The volcano Popocatépetl, situated 72 km. southeast of the City of Mexico (Fig. 1, p. 391), is one of several volcanoes along the east edge of the Valley of Mexico. The highest point, on the west crater rim, is 5452 m. (17,887 ft.). The volcanic range of Iztaccíhuatl is 14 km. north of Popocatépetl and is 5288 m. (17,342 ft.). These two volcanoes are the southernmost of six peaks comprising the Sierra Nevada, a mountain range bordering the Valley of Mexico on the east (Fig. 1). Popocatépetl rises 3195 m. above the Valley of Mexico, and also rises 4550 m. above the tropical region of the Balsas Basin several kilometers to the south. The volcanic cone with a surface slope of 27 to 30 degrees on which the firn field lies is symmetrical, yet the conical area of the summit is small owing to a deep vertical-walled crater with a rim circumference of about 2.4 km. (1.5 miles).
Fig. 1. (Above, left) Index Map of Mexico. Drainage Map of the Sierra Nevada and vicinity. (Below, left) Map of volcano Popocatépetl, indicating firn limit around crater
During geologic investigations of the late Pleistocene history of the volcano in 1949, 1950 and 1953, observations herein recorded were made on its firn field.
Description of the Firn Field
The north, northwest and west sides of the cone are covered with a permanent firn field. This extends around the north rim of the crater from the east side to the highest peak on the west side, and beyond to the southwest (Fig. 1, p. 391; Fig. 2, p. 393). As determined from vertical aerial photographs taken in November 1945, and from personal observations, the firn limit on the east side is at approximately 4950 m., on the northeast side from 4800 to 4900 m., varies on the north side from 4700 to 4573 m. (15,000 ft.) at the end of a tiny glacier, and on the west side from 4700 to 4800 m. Firn is absent on the south side; although snow falls there, it melts rapidly. Snow that blows over to lower altitudes on the west side remains for several months longer (Fig. 3, p. 393).
Fig. 2 View of north side of Popocatépetl, taken at 3882 m., 27 July 1949. Two small glaciers not present in 1945 are in center of photograph. Crevasses shown here were not present in 1953
Fig. 3 View of west side of Popocatepetl, taken at 4154 m., 31 July 1950. Recent snowfall hides the lower limit of firn field above the vertical lava cliffs
A deep theaterlike valley or barranca is eroded into the older northwest side of the volcano. Cliffs comprising the headwall of this older barranca are separated from the northwest side of the higher and younger cone by a col at 4827 m. In November 1945, when the photographs were taken, no snow in this col existed. In July and August 1950 during the summer period of accumulation, snow was 1–5 m. thick in the col, on the lava cliffs below to the west and at the base of these cliffs (Fig. 3). Snow was absent in the col in May 1953; probably this snow melted completely during the first intense ablation period (February through May 1951).
The 1945 photographs show no expansion of the firn edge as small glaciers on the north side of the cone, yet in 1949 two conspicuous glacier tongues extended down into the head of the largest and longest of three valleys, the Ventorrillo Valley (Figs. 1 and 2). The glacier tongue (measured in August 1950) was about 100 m. long, 15 m. wide, and was composed of firn of irregular-shaped ice crystals, 0.5–1.2 cm. in length. Miniature lateral moraines of volcanic rubble 1–2.5 m. wide and 1.5–2.5 m. high were pushed up on both sides of the glacier. The end of this glacier in May 1953 (Fig. 4, p. 393) was melted back 4 m. from its 1950 terminal position at 4573 m. altitude. The measurement made in 1950, however, was near the end of the summer accumulation period, while that made in 1953 was near the end of the ablation period. These different terminal positions of the little glacier represent the expected seasonal change or annual change. Significant figures regarding recession or advance will be available after additional observations are made during the next decade.
