Melt Channels Research Articles

Compositional variability in mafic arc magmas over short spatial and temporal scales: Evidence for the signature of mantle reactive melt channels

Understanding arc magma genesis is critical to deciphering the construction of continental crust, understanding the relationship between plutonic and volcanic rocks, and for assessing volcanic hazards. Arc magma genesis is complex. Interpreting the underlying causes of major and trace element diversity in erupted magmas is challenging and often non-unique. To navigate this complexity mafic magma diversity is investigated using sample suites that span short temporal and spatial scales. These constraints allow us to evaluate models of arc magma genesis and their geochemical implications based on physical arguments and recent model results. Young volcanic deposits (≲18 kyr) are analysed from the Southern Volcanic Zone (SVZ), Chile, in particular suites of scoria cones on the flanks of arc stratovolcanoes that have erupted relatively primitive magmas of diverse compositions. Our study is centred on the high-resolution post-glacial tephrochronological record for Mocho-Choshuenco volcano where tight age constraints and a high density of scoria cones provide a spatially well-resolved mafic magma dataset. Two compositional trends emerge from the data. Firstly, magmas from cones on the flanks of the main edifice become more mafic with distance from the central vent. This is attributed to fractional crystallisation processes within the crust, with distal cones sampling less differentiated magmas. Secondly, there is a set of cones with distinct major and trace element compositions that are more primitive but enriched in incompatible elements relative to the central system and other ‘normal SVZ’ magmas. This distinct signature – termed the ‘Kangechi’ signature – is observed at three further clusters of cones within the SVZ. This is attributed to greater preservation of the enriched melt signature arising from reactive melt transport within the mantle wedge. Our model has important implications for arc magma genesis in general, and in particular for the spatial and temporal scales over which compositional variations are preserved in erupted magmas.

Modeling Antarctic subglacial lake filling and drainage cycles

Abstract. The growth and drainage of active subglacial lakes in Antarctica has previously been inferred from analysis of ice surface altimetry data. We use a subglacial hydrology model applied to a synthetic Antarctic ice stream to examine internal controls on the filling and drainage of subglacial lakes. Our model outputs suggest that the highly constricted subglacial environment of our idealized ice stream, combined with relatively high rates of water flow funneled from a large catchment, can combine to create a system exhibiting slow-moving pressure waves. Over a period of years, the accumulation of water in the ice stream onset region results in a buildup of pressure creating temporary channels, which then evacuate the excess water. This increased flux of water beneath the ice stream drives lake growth. As the water body builds up, it steepens the hydraulic gradient out of the overdeepened lake basin and allows greater flux. Eventually this flux is large enough to melt channels that cause the lake to drain. Lake drainage also depends on the internal hydrological development in the wider system and therefore does not directly correspond to a particular water volume or depth. This creates a highly temporally and spatially variable system, which is of interest for assessing the importance of subglacial lakes in ice stream hydrology and dynamics.

Evidence for magma entrapment below oceanic crust from deep seismic reflections in the Western Somali Basin

Our understanding of melt generation, migration, and extraction in the Earth’s mantle beneath mid-oceanic ridges is mostly derived from geodynamic numerical models constrained by geological and geophysical observations at sea and field investigations of ophiolites, and is therefore restricted to the oceanic crust and the shallow part of the mantle. Here we use a >200-km-long, deep seismic reflection section to image with high resolution the sub-oceanic lithosphere within the Western Somali Basin (offshore eastern Africa) where spreading ceased at ca. 120 Ma. The location of the failed spreading axis is inferred from both seismic data and gravity data. Several groups of strong reflections are imaged to depths of >30 km below the top of the oceanic crust. We interpret the deepest reflectors, within the mantle, as resulting from frozen melt bodies which may be relicts of a paleo–melt channel system located at the base of the lithosphere and formerly feeding the failed ridge axis. Other reflectors within the mantle may correspond to melt bodies injected into major shear zones along the Davie fracture zone. Another group of reflectors, located below a 8–5-km-thick oceanic crust, is interpreted as marking a fossil melt-rich crust-mantle transition zone as much as 3 km thick. This interpretation implies an inefficient extraction of melt out of the mantle, which is favored by the combination of a slow spreading rate and a high magma budget.

