Looking at recent satellite images of the Southern Patagonian Icefield, my attention was drawn to a remarkable debris lump on the surface of Glaciar Río Túnel Inferior, a relatively small glacier in Argentina just East of the icefield (see Glaciar Río Túnel in OpenStreetMap). As I had not seen this debris patch before, I did a bit of research on recent and older satellite images to learn more about it. This is nowadays a fairly straightforward task thanks to the conspicuous archive of freely available satellite images from programs like USGS Landsat and ESA Copernicus (Sentinel).
On the most recent Sentinel 2 image (from 2021-03-03), the debris patch is approximately 600 m wide by 700 m long (in the direction of flow), with an area of about 0.3 km2, and a shape coarsely resembling a drop. An approximately 50 m long apron is visible in the most distal parts (closest to the tongue), where it is apparently steepest. This apron is apparently shallower and less prominent in the proximal parts (towards the head of the glacier). There seems to be a small pond near the center. It is not possible with the data at hand to calculate the volume of rock involved here, but a very crude estimate can probably be made by assuming that the debris thickness may vary between 0.1 and 1 m, with a geometric mean of 0.3 m. This gives a coarse estimated volume of the order of 100000 m3.
Looking back in time with the help of the satellite data, it is possible to track the evolution of this debris lump as it rode down the glacier with the flow during the last 11 years. The furthest back in time I could clearly observe the lump in the accumulation area of the glacier is on a Landsat 7 image of 2010-03-29 or, with some effort, on a Landsat 7 image from from 2010-01-24, although in this case some freshly fallen snow makes detection difficult. In the Landsat 7 image taken on 2010-03-29, the lump seems to appear just out of the accumulation area before the crevasse zone near the snowline.
How did this debris accumulation form? The simplest and more straightforward hypothesis is that at some point before 2010 a rock avalanche near the head wall of the glacier deposited this material in the upper accumulation zone. Ensuing snowfalls may have buried the debris, which later on appeared down stream of the glacier as the ice flow transported the patch into the ablation zone, where it became visible.
However, I get the impression that a rockfall from one of the head-walls and involving such a fairly large amount of material might have spread debris into a wider area, instead of leaving a quite coherent heap. It is thus possible that it wasn’t an avalanche at all, but perhaps a gentler rock topple, or a rock slide that laid the material rather softly on the accumulation zone. Another interesting hypothesis could be that this lump was created out of material incorporated from the bed of the glacier.
I have searched all satellite images I could find from 1984 on-wards and could not find any trace of avalanche or rock deposition. This is not so significant as it seems, after all the coverage by sequential satellite images available before 2005 or 2000 is sparse and leaves long periods of time between images. The question then remains open until more evidence is available.
But whatever the explanation for the origin it will be an interesting and fun exercise to continue monitoring this debris lump as it makes its way towards the tongue in the coming years.
Thanks for reading.