Fig. 4 Small glacier extending into head of Ventorrillo Valley from firn field, to altitude of 4569 m. on 16 May 1953. Person circled provides scale
The thickest part of the firn field is on the northwest side of the cone where, as determined by depth of crevasses open down to bedrock, it was approximately 40 m. thick in 1951. Oblique aerial photographs taken in 1942 reveal no crevasses through the snow on the firn. The 1945 photographs show that one crevasse had opened; it was 225 m. long. In July 1949 four crevasses approximately 300–400 m. long appeared, as well as several smaller ones. By August 1950 most of the smaller ones were bridged by snow; two of the larger crevasses were too wide and deep to be filled by the end of the summer accumulation period. In May 1953, however, the lower crevasses had disappeared and new wide ones opened in the firn up on the north rim of the cone.
Precipitation Regimen on the Cone
The climate of Popocatépetl is determined in part by its latitude and its altitude. Four climatic zones ring the Sierra Nevada. At the lower elevations of the Valley of Mexico to the west (2257 m.) and the Valley of Puebla to the east (2162 m.), a warm temperate plateau climate exists with dry winters and cool summers. A warm, temperate, rainy climate, with no conspicuous dry season, prevails around the slopes of the Sierra Nevada. The upper slopes of Iztaccíhuatl and the cone of Popocatépetl have a tundra climate and an ice cap climate.Reference Tamayo 1
Winter accumulation added to summer accumulation enables a firn field to remain and flourish on Popocatépetl throughout the year. Based upon evaporation records for Iztaccíhuatl, the City of Mexico and of Puebla, maximum evaporation on the cone of Popocatépetl occurs in March, April and May.Reference Ward, Brooks, Köppen and Geiger 2 Owing to summer cloudiness, the least amount of evaporation takes place in July, August and September. Data on precipitation, temperature and cloudiness for Iztaccíhuatl,Reference Robles Ramos 3 as well as data on personal observations, indicate that loss of firn and snow from melting increases on Popocatépetl from January through May, with the greatest loss in May. Penitentes appear most magnificent in late May (Fig. 5, p. 393). Greater cloudiness and snowfall reduce melting June through January, except for October when skies are clearer with less precipitation.
Fig. 5 Penitentes one meter high in firn field on east side of cone of Popocatépetl, at altitude of about 5030 m., 17 May 1953.
Maximum accumulation occurs in November, December and January, with additional nourishment in the summer from early June through September. The chief form of nourishment is snow, although much hail accumulates in June through August, and there may be some secondary accumulation produced by refreezing meltwater. Most storms are easterly and blow snow westward across the cone. This blown snow accumulates on the north, northwest and west sides. Snow also falls at lower altitudes, down to 3870 m., but it melts. Some falls into the huge crater and is melted. In 1945 a small lake was in the crater bottom; in 1953 it was absent. The sun’s rays fall onto the north side of the cone at an acute angle for most of the year. The south side receives the sun’s rays more normal to its slopes for most of the year except in summer. As a result accumulation of snow is hindered on the south side, and the great amount of melting possible during the summer months on the north side is reduced due to the greater amount of cloudiness at that time. The amount of firn or snow melted by internal volcanic heat today is not known, but is considered insignificant. Fig. 6 (p. 391) presents an estimate of the precipitation regimen for the cone of Popocatépetl. Accumulation may exceed ablation during the winter months; from February through May ablation exceeds accumulation. Additional accumulation from early June to mid-September, added to that for the winter, results in annual accumulation more than equalling annual wastage and in a permanent firn field on the mountain top.
