Santa María, Guatemala: Bimodal soda-rich calc-alkalic stratovolcano
Santa María has a three-phase history of activity: a long period of composite cone growth, a dramatic plinian eruption and a dome-extrusion period. The volcano has produced approximately equal volumes of basaltic andesite and dacite, with a clear compositional gap. The basaltic andesite was mainly erupted at slow rates in the cone-building stage, as a sequence of lavas which generally became more silicic and more enriched in incompatible elements. The plinian eruption of 1902 was a brief homogeneous outpouring of dacite with a very minor proportion of discrete basaltic andesite magma, which followed an exceptionally long repose. The dome-building period has been a pulsating series of extrusions of compositionally monotonous, degassed dacite which has continued to the present. The extrusives of each phase share common characteristics of relatively high Na2O, and identical Sr isotope ratios. The cone-building period of Santa María is thought to represent the development of an open-system magma body which grew in size because of increasing inhibition of extractions of magma to the surface. This large magma body may have resided at the base of the crust, and transmitted magma in batches to the near-surface region where more fractionation often occurred before eruption. The long repose before the plinian eruption probably stabilized conditions at the top of the deep open-system magma body and caused development of a silicic cap. The overall rate of magma production in the mantle below Santa María may be constant at about 1.5–2 km3/1000 y., and the key element which influences the style and composition of lavas that reach the surface is the tendency for magma to reside for long periods at the crust/mantle interface. As the cone at the surface grew, the inhibition of surface eruptions increased and finally became complete. Detachment of the silicic cap and its eventual rise renewed activity in the plinian and dome-building phases
Journal of Volcanology and Geothermal Research
Rose, William I., "Santa María, Guatemala: Bimodal soda-rich calc-alkalic stratovolcano" (2003). Department of Geological and Mining Engineering and Sciences Publications. 122.