Experimental constraints on subglacial rock friction
Experimental constraints on subglacial rock friction
复制标题
冰下岩石摩擦力的实验限制
作者：
D. Hansen;L. Zoet
冰下岩石摩擦是控制硬层冰川滑动动力学和侵蚀潜力的重要因素，但在很大程度上仍不受约束。为了探索基础熔融速率、有效应力和冰温对摩擦阻力的相对影响，我们进行了石灰岩层在固定冰板下滑动的磨损试验。剪切应力与熔融速率和低温应力成线性关系，证实了粘性阻力和有效应力都是排出条件下接触力的一阶控制因素。此外，冰层中的温度梯度增加了粘性阻力对底部剪应力的贡献。在所有实验中，熔体速率和剪切应力之间的关系可以用一个模型来最好地解释，该模型考虑了回生和粘性蠕变对床面法向阻力的影响。我们将其解释为，即使在零度以下的温度下，围绕夹带碎屑的流体流动也会产生底部阻力。将预熔化动力学引入到Watts/Hallet冰下岩石摩擦模型中，我们发现预测的碎屑床阻力减少了大约一个数量级，而过渡半径相应增加了约3.5倍。这比我们观察到的略低于压力熔点的冰要低。
Abstract Subglacial rock friction is an important control on the sliding dynamics and erosive potential of hard-bedded glaciers, yet it remains largely unconstrained. To explore the relative influence of basal melt rate, effective stress and ice temperature on frictional resistance, we conducted abrasion experiments in which limestone beds were slid beneath a fixed slab of ice laden with granitic rock fragments. Shear stress scales linearly with melt rate and cryostatic stress, confirming that both viscous drag and effective stress are first-order controls on the contact force in drained conditions. Furthermore, temperature gradients in the ice increase the contribution of viscous drag on basal shear stress. In all experiments, the relationship between melt rate and shear stress is best explained by a model that accounts for the effects of regelation and viscous creep on the bed-normal drag force. We interpret this to mean fluid flow around entrained clasts contributed to basal drag even at subfreezing temperatures. Incorporating premelting dynamics into the Watts/Hallet model for subglacial rock friction, we find that the predicted debris-bed drag decreases by approximately an order of magnitude, with a corresponding ~3.5 × increase in the transition radius. This is lower than we observe for ice slightly below the pressure melting point.