The ice reaches thicknesses of only and areal extents of a few hundred metres. Meltwater feeds streams. Climbing parties in 1956 reported two glaciers on the northwestern slope, a 1958 report indicated that an ice body at elevation descends into two branches and is followed at lower elevation by another glacier also with two branches—but in neither case with any evidence of movement—and in 2014 there was ice in the summit crater and substantial glaciers on the eastern and southern slopes, which reached elevations below . There have been increases in ice area between 1974 and 1983 but between 1986 and 2000, ice area decreased by 40%. The melting of the ice is expected to produce an increased discharge at first, but eventually ice diminishes to the point that runoff will decline. Penitentes have been encountered by climbers as early as 1937, in 1949 there were reportedly high penitentes on Ojos del Salado. Penitentes are high ice spires which form when ice sublimates in the intense insolation. Subsurface ice Ice buried beneath sand and encased in moraines is more important than surface ice at Ojos del Salado. It is retreating but the insulating effect of the cover slows the retreat. Cryokarst, erosional gullies and so-called "infilled valleys" have been observed; they most likely form when buried ice and snow melt. The combined effect of erosion by the meltwater and the disappearance of ice volume creates cavities that collapse and form the valleys and pseudokarst landforms. Pseudokarst landforms and dolines are other structures generated by the melting of buried ice. Past glaciation Lateral moraines altered by wind erosion occur north of Ojos del Salado and some lava flows bear traces of glaciation. Research published in 2019 found cirques and U-shaped valleys on Ojos del Salado. However, there is no evidence of Pleistocene glacier advances in the region nor any indication of a Pleistocene snowline, although cirques have been reported from Nevado Tres Cruces and some sources propose the existence of glaciers 19,000 years ago. The monsoon reached farther south during the Pleistocene but did not reach Ojos del Salado, allowing the development of glaciers only at more northern latitudes. Westerly winds did not regularly influence the climate at the volcano, either. Geology In South America, there are about 200 volcanoes with evidence of eruptions during the Pleistocene and Holocene along the western coast, where the Nazca Plate and Antarctic Plate subducts