Ice shelf
An ice shelf is a thick, floating slab of ice formed by the seaward extension of a glacier or ice sheet from coastal land into the ocean, typically reaching thicknesses of several hundred meters.[1][2] These features are predominantly located around Antarctica, where they fringe much of the continent's coastline and cover a total area exceeding 1.5 million square kilometers, with the Arctic hosting smaller examples near Greenland.[3][4] Ice shelves serve as dynamic interfaces between land-based ice masses and the ocean, acting as buttresses that resist the outward flow of upstream glaciers and thereby stabilize the Antarctic Ice Sheet's contribution to global sea level.[4][5] The Ross Ice Shelf, the largest by area at approximately 487,000 square kilometers—roughly the size of France—exemplifies their scale and persistence, extending hundreds of kilometers into the Ross Sea.[6][4] While basal melting from ocean currents and iceberg calving represent primary thinning mechanisms, empirical measurements indicate variability in mass balance, with recent surface accumulation from snowfall often exceeding losses in some sectors, countering narratives of uniform retreat.[7][8][7]Definition and Formation
Definition
An ice shelf is a thick, floating extension of a continental ice sheet or glacier that protrudes over the ocean while remaining attached to the grounded ice or coastline.[9] These formations occur primarily in polar regions where sufficiently cold ocean temperatures prevent rapid basal melting, allowing the ice to extend seaward for distances up to hundreds of kilometers.[10] Ice shelves are distinguished from sea ice by their origin as consolidated snow and glacial ice from land, rather than frozen seawater, resulting in lower salinity and denser structure.[11] Typical ice shelves range in thickness from approximately 50 to 600 meters, with surface areas varying widely depending on the feeding glacier or ice sheet.[9] They are nourished primarily by the inflow of ice from upstream terrestrial sources, supplemented by surface snow accumulation, and experience losses through calving of icebergs at their margins and submarine melting at their bases.[4] In Antarctica, ice shelves collectively cover over 1.5 million square kilometers, representing about 11% of the continent's total ice extent.[12] While smaller or absent in the Arctic due to warmer ocean conditions and different topography, analogous features exist elsewhere, such as in Greenland's fjords.[3]
Formation Mechanisms
Ice shelves primarily form through the extension of grounded glaciers or ice sheets from continental interiors into adjacent ocean basins, where the ice transitions from grounded to floating due to buoyancy arising from the density contrast between ice (approximately 917 kg/m³) and seawater (around 1025–1030 kg/m³).[4] This process occurs in regions where cold coastal waters border large ice masses, allowing the ice front to advance seaward under gravitational driving stress until equilibrium is reached with resistive forces such as lateral shear and basal drag near grounding lines.[11] The resulting structure is a thick, relatively flat slab of ice, often hundreds of meters thick, extending kilometers to hundreds of kilometers offshore.[13] Secondary formation pathways involve the progressive thickening of sea ice fastened to coastal promontories or islands, where multi-year accumulation and deformation under wind and ocean forces build sufficient draft for permanence, though such shelves are typically thinner and smaller than those fed by continental ice.[13] Local snowfall accumulation on nascent floating ice contributes to vertical growth, with compaction and metamorphosis converting firn to solid ice over timescales of decades to centuries, enhancing stability against tidal flexure and ocean currents.[3] In select environments, marine ice accretion at the ice-ocean interface—driven by supercooling of seawater and platelet ice formation—can supplement mass balance, particularly in high-latitude cavities with freshwater influx from surface melt or sea ice melt.[4] The interplay of these mechanisms is governed by ice rheology, where viscous flow accommodates extension, and by environmental controls such as air temperatures below -10°C to minimize surface ablation and ocean temperatures conducive to freezing rather than melting at the base.[11] Empirical observations, including radar profiling of internal layers, confirm that most Antarctic ice shelves exhibit isochronal stratigraphy reflecting upstream flow trajectories, underscoring the dominance of advective transport from inland sources over in-situ marine or atmospheric buildup.[14] Hybrid formations, combining glacier outflow with consolidated sea ice, occur where fast ice dams inhibit calving, allowing gradual merging and lateral expansion.[3]Physical Properties and Dynamics
Physical Characteristics
