The Caliente vent at Santiaguito continues to be in a state of unusually strong pyroclastic activity, a condition that began in April 1975 and has been confirmed by every reported observation since that date. The intensity and number of the explosions has varied, but observations are too infrequent to be sure of trends. The volcano's lack of visibility from inhabited locations has limited recorded observations to about 30 different days since early 1975. On all of these dates, pyroclastic activity from the Caliente vent was noted. The frequency of explosions was typically 0.2-4.0/hour and the heights of ash clouds ranged from 300 to more than 6,000 m.

Especially large ash eruptions, with clouds to heights of more than 6 km, were observed on 7 and 9 May, and in early June (exact date unrecorded) 1976, and on 9 and 21 February and 14 and 19 March 1977. Most of these larger events resulted in ash fallout at nearby towns and cities. No nuée ardente activity has been reported in the recent activity period. Lava and/or dome extrusion at the El Brujo vent has continued, but has slowed since 1975.

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: W. Rose, Jr., Michigan Tech. Univ.