Fig. 6 Diagrammatic estimate of precipitation regimen on the cone of Popocatépetl
Historic Fluctuations of the Firn Field
The firn field undoubtedly is one of those in North America that did not survive the post-Wisconsin xerothermic interval (Climatic Optimum).Reference Matthès and Meinzer 4a , Reference Matthès b That it did exist at least by pre-Spanish conquest times is suggested in Toltec and Aztec sketches of the mountains which show much ice or snow on both Iztaccíhuatl and Popocatépetl. Its existence as early as 1519 is known through the works of the historian Prescott and others.Reference Dollfus, Montserrat and Pavie 5 , Reference Weitzberg 6 Comments made by mountain climbers are useful in firn field reconstruction, as, for instance, those by Packard,Reference Packard 7 who stated that he found in 1885 a stream flowing in the largest valley on the north side of the cone “fed by the snows of the peak.” No stream flows in that valley today, the Ventorrillo Valley, except on sunny days from midday to late afternoon. By 1906, when Tempest AndersonReference Anderson 8 climbed the mountain, he found the glacier in the Ventorrillo Valley nearly down to about 4335 m., as near as can be estimated from his photograph. A photograph of the north side of the volcano taken by Hugo Brehme of the City of Mexico in 1910 shows the glacier in Ventorrillo Valley at a point estimated to be 4390 m. WeitzbergReference Weitzberg 6 pictures the glacier in Ventorrillo Valley in a photograph taken just prior to the 1920–21 eruptions of the volcano, at an altitude estimated here to be at approximately 4435 m.
The 100 m. recession in altitude of the firn field from 1906 to 1920 is in agreement with field evidence of recent glaciation in the upper part of Ventorrillo Valley. This recession is distinguished by (1) fresh striations and glacial polish on knobs of bedrock on the valley floor at 4440 m. and also throughout the upper part of the valley, and (2) comparison of these fresh striae with weathered and disintegrated striated surfaces on bedrock knobs on the valley floor at 4335 m. and 4236 m. All this glaciation is after the post-Wisconsin xerothermic interval, even that below the point where Anderson found the glacier in 1906. The evidence of much older glaciation, probably late Wisconsin in age, in Ventorrillo Valley down to where the valley emerges from the steeper part of the cone and beyond a few hundred meters onto the north flanks of the volcano to an altitude of 4150 m., reported elsewhere,Reference White 9 is in the form of (1) striated and polished surfaces on lava flows on the valley floor, (2) a ridge of volcanic rubble (an end moraine) that almost crosses the valley, (3) an inner ridge (a lateral moraine) on the east side of the valley, and (4) striated blocks in the ash of a dissected alluvial fan a few meters north of the moraine.
When WaitzReference Waitz 10 climbed the mountain in October 1920 during one of the 1920–21 eruptions, he saw the glacier still well preserved in Ventorrillo Valley and attributed its salvation to its sheltered position. He estimated the lower limit of ice at that time to be approximately 4800 m. How much of that recession might be due to volcanic activity is not known, but the growth again and continued presence of the firn field since 1921 attests to an annual surplus of accumulation over ablation. From 1922 to 1942 the status of the firn field, as might be ascertained from written or pictorial records or from communications with numerous mountain climbers, is not yet determined.
As expected, the margin of the firn field on Popocatépetl fluctuates seasonally and annually. Marginal fluctuations on a larger scale occurred in the historic past and in the geologic past. The present climatic situation is sufficient to promote a firn field which appears to prosper. Its current expansion is evident in the appearance of two small glaciers on the north side since 1945, and in the growth, movement, disappearance and replacement of crevasses. Barring volcanic interruptions, the balance between accumulation and ablation either in favor of advance or of recession will be revealed in the future subsequent to additional observations and to application of glaciological studies.
Acknowledgements
The 1950 field season was partially financed by the Bache Fund of the National Academy of Sciences. Both 1949 and 1950 seasons were sponsored by Syracuse University Department of Geology (Syracuse, N.Y.), Syracuse Museum of Natural Science, and Instituto de Geología, Mexico. The help of E. T. Apfel of Syracuse University is appreciated. J. L. Lorenzo of Mexico assisted in 1949; J. W. Blagbrough and R. F. Novotny, both then of Syracuse University, assisted in 1950; E. de Mária y Campos of Mexico determined depth of crevasses in 1951.
