Occurrence Of Melting Research Articles

Petrogenesis of mafic collision zone magmatism: The Armenian sector of the Turkish–Iranian Plateau

The Turkish–Iranian Plateau grew after the Middle Miocene following the initial Paleogene Arabia–Eurasia collision. Authors attribute uplift to break-off of the southern Neo-Tethys slab beneath the Bitlis–Zagros Suture at ~15–10Ma, coupled with continued plate convergence and regional crustal shortening. Since this time there has been an upsurge in mantle-derived collision magmatism over large parts of NW Iran, Eastern Anatolia and the Lesser Caucasus, potentially hundreds of kilometres from the site of southern Neo-Tethys slab break-off, >10Myr after the proposed break-off event. Whole rock elemental and Sr–Nd–Pb–Hf isotope data are presented for <3Ma trachy-basalt to trachy-basaltic andesite lavas erupted in Armenia in the South Caucasus. Samples formed by <5% melting of fertile subduction-modified spinel-facies lithospheric mantle, and few display elemental or isotopic evidence for contamination by the 45-km thick Mesozoic–Paleogene arc crust or South Armenian Block continental crust. Recent magmatic activity in Armenia may not be a direct consequence of southern Neo-Tethys slab break-off 300–450km away, beneath the Bitlis Suture. Late Miocene break-off of a second (northern Neo-Tethys) slab beneath the Pontide Arc may have allowed asthenospheric upwelling over a wider area than was affected by southern Neo-Tethyan break-off. However, whole-scale delamination of mantle lithosphere is ruled out due to the modest degrees of partial melting, a lack of asthenospheric components and limited crustal involvement in magmatism. Small-scale sub-lithospheric convection may be complementary to break-off, causing localised removal of lithospheric mantle and aiding the occurrence of melting for a significant time interval after the break-off event(s). Collision magmas such as those in Armenia represent mantle-derived additions to continental crust, enriched in incompatible elements but with Th/La ratios≤0.2, much lower than those calculated for continental crust (0.25–0.3). Collision magmatism in Turkic-style orogens must be balanced by infra-crustal recycling and delamination to produce bulk continental crust.

Brazing Operation for Aluminium Semi-Products by Heat Treatment

Nowadays, there is an effort to increase the economic effectiveness, including manual work, from the aspect of the modernization of production technologies. The brazing operation is one of the operations requiring skills and experience and therefore, the paper deals with modern brazing technology of aluminium alloy semi-products which are heated only in the electric furnaces. Application of heat can lead to the formation of the perfect join of semi-products based on such materials as A 3004 and AA 4045. A thin layer called Clad is deposited on one component of the AA 4045 material while the mentioned thin layer is molten to the prescribed temperature in the furnace but the most important fact is that there is not occurrence of melting in relation to the base material. After cooling, Clad creates a perfect metallurgical join between the joined surfaces. The resulting microstructure corresponds to silumin alloy. The transition between silumin alloy and aluminium matrixes of the component is continuous and without any defects in the form of discontinuities or pores and this fact was confirmed by the evaluation of the microstructure. This technology is used in the production of battery coolers in electric hybrid cars. Tightness of brazed join is controlled by pressure and helium tests. Analysed cooler was suitable from the aspect of the performed testing procedures.

Hot compression behavior of the ignition-resistant Mg–5Y–2.5Zn–1.2Ca alloy with long-period stacking ordered structures

The effect of addition of Ca to the hot compressive deformation and dynamic recrystallization (DRX) behaviors of the Mg–5Y–2.5Zn alloy was studied for the development of Mg alloys with long-period stacking ordered (LPSO) structures that have high ignition-resistance and good forgeability. The addition of Ca by 1.2wt.% markedly increased the ignition temperature (by 323K), from the baseline value of 1150 to 1473K. Most of the Ca in the alloy was in the form of Mg2Ca, which coexists preferentially with the LPSO phase. Due to the brittleness of Mg2Ca and the occurrence of eutectic melting of α-Mg/Mg2Ca, the temperature and strain rate ranges for hot deformation of the Mg–5Y–2.5Zn–1.2Ca alloy were narrower compared with those for the Mg–5Y–2.5Zn alloy. Analyses and comparison of the DRX behavior and processing maps of the two alloys, however, indicated that there is a window (650–723K and 1×10−3–1s−1) within which the addition of Ca promotes workability. This result can be attributed to the formation of a higher DRX fraction by the presence of Mg2Ca phase that causes more effective breaking of LPSO phase during hot deformation and thereby provides more sites for nucleation of DRXed grains.

Influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane

The influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane was investigated using small-angle X-ray scattering, temperature-modulated differential scanning calorimetry and scanning electron microscopy. It is found that with increasing annealing temperature from 105 to 145 degrees C, the main lamella melting peak combines with that from connecting bridges and a uniform pore arrangement is obtained in the microporous membrane. The annealed lamella thickness is increased and lamellar structure is improved, due to the occurrence of melting and recrystallization during annealing. At the same time, more secondary crystals are formed. The melting and recrystallization and secondary crystals contribute to the appearance of an annealing peak in the differential scanning calorimetry curve of annealed film. During the following cold and hot stretching, the secondary crystals disappear and convert to initial connecting bridges. The improved lamellar structure can support the scaffold of pore structure, resulting in a uniform connecting bridge arrangement. But further increasing the temperature to 150 degrees C degrades the initial lamellar structure, leading to a decrease of pore arrangement in the stretched microporous membrane. Annealing leads to the difference of lamellar structure: the initial lamellar structure is improved and some weak secondary crystals are formed in the amorphous region. (c) 2014 Society of Chemical Industry

Using daily air temperature thresholds to evaluate snow melting occurrence and amount on Alpine glaciers by &amp;lt;i&amp;gt;T&amp;lt;/i&amp;gt;-index models: the case study of the Forni Glacier (Italy)

Abstract. Glacier melt conditions (i.e., null surface temperature and positive energy budget) can be assessed by analyzing data acquired by a supraglacial automatic weather station (AWS), such as the station installed on the surface of Forni Glacier (Italian Alps). When an AWS is not present, the assessment of actual melt conditions and the evaluation of the melt amount is more difficult and simple methods based on T-index (or degree days) models are generally applied. These models require the choice of a correct temperature threshold. In fact, melt does not necessarily occur at daily air temperatures higher than 0 °C. In this paper, we applied both energy budget and T-index approaches with the aim of solving this issue. We start by distinguishing between the occurrence of snowmelt and the reduction in snow depth due to actual ablation (from snow depth data recorded by a sonic ranger). Then we find the daily average temperature thresholds (by analyzing temperature data acquired by an AWS on Forni Glacier) which, on the one hand, best capture the occurrence of significant snowmelt conditions and, on the other, make it possible, using the T-index, to quantify the actual snow ablation amount. Finally we investigated the applicability of the mean tropospheric lapse rate to reproduce air temperature conditions at the glacier surface starting from data acquired by weather stations located outside the glacier area. We found that the mean tropospheric lapse rate allows for a good and reliable reconstruction of glacier air temperatures and that the choice of an appropriate temperature threshold in T-index models is a very important issue. From our study, the application of the +0.5 °C temperature threshold allows for a consistent quantification of snow ablation while, instead, for detecting the beginning of the snow melting processes a suitable threshold has proven to be at least −4.6 °C.

Structural changes of potassium-saturated smectite at high pressures and high temperatures: Application for subduction zones

In a subduction zone, water-rich pelagic sediments overlying the subducting oceanic plate are accumulated, forming an accretionary wedge. A portion of these sediments, primarily smectites, can subduct and lose the water present in their pores, but the mechanism of the clay structural transformation and its composition are not well-defined. Smectite clay minerals can remain stable, carrying water at pressures corresponding to the upper mantle and thus contributing to the occurrence of melting and metasomatic processes in the mantle. In this work, we performed several high-pressure and high-temperature experiments to verify the structural behavior of a potassium-saturated smectite under various pressure and temperature conditions: (1) atmospheric pressure at two different temperatures (100°C to 700°C); (2) room temperature (25°C) and pressure up to 11.5GPa — diamond anvil cell (DAC) experiments; (3) under pressures of 2.5GPa and 4.0GPa and temperatures from 200°C to 700°C. Scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and infrared spectroscopy (FTIR) analysis on all samples studied suggest that, under elevated pressures (2.5GPa, which corresponds to a depth of approximately 75km, and 4.0GPa, equivalent to a depth of approximately 120km), smectite remains stable until 200°C. With increasing temperature, the initial structure partially loses its interlayer H2O and transforms into a mixed layer of illite/smectite, which is maintained until 400°C under 2.5GPa and until 300°C under 4.0GPa. Beyond these conditions, with increasing temperature, the smectite loses all of its interlayer H2O and transforms into muscovite. Additionally, at room temperature, smectite is stable under nearly 12GPa. These results contribute significantly to the understanding of how pelagic sediment dehydration and transformation occur in the subduction process, as well as the behavior of smectite under the influence of increasing pressure and temperature.

