Advancing understanding of Holocene rock glacier dynamics

Rock glaciers, major cryospheric features in alpine landscapes, pose formidable challenges in extracting climatic information over recent to Holocene timescales. This presentation delves into an integrative multi-method approach, striving to replicate modern motion through feature tracking, exposure ages from 10Be concentrations, and observations of rock glacier morphology. Applying a novel numerical model for rock-glacier dynamics, our study focuses on the Holocene to modern activity of a prominent rock glacier flowing northeast from a 300-m tall headwall on the Mt. Sopris (West Elk Mountains, Colorado USA).

The Mt. Sopris rock glacier spans 2 km from its headwall avalanche source cone to a 25 m tall terminus, adorned with metric size granitic blocks exhibiting systematic variations in lichen cover and weathering. Fine-grained material fills voids between blocks in the lowermost reaches, supporting tree clusters. The ¹⁰Be-based exposure ages of block surfaces range from 1.5 to 12 kyr, with ages older than 6 kyr being compressed into the bottom quarter of the rock glacier. Modern rock-glacier surface velocities, ranging from 0.6 to 2 m/yr, can be explained by the internal deformation of a 25-m thick ice core beneath the rocky surface. However, interpreting the ¹⁰Be exposure age profile proves challenging, leading to the development of a new numerical model for rock-glacier dynamics.

Our model simplifies the mass balance to an avalanche cone accumulation zone, and the rock cover is assumed to damp melting of underlying ice over the remaining areas of the rock glacier. Climate forcing is achieved through a proposed history of the snow avalanche activity. The rock glacier velocity is calculated assuming Glen’s flow law in the interior ice and acknowledges the role of debris cover in augmenting the stress profile throughout. Preliminary modeling suggests that an avalanche cone history with two independent pulses, one in the early Holocene and the other simulating the Neoglacial, captures dominant features of the ¹⁰Be exposure age structure. The first manifestation of the rock glacier extends to approximately 1.5 km in lengths, then extends, thins, and slows over the mid-Holocene lull in input, before being overtaken and re-accelerated by the Neoglacial pulse.

This study contributes new insights into rock glacier dynamics, bridging multiple timescales and quantitatively assessing physical processes in action. Rock glaciers, key players in alpine landscape evolution, exhibit a response to climate that differs from typical glacier systems in that they never retreat, and can survive long periods of low snow input. Our numerical simulations allow investigation of dynamic responses to variations in both climate and headwall backwearing erosion. Success of our approach on the Mt. Sopris rock-glacier system suggests its utility in developing a deeper understanding of how different high mountain landscapes respond to climatic fluctuations over Holocene timescales.