Diamonds and Other Exotic Minerals Recovered from Peridotites of the Dangqiong Ophiolite, Western Yarlung‐Zangbo Suture Zone, Tibet

AbstractVarious combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish‐orange to colorless. The grains are all 100–200 µm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm−1 and 1333 cm−1, mostly at 1331.51 cm−1 or 1326.96 cm−1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi‐stage formation for this body and similar ophiolites in the Yarlung‐Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones (SSZ), where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.

Genesis and tectonic setting of ophiolitic chromitites from the Dehsheikh ultramafic complex (Kerman, southeastern Iran): Inferences from platinum-group elements and chromite compositions

Chromitite bodies of various sizes associated with dunite envelopes have been found in the Dehsheikh ultramafic massif, in the southeastern part of the outer Zagros ophiolite belt. The chromitites occur as layered and lenticular bodies, and show both magmatic and deformational textures, including massive, disseminated, banded and nodular types. The Dehsheikh chromitites display a variation in Cr# [100×Cr/(Cr+Al)] from 69 to 78, which is typical of high-Cr chromitites. The Al2O3 and TiO2 contents of chromites range from 10.3wt.% to 16.9wt.% and 0.12wt.% to 0.35wt.%, respectively. The Al2O3, TiO2, and FeO/MgO values calculated for parental melts of Dehsheikh chromitites are within the range of boninitic melts. Chondrite-normalized distribution patterns of platinum-group elements show relative enrichments in Ru, Ir, and Os, and depletions in Rh, Pd, and Pt that are typical of chromitites associated with ophiolites formed by high degrees of mantle partial melting. The presence of Na-rich amphibole inclusions in chromite grains, together with the mineralogical and chemical composition of the chromitites and estimates of their parental melt compositions are used to help establish the tectono-magmatic setting. It is shown that the Dehsheikh massif is an ophiolite formed in a suprasubduction zone setting. We suggest that the composition of the rocks in this section was influenced by hydrous partial melts which might be formed in the subduction zone. Variable melt/rock interaction produced melt channel networks in the dunite which allowed the parental melt of the chromitite to percolate through them. Similar characteristics have been observed in other ophiolite complexes from the outer Zagros Iranian ophiolitic belt; these are believed to be the product of magmatism in a fore-arc environment.

Modeling 3D Magma Dynamics Using a Discontinuous Galerkin Method

AbstractDiscontinuous Galerkin (DG) and matrix-free finite element methods with a novel projective pressure estimation are combined to enable the numerical modeling of magma dynamics in 2D and 3D using the library deal.II. The physical model is an advection-reaction type system consisting of two hyperbolic equations to evolve porosity and soluble mineral abundance at local chemical equilibrium and one elliptic equation to recover global pressure. A combination of a discontinuous Galerkin method for the advection equations and a finite element method for the elliptic equation provide a robust and efficient solution to the channel regime problems of the physical system in 3D. A projective and adaptively applied pressure estimation is employed to significantly reduce the computational wall time without impacting the overall physical reliability in the modeling of important features of melt segregation, such as melt channel bifurcation in 2D and 3D time dependent simulations.

Dual-scale phase-field simulation of Mg-Al alloy solidification

Phase-field simulations of the nucleation and growth of primary α-Mg phase as well as secondary, β-phase of a Mg-Al alloy are presented. The nucleation model for α- and β-Mg phases is based on the “free growth model” by Greer et al.. After the α-Mg phase solidification we study a divorced eutectic growth of α- and β-Mg phases in a zoomed in melt channel between α-phase dendrites. The simulated cooling curves and final microstructures of α-grains are compared with experiments. In order to further enhance the resolution of the interdendritic region a high-performance computing approach has been used allowing significant simulation speed gain when using supercomputing facilities.