Occurrence and mechanisms of impact melt emplacement at small lunar craters

Using observations from the Lunar Reconnaissance Orbiter Camera (LROC), we assess the frequency and occurrence of impact melt at simple craters less than 5km in diameter. Nine-hundred-and-fifty fresh, randomly distributed impact craters were identified for study based on their maturity, albedo, and preservation state. The occurrence, frequency, and distribution of impact melt deposits associated with these craters, particularly ponded melt and lobate flows, are diagnostic of melt emplacement mechanisms. Like larger craters, those smaller than a few kilometers in diameter often exhibit ponded melt on the crater floor as well as lobate flows near the crater rim crest. The morphologies of these deposits suggest gravity-driven flow while the melt was molten. Impact melt deposits emplaced as veneers and “sprays”, thin layers of ejecta that drape other crater materials, indicate deposition late in the cratering process; the deposits of fine sprays are particularly sensitive to degradation. Exterior melt deposits found near the rims of a few dozen craters are distributed asymmetrically around the crater and are rare at craters less than 2km in diameter. Pre-existing topography plays a role in the occurrence and distribution of these melt deposits, particularly for craters smaller than 1km in diameter, but does not account for all observed asymmetries in impact melt distribution. The observed relative abundance and frequency of ponded melt and flows in and around simple lunar craters increases with crater diameter, as was previously predicted from models. However, impact melt deposits are found more commonly at simple lunar craters (i.e., those less than a few kilometers in diameter) than previously expected. Ponded melt deposits are observed in roughly 15% of fresh craters smaller than 300m in diameter and 80% of fresh craters between 600m and 5km in diameter. Furthermore, melt deposits are observed at roughly twice as many non-mare craters than at mare craters. We infer that the distributions and occurrences of impact melt are strongly influenced by impact velocity and angle, target porosity, pre-existing topography, and degradation. Additionally, areally small and volumetrically thin melt deposits are sensitive to mixing with solid debris and/or burial during the modification stage of impact cratering as well as post-cratering degradation. Thus, the production of melt at craters less than ∼800m in diameter is likely greater than inferred from the present occurrence of melt deposits, which is rapidly affected by ongoing degradation processes.

Experimental determination of melting in the systems enstatite-magnesite and magnesite-calcite from 15 to 80 GPa

Pressure-temperature melting curves in two carbonate-bearing systems of relevance to Earth’s mantle have been determined using the laser-heated diamond-anvil cell (LH-DAC). The solidus along the MgSiO3-MgCO3 join in the MgO-SiO2-CO2 system (MS-CO2) was defined from 15 to 80 GPa using in situ melting criteria, reaching a maximum temperature of ~2340 K at 80 GPa. The occurrence of melting has been confirmed with ex-situ textural and chemical analysis of recovered samples. The melting curve has a negative d T /d P slope at pressures between ~15 and 23 GPa resulting from the subsolidus phase transition of ilmenite- to perovskite-structured MgSiO3. The shallow slope of the melting curve at pressures higher than this transition indicate that for plausible mantle geotherms carbonate-bearing silicate lithologies will melt throughout the lower mantle. The solidus of a mixture along the MgCO3-CaCO3 join was determined as a proxy for alkali-free carbonate lithologies. Melting temperatures increase from 1860 K at 16 GPa to ~2100 K above 35 GPa, where the melting curve flattens. The melting reaction magnesite + post-aragonite (high-pressure CaCO3) = melt was confirmed using an in situ experiment. We conclude that crystalline Mg and Ca carbonate mixtures are unstable with respect to molten carbonate at conditions of the convective lower mantle. The flat melting curves at high pressures in both systems suggests that subducted carbonates will undergo melting at lower mantle conditions, a process that may be important for superdeep diamond formation and carbon storage in the deep mantle.

