Recent activity at Santa María has been characterized by weak-to-moderate explosions producing ash, crater-rim collapses and avalanches of block lava and ash, pyroclastic flows, and an active lava flow (BGVN 28:10). Activity was similar from October 2003 to June 2004, consisting mostly of explosions from Santiaguito, a lava-dome complex that includes the Caliente vent. The explosions produced ash plumes, and there were numerous block-lava-and-ash avalanches from Caliente collapses.

Activity during October-November 2003. Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hidrologia (INSIVUMEH) reported frequent explosions during October 2003 (BGVN 28:10). The Washingon VAAC noted low-level ash plumes visible in 31 October satellite imagery.

As of 17 November, according to INSIVUMEH, several weak-to-moderate eruptions from the lava dome complex sent plumes to ~ 700 m above the crater that drifted SW. According to the Washington VAAC, a pilot saw a plume above Santa María on 16 November; the narrow plume was visible on satellite imagery extending ~ 35 km W. Small eruptions on 18 and 23 November produced local tephra fall. Small avalanches occurred on 18 November. On 24 November five explosions occurred at 1-minute intervals, producing an ash-and-gas plume that rose to 2 km above the crater and dispersed up to 12 km SSW.

On 28 November the seismic network recorded several explosions. INSIVUMEH noted that many of the explosions were followed by block-and-ash avalanches, which traveled SW and S down the Caliente dome. At least five collapses of megablocks from the S rim of the active vent generated short pyroclastic flows to the base of the Caliente dome. On 1 December ash emissions drifted SE and nearly constant avalanches occcurred in the active lava-flow area.

Activity during December 2003. During 7-9 December, frequent, small explosive eruptions expelled ash to less than 1 km above the crater that dispersed to the NW. Moderate-sized avalanches from the S and SE sides of the dome were recorded during the same time period. Weak-to-moderate explosions continued during 10-16 December. On 10 December ash mainly drifted SE toward Santa María de Jesús and las Majadas. Avalanches traveled S and SW from the fronts of lava flows. According to the Washington VAAC, on 12 December ash clouds were visible on satellite imagery at an altitude of ~ 4.5 km, drifting SW.

During 18-22 December, weak-to-moderate explosions caused plumes to drift mainly S and SE towards the Monte Claro, Monte Bello, La Florida, and El Faro fincas (ranches). Nearly constant avalanches traveled S and SW from the fronts of lava flows. Based on information from Retalhuleu airport, the Washington VAAC reported a minor emission on 18 December. No ash was visible on satellite imagery.

On 30 December more weak-to-moderate explosions sent ash-and-gas plumes 500-700 m high. They drifted SW and deposited fine ash in a mountainous region with several ranches. Avalanches continued to spall off of lava-flow fronts on the volcano's SW and S flanks and occasionally from the Caliente dome.

Activity during January 2004. According to seismic data, during 1-5 January weak-to-moderate explosions occurred, causing block-and-ash avalanches to travel 100-250 m down the volcano's SW and S flanks and the Caliente dome. Small amounts of ash fell around the volcano.

During 7-12 January, several weak-to-moderate explosions and avalanches occurred. A partial lava-dome collapse on 7 January produced avalanches down the SW flank. Many of the avalanches were moderate to strong, lasting 1-2 minutes as they traveled SW and S down Caliente dome. Explosions on 12 January produced plumes to ~ 500 m above the volcano. Ash plumes were also visible on satellite imagery several days during the report period.

On the morning of 15 January a moderate explosion at the dome caused a collapse at the edge of the crater. Volcanic material traveled down the SW flank, reaching the base. Ash rose ~ 900 m above the crater and fell on the observatory. Weak avalanches occurred in the SE portion of the lava dome. On 19 January moderate explosions occurred and avalanches descended the lava dome. The plumes produced from the explosions traveled E, depositing small amounts of fine ash around the volcano, including on the ranches of San Jose, Quina, and San Juan Patzulín.

During 21-27 January, weak-to-moderate explosions continued. Avalanches of blocks of lava and ash descended the S and SW flanks of the Caliente dome and explosions produced low-level ash plumes. Small-to-moderate explosions continued during 28 January to 2 February. During 31 January to 2 February, collapses occurred at the SW edge of the lava dome within the Caliente dome. Ash plumes rose to ~ 1 km above the lava dome, accompanied by small avalanches of blocks and ash. According to the Washington VAAC, on 2 February ash plumes were visible on satellite imagery rising to ~ 1 km above the volcano.

Activity during February 2004. During 4-9 February, small-to-moderate explosions occurred, and relatively weak avalanches traveled down Santa María's SW flank. According to the Washington VAAC, ash plumes were visible on satellite imagery on 5 February ~ 2.3 km above the volcano. INSIVUMEH reported that on the morning of 8 February, an explosion produced an ash-and-gas cloud that rose 1-1.3 km above the volcano and drifted WSW.

