Formation of Tunnel Valleys/channels
Although there is agreement on the role of meltwater in creation of tunnel valleys, several theories are still under consideration for the role of that meltwater:
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- Steady state theory - Boulton and Hindmarsh propose a steady state theory. They suggest tunnel valleys form in unconsolidated sediment when meltwater flows under pressure through an initially narrow subglacial conduit. With progressive removal of sediment by meltwater, ice deforms under its own weight into the cavity to create a tunnel valley through a positive feedback mechanism.
- Jökulhlaup driven erosion - Piotrowski argues that ice sheets may, in some instances, be cold-based; that is they contact land that is frozen (permafrost) and they freeze to the permafrost. Meltwater builds up behind this frozen ice terminus until it generates sufficient pressure to lift the ice and break the bond, with a catastrophic meltwater release such as is seen with the Icelandic jökulhlaup. As a consequence of this jökulhlaup a tunnel valley is formed.
- Upglacier erosion - Wingfield proposes that tunnel valleys form gradually, with the valley head cutting progressively back toward the source up-glacier during deglaciation.
Periodic outbursts of subglacial water have been observed moving subglacial water between subglacial lakes beneath the East Antarctic Ice Sheet. Satellite data recorded a subglacial discharge totaling 2 km3 (0.48 cu mi) traveling ~260 km (160 mi) over a period of less than a year. As the flow subsided, the weight of ice closed the tunnel and sealed the lake again. The water flow was modeled satisfactorily with channeling in ice and in sediment. The analytic model shows that over some regions, the ice-bedrock geometry included sections which would have frozen, blocking off flow, unless erosion of the sedimentary substrate was the means of creating a channel and sustaining the discharge. Hence, combining this data and analysis with Icelandic jökulhlaup observations, there is experimental evidence that some form of the jökulhlaup hypothesis with features of the steady state model is correct.
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