Effect of pulse to pulse interactions on ultra-short pulse laser drilling of steel with repetition rates up to 10 MHz

We report on the effect of pulse to pulse interactions during percussion drilling of steel using high power ps-laser radiation with repetition rates of up to 10 MHz and high average powers up to 80 W. The ablation rate per pulse is measured as a function of the pulse repetition rate for four fluences ranging from 500 mJ/cm2 up to 1500 mJ/cm2. For every investigated fluence an abrupt increase of the ablation rate per pulse is observed at a distinctive repetition rate. The onset repetition rate for this effect is strongly dependent on the applied pulse fluence. The origin of the increase of the ablation rate is attributed to the emergence of a melt based ablation processes, as Laser Scanning Microscopy (LSM) images show the occurrence of melt ejected material surrounding the drilling holes. A semi empirical model based on classical heat conduction including heat accumulation as well as pulse-particle interactions is applied to enable quantitative conclusions on the origin of the observed data. In agreement with previous studies, the acquired data confirm the relevance of these two effects for the fundamental description of materials processing with ultra-short pulsed laser radiation at high repetition rates and high average power.

Influence of heat treatment on microstructure of Ni3Al based single crystal superalloy

The effect of heat treatment (solution and age treatment) on Ni3Al based single crystal superalloy was investigated in this study. The solution temperatures of the experimental alloy were 1315–1335°C, 1320–1340°C and 1325–1345°C, and all the solution treatments could eliminate dendritic microsegregation in the occurrence of incipient melting. The primary age temperatures and times range was 1100–1130°C and 1–4 h, while the secondary age times were 16–40 h at 870°C. There was a comprehensive consideration of microstructure, actual working status of tube furnace and cost factor, and the optimum heat treatment was 1320–1340°C/16 h, air cooling (AC)+1120°C/3 h, AC+870°C/32 h, AC.

Detection of melting by X-ray imaging at high pressure.

The occurrence of partial melting at elevated pressure and temperature is documented in real time through measurement of volume strain induced by a fixed temperature change. Here we present the methodology for measuring volume strains to one part in 10(-4) for mm(3) sized samples in situ as a function of time during a step in temperature. By calibrating the system for sample thermal expansion at temperatures lower than the solidus, the onset of melting can be detected when the melting volume increase is of comparable size to the thermal expansion induced volume change. We illustrate this technique with a peridotite sample at 1.5 GPa during partial melting. The Re capsule is imaged with a CCD camera at 20 frames/s. Temperature steps of 100 K induce volume strains that triple with melting. The analysis relies on image comparison for strain determination and the thermal inertia of the sample is clearly seen in the time history of the volume strain. Coupled with a thermodynamic model of the melting, we infer that we identify melting with 2 vol.% melting.

Optimizing the tensile properties of Al–Si–Cu–Mg 319-type alloys: Role of solution heat treatment

The work presented in this study was carried out on Al–Si–Cu–Mg 319-type alloys to investigate the role of solution heat treatment on the dissolution of copper-containing phases (CuAl2 and Al5Mg8Cu2Si6) in 319-type alloys containing different Mg levels, to determine the optimum solution heat treatment with respect to the occurrence of incipient melting, in relation to the alloy properties. Two series of alloys were investigated: a series of experimental Al–7wt% Si–3.5wt% Cu alloys containing 0, 0.3, and 0.6wt% Mg levels. The second series was based on industrial B319 alloy. The present results show that optimum combination of Mg and Sr in this study is 0.3wt% Mg with 150ppm Sr, viz. for the Y4S alloy. The corresponding tensile properties in the as-cast condition are 260MPa (YS), 326MPa (UTS), and 1.50% (%El), compared to 145MPa (YS), 232MPa (UTS), and 2.4% (%El) for the base alloy with no Mg. At 520°C solution temperature, incipient melting of Al5Mg8Cu2Si6 phase and undissolved block-like Al2Cu takes place. At the same time, the Si particles become rounder. Therefore, the tensile properties of Mg-containing alloys are controlled by the combined effects of dissolution of Al2Cu, incipient melting of Al5Mg8Cu2Si6 phase and Al2Cu phase, as well as the Si particle characteristics.

In-situ FTIR imaging on the plastic deformation of iPP thin films

The plastic deformation behavior of isotactic polypropylene (iPP) film is studied with in-situ Fourier transformation infrared microspectroscopic imaging (FTIRI). During uniaxial tensile test, spatial distributions of crystallinity and orientations are obtained in necking region (NR), transition front (TF) and non-necking region (NNR). A low valley of crystallinity exists at TF, while both NNR and NR have crystallinities at a high plateau. This provides a direct evidence of deformation-induced melting–recrystallization mechanism of plastic deformation. The non-monotonic evolution of amorphous orientation from NR, TF to NNR further supports the occurrence of deformation-induced melting–recrystallization. The decrease of amorphous orientation behind TF is attributed to recrystallization.

