There have been many theories of why glacial surges occur. Hydrological control Surges may be caused by the supply of meltwater to the base of a glacier. Meltwater is important in reducing frictional forces to glacial ice flow. The distribution and pressure of water at the bed modulates the glacier's velocity and therefore mass balance. Meltwater may come from a number of sources, including supraglacial lakes, geothermal heating of the bed, conduction of heat into the glacier and latent heat transfers. There is a positive feedback between velocity and friction at the bed, high velocities will generate more frictional heat and create more meltwater. Crevassing is also enhanced by greater velocity flow which will provide further rapid transmission paths for meltwater flowing towards the bed. However, Humphrey found no precise correlation between ice-slow down and the release of water inside of a glacier. The evolution of the drainage system under the glacier plays a key role in surge cycles. Thermal regime Glaciers that exhibit surges like those in Svalbard; with slower onset phase, and a longer termination phase may be thermally controlled rather than hydrologically controlled. These surges tend to last for longer periods of time than hydrologically controlled surges. Deformable bed hypothesis In other cases, the geology of the underlying country rock may dictate surge frequency. For example, poorly consolidated sedimentary rocks are more prone to failure under stress; a sub-glacial "landslip" may permit the glacier to slide. This explains why surging glaciers tend to cluster in certain areas. Critical mass Meier and Post suggest that once mass accumulates to a critical point, basal melting begins to occur. This provides a buoyancy force, "lifting" the glacier from the bed and reducing the friction force. References Bibliography [Dowdeswell,J.A., B. Unwin, A. -M. Nuttall and D. J. Wingham. 1999. Velocity structure, flow instability and mass flux on a large Arctic ice cap from satellite radar interferometry. Elsevier Science B.V.] [Humphrey, Neil Frank. Basal Hydrology of a Surge-Type Glacier: Observations and Theory Relating to Variegated Glacier. University of Washington, 1987.] [Jiskoot H, DT Juhlin. 2009. Surge of a small East Greenland glacier, 2001–2007, suggests Svalbard-type surge mechanism. Journal of Glaciology, Vol. 55, No. 191., pp. 567–570.] https://web.archive.org/web/20050323125548/http://users.aber.ac.uk/kak3/glacier_surges.htm http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%208.htm http://www.bgrg.org/pages/education/alevel/coldenvirons/Lesson%208.htm [Murray, T., T. Strozzi, A. Luckman, H. Jiskoot, and P. Christakos (2003), Is there a single surge mechanism? Contrasts in dynamics between glacier surges in Svalbard and other regions, J. Geophys. Res., 108(B5),