Pink granite boulders scattered across the rugged volcanic peaks of Antarctica's Hudson Mountains have long intrigued geologists. These distinctive erratics, standing out against the dark basalt, hinted at geological secrets buried deep beneath the ice. A recent UK-led study has finally unraveled their origin, revealing a colossal granite formation lurking under the Pine Island Glacier—a discovery that reshapes our understanding of Antarctic geology and ice dynamics.
The research, published in Communications Earth & Environment, a Nature portfolio journal, integrates petrology, geochronology, and aerogeophysics to map subglacial bedrock. Lead author Dr. Tom Jordan from the British Antarctic Survey (BAS) describes it as a "hidden giant," spanning nearly 100 kilometers wide and 7 kilometers thick—roughly half the size of Wales. This Middle Jurassic granite, dated to about 175 million years ago, was exposed through meticulous analysis of surface rocks transported by ancient ice flows.Read the full study here.
The Enigma of the Pink Erratics
Glacial erratics are rocks transported by ice and deposited far from their source, serving as geological breadcrumbs. In the Hudson Mountains, these pink granitoids—alkali granites and syenites rich in feldspar—were spotted high on nunataks, isolated peaks piercing the ice sheet. Their presence puzzled scientists for decades: how did these light-colored boulders end up amid dark volcanic terrain?
Fieldwork during the International Thwaites Glacier Collaboration collected samples from sites like Sif Island. Petrographic analysis revealed coarse-grained textures with pink potassium feldspar, distinguishing them from local volcanics dated 3-8 million years old. Striations on nearby bedrock indicated northward ice flow during the Last Glacial Maximum (LGM), around 20,000 years ago, when the ice sheet was thicker and more erosive.
This transport mechanism—plucking from the bed and deposition—links surface clues to subglacial realms inaccessible by drilling.
Advanced Techniques Unlock Subglacial Secrets
Unraveling the mystery required multidisciplinary tools. Researchers employed U-Pb zircon geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Zircons from erratics yielded crystallization ages clustering at 179-171 million years, confirming Middle Jurassic origins. Additional apatite fission-track (AFT) and (U-Th-Sm)/He (AHe) thermochronology traced cooling histories: rapid mid-Cretaceous exhumation (130-90 Ma AFT, 73-95 Ma AHe), followed by slow erosion.
Aerogeophysical surveys were pivotal. Data from BAS Twin Otter flights, NASA's Operation IceBridge, and AGASEA compiled gravity and magnetic anomalies. Negative Airy isostatic gravity anomalies (~20 mGal) signaled low-density granite beneath higher-density volcanics. Forward modeling with GMGPY software estimated a 7-8 km thick granitic body overlain by 1-2 km volcanics.
- Gravity data: Bouguer-corrected to isolate bedrock signals.
- Magnetic anomalies: Positive highs from mafic intrusives in Hudson Mountains.
- Integration: Erratic ages matched geophysical models, pinpointing the source.
Dr. Joanne Johnson, BAS geologist, notes rocks as a "treasure-trove" for hidden landscapes.BAS press release details.
Dimensions and Geological Significance
The inferred granite pluton extends under Pine Island Glacier (PIG), a fast-flowing outlet contributing significantly to sea level rise. Its vast scale—100 km laterally, 7 km vertically—expands known Jurassic magmatism in West Antarctica, linking to Gondwana breakup.
Two erratic provinces emerge: southern Hudson Mountains dominated by pink ~175 Ma granites; northern by mid-Cretaceous intrusives. Xenoliths in volcanics confirm pre-volcanic erosion. This bedrock heterogeneity influences basal conditions, from sliding to hydrology.
Revealing Ancient Ice Dynamics
Past ice flow diverged from today. LGM reconstructions from striations and surface slope show northward streaming over Hudson Mountains, sourcing erratics from PIG flanks. Modern flow is east-west, channeled post-7-8 ka deglaciation.
These pathways refine ice-sheet models like PISM, incorporating erodibility and topography. Thicker LGM ice plucked deep bedrock, depositing large clasts (>1 m), absent in thinner Holocene flows.
Implications for Climate Modeling and Sea Level Rise
Pine Island Glacier loses ice rapidly—up to 100 Gt/year recently—due to ocean-driven melting and basal sliding. Granite's smoother, less deformable surface versus sediments affects friction and meltwater routing, key for projections.
Enhanced models predict West Antarctic Ice Sheet (WAIS) vulnerability. Accurate paleo-reconstructions reduce uncertainties in IPCC forecasts, where WAIS contributes meters to potential rise. This study bolsters basal geology parameterization.
UK Academic Contributions to Polar Science
While BAS led, universities shone in geochronology. Ethan Conrad and Andrew Carter from University College London's (UCL) London Geochronology Centre provided precise dating. Carter's dual affiliation with Birkbeck, University of London, underscores interdisciplinary expertise.
UCL's facilities enabled high-resolution LA-ICP-MS, vital for zircon analysis. Such collaborations exemplify UK higher education's polar research prowess, funded by NERC. BAS scientists like Jordan and Johnson, often PhD-trained at universities, bridge fieldwork and academia.
Broader Context in Antarctic Research
This builds on ITGC efforts mapping Thwaites and PIG, vulnerable sectors. Complementary studies reveal subglacial lakes, rivers, and topography via satellite altimetry (e.g., 85 new lakes detected 2025). Granite discovery complements bedrock maps enhancing ice stability insights.
Historical context: Jurassic granites tie to Ferrar Large Igneous Province, but this alkali suite suggests distinct tectonics.
Future Directions and Technological Horizons
Ongoing aerogeophysics and machine learning on erratics promise bedmaps continent-wide. Drilling (e.g., Beyond EPICA) may sample granite directly. Radar and seismics probe hydrology over this pluton.
Challenges: logistical extremes, data integration. Opportunities: AI for anomaly detection, expanding erratic databases.
Careers in Glacial Geology and Geophysics
This study highlights demand for experts in thermochronology, aerogeophysics. UK universities offer MSc/PhDs in Earth Sciences, with BAS placements. Roles span postdocs analyzing datasets to field geologists on Twin Otters.
Skills: GIS, MATLAB, geochron labs. Impacts global challenges like sea level rise, attracting funding.
