Joe MacGregor on Mapping the Geology of Greenland Below the Ice Sheet
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A thick ice sheet covers 80 percent of Greenland. Until recently, our best geological maps of the island were based on the exposed periphery, with the subglacial geology inferred by extrapolating from the edges and using educated guesswork. In April 2024, Joe MacGregor and his colleagues assembled new data about the rocks below Greenland’s ice from seismic, gravity, magnetic, and topographic surveys. The result is a new geological map that, for the first time, features several geological provinces that bear no relation to those exposed on Greenland’s ice-free periphery. His team also uncovered networks of long and straight subglacial valleys.
MacGregor is a Research Physical Scientist at NASA’s Goddard Space Flight Center.
Podcast Illustrations
Prior Geological Map of Greenland (2009)
The best geological map of Greenland prior to the new map discussed in the podcast. The map relied on extrapolating the geology of the ice-free exposures of rock around Greenland’s periphery and correlating rocks from one coast to another, as well as information from drill sites, nunataks (mountains and ridges that protrude from the ice cap), glacial erratics, provenance studies of detritus in sedimentary rocks, and, to a very limited extent, geophysical data.
Dawes, P.R. (2009), Geological Survey of Denmark and Greenland Bulletin 17, 57 https://doi.org/10.34194/geusb.v17.5014
Geophysical Surveys
Much of airborne geophysical survey work used to prepare the new geological map of Greenland was performed using the Lockheed P-3 Orion aircraft. A special feature is the tail boom housing a magnetometer designed to capture magnetic anomalies in the Earth’s magnetic field caused by submarines.
Jeremy Harbeck, NASA
Annotated diagram of the P-3 Orion, showing the location of the instruments used to capture the geophysical data discussed in the podcast.
NASA
Arctic survey flights conducted as part of NASA’s Operation IceBridge. Since 1993, NASA has flown over a million kilometers over the Arctic to conduct geophysical surveys.
NASA
The New Geological Map of Greenland
MacGregor, J.A. et al. (2024), Geophysical Research Letters 51, e2023GL107357.
In the podcast, MacGregor describes a new method of visualizing the surface of the ice sheet in which the simulated illumination of a digital elevation model varies according to the direction of surface ice flow. (A) Map of Greenland’s surface shaded using flow-aware hillshade, in which the artificial illumination direction (and corresponding shadows) at each pixel is 90 degrees counterclockwise to the ice-flow direction. The resulting shading emphasizes small-scale variability in the ice-sheet surface slope, which is primarily related to variability in the topography under the ice. The change in shading from the middle of the ice sheet to the periphery reflects the smaller slopes in the interior and the smaller surface bumps induced by subglacial topography, because as the ice thickness decreases, the surface becomes more sensitive to what is going on below. The map is based on the laser altimetry data. (B) The green lines are manual tracings of linear features in the map. GrIMP: Greenland Ice Mapping Project.
Results from each of the individual geophysical data types surveyed with hand-drawn boundaries. (A, B) Depth to the Moho discontinuity and shear-wave speed anomaly at 10 km depth respectively from seismic tomography. As mentioned in the podcast, the shallower depths and lower shear-wave velocities are pronounced near the central east coast. (C) Gravity anomaly. (D) Earth Magnetic Anomaly Grid. (E) Topography assuming ice‐free conditions and rebounded to adjust for isostatic sinking of surface under the ice-sheet weight. The map is based on the radar sounding measurements discussed in the podcast. (F) Ice surface with flow-aware hillshade with geological provinces on the ice-free surface. (G) Superposition of all the traced boundaries from each of the geophysical surveys.
Credits for individual surveys appear in MacGregor, J. A. et al. (2024)
(A) This is the new geological map. It is based on the set of geophysical surveys whose results are summarized in the maps in the previous figure. The geological provinces are colored by age. Three newly identified provinces (unshaded Regions A, B, and C) do not correspond to known provinces of the exposed periphery of Greenland. (B) Map showing the major ice flows and newly discovered network of long straight valleys (purple lines labeled as surface linations in the legend) discussed in the podcast. The dotted line shows the hypothesized track of the hotspot currently under Iceland (see next figure). (C, D) Zoom-ins on the Petermann Glacier and the Northeast Greenland Ice Stream (NEGIS). These originate close to the boundaries of the newly identified geological provinces. See below for images of these glaciers.
Suggested track of the Iceland hot spot based on variations in the Earth’s magnetic field. This serves as an indicator of the amount of heat being supplied to base of the Greenland Ice Sheet from the Earth’s interior. A band of warmer-than-expected rock stretching from northwest to southeast Greenland, together with models of the lithosphere derived from gravity data, is interpreted to be the scar left as the Greenland tectonic plate moved over the hot spot.
Martos, M. M. et al. (2018) Geophysical Research Letters 45, 8214
An iceberg near the calving front of Zachariæ Isstrøm. As mentioned in the podcast, this glacier flows into the sea at the northeast end of the the Northeast Greenland Ice Stream that starts at a point near the boundary of one of the newly discovered inland geological provinces.
NASA, Jeremy Harbeck
One of the largest in Greenland, the Petermann glacier flows into the sea at the northwest end of an expanding zone of fast-flowing ice.
Copernicus Sentinel-2 satellite, 2022