Twenty-six Pleistocene tephra layers, four ash-flow tuff deposits, and four fluvial-lacustrine sediment sequences, interstratified with paleosols, occur in an area of 1,500 km2 between Guatemala City and Lake Atitlán. Most of the tephra layers and one of the ash flows can be individually recognized in the field by mineralogy of Fe-Mg phenocrysts, texture, pumice color, and stratigraphic position.

The texture, thickness, and distribution of the tephra layers indicate that they originated from five separate centers: the Pacaya volcanic complex, Agua Volcano, Acatenango or Fuego Volcanoes, the Lake Atitlán area, and the Laguna de Ayarza area. The exact sources of the ash flows are not known. Size of pumice clasts, pumice texture, and mineral content show that the basin-filling ash-flow tuff units are of primary origin and could not have resulted from inwash of previously deposited pumiceous tephra. Each ash-flow tuff apparently was deposited shortly after a tephra eruption.

Reconnaissance work indicates that the H ash-flow tuff unit and its underlying tephra cover a major portion of the volcanic highlands — at least 16,000 km2 and 7,500 km2, respectively. Their source probably lies close to the Lake Atitlán area. These units should be extremely useful as time-stratigraphic marker horizons in other parts of the Guatemalan highlands.

Radiometric dating indicates that most major tephra and ash-flow deposits were deposited between 40,000 yr and 1.84 m.y. ago. Were similar eruptions to occur today, widespread devastation and loss of life could result.

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