Thermal conditions and movement of rock glaciers in the North Cascades, Washington

Rock glaciers are a largely unrecognized phenomenon in the North Cascades. In part this reflects their scarcity there. Additionally, because rock glaciers are widely held to be the product of permafrost conditions, the dearth of literature regarding North Cascade rock glaciers also reflects the notion that active rock glaciers should not exist at all in such temperate mountain ranges. Rock glaciers have been linked to specific air temperature conditions ( < -2 degreesC), and, based on that link, are often used as visual indications of mountain permafrost. The North Cascades, a maritime mountain range with high snowfall and relatively warm climate, are a good location to test the permafrost-rock glacier link. Review of aerial photography and satellite imagery, however, reveals at least ten morphologically active rock glaciers and even more that appear inactive. To test the activity and possible link to permafrost conditions, I selected two of the active-looking rock glaciers for movement monitoring and thermal investigation. Movement monitoring was accomplished by conducting repeat scans with a terrestrial laser scanner; this investigation represents the first attempt to use this technique on rock glaciers in North America. The Craggy Peak rock glacier was shown to be moving downslope at a rate of 5 to 10 cm per year. Movement vectors toward the top of the rock glacier suggested deflation, while vectors toward the toe indicated a slight inflation. Flow toward the top and center of the rock glacier also was faster reflecting the steeper slope while flow toward the toe slowed and vectors radiated out. Movement was not detectable on second rock glacier, Star Peak, due mainly to lack of control points located on and around the scan target. Moreover, lack of a good vantage point at the site limited the scan coverage, inhibiting data processing. Because the North Cascades are a maritime mountain range with climate conditions thought to be too warm and wet to support rock glaciers, I also deployed miniature temperature data loggers in both rock glaciers to record air temperature at the surface and within the rubble. Three logger strings were deployed with three loggers. Each string contained one surface logger, one logger of intermediate depth and one logger that was between 1.5-2.3 meters deep in the rubble (depending on the string). One year of data has revealed that average ground temperature on the rock glaciers is probably near -1 ± 1 degrees C and modeled near-surface air temperature above them is 0.0 ± 1.6 degrees C. Air temperature is marginally to warm to support permafrost, though a more lengthy study period is needed. Thermal exchange during the summer appears to be governed by conductive processes in the form of rain water and solar heating. Moreover, forced convection occurs when wind pumps air into the regolith. During the fall, I document at least one instance where the data loggers capture natural convection when relatively warm air evacuated the regolith. Natural convection occurs when cold air overlays warm air and the subsequent density driven inversion results in warm air escaping into the air and cold air settling into the regolith.