-Barry Cameron, United States

-Michael Jackson, United States

-Luz Luna, El Salvador

-Jim Walker, United States

The primary focus of our group will be the 1902 eruption of Santa María. The eruption involved two very distinct types of magma: one basic (ca. 53 wt.% SiO2), and one silicic (ca. 66 wt.% SiO2). This is a common theme of the largest explosive eruptions in Guatemala. Explosive eruptions at Ayarza Caldera, Atitlán Caldera (the Los Chocoyos eruption), and Cerro Quemado, all involved at least two distinct types of magma. Hence, it is very important to understand such mixed eruptions and how they develop because they are so potentially dangerous.

In detail, we hope to learn whether the distinct magmas coexisted for a long time in a chamber beneath the volcano, and are petrogenetically related, or whether the mixing event involved two unrelated magmas and actually triggered the eruption.

These two contrasting scenarios in turn, have very different implications for dome growth and development at Santiaguito, as well as volcanic gas emission, magma movement and storage, volcanic seismicity and petrologic monitoring.

Existing chemical data on the two types of magma include major and trace element analyses on 1902 pumice fragments. We will add Sr, Nd, and Pb isotopic data on pumices, as well as microprobe analyses on minerals in the pumices. The latter will include detailed study of plagioclase zonation, amphibole occurrence and composition, and their significance for magma mixing.

We think it is important as part of the investigation to include study of the "mafic xenoliths" in the dome lavas of Santiaguito as indicators (residues?) of magma mixing, and of the pre-1902 pyroclastics of Santa María for evidence of earlier mixing events. Major element, trace element, mineralogic and Sr, Nd and Pb isotopic study of the products of both pre- and post-1902 eruptions will complete our understanding of magma mixing and its influence on explosive eruptions at Santa María/Santiaguito.

We strongly recommend such a petrologic/geochemical study of the 1902 eruption and further recommend the following petrologic/geochemical studies to complement existing information and add to our knowledge of how Santa María works:

1) A melt inclusion study of the 1902 magmatic components, and of the Santiaguito lavas and pyroclastics to determine pre-eruptive volatile contents, and how they change with time.

2) A crystal growth study of plagioclase in the domal lavas of Santiaguito to estimate magmatic residence time below the surface.

3) A U-Th disequilibrium study of the phenocrysts in the lavas and pyroclastics of Santa María/Santiaguito to estimate magmatic residence time below the surface.

4) A complete major element, trace element, isotopic, and geochronologic study of the lavas and pyroclastics of the older volcanic foundation of Santa María in order to completely understand the magmatic evolution of the region.


5) A combined radiogenic/stable isotopic study of the lavas and pyroclastics of Santa María in order to completely understand the magmatic evolution of Santa María and to explain its sodium-rich nature.

6) Continued monitoring of the petrology/geochemistry of the products of Santiaguito to hopefully anticipate any chemical changes foretelling changes in the magmatic substructure.

7) A Nd isotopic study of the products of Santa María, seeking out evidence of isotopic changes before large explosive eruptions.

8) A geochemical comparison of Santiaguito and the dome at Cerro Quemado, including the use of melt inclusions, to ascertain what, if any, chemical characteristics influence endogenous versus exogenous dome growth.