De- icing progression of ice-cored moraines in a humid, subpolar climate, Kötlujökull, Iceland

Modes and rates of de-icing ice-cored moraines under humid subpolar conditions have been determ ined for two dead-ice fields of different age representing a mature and a final stage of collapse in the Kötlujökull terminus region, south Iceland. The melting processes considered are backwasting, defined as the lateral retreat of near-vertical ice walls, or steep, ice-cored slopes, and downwasting, defined as the thinning of the ice core by melting along the top and bottom surfaces. In the mature stage of collapse, where there is less than 1 m of sediment cover, major faces of ice exposed for rapid melting are ordinarily significant and long-lived due to intense precipitation. In this fully ice-cored terrain the predominating processes are backwasting of exposed ice walls and downwasting by bottom melt, each contributing to the annual surface lowering of about 0.5 m, or 35%; the total annual surface lowering (melt rate) due to all processes averages 1.4 m. In those parts of the fully ice-cored dead-ice field where ice-degradation is at a more advanced stage and free-faces of ice are ordinarily absent, the melt rate averages 0.8 m. In the final stage of collapse, where the ice mass is disintegrated into isolated dead-ice blocks buried beneath multiple resedimented deposits of 1–3 m, the annual rate of surface lowering of ice cores has decreased to 0.3 m. Factors contributing to this are a thicker sediment cover, insulation by an increased amount of vegetation, and decreased amount of running water beneath the ice blocks. It is concluded that in the current climate it takes around 50 years to melt down 40 m of stagnant dirty glacier ice; about 20 years are required to disintegrate the ice into isolated dead-ice blocks, and another 30 years to entirely melt out the ice cores.