Mantle plume capture, anchoring, and outflow during Galápagos plume‐ridge interaction

AbstractCompositions of basalts erupted between the main zone of Galápagos plume upwelling and adjacent Galápagos Spreading Center (GSC) provide important constraints on dynamic processes involved in transfer of deep‐mantle‐sourced material to mid‐ocean ridges. We examine recent basalts from central and northeast Galápagos including some that have less radiogenic Sr, Nd, and Pb isotopic compositions than plume‐influenced basalts (E‐MORB) from the nearby ridge. We show that the location of E‐MORB, greatest crustal thickness, and elevated topography on the GSC correlates with a confined zone of low‐velocity, high‐temperature mantle connecting the plume stem and ridge at depths of ∼100 km. At this site on the ridge, plume‐driven upwelling involving deep melting of partially dehydrated, recycled ancient oceanic crust, plus plate‐limited shallow melting of anhydrous peridotite, generate E‐MORB and larger amounts of melt than elsewhere on the GSC. The first‐order control on plume stem to ridge flow is rheological rather than gravitational, and strongly influenced by flow regimes initiated when the plume was on axis (>5 Ma). During subsequent northeast ridge migration material upwelling in the plume stem appears to have remained “anchored” to a contact point on the GSC. This deep, confined NE plume stem‐to‐ridge flow occurs via a network of melt channels, embedded within the normal spreading and advection of plume material beneath the Nazca plate, and coincides with locations of historic volcanism. Our observations require a more dynamically complex model than proposed by most studies, which rely on radial solid‐state outflow of heterogeneous plume material to the ridge.

Divorced Eutectic Solidification of Mg-Al Alloys

We present simulations of the nucleation and equiaxed dendritic growth of the primary hexagonal close-packed $$\mathrm{\alpha} $$ -Mg phase followed by the nucleation of the $$\mathrm{\beta} $$ -phase in interdendritic regions. A zoomed-in region of a melt channel under eutectic conditions is investigated and compared with experiments. The presented simulations allow prediction of the final properties of an alloy based on process parameters. The obtained results give insight into the solidification processes governing the microstructure formation of Mg-Al alloys, allowing their targeted design for different applications.

Partial melting of deeply subducted eclogite from the Sulu orogen in China.

We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U–Pb dating and petrological analyses, that partial melting occurred at 228–219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust.

The origin and significance of crustal minerals in ophiolitic chromitites and peridotites

Various combinations of zircon, quartz, corundum, K-feldspar, plagioclase, apatite, amphibole, rutile, titanite, almandine garnet, kyanite, andalusite, and coesite have been recovered from podiform chromitites of the Luobusa and Dongqiao ophiolites in Tibet, the Oman ophiolite and the Ray–Iz ophiolite in the Polar Urals, Russia. Chromitites in all four ophiolites also contain moissanite and the Luobusa and Ray–Iz ophiolites contain in-situ diamonds. Most of the recovered zircons are sub-rounded grains with complex internal structures indicating polyphase growth. Trace element contents and a low-pressure inclusion assemblage (quartz, muscovite, K-feldspar, apatite, ilmenite, rutile) indicate a continental crustal origin for the zircons. They have SIMS U–Pb ages that are generally much older than the host body (total range: 90 to 2500Ma). The presence of numerous crustal minerals, particularly zircon, suggests derivation from metasedimentary rocks subducted into the mantle. Their preservation in chromitites and peridotites implies effective isolation from the mafic melts that formed the ophiolites. We suggest that most of these minerals were derived from the crustal parts of subducted slabs and were encapsulated into chromite grains precipitated from rising asthenospheric and suprasubduction magmas. The chromite grains were carried in melt channels to shallow levels in suprasubduction mantle wedges and then deposited as podiform chromitites near the Moho. The rise of asthenospheric peridotites into suprasubduction zones was facilitated by subduction initiation and possibly by slab tear allowing mixing of the UHP, highly reduced minerals and crustal minerals now found in ophiolitic chromitites and peridotites.