Effect of Mg addition of microstructure of 319 type alloys

The present study was undertaken to investigate the effect of Mg on the occurrence of incipient melting in experimental and industrial 319 alloys using thermal analysis, tensile testing, microstructural analysis and porosity measurements. Castings were prepared from experimental and industrial alloy melts containing Mg levels of 0–0·6 wt-%. The addition of Mg leads to the segregation of the copper phase, resulting in the formation of the block-like form of the CuAl2 phase rather than its finer eutectic-like form. This makes it more difficult to dissolve the CuAl2 phase during solution heat treatment. The addition of Mg to 319 alloy, irrespective of the alloy source, modifies the Si particle morphology. This effect is observed very clearly at 0·6 wt-%Mg, with a corresponding decrease in the Al–Si eutectic temperature compared to the base alloy. As expected, the modification effect of Mg is not very obvious at low Mg addition. The addition of Mg also leads to the precipitation of the Al5Mg8Cu2Si6 phase. This phase normally precipitates after the CuAl2 phase. Nevertheless, when the addition of Mg exceeds 0·4 wt-%, the precipitation of the Al5Mg8Cu2Si6 phase also takes place in another reaction, before the precipitation of the CuAl2 phase. The morphology of the Al5Mg8Cu2Si6 phase particles in this case is script-like rather than the irregular shaped particles normally observed.

Impact of tidal heating on the onset of convection in Enceladus’s ice shell

By performing 3D simulations of thermal convection and tidal dissipation, we investigated the effect of tidal heating on the onset of convection in Enceladus’s ice shell. We considered a composite non-Newtonian rheology including diffusion, grain-size-sensitive and dislocation creeps, and we defined an effective tidal viscosity reproducing the dissipation function as predicted by the Andrade rheology. For simulations with no or moderate tidal heating, the onset of convection requires ice grain sizes smaller than or equal to 0.5–0.6mm. For simulations including significant tidal heating (>10−6Wm−3), the critical grain size for the onset of convection is shifted up to values of 1–1.5mm. Whatever the width of the internal ocean, convection is initiated in the polar region due to enhanced tidal dissipation at high latitudes. For a given eccentricity value, the onset of convection depends on the ocean width, as tidal flexing and hence tidal heat production is controlled by the ocean width. For heating rates larger than 5–9×10−7Wm−3, we systematically observe the occurrence of melting in our simulations, whatever the grain size and for both convecting and non-convecting cases. Grain sizes smaller than 1.5mm, required to initiate convection, may be obtained either by the presence of a few percent of impurities limiting the grain growth by pinning effects or by the increase of stress and hence dynamic recrystallization associated with tidally-induced melting events.

Evaluation of a novel inversion model for surface melt magnitude over the Greenland ice sheet during the 2002 ablation season

This work evaluates the performance of an empirical inversion model for surface liquid water fraction (LWF) or melt magnitude, using data from the Moderate Resolution Imaging Spectroradiometer (MODIS). Estimates of melt magnitude were evaluated through quantitative comparison to surface melt occurrence and duration derived from the cross-polarization gradient ratio (XPGR) and diurnal amplitude variation (DAV) passive microwave melt indices over the 2002 melt season. Longer melt durations derived from XPGR tend to correspond to higher LWF amounts at lower elevations, where temperatures are warmer than at higher elevations. DAV indicates melt occurrence at higher elevations, with lower durations in melt corresponding to small amounts of LWF. The LWF inversion model demonstrates that it is sensitive to both high melt duration events within the ablation zone, yet capable of capturing greater spatial variability in melt conditions broadly over the entire sheet.

Defects related to incipient melting in Al–Si–Cu–Mg alloys

The present study was undertaken to investigate the effect of Mg on the occurrence of incipient melting in experimental and industrial 319 alloys, using porosity measurements. Castings were prepared from experimental and industrial alloy melts containing Mg levels of 0–0.6wt%. Tensile test bars were cast using an ASTM B-108 type permanent mold. The bars were solution heat treated in the range of 490–540°C for the single step solution treatment, and at 505°C followed by 520°C or 530°C for the two-stage solution heat treatment. Porosity (arising due to incipient melting) was also measured to monitor the occurrence of the incipient melting. Optical microscopy technique was used for microstructural analysis, and quantification, of porosity. The results showed that both Mg concentration and solution temperature play an important role in the occurrence of incipient melting. The porosity measurements were found to be in accordance with the tensile properties and confirmed the results obtained in terms of the incipient melting observed for each alloy/solution treatment condition. Compared to the experimental alloys, the industrial alloy shows a greater resistance to incipient melting, which may be accounted for in terms of the reaction between Cu and trace elements such as Fe and Ni present in the alloy, leading to an increase in the incipient melting temperature.

