HIAWATHA IMPACT CRATER
by: Charles O’Dale
Data references from: Science Advances 14 Nov 2018:
- Type: Complex
- Age: 2.5ma to 11.7ka (Geological dating)ab
- Diameter: 31 km
- Location: N 79° W 65°
a Formed within the same types of highly metamorphosed 2.5 million year Paleoproterozoic terrain as mapped across most of Inglefield Land. The crater includes ice from the Last Glacial Period (LGP; ~115 to 11.7 ka ago). In radar profiles in the northeast corner of the study area, outside the crater, this unit corresponds to late LGP ice exposed at the surface. To the northeast of and within the structure, this unit sits conformably below the Holocene unit, but within the structure, it does not contain any reflection-rich BøllingAllerød ice (14.7 to 12.8 ka ago), from the period immediately before the Younger Dryas, or the trio of distinct LGP reflections observed throughout the northern Greenland Ice Sheet, the youngest of which is ~38 ka old. (Kurt H. Kjær et al Nov 2018)
b The crater is exceptionally well-preserved, and that is surprising, because glacier ice is an incredibly efficient erosive agent that would have quickly removed traces of the impact. But that means the crater must be rather young from a geological perspective. (Natural History Museum of Denmark)
A large impact crater beneath Hiawatha Glacier in northwest Greenland
(Kurt H. Kjær et al Nov 2018)
We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.
Image courtesy of: BRIAN T. JACOBS, National Geographic STAFF
This structure is covered by up to 930 m of ice but has a clear circular surface expression. An elevated rim in the bed topography encloses the relatively flat depression with a diameter of 31.1 ±0.3 km and a rim-to-floor depth of 320 ± 70 m. In the center of the structure, the bed is raised up to 50 m above the surrounding topography, with five radar-identified peaks that form a central uplift up to 8 km wide. The overall structure has a depth-to-diameter ratio of 0.010 ± 0.002 and is slightly asymmetric, with a gentler slope toward the southwest and maximum depth in the southeast of the structure. Two winding subglacial channels, up to ~500 m deep and ~5 km wide, intersect the southeast flank of the circular structure. Before entering the structure, the northern channel merges with the southern channel and then spills over a large breach in the structure’s rim upon entering the main depression. These channels do not have a recognizable topographic expression within the structure. On the downstream side of the structure, there is a second smaller breach in the northwestern portion of the structure’s rim. Ice flows through this second breach to form the tongue-shaped terminus of Hiawatha Glacier. The present ice-sheet margin lies ~1 km past this northwestern rim, and it is the circular depression itself that contains the semicircular ice lobe that extends conspicuously beyond the straighter ice-sheet margin farther southwest. (Kurt H. Kjær et al Nov 2018)
In this sample (glaciofluvial sedimen collected from the active floodplain), we found angular quartz grains displaying shock-diagnostic planar deformation features (PDFs). These PDFs are straight, generally penetrative, and spaced down to less than 2 mm. Only a few are decorated by small fluid inclusions, whereas toasting occurs in some grains, i.e., a brown coloration due to intense post-shock hydrothermal alteration of the shock lamellae. The orientations of 37 PDF sets in 10 quartz grains were measured with a five-axis Leitz universal stage. Up to seven different orientations per grain were observed. This distribution is similar to the distribution observed in the central uplifts of large Canadian impact structures, where a threshold shock pressure of >16 GPa was inferred from the presence of PDFs. (Kurt H. Kjær et al Nov 2018)
If this crater could be dated to 12,800 years old, it could certainly be credited as the Younger Dryas instigator, and would end this decades-long debate.
What’s more, because of the crater’s location on Greenland’s ice sheet, it’s possible that the impact could’ve caused exactly the kind of massive influx of freshwater to the North Atlantic that the Younger Dryas-flood proponents stand behind.
Hitting an ice sheet with a meteorite could cause a number of water-related effects. According to Allen West, retired geophysicist, the impact could vaporize ice, releasing water molecules into the air that would eventually rain back down; it could destabilize the ice such that it slides into the water; it could create icebergs. Any of these, or a combination of them, could have led to a flood of freshwater into the North Atlantic. (Discover 2018)
Kurt H. Kjær, Nicolaj K. Larsen, Tobias Binder, Anders A. Bjørk, Olaf Eisen, Mark A. Fahnestock, Svend Funder, Adam A. Garde, Henning Haack, Veit Helm, Michael Houmark-Nielsen, Kristian K. Kjeldsen, Shfaqat A. Khan, Horst Machguth, Iain McDonald, Mathieu Morlighem, Jérémie Mouginot, John D. Paden, Tod E. Waight, Christian Weikusat, Eske Willerslev, Joseph A. MacGregor. A large impact crater beneath Hiawatha Glacier in northwest Greenland. Science Advances, 2018; 4 (11): eaar8173 DOI: 10.1126/sciadv.aar8173
Following the discovery of the Hiawatha impact crater beneath the northwest margin of the Greenland Ice Sheet, we explored satellite and aerogeophysical data in search of additional such craters. Here we report the discovery of a possible second subglacial impact crater that is 36.5‐km wide and 183 km southeast of the Hiawatha impact crater. Although buried by 2 km of ice, the structure’s rim induces a conspicuously circular surface expression, it possesses a central uplift, and it causes a negative gravity anomaly. The existence of two closely spaced and similarly sized complex craters raises the possibility that they formed during related impact events. However, the second structure’s morphology is shallower, its overlying ice is conformal and older, and such an event can be explained by chance. We conclude that the identified structure is very likely an impact crater, but it is unlikely to be a twin of the Hiawatha impact crater.