Origin of ultrahigh pressure and highly reduced minerals in podiform chromitites and associated mantle peridotites of the Luobusa ophiolite, Tibet

Podiform chromitites and their host peridotites in the Kangjinla mining district of the Luobusa ophiolite contain similar collections of ultrahigh pressure (UHP), highly reduced and crustal-type minerals. Abundant diamonds have been recovered from both lithologies and these are associated with a wide range of base metal alloys, native elements, carbides, oxides, silicates and others. The presence of UHP and highly reduced minerals in these rocks indicates that at least some of the chromite must have crystallized deep within the mantle as well as in a shallow mantle wedge in a suprasubduction zone (SSZ) environment. The unusual minerals were encapsulated in chromite grains and carried upward by mantle convection. The peridotite of Luobusa was trapped in the mantle wedge where it was modified by SSZ fluids and melts. Partial melting and mobilization of the chromite grains allowed them to be carried to shallow levels in melt channels and eventually deposited as chromitites near the crust mantle boundary. The unusual minerals were preserved during this process because they were encapsulated in chromite grains, either during crystallization or by later fluid fluxing.

Origin of podiform chromitite, a new model based on the Luobusa ophiolite, Tibet

Podiform chromitites have been interpreted as the result of melt–rock reaction and related melt mixing in upper mantle sections of ophiolites. However, the discovery of ultrahigh-pressure (UHP) minerals, especially diamond and coesite, in many podiform chromitites and host peridotites, raises fundamental questions about the validity of this model. Chromitites in the Luobusa ophiolite of Tibet range from massive, to nodular to disseminated. Chromite grains in both the chromitites and peridotites have variable but relatively high MgO and are classified as magnesiochromite. Many magnesiochromite grains in the massive chromitites contain inclusions of forsterite and pyroxene, as well as diamonds and other unusual minerals. Forsterite inclusions have Fo numbers of 97–99 and NiO contents of 1.11–1.29wt.%. Mg#s (=100∗Mg/(Mg+Fe)) of clinopyroxene inclusions are 96–98 and those of orthopyroxene are 96–97. X-ray studies show that the olivine inclusions have very small unit cells and short cation–oxygen bond distances, suggesting crystallization at high pressure. In contrast, magnesiochromite grains in nodular and disseminated chromitites lack pyroxene inclusions and their olivine inclusions have lower Fo numbers of 94–96 and lower NiO contents of 0.35–0.58wt.%. In addition, magnesiochromite in massive ores has higher Fe3+/Fetotal (0.42) than that in nodular and disseminated ores, which have ratios of 0.22. Disseminated chromitites also show systematic changes in olivine and magnesiochromite compositions from the dunite envelope to the massive ore, indicating melt–rock reaction. These observations suggest that the formation of podiform chromitites is a multi-stage process. Magnesiochromite grains and perhaps small bodies of chromitite crystallize deep in the mantle under low ambient ƒO2 from partial melts of peridotite. UHP minerals and highly magnesian olivine and pyroxene inclusions are trapped in these magnesiochromite grains. When oceanic crustal slabs are trapped in suprasubduction zones (SSZ), they are modified by island arc tholeiitic and boninitic magmas, which change the magnesiochromite compositions and deposit chromitite ores in melt channels.

Ductile compaction of partially molten rocks: the effect of non-linear viscous rheology on instability and segregation

Abstract The segregation of melt from a linear viscous matrix is traditionally described by McKenzie's compaction theory. This classical solution overlooks instabilities that arise when non-linear solid matrix behaviour is considered. Here we report a closed form 1-D solution obtained by extending McKenzie's theory to non-linear matrix behaviours. The new solution provides periodic stress singularities, acting as high porosity melt channels, to be the fundamental response of the compacted matrix. The characteristic length controlling the periodicity is still McKenzie's compaction length $\bar{\delta }_{\rm c}$, adjusted for non-linear rheologies.