During 11-16 February, small-to-moderate explosions produced ash plumes to a maximum height of 1.4 km above Santa María. In addition, avalanches went down the volcano's SW flank. Explosions on 16 February deposited fine ash up to 12 km SW. Moderate explosions continued on 19 February. Plumes rose 0.7-1 km above the volcano and mainly drifted SSW as fine ash fell in the mountainous region around the volcano. On 23 February, avalanches of lava blocks and derived ash moved SW down the dome.

During 25 February to 2 March, weak-to-moderate explosions continued. Ash-and-gas plumes rose to ~ 1.4 km above the crater, and ash fell in the mountainous region around the volcano. Weak-to-moderate avalanches of volcanic material was shed from lava-flow fronts.

Activity during March 2004. During 4-9 March, small-to-medium explosions occurred, producing ash-and-gas plumes to 1.5 km above the crater. Avalanches traveled S and SW. Small-to-medium explosions continued during 10-15 March, producing ash-and-gas plumes to ~ 1.3 km above the crater. A small partial collapse on 10 March sent pyroclastic flows down the SSW flank. During the rest of the period, weak avalanches traveled S and SW.

During 15-23 March, several small-to-medium explosions produced ash-and-gas plumes to ~ 1.5 km above the crater. Incandescent avalanches traveled SW from the lava dome. In addition, ash fell in proximal areas. A partial lava-dome collapse on 17 March sent a pyroclastic flow down the volcano's flanks. Weak to moderate explosions produced plumes up to 1 km high during the week of 24-30 March. Light ashfall occurred in nearby areas on several occasions. On 25 March incandescent avalanches from the S flank of the Caliente dome flowed to the SE. Lahars descended the Nimá I river on 28 March and the Nimá I and Nimá II rivers on the evening of 29 March.

Activity during April 2004. During 31 March to 6 April, weak-to-moderate explosions continued, producing plumes to 1.3 km above the volcano. Several partial lava-dome collapses produced avalanches down the S flank. A strong explosion on 1 April caused a collapse and produced a pyroclastic flow that moved ~ 4 km SW toward the Nimá II river. On 12 April weak-to-moderate explosions sent plumes 500-800 m above the volcano. Avalanches of lava blocks and ash traveled down the S flank.

On 18 April, explosions at the lava dome produced ash-and-gas plumes that rose up to ~ 0.8 km above the vent. Small avalanches of incandescent lava also descended the SW side of the Caliente dome. On 19 April, an ash-and-gas plume rose to ~ 4.5 km altitude and drifted SW.

During 22 April-4 May, explosions produced ash-and-gas plumes that rose to ~ 1 km above the crater. Small incandescent avalanches descended the SW side of the Caliente dome. An explosion on 27 April produced a pyroclastic flow that traveled ~ 3 km to the SW.

Activity during May 2004. During 5-7 May, weak-to-moderate explosions sent ash-and-gas plumes to ~ 900 m above the crater. Small partial collapses at the edge of the Caliente dome produced incandescent avalanches to the SW. Weak-to-moderate explosions continued during 10-17 May, producing ash-and-gas plumes that rose to ~ 1 km above the crater. Small partial collapses at the edge of the Caliente dome produced incandescent avalanches to the SW. On 17 May a lahar traveled S down Nimá River I.

During 18-21 May, weak-to-moderate explosions produced ash-and-gas plumes that rose to ~ 1 km above the crater. Many of the moderate explosions were accompanied by incandescent avalanches. On 20 May aa small partial collapse at the edge of the Caliente dome produced an incandescent avalanche to the SW base of the dome. Weak-to-moderate explosions during 31 May-1 June produced ash-and-gas plumes that rose ~ 1.5 km above the crater. Small collapses at the edge of the dome sent avalanches of incandescent material down the SW flank.

Activity during June 2004. On 1 June, 33 weak to moderate explosions producing plumes up to 1.5 km above the summit were recorded. Collapses on the SW side of Caliente produced small pyroclastic flows that descended to the base of the Caliente and La Mitad domes. During 6-8 June, many weak to moderate explosions sent ash-and-gas plumes up to ~ 1.5 km above the Caliente dome, along with some avalanches and flank collapses. Moderate-volume lahars descended the Nimá Segundo river and San Isidro ravine on 1 and 6 June, respectively.

INSIVUMEH reported that on 18 June weak-to-moderate explosions sent ash plumes to 0.4-1 km above the crater. The plumes drifted W, depositing fine ash. According to the Washington VAAC, satellite imagery showed three ash emissions on the 18th that rapidly moved W, becoming more diffuse near the Mexican border. Weak-to-moderate explosions occurred during 25-29 June. Plumes rose to ~ 1 km above the crater and there were sporadic, weak avalanches. On 28 June a partial collapse sent material down the W side of Caliente dome for ~ 40 minutes.

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: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hidrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).