Moulin

A moulin (from French moulin, mill) is a vertical or nearly vertical shaft in a glacier or ice sheet through which water descends from the surface to the bed. Moulins are the primary drainage pathways for surface meltwater on the Greenland Ice Sheet and other glaciers, and they play a critical role in ice dynamics by modifying basal lubrication and heat transfer.

Formation and Structure

Moulins form when surface meltwater accumulates in crevasses or supraglacial lakes and drains through cracks in the ice. The water bores a vertical shaft — sometimes tens or hundreds of metres deep — that can remain open for an entire melt season or, in some cases, persist for multiple years. The shaft diameter varies from a few metres to over ten metres, and the water flow within can reach velocities of several metres per second.

Role in Ice Dynamics

The most important consequence of moulins for ice sheet dynamics is the delivery of surface meltwater to the bed of the ice sheet. This water lubricates the ice-bed interface, reducing basal friction and enabling the ice to flow faster. On the Greenland Ice Sheet, this effect is most pronounced in the ablation zone, where summer melt is concentrated and moulins are most numerous.

The lubrication effect is seasonal: glaciers accelerate in summer when meltwater is abundant and decelerate in winter when drainage freezes or slows. But there is growing concern that sustained warming could expand the area of the bed affected by meltwater, potentially activating basal sliding in regions that were previously frozen to the bed. If this occurs, the ice sheet's dynamic response to warming would accelerate, because basal sliding reduces the effective viscosity of the ice and allows faster flow even without additional thinning.

Moulins thus represent a critical coupling mechanism between the surface climate and the interior dynamics of ice sheets — a small-scale feature with potentially large-scale consequences.