Soru

Passage:
Glaciologists investigating the dynamics of ice-sheet motion have long recognized that basal sliding—the movement of a glacier over its underlying bedrock or sediment—is intimately linked to subglacial hydrology. Early empirical models posited a straightforward linear relationship: higher volumes of surface meltwater penetrating to the glacier bed were assumed to invariably accelerate ice velocity by lubricating the ice-bed interface. However, continuous GPS monitoring of the Greenland Ice Sheet has revealed a paradoxical seasonal deceleration during periods of peak mid-summer melting, demonstrating that the architecture of subglacial drainage conduits, rather than the raw volume of meltwater alone, governs basal lubrication.

The subglacial hydrological network operates primarily in two distinct morphological regimes: distributed networks and channelized systems. At the onset of the summer melt season, when surface melt first reaches the bed through vertical fractures known as moulins, the subglacial water is routed through a distributed network of interconnected cavities. These linked cavities, formed in the lee of bedrock protrusions as ice slides over irregular topography, exhibit high hydraulic resistance and low conveyance capacity. Because water cannot rapidly escape through these tortuous orifices, influxes of meltwater induce extreme basal water pressures that approach or exceed the overlying ice overburden pressure. This elevated hydrostatic pressure reduces the effective normal stress (the net downward force pressing the ice against the bedrock), effectively decoupling portions of the glacier from its bed and triggering transient surges in sliding velocity.

As the melt season progresses, however, sustained high-volume discharge drives a fundamental structural transition. The frictional heat generated by the turbulent flow of water begins to melt the conduit walls faster than the inward creep of ice can close them. Consequently, the inefficient distributed cavity network rapidly coalesces into discrete, highly efficient conduits known as Röthlisberger channels (R-channels). Paradoxically, because these mature channelized systems possess immense hydraulic capacity and evacuate water rapidly to the ice margin, they operate at significantly lower internal water pressures than the distributed networks they replace. The resulting drop in subglacial water pressure drains adjacent cavities, restoring high effective normal stress across the surrounding bedrock and substantially increasing frictional resistance. Thus, even under conditions of peak surface runoff, the emergence of an efficient channelized network leads to a marked reduction in basal sliding rates.

According to the passage, which of the following explicitly explains why an increase in surface meltwater during mid-summer can lead to a reduction in ice-sheet basal sliding velocity?