Internal structure of the Green Lake 5 rock glacier, Colorado Front Range, USA

Information about the internal structure of rock glaciers is needed to understand their reaction to ongoing climate warming. Three different geophysical techniques-shallow seismic refraction, ground-penetrating radar (GPR) and electrical resistivity tomography-were used to develop a detailed subsurface model of the Green Lake 5 rock glacier in the Colorado Front Range, USA. Below a thin zone of fine sediments and soils (0.7-1-m thickness; 0-20 kΩm and 320-370ms^-1), a 1-3-m thick zone with low p-wave velocities (790-820ms^-1) and high electrical resistivity (20-100 kΩm) is interpreted as the ice-free, blocky active layer with large void spaces. The data corroborate strong reflections of the GPR signals which travel at this depth at 0.11mns^-1. A third layer that extends from depths of 1-3m to about 5m is characterised by lower electric resistivities (5-20 kΩm) and has lower electromagnetic wave velocities (0.65mns^-1), representing unfrozen, finer and wetter sediments. At around 5-m depth, the measured physical parameters change drastically (vp=3200-3300ms^-1, 50-150 kΩm, vGPR=0.15mns^-1), showing an ice-rich permafrost zone above the bedrock. This model of the internal structure was used to evaluate an existing hydrological flowpath model based on the hydrochemical properties of water outflow from the rock glacier.