The Caliente lava dome at the Santiaguito dome complex continued actively erupting during the first half of 2015. There were at least 50 reports of ash explosions during January-June 2015 from Guatemala's INSIVUMEH (Instituto Nacional de Sismologia, Vulcanologia, Meterologia e Hidrologia) or the Washington VAAC (Volcanic Ash Advisory Center). There were also numerous avalanches, several reports of incandescence and audible degassing from the crater and the lava flow, and continued growth of the dacitic lava flow that first erupted in May 2014.

Explosions, ash plumes, and ashfall. Three times in January 2015 INSIVUMEH reported explosions generating ash plumes that rose from 500 to 700 m above the dome. Plumes drifted SW, and minor ashfall was reported each time in Palajunoj (20 km SSW) and once in the village of San Marcos (11 km SW). Washington VAAC reported three additional ash plumes. Each was observed by satellite, rose to an altitude of 4,300 m, (1,800 m above the dome) and drifted 18- 24 km SW or SE.

In February, Washington VAAC reported one ash plume at 4,300 m (1,800 m above the dome) drifting SE, and INSIVUMEH reported five additional days with explosions sending ash-bearing plumes to heights of 200- 800 m above the dome. Ashfall was reported 5 km E; 9-19 km SW in Loma Linda (9 km), San Marcos (11 km) and Palajunoj (twice); approximately 6 km SSE at the Santiaguito Observatory and at the Patzulin farm; and 5 km SE to the village of Santa Maria de Jesus. Generally constant emissions of white steam rose 300 m above the cone and smaller explosions that did not generate ash plumes were also frequently reported.

For March and April, ash-bearing plumes were reported at least 11 times. Two Washington VAAC reports had plumes drifting W at 4,900 m and 4,600 m altitude (2,400 m and 2,100 m above the dome) as far as 18 km from the dome, and one drifting W at 3,400 m altitude (900 m above the dome). INISVUMEH reported plumes rising to between 300 and 800 m above the dome, and producing minor ashfall 20 km SSW in the Palajunoj area seven times; once as far SSW as Retalhuleu (27 km); and SE about 5 km to San Jose Maria farm.

Explosive activity at Caliente increased in May with 13 days of ash-bearing explosions, and many small explosions within the crater and avalanches around the crater rim. INSIVUMEH reported all ash plumes rising to either 500 or 600 m, and drifting from SE to SW, generally up to 20 km. Ashfall was reported in the villages of Calaguache 10 km S, Santa Maria de Jesus 5 km SE, and Palajunoj, and their surrounding farms. The ash plumes reported by Washington VAAC extended 24 km SE, 18 km SSE, 20 km SSW, 16 km SW, and 24 km WNW at altitudes that were 4,300 to 4,600 m (1,800 to 2,100 m above the dome).

Strong, audible degassing from the crater was reported several times during June by INSIVUMEH. They noted explosions with ash-bearing plumes seven times; the plumes rose to between 300 and 800 m above the summit of Caliente, and ashfall was reported six times in the area of Palajunoj, and once in Quetzaltenango 10 km to the N. Washington VAAC reported three additional ash plumes rising to 4,600 m altitude (2,100 m above the dome) and drifting up to 55 km W and WNW.

Dacitic lava flow. The lava flow that began during the 9 May 2014 eruption had traveled more than 3.5 km from the summit and was still incandescent at the front of the flow into early February 2015. Weak to moderate avalanches from the flow were observed through February, after which reported activity tapered off. MODIS/MODVOLC (Moderate Resolution Imaging Spectroradiometer) thermal imaging data also support the observations of a decrease in activity and cooling of the lava flow by March 2015. Thirteen pixels of thermal anomalies were recorded over the volcano in January, with only three in February, two in March, and one in April. On 5 May 2015, INSIVUMEH noted avalanches generated from a possible new flow emerging from the top of the cone, but no MODIS/MODVOLC anomalies were recorded for May.

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 Hydrologia (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/inicio.html); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).