Seismometers near the volcano detected rates of rockfalls and explosions that were markedly lower in January and February (BGVN15:11 for a description of the Observatory, including material originally in BGVN 16:02]. More than 20 pyroclastic flows and lateral blasts were observed between April and early December 1990, but none have been reported since then. SO₂ emission measured by COSPEC (from 3.4 km S of the dome's Caliente vent) was 35 ± 7 t/d on 8 February and 62 ± 24 t/d the next day, similar to the 48 ± 15 t/d of 22 February 1990.

Figure 19. Santiaguito Dome complex (left background) with a small plume emerging from Caliente Vent at left-center. Santa María volcano is at center background, partially obscured by clouds. At right foreground is the new Santiaguito Volcano Observatory, at Finca El Faro, [7] km S of the dome.

Geological Summary. Symmetrical, forest-covered Santa María volcano is part of a chain of large stratovolcanoes that rise above the Pacific coastal plain of Guatemala. The sharp-topped, conical profile is cut on the SW flank by a 1.5-km-wide crater. The oval-shaped crater extends from just below the summit to the lower flank, and was formed during a catastrophic eruption in 1902. The renowned Plinian eruption of 1902 that devastated much of SW Guatemala followed a long repose period after construction of the large basaltic-andesite stratovolcano. The massive dacitic Santiaguito lava-dome complex has been growing at the base of the 1902 crater since 1922. Compound dome growth at Santiaguito has occurred episodically from four vents, with activity progressing E towards the most recent, Caliente. Dome growth has been accompanied by almost continuous minor explosions, with periodic lava extrusion, larger explosions, pyroclastic flows, and lahars.

Information Contacts: Rodolfo Morales and Gustavo Chigna, INSIVUMEH; William I. Rose, Robert Andres, and Kimberly Kogler, Michigan Tech.