Supraglacial melt channel networks in the Jakobshavn Isbræ region during the 2007 melt season

Supraglacial channels are an important mechanism for surface water transport over the ablation zone of western Greenland. The first assessment of the spatio-temporal distribution of surface melt channels and their relationship to supraglacial lakes over the Jakobshavn Isbræ region of Western Greenland was analysed using Landsat Enhanced Thematic Mapper Plus panchromatic images during the 2007 melt season. A total of 1188 melt channels were delineated and show an increase in the number of melt channels throughout the season, reaching a peak on 9 August. Water-filled melt channels advanced to a maximum elevation of 1647 m on 9 August and attained a minimum average slope of 0.009 on 8 July. The ablation zone demonstrates two hydrologic modes, where crevasse and moulin terminating channels dominate at elevations <800 m and higher-order channel networks >800 m. Development of higher-order networks is interrupted by flow divergence due to partitioning of melt water into vertical infiltration through moulins and crevasse fields prevalent at lower elevations. Tributary and connector networks between 800 and 1200 m in elevation are correlated with fewer lake occurrences, lower surface velocities (~50 m a−1), and ice flow dominated by internal deformation over basal sliding. High-order channels are associated with lake basins that exceed melt water storage capacity. Evolution of channel networks is coupled to changes in melt water production, runoff, and ice dynamics with implication for the englacial and subglacial environments. © 2013 The Authors. Hydrological Processes Published by John Wiley & Sons, Ltd.

Relationship between syndeformational partial melting and crustal-scale magmatism and tectonism across the Wet Mountains, central Colorado

In the Wet Mountains of central Colorado, we document evidence for increasing metamorphic grade and associated higher amounts of partial melting along a transect from northwest to southeast. Field observations of structural orientation and style, qualitative assessment of strain intensity, analysis of metamorphic mineral assemblages, and macroscopic identification of leucosomes and migmatites are complemented by the use of melt microstructures to carefully document the presence and locations of former partial melt and to identify melt-producing reactions. In the northwest Wet Mountains, migmatitic foliation is moderately well developed, and partial melting occurred via granite wet melting and muscovite-dehydration melting, with rare melt pseudomorphs remaining. At Dawson Mountain in the central part of the range, inferred former melt channels are preserved along grain and subgrain boundaries, deformation appears more intense, and anatexis occurred through biotite-dehydration melting. Farthest to the south, the highest intensity of strain is inferred, with very closely spaced foliations, abundant dynamic recrystallization, and local mylonitization occurring in rocks of granitic composition, and partial melting occurring via granite wet melting. Metapelitic rocks experienced biotite-dehydration melting and contain garnet with Mn-rich rims and Mn-poor cores mantled by plagioclase, decussate biotite, and quartz, textures indicating back-reaction between melt and garnet. These textures indicate there was abundant melt within these highest-grade rocks and extensive melt-rock interaction. Throughout the Wet Mountains, deformation is concentrated in areas where melt-producing reactions occurred, and melt appears to be localized along deformation-related features, suggesting a correlation between partial melting and deformation. The northern Wet Mountains contain few plutons, whereas the central and southern portions of the Wet Mountains contain more pervasive dikes and sills and may contain more former melt as a result of both higher metamorphic grade and widespread thermal insulation. The Wet Mountains represent an exhumed section of formerly molten middle crust located at the transition between upper and lower crust and provide insight into processes ongoing at depth in modern orogenic belts. The microstructures indicative of former partial melt, textures associated with melt-rock interaction, and melting reactions we have identified in the Wet Mountains will greatly facilitate the recognition of other such exhumed sections.

