of ice streams, and possibly trigger a collapse of the inland ice reservoir.
The Nisqually Glacier, Mount Rainier, Washington, 1857-1979: A summary of the long-term observations and a comprehensive bibliography
Nisqually Glacier on Mount Ranier, Washington has a long record of terminus position observations and ice-surface altitude measurements along specific profiles, and has been the topic of numerous scientific studies. From the earliest observations in 1857 to the present many individuals and several different organizations have been involved in data collection at Nisqually Glacier. In order to preserve the long-term data, it was assembled and reduced to a standard format for this report. A comprehensive bibliography of scientific publications relating to the glacier is included. Between 1857 and 1979, Nisqually Glacier receded a total of 1,945 meters and advanced a total of 294 meters. Advances occurred from 1963-68 and from 1974-79. Ice-surface altitude changes of as much as 25 meters occurred between 1944 and 1955.
Columbia Glacier progress report; December 1977
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