The influence of microwave-assisted hydrogen plasma treatment on electroless Ni–P coatings

As-deposited electroless Ni–P coatings were heated in air and subsequently treated by microwave-assisted hydrogen plasma with various powers. The phase evolution and microstructure of the Ni–P coatings were studied in detail. The results indicate that the amorphous Ni–P coatings were first transformed into Ni, Ni3P, and the residual amorphous phase after heating at 300 °C in air. The NiO, Ni2P, and Ni12P5 phases with a half size of an island at the as-deposited state were formed after heating at 400 °C for 3 h. Furthermore, the 600 W or higher hydrogen plasma power was required to obtain the Ni and Ni3P phases. A short hydrogen plasma treatment of 1 min at 600 W can reduce NiO into Ni and transform Ni2P and Ni12P5 into larger N3P grains. The NiO phase was first transformed into the Ni phase, which subsequently reacted with the Ni2P into Ni3P phases. In addition, no phosphorus loss was observed. For longer plasma treatment (10 min), the nickel phase dissolved into the lattice of the Ni3P phase, and the surface morphology of the Ni–P coatings changed considerably because of the occurrence of phase transformation and partial melting.

Dendritic Crystallization: A Single Process for all the Textures of Olivine in Basalts?

The olivine macrocrysts found in oceanites, picrites and magnesian basalts erupted at hotspot volcanoes are generally interpreted either as phenocrysts crystallized from the magma or as xenocrysts extracted from a deforming cumulate. To constrain the origin of these crystals we studied their texture and composition at Piton de la Fournaise volcano, La Reunion. We show that macrocrysts are organized and subdivided into parallel units; this suggests a crystallization by dendritic growth and ripening rather than by a complex combination of paired nucleation, crystal aggregation or synneusis. Dendritic growth is also evidenced by the occurrence of hollow faces, P-rich zones, melt and Cr-spinel inclusions formed from the accumulation of slow diffusing impurities (P, Cr, Al) in the liquid at the contact with rapid-growing olivine. We suggest that early dendritic crystallization may even cause branch misorientations and lattice mismatches, yielding subgrain boundaries, dislocation lamellae and to a certain extent undulose extinction, which have all been formerly interpreted in terms of plastic intracrystalline deformation. We interpret olivine macrocrysts as phenocrysts crystallized under a strong degree of undercooling (-ΔT > 60°C), and derived from a harrisitic mush formed on the cold walls of the magma reservoir. Given the growth shapes indicated by P zoning patterns and external faces, the olivine macrocrysts (which consist of groups of several subcrystals) have grown in suspension within the liquid and were neither aggregated into a dense cumulate nor corroded, shocked or deformed before or during their transport to the surface. The major consequence of our study is that most olivine macrocrysts are not xenocrysts, and very few of them, if any, have experienced intracrystalline deformation. The importance of deforming (creeping) cumulate bodies, thought to accommodate the spreading of basaltic volcanoes in La Reunion and Hawaii, may hence have been overestimated.

The extreme melt across the Greenland ice sheet in 2012

The discovery of the 2012 extreme melt event across almost the entire surface of the Greenland ice sheet is presented. Data from three different satellite sensors – including the Oceansat‐2 scatterometer, the Moderate‐resolution Imaging Spectroradiometer, and the Special Sensor Microwave Imager/Sounder – are combined to obtain composite melt maps, representing the most complete melt conditions detectable across the ice sheet. Satellite observations reveal that melt occurred at or near the surface of the Greenland ice sheet across 98.6% of its entire extent on 12 July 2012, including the usually cold polar areas at high altitudes like Summit in the dry snow facies of the ice sheet. This melt event coincided with an anomalous ridge of warm air that became stagnant over Greenland. As seen in melt occurrences from multiple ice core records at Summit reported in the published literature, such a melt event is rare with the last significant one occurring in 1889 and the next previous one around seven centuries earlier in the Medieval Warm Period. Given its rarity, the 2012 extreme melt across Greenland provides an exceptional opportunity for new studies in broad interdisciplinary geophysical research.