Iztaccihuatl is about 40 miles (60 km) southeast of Mexico City. It is in the Trans-Mexico volcanic belt. Iztaccihuatl is a stratovolcano made of layers of viscous lava flows, flow breccias, and tephra. Andesite and dacite are the common rock types.
The volcano began to form about 900,000 years ago. Growth of Iztaccihuatl was in two phases. The older phase (900,000 to 600,000 years ago) constructed a large shield volcano with a summit caldera. Cones and lava flows erupted on the flanks of the shield. The younger phase (younger than 600,000 years ago) consists mostly of lava flows and pyroclastic material erupted from the summit of Iztaccihuatl and on the flanks of the volcano. Volcanism ended about 80,000 years ago. The photo above shows Iztaccihuatl (bottom right) and Popocatepetl (top left).
Sources of Information:
Photographs copyrighted and provided by Steve O'Meara of Volcano Watch International. Nixon, G.T., 1989, The geology of Iztaccihuatl volcano and adjacent areas of the Sierra Nevada and Valley of Mexico: Geological Society of America Special Paper 219, 58 p.
Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.
White, S.E., Iztaccihuatl, Mexico: Volcano News, 23, p. 1-3.