Ice shelves are expansive floating platforms composed primarily of glacier ice derived from continental ice sheets, overlain by firn (partially compacted snow) and recent snow accumulation. The ice structure features stratified layers reflecting annual accumulation cycles, with denser basal ice transitioning upward to porous firn zones; pure glacier ice has a density of approximately 917 kg m⁻³, while firn densities range from 500 to 800 kg m⁻³ depending on compaction and depth.[15][16] Crevasses and rifts form due to extensional stresses from ice flow divergence, particularly near grounding lines and calving fronts, creating brittle fractures that can extend tens to hundreds of meters deep and influence structural integrity.[17] Thickness varies markedly across an ice shelf, typically averaging 300–500 meters but reaching 800–1,200 meters near the grounding line where ice transitions from grounded to afloat, and thinning to 200–400 meters at the marine front exposed to calving.[18][19] For instance, the Ross Ice Shelf maintains an average thickness of about 350 meters across its 487,000 km² extent, with maximum values exceeding 1,000 meters inland.[19] The ice is predominantly meteoric (freshwater-derived), exhibiting very low salinity (<0.1 g kg⁻¹) compared to sea ice, which enables buoyancy despite partial submersion—approximately 90% of the thickness lies below the ocean surface due to the density contrast with seawater.[20] Temperatures within ice shelves decrease from near-surface values (often -20°C to -30°C in Antarctic winter) to the pressure-dependent melting point at the base, around -2°C to -1°C, with steeper gradients (up to -0.36°C m⁻¹) in zones of basal melting influenced by ocean heat.[21] The upper surface remains relatively flat over broad areas but undulates with underlying bedrock topography and flow-induced flexure, while the basal interface is irregular, featuring channels and roughness elements that interact with subsurface ocean currents.[22] These properties collectively determine an ice shelf's capacity to resist fracturing and support upstream ice flow, with thinner, crevassed margins more prone to instability.Internal Dynamics and Stability Factors
Ice shelves undergo internal dynamics dominated by viscous creep, where ice deforms slowly under its own weight and gravitational driving stress from the adjacent ice sheet, resulting in divergent flow patterns that promote longitudinal extension and transverse compression.[4] This creep is governed by Glen's flow law, with rates increasing nonlinearly with stress and inversely with ice viscosity, which varies with temperature and crystal fabric.[23] Fractures and rifts form where extensional stresses exceed the ice's tensile strength, often at shear margins or suture zones, altering local stress fields and facilitating further deformation.[24] Damage from crevasses and rifts reduces effective ice viscosity by introducing voids and facilitating recrystallization, which enlarges grains and enhances creep susceptibility, thereby diminishing the shelf's capacity to resist flow.[25] Ocean tides modulate these dynamics by inducing flexure that propagates rifts, with semi-diurnal cycles accelerating propagation rates by up to 10 meters per day in vulnerable shelves like those bordering the Weddell Sea.[26] Stability hinges on the buttressing effect, wherein the ice shelf exerts lateral and longitudinal backstress on tributary glaciers, restraining their discharge; reductions in buttressing from thinning or fracturing can accelerate grounding line retreat by 2-5 times in models of damaged shelves.[23] Mass balance factors include surface accumulation from snowfall, averaging 200-500 kg m⁻² yr⁻¹ across Antarctic shelves but insufficient to offset basal melt rates exceeding 1-10 m yr⁻¹ in warm-water cavities.[27][28] Basal melting, driven by Circumpolar Deep Water incursion, undercuts the shelf and promotes hydrofracture from surface melt ponds, while grounding zone processes like tidal pumping amplify erosion at the ice-ocean interface.[29][30] These internal processes interact such that localized thinning propagates instability upstream, as observed in finite element analyses showing damage-induced stress perturbations extending hundreds of kilometers inland.[31]Environmental Interactions
