Characterization of a tropical ice body on Iztaccihuatl volcano, Mexico

In the last century ice bodies have changed significantly worldwide in response to climatic changes, apparently enhanced during the last decades, presumably owing to anthropogenic influence. Shrinkage of glaciers in the northern latitudes has been extensively documented; however, the behavior of glaciers in tropical or inter-tropical latitudes has been difficult to document. In Mexico, small glaciers are present in the highest mountains: Citlaltepetl (5675 in), Popocatepetl (5452 in) and Iztaccihuatl (5286 in), all of them volcanoes. The glaciers of Mexico offer important clues about climate change at this latitude. Current inventory and monitoring of Mexican glaciers includes determination of glaciated areas and volumes. A survey has been carried out using ground-penetrating radar in order to get a figure of thickness and a preliminary volumetric estimate of an ice body of Iztaccihuatl volcano named Ayoloco glacier. A radar profile 415 in in length was obtained at a mean altitude of 5000 in with an azimuth of 190degrees, in a relatively flat area known as La Panza on top of the mountain. 100 Mhz antennas were used in the reflection mode with a separation of 2.5 in and a step-size of 2.5 in. A common mid-point survey was performed comprising 12 stations, at 0.5 in steps, to determine the wave velocity propagation in the ice-rock mixture that constitutes the glacier. Results yield a value of 0.17 m/ns, which is close to the reported value for ice of 0.16 m/ns. We find a layered structure perturbed by faulting at various inclinations, and an interface between the glacier and the underlying rock along a concave surface, shallow at the profile's extremes. There appears to be a transition at 50 in depth, from brittle to ductile ice. Since Iztaccihuatl volcano has many craters near the summit, we infer that this profile reveals the shape of one of the craters, which has been filled by the glacier's ice and a mixture of rock fragments and volcanic debris. The estimated upper volume of the Ayoloco glacier is 10.04 X 10(6) m(3); this figure will have to be revised when more, perpendicular GPR transects are performed.