Extracted from Azócar (2013).

Rock glaciers in air photos and satellite images present particular visual features and distribution patterns that have been used by several authors to identify rock glaciers in mountain areas (Barsch, 1996; Roer & Nyenhuis, 2007):

Even though rock glaciers can be easily detected visually, classification of their dynamic status (see section as active, inactive and relict requires a more detailed analysis of several geomorphological and environmental characteristics. In general, the dynamic status of rock glaciers has been evaluated based on geomorphological criteria (i.e., surface relief, appearance of the rock glacier front), environmental attributes (i.e., the presence of vegetation) and direct measurements of velocity and thermal conditions (Janke et al., 2013). A steep front (>35°) with unstable rocks and without vegetation has usually been used as a characteristic indicative of an active rock glacier; in contrast, a smooth front slope with stable boulders indicates that a rock glacier is inactive (Burger et al., 1999). On the other hand, an irregular and collapsed surface due to thawing of the ice commonly indicates that a rock glacier is a relict form (Putnam & David, 2009).

In-situ measurements of surface velocity through GPS survey and photogrammetry permit the quantification of rock glacier creep. Based on this kinematic information, it is possible to distinguish an active glacier from an inactive or relict forms very easily; however, GPS measurements are not suitable for making a clear distinction between inactive and relict rock forms. BTS measurement and monitoring of GST are appropriate methods to distinguish between intact and relict forms but not between active and inactive forms (see section 2.9).

For this study, the relevance of different geomorphological, geomorphometric and environmental characteristics that indicate the dynamic status of rock glaciers is summarized in Table 2, based mainly on the studies of Roer & Nyenhuis (2007), Barsch (1996), Burger et al. (1999) and Janke et al. (2013). Each criterion presented in Table 2 can be used to evaluate a rock glacier’s dynamic status. The characteristics criteria were adapted for the specific environmental conditions of rock glaciers in the semi-arid Andes.

Table 1. Evaluation of geomorphological, geomorphometric and environmental characteristics for the determination of rock glacier activity in the semi-arid Chilean Andes (Slightly modified after Roer & Nyenhuis, 2007).

Method / indicator Determined by Data type Active vs. inactive Inactive vs. relict Acttive vs. relict
Slope angle of rock glacier front Slope angle: steep/flat Quantitative Not suitable Deficient Good
Geomorphological appearance of rock glacier front Micro-scale geomorphic forms indicating movement Descriptive Very good Deficient Very good
Tonal appearance of rock glacier front on air-photos or satellite images Presence of light tones on slope front Descriptive Very good Good Very good
Vegetation or lichen abundance Spatial distribution Descriptive Not suitable Not suitable Not suitable
Geomorphological appearances of the surface relief Presence of ridges and furrows Descriptive Deficient Deficient Good
Appearance of rocks on rock the rock glacier surface Degree and position of rock weathering Descriptive Deficient 1 Good Very good
The stability of large rocks on the rock glacier surface Large rocks moveable by hand Descriptive Deficient 2 Good Very good
Ocurrence of ice outcrops Location of feature Descriptive Not suitable Very good 3 Very good
Occurrence of thermokarst Location of feature Descriptive Not suitable 4 Very good Very good
Basal Temperature of snow(BTS) Ground Surface Temperature (GST) Temperature measurements under a cover snow of at least 0.8 m Quantitative Not suitable 5 Good
Measurements of velocity GPS survey Quantitative Good Very good Very good
Perennial snow patches Location of feature Descriptive Not suitable 6 Not suitable / Good Not suitable
Measurements of water temperature coming from the rock glacier Temperature measurements of spring water Descriptive Not suitable 7 Good Good

Main Citations

  1. In general, active and inactive rock glaciers tend to have rocks that do not appear weathered; moreover, there are clear signs of overturning on the rock surface. On the other hand, relict rock glaciers have rocks fragments that appear to be weathered and have lichen growth.

  2. In active and inactive rock glaciers, it is possible that some large rocks can be moved by hand; in contrast, in a relict rock glacier, large rocks have settled and are impossible to move with the force of one person.

  3. The occurrence of ice shows that a rock glacier is not a relict but active or inactive; in contrast, the absence of ice outcrops is irrelevant to state rock glacier activity.

  4. The absence of thermokarst does not necessarily mean that the rock glacier is active or inactive; in contrast, the presence of thermokarst might indicate that a rock glacier is active or inactive, but not a relict.

  5. In the Alps, BTS >-2?C indicates the absence of permafrost conditions; in contrast, BTS <-3?C indicates the probable presence of permafrost. Thus, BTS can be used as indicator to distinguish active-inactive rock glaciers from relict forms; however, the interpretation of BTS and GST temperature thresholds in the semi-arid Andes should be calibrated in the context of local conditions (Lewkowicz & Ednie, 2004; Brenning et al., 2005).

  6. The absence of perennial snow patches does not necessarily indicate the dynamic status of rock glaciers; however, perennial snow patches are indicators of permafrost conditions and thus of active-inactive rock glaciers (Haeberli, 1975).

  7. A temperature near 0?C implies that water is flowing over ice; thus, it indicates an active or inactive rock glacier’s dynamic status; however, a high temperature does necessarily mean that there is no ice within the rock glacier (Haeberli, 1975).