SIR-B Radar Imagery of Volcanic Deposits in the Andes

The Andean Cordillera of western South America between 16°S and 28°S and south of 34°S latitude have been two of the most volcanically active areas in the world for the last 25 million years (Ma). Synthetic-aperture radar imagery from the Shuttle Imaging Radar-mission B (SIR-B) was collected in October 1984 over the central Andes between 20°S and 24°S and also south of 42 °S. Despite signal-strength problems that drastically reduced the signal-to-noise (S/N) ratio of the images, volcanic features of both areas show up well. In particular, ignimbrite sheets formed by large explosive eruptions stand out as very strong radar reflectors due to their characteristically rough erosional surface. High backscatter is apparently caused by erosional relief on the ignimbrites at scales ranging from the radar wavelength (23 cm for SIR-B) to the 30-200 m scale of quebradas (gullies and canyons). The quebradas are steep-sided and straight, and have a similar southeast orientation in ignimbrites separated by as much as 450 km spatially and 6 Ma temporally. The consistent regional erosional pattern appears unrelated to the emplacement of the ignimbrites, and is probably caused by preferential eolian erosion in the direction of the prevailing wind. Hand-held space photographs, ground observations, and Landsat Thematic Mapper (TM) imagery support the interpretation of the ignimbrite radar signature. The Chilean volcano Michinmahúida (43°S) was imaged by four radar data takes at different incidence angles, which show tectonic, glacial, and volcanic features of that nearly inaccessible and often cloud-covered region.