Basal Drainage System Response to Increasing Surface Melt on the Greenland Ice Sheet

Surface meltwater reaching the bed of the Greenland ice sheet imparts a fundamental control on basal motion. Sliding speed depends on ice/bed coupling, dictated by the configuration and pressure of the hydrologic drainage system. In situ observations in a four-site transect containing 23 boreholes drilled to Greenland's bed reveal basal water pressures unfavorable to water-draining conduit development extending inland beneath deep ice. This finding is supported by numerical analysis based on realistic ice sheet geometry. Slow meltback of ice walls limits conduit growth, inhibiting their capacity to transport increased discharge. Key aspects of current conceptual models for Greenland basal hydrology, derived primarily from the study of mountain glaciers, appear to be limited to a portion of the ablation zone near the ice sheet margin.

Basal channels on ice shelves

Recent surveys of floating ice shelves associated with Pine Island Glacier (Antarctica) and Petermann Glacier (Greenland) indicate that there are channels incised upward into their bottoms that may serve as the conduits of meltwater outflow from the sub‐ice‐shelf cavity. The formation of the channels, their evolution over time, and their impact on ice‐shelf flow are investigated using a fully‐coupled ice‐shelf/sub‐ice‐shelf ocean model. The model simulations suggest that channels may form spontaneously in response to meltwater plume flow initiated at the grounding line if there are relatively high melt rates and if there is transverse to ice‐flow variability in ice‐shelf thickness. Typical channels formed in the simulations have a width of about 1–3 km and a vertical relief of about 100–200 m. Melt rates and sea‐water transport in the channels are significantly higher than on the smooth flat ice bottom between the channels. The melt channels develop through melting, deformation, and advection with ice‐shelf flow. Simulations suggest that both steady state and cyclic state solutions are possible depending on conditions along the lateral ice‐shelf boundaries. This peculiar dynamics of the system has strong implications on the interpretation of observations. The richness of channel morphology and evolution seen in this study suggests that further observations and theoretical analysis are imperative for understanding ice‐shelf behavior in warm oceanic conditions.

Volatiles contents, degassing and crystallisation of intermediate magmas at Volcan de Colima, Mexico, inferred from melt inclusions

In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical infor- mation on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly H2O) and evolution of intermediate arc magmas at Volcan de Coli- ma, Mexico. The melt inclusions have maximal H2O contents (B4 wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxene- hosted melt inclusions can preserve H2O contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6 m 3 s -1 ). The absence of noticeable H2O loss in some of the inclusions requires post-entrapment diffusion coefficients (B1 9 10 -13 m 2 s -1 ) at least several order of magnitude smaller than experimentally determined H ? diffusion coefficient in pyroxenes. The H2O content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive H2O reequil- ibration, are observed around 1 and 0.2 wt%. H2O diffusive loss is also consistent with the manifest lack of correlations between H2O and CO2 or S contents. The absence of tex- tural evidence supporting post-entrapment H2O loss sug- gests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion popu- lation accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that H2O diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapour- saturated magmas.

Large melt channels discovered underneath Antarctic ice shelves

New radar observations reveal melt channels 500 meters to 3 kilometers wide and up to 200 meters deep underneath the ice shelf buttressing the Pine Island Glacier in West Antarctica; researchers suggest that these subglacial channels could be a “prelude to eventual collapse” of the ice shelves, which cover about 10% of the areal extent of the West Antarctic Ice Sheet (WAIS). A complete collapse of WAIS could increase global sea level by at least 3–4 meters. Taking advantage of a dense network of radar lines designed to penetrate ice thicknesses of more than 2 kilometers, Vaughan et al. combine indirect satellite observations with direct echo‐sounding measurements from submersibles to show what scientists have suspected: Thinning ice shelves lead to crevassing of the ice shelves. Echo sounding reveals 50‐to 100‐meter‐wide crevasses aligned with the subglacial channels; the basal crevasses penetrate up to one third of the ice thickness in the shelves buttressing the Pine Island Glacier, even leaving their mark on the surface of the glacier. Melting at the base of floating ice shelves creates subglacial channels. Using stress models, the authors show that when the subglacial channels form sufficiently rapidly, they create crevasses, which the authors found on the surface and at the base of the ice shelf bordering the Pine Island Glacier.