Carrasco-Núñez et al. (1993) proposed that the presence of glacier ice and a very active hydrothermal system during late Pleistocene time provided a constant supply of pore water, which enhanced the hydrothermal alteration of the summit of Citlaltépetl and was the origin of most of the water for the lahar. The intense hydrothermal alteration seems to be related to an acid-sulfate leaching process where sulfates are added, while mobile elements are removed from the surroundings rocks to form clay, silica, and sulfate minerals.
	Plot of the degree of hydrothermal alteration (area and intensity) versus and the area covered by glacial ice for the Cascade volcanoes and Citlaltépetl (from Carrasco-Núñez et al., 1993). 0-no alteration; 1- small areas of moderate alteration; 2- moderate alteration; 3- large areas of moderate alteration; 4- large intensely-altered rocks. C-Citlaltépetl, GP- Glacier Peak; MA-Mount Adams; MB-Mount Baker; MH-Mount Hood; MJ- Mount Jefferson; ML- Mount Lassen; MR-Mount Rainier; MS-Mount Shasta; MSH- Mount St. Helens; TS-Three Sisters.