Ice shelves engage in dynamic exchanges with the surrounding ocean, primarily through basal melting and freezing at their undersides. Intrusions of relatively warm, modified Circumpolar Deep Water (mCDW) into sub-ice cavities drive turbulent heat fluxes, with basal melt rates ranging from negligible to exceeding 100 meters per year in hotspots like the Amundsen Sea sector.[32] [33] These processes are governed by cavity geometry, ocean stratification, and upwelling mechanisms, where meltwater plumes enhance vertical mixing and advective heat transport, potentially amplifying rates by factors of two or more under increased subglacial discharge.[34] Conversely, in regions with colder shelf waters, basal freezing can occur, contributing to ice shelf thickening and stability.[35] Such ocean-ice coupling influences broader Southern Ocean circulation by injecting freshwater, which alters density gradients and restrains dense shelf water formation.[36] [37] Atmospheric interactions predominantly affect ice shelf surfaces via mass exchange processes that determine net surface mass balance (SMB). Precipitation, mainly as snow, provides the dominant input, with East Antarctic shelves experiencing accumulation rates that have increased by up to 10-20% since the 1980s due to enhanced moisture transport from warmer atmospheres.[38] [39] Sublimation and wind-driven erosion represent losses, while episodic surface melting during austral summers—triggered by föhn winds or blocking highs—forms melt ponds that percolate through crevasses, potentially initiating hydrofracture and accelerating calving.[11] These atmospheric forcings exhibit variability tied to large-scale modes like the Southern Annular Mode, which modulates precipitation and temperature anomalies across shelves.[40] Overall, SMB remains positive for many Antarctic ice shelves, counterbalancing basal losses in mass budget assessments from 1979-2017.[39] Ice shelves also host and shape biological communities, particularly in sub-ice environments that shelter diverse marine ecosystems from surface disturbances. Benthic assemblages beneath Antarctic shelves include suspension-feeding sponges, cnidarians, and polychaetes, thriving on particulate organic matter advected by cavity currents and minimal light penetration.[41] Recent explorations, such as those under the George VI Ice Shelf, reveal high biodiversity comparable to open-shelf habitats, with productivity sustained by nutrient recycling rather than primary production.[42] These under-ice niches support foundational Southern Ocean food webs indirectly by stabilizing calving regimes that release icebergs—hotspots for enhanced primary production via iron fertilization—and by influencing sea ice formation critical for krill and higher trophic levels.[43] Perturbations from melting can disrupt these habitats, though empirical data indicate resilience in undisturbed cavities persisting for millennia.[41]Geographical Distribution
Antarctic Ice Shelves
Antarctic ice shelves form extensive floating platforms along approximately 75% of the continent's 17,968-kilometer coastline, primarily occupying deep embayments and seas where the grounded ice sheet transitions to flotation. These shelves are distributed across East Antarctica, West Antarctica, and the Antarctic Peninsula, with the greatest concentrations in the Weddell Sea, Ross Sea, and along the East Antarctic margin. Antarctica encompasses roughly 15 major ice shelves and over 100 smaller ones, collectively covering more than 1.5 million square kilometers, representing the vast majority of global ice shelf extent.[9] In West Antarctica, the Ross Ice Shelf dominates, extending into the Ross Sea between the Transantarctic Mountains and the Marie Byrd Land coast, with an area of approximately 487,000 square kilometers and thicknesses reaching up to 900 meters. The Ronne-Filchner Ice Shelf, the second largest, occupies the southern Weddell Sea, fed by outlet glaciers from the East and West Antarctic Ice Sheets, spanning about 430,000 square kilometers. Smaller shelves like the Getz and Pine Island in the Amundsen Sea sector connect to rapidly flowing glaciers, while the Thwaites Glacier margin features a fragmented shelf prone to calving.[44][45][9] East Antarctica hosts more stable and extensive shelves, including the Amery Ice Shelf in Prydz Bay, which covers around 62,000 square kilometers and calves large tabular icebergs periodically, and the Riiser-Larsen Ice Shelf along the Princess Martha Coast, noted for areal growth in recent decades. Other notable East Antarctic features include the Fimbul Ice Shelf near the Sør Rondane Mountains and the Nivlisen Ice Shelf in the Lazarev Sea. These shelves generally experience lower basal melting rates due to colder ocean waters compared to West Antarctic counterparts.[44][46] Along the Antarctic Peninsula, shelves are smaller and more vulnerable to atmospheric warming, with the Larsen Ice Shelves (A, B, C) extending into the Weddell Sea from the eastern peninsula, historically covering up to 11,000 square kilometers before partial disintegrations. The Wilkins Ice Shelf, on the western peninsula facing the Bellingshausen Sea, exemplifies thinner, more dynamic margins influenced by surface crevassing and ocean upwelling.[9]| Major Antarctic Ice Shelf | Location | Approximate Area (km²) |
|---|---|---|
| Ross | Ross Sea, West Antarctica | 487,000[44] |
| Ronne-Filchner | Weddell Sea | 430,000[45] |
| Amery | Prydz Bay, East Antarctica | 62,000[44] |
| Larsen C | Weddell Sea, Antarctic Peninsula | ~50,000 (pre-2002)[9] |