Santa María is estimated to be about 30,000 years old, based on a dated paleomagnetic anomaly which occurs on the composite cone lavas that may correlate with a dated anomaly in Mexico (Rose et al., 1977). Santiaguito dome was formed in 1922 as a result of the eruption on the southwestern flank of Santa María.

Correlating the Santa María waveforms with that of the Mono Lake excursion indicates that the latter 40% of the cone's growth occurred in a period of 1000 to 3000 yr, and that the bulk of the cone was emplaced prior to 25 ka. The rates of flux of the volcano suggest steady-state conditions. Assuming this, between 2500 and 7500 yr were required to construct the Santa María composite volcano. Therefore, the onset of volcanism would have begun between 27.5 to 32.5 ka. Rose et al. (1977) suggest that the cone was emplaced over the course of 30 ka; however, this evidence indicates that cone construction was completed at 25 ka. The cone construction was followed by a long period of repose punctuated by rare extrusion of small-volume lava flows from flank vents.(Conway et al, 1993)

The 1902 eruption (see eruptive history) brought as much magma to the surface as the cone building (~20 km³) and the Santiaguito dome (~1 km³) combined. The Santiaguito dome was built through eruptions from 1922 to the present.

Santa Maria Volcano: Steady State Model.
(Conway et al., 1984)

Interior of the 1902 explosion crater of Santa María, view from the slope of Santiaguito, looking NW. Light colored layers are flow interiors, darker material is fragmental rock. Photo by Bill Rose, January 1974.