Glaciers and frozen‐debris landforms have coexisted and episodically or continuously interacted throughout the Holocene at elevations where the climate conditions are cold enough for permafrost to occur. In the European Alps, the Little Ice Age (LIA) characterized the apogee of the last interaction phase. In areas of consecutive post‐LIA glacier shrinkage, the geomorphological dominant conditioning of the ongoing paraglacial phase may have transitioned from glacial to periglacial and later even shifted to post‐periglacial. Such transitions can be observed through the morphodynamics of glacitectonized frozen landforms (GFLs), which are permafrost‐related pre‐existing frozen masses of debris deformed (tectonized) by the pressure exerted by an interacting glacier. This contribution aims at evidencing the processes driving the ongoing morphodynamical evolution of an actively back‐creeping GFL within the LIA forefield of the Aget glacier on the basis of long‐term time series of ground surface temperature, and in‐situ geodetic and geoelectrical measurements. Our observations for the last two decades (1998–2020), which have been the warmest since the LIA, reveal a resistivity decrease in the permafrost body and a surface subsidence of up to a few centimeters per year. The former indicate a liquid water‐to‐ice content ratio increase within the permafrost body and the latter a ground ice melt at the permafrost table, both processes having taken place heterogeneously at the scale of the landform. The absence of acceleration of landform motion during that period despite a probable warming trend of the frozen ground may indicate that the ongoing degradation is reaching a tipping point at which processes related to interparticle friction and thinning of the permafrost body contribute to gradually inactivate the mechanism of permafrost creep.

<p>Glaciers and frozen-debris landforms have coexisted and episodically or continuously interacted throughout the Holocene at elevations where the climate conditions are cold enough for permafrost to occur. In the European Alps, the Little Ice Age (LIA) characterized the apogee of the last interaction phase. In areas of consecutive post-LIA glacier shrinkage, the geomorphological dominant conditioning of the ongoing paraglacial phase may have transited from glacial to periglacial and later even shifted to post-periglacial. Such transitions can be observed through the morphodynamics of glacitectonized frozen landforms (GFL), which are permafrost-related pre-existing frozen masses of debris deformed (tectonized) by the pressure exerted by an interacting glacier.</p><p>This contribution aims at evidencing the processes driving the ongoing morphodynamical evolution of two actively back-creeping GFL and a debris-covered glacier within the LIA forefield of the Challand and Epée glaciers on the basis of long-term time series of ground surface temperature, in-situ geodetic and geophysical measurements. Our observations for the last two decades (1997-2021), which have been the warmest since LIA, reveal a resistivity decrease in the permafrost body of the two GFL and a surface subsidence of a few centimetres per year up to locally a few decimetres per year. The former indicate a liquid water-to-ice content ratio increase within the permafrost body and the latter a ground ice melt at the permafrost table, both processes having taken place heterogeneously at the scale of the two GFL. These observations can be interpreted as the system is entering post-periglacial conditioning. In comparison, the still widespread debris-covered tongue of the Epée glacier, almost immobile and disconnected from the glacier active part, suffers a melt-induced subsidence of about 50 centimetres per year, indicating the insufficiency of the debris cover thickness to insure a long-term preservation of the ice under current climate conditions.</p><p>This study enhances the importance of decadal-scale and multi-disciplinary approach in understanding driving processes contributing to surface elevation changes due to ice melt or thawing frozen ground in permafrost-prone alpine LIA glacier forefields and to the dynamics of associated GFL.</p>

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Abstract. Viscous flow features in perennially frozen talus/debris called rock glaciers are being systematically inventoried as part of global climate-related monitoring of mountain permafrost. In order to avoid duplication and confusion, guidelines were developed by the International Permafrost Association for discriminating between the permafrost-related landform “rock glacier” and the glacier-related landform “debris-covered glacier”. In two regions covered by detailed field measurements, the corresponding data- and physics-based concepts are tested and shown to be adequate. Key physical aspects, which cause the striking morphological and dynamic difference between the two phenomena/landforms concern: • tight mechanical coupling of the surface material to the frozen rock-ice mixture in the case of rock glaciers as contrasting with essential non-coupling of debris to glaciers they cover; • talus-type advancing fronts of rock glaciers exposing fresh debris material from inside the moving frozen bodies as opposed to massive surface ice exposed by advancing fronts of debris-covered glaciers; and • increasing creep rates and continued advance of rock glaciers as convex landforms with structured surfaces versus predominant slowing down and disintegration of debris-covered glaciers as concave landforms with primarily chaotic surface structure. Where debris-covered surface ice is, or has recently been, in contact with thermally-controlled subsurface ice in permafrost, complex conditions and interactions can develop morphologies beyond simple “either-or”-type landform classification. In such cases, remains of buried surface ice mostly tend to be smaller than the lower size limit of “glaciers” as applied in glacier inventories, and to be far thinner than the permafrost in which they are embedded.