Search

Satellite images and ground-based observations were used to investigate the morphology and origin of valleys and ridges across large ignimbrite provinces around La Pacana Caldera, N. Chile. We studied 12 separate sites that included five different ignimbrite units that range in age from ∼ 5.6 ± 0.5 Ma to 1.35 ± 0.15 Ma. Over 400 features from within these study sites were quantified. Study sites typically have dimensions of 25 × 35 km. Measurements were made using Landsat TM, ETM+, and SRTM-derived DEM and Shaded Relief images. Morphological measurements were made of the ridges and channels, including the direction of regional slope, length, orientation and gradient of each feature. A classification of the different ignimbrite erosion influences, resulting in varying landscape morphologies identified 4 landscape types; Fluvial, Eolian, Modified and Complex. We conclude that the most prevalent features are fluvial-derived channels but the most distinctive are linear, wind-derived ridges. The orientation of these ridges suggests that strong uni-directional winds from the northwest have been constant over a long period of time (> 1 My).

This paper re-examines recent ground-deformation measurements at Volcan de Colima, Mexico, to test the hypothesis that the observed movements are in response to pressure changes within a sub-volcanic magma chamber, as suggested for other volcanoes by [Mogi (1958) Earthq. Res. Inst. 36, 99-134]. Measurements of vertical ground deformation across the summit dome complex of Volcan de Colima from a precise levelling network between 1982 and 1999, together with vertical and horizontal displacements derived from dual-frequency GPS measurements in 1994 and 1997, show continuous subsidence. The deformation pattern derived from the levelling shows that subsidence increases towards the summit. The closest stations to the summit (1.1 km distant) show a mean subsidence rate of 5 mm per year compared to the reference station at 2.3 km distance, which may be subsiding itself. Vertical displacements of individual stations since 1982 show that the subsidence has been fairly continuous. The GPS stations, which are distributed more widely and include some close to the edge of the active dome, confirm summit subsidence. The largest measured value, a decrease of 280 mm, or 93 mm subsidence per year, was obtained at the edge of the dome. Horizontal displacements measured during 1994–1997 also show the largest values at the summit, but these are much smaller than the vertical displacements, with the maximum rate of 23 mm per year recorded close to the dome. It is conclusively shown that these measured movements cannot be due to deflation of a buried Mogi source, as vertical and horizontal displacements for some stations are in contrary directions to those predicted by the model, and there is no consistent pattern to the horizontal movements. We attribute the measured deformation to downslope creep, settling and compaction of the edifice, gravitational spreading, or a combination of these processes.