Based on notices from JMA, the Tokyo VAAC reported that during 18-22 May ash plumes from Aso rose to altitudes of 1.5-2.1 km (5,000-7,000 ft) a.s.l. and drifted N, NE, and SE. A pilot noted on 18 May that an ash plume rose to an altitude of 3 km (10,000 ft) a.s.l.

Source: Tokyo Volcanic Ash Advisory Center (VAAC)

CVGHM reported that during 18-22 May seismicity at Dieng Volcanic Complex increased. On 22 May diffuse white plumes rose 20 m from the Timbang cone; plumes from the cone had not been previously observed. The next day carbon dioxide gas emissions increased. CVGHM raised the Alert Level to 2 (on a scale of 1-4).

Source: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)

According to scientists from the Institute of Earth Sciences at the University of Iceland and the Icelandic Meteorological Office, an eruption from the subglacial Grímsvötn volcano began on 21 May following about an hour of tremor. A GPS-station on the rim of the Grímsvötn caldera had revealed continuous inflation and expansion of few centimeters per year since the 2004 eruption, interpreted as inflow of magma to a shallow chamber. Other precursors over the previous few months were increased seismicity, including some bursts of tremor, and increased geothermal activity.

The eruption began at 1630 on 21 May, and at 2000 the eruption plume rose to an altitude over 20 km (65,000 ft) a.s.l. The plume altitude fell to 15 km (49,200 ft) a.s.l. during the night but occasionally still rose to 20 km (65,000 ft) a.s.l. Ash from the lower part of the eruption plume drifted S and at higher altitudes drifted E. A few hours after the onset of the eruption ashfall began over areas S of the Vatnajökull ice cap, more than 50 km from the eruption site. Earthquake locations and limited observations during an initial overflight suggested that the eruption site was in the SW part of the caldera, where the 2004 eruption was located. According to news articles, the road in Skeidarársandur, S of Vatnajökull and part of the ring road around Iceland, closed and remained closed through 24 May.

During the morning of 22 May the plume rose to an altitude of 10-15 km (32,800-49,200 ft) a.s.l. The color of the plume was brown-to-grayish and sometimes black close to the source. Most of the plume drifted S, but lower parts traveled SW. Tephra fall was concentrated to the S and to a lesser extent N and E. In the afternoon lightning strikes ranged from 60-70 per hour (up to 300 during one hour) and were most frequent in the ash plume to the S. News outlets noted that the Keflavík airport closed. Ashfall was reported from the Reykjavík area in the SW to Tröllaskagi Peninsula in the N. An article also stated that, according to a geophysicist, the eruption was the largest for Grímsvötn in 100 years, was similar to the eruption of 1873, and was larger than the Eyjafjallajökull eruption of 2010.

During 22-23 May the ash plume rose to altitudes of 5-10 km (16,400-32,800 ft) a.s.l. and drifted S at lower altitudes and W at altitudes 8 km (26,200 ft) a.s.l. and higher. Ashfall was detected in several areas throughout Iceland except in some areas to the NW. On 24 May the ash plume was estimated to be mostly below 5 km (16,400 ft) a.s.l. because meteorological clouds over the glacier were at 5-7 km (16,400-23,000 ft) a.s.l. and the plume only briefly rose above the cloud deck. Satellite images showed the plume extending over 800 km from the eruption site towards the S and SE. News articles reported that dozens of carriers rerouted or cancelled flights in Norway, Denmark, and Scotland.

Sources: Icelandic Meteorological Office (IMO), Institute of Earth Sciences, Iceland Review, Associated Press, The Local (Sweden)

INETER reported that during April seismicity at Telica increased, with earthquake magnitudes ranging up to 3.3 and some explosions detected. On 14 May at approximately 2310 a series of explosions were detected by the seismic network and were accompanied by ash and gas emissions. Tephra fall was reported in La Quemada, 4 km N. During the previous few days residents in nearby communities observed reddish colored gas plumes, and those living on the flanks felt earthquakes. On 16 May the seismic network registered three explosions that were accompanied by gas-and-tephra emissions. One plume rose 1.2 km above the crater. Nearby communities to the SE, including Cristo Rey, Las Colinas, and La Quemada, were affected by the emissions.

During 17-18 May another series of small explosions was detected. A gas-and-ash plume rose 600 m above the crater. On 18 May an explosion that lasted 6 minutes produced an ash plume that rose 2.6 km above the crater. Fifteen explosions during 18-19 May were accompanied by gas-and-ash plumes that rose 500-800 m high. Women and children living on the flanks were evacuated. Explosions continued to be detected during 19-20 May with gas-and-ash plumes again rising 500-800 m above the crater. Residents on the N flank in the community of El Ñajo reported that new fumaroles had opened, prompting authorizes to plan for monitoring all wells within a 5-km-radius of Telica. During 20-21 May activity decreased. An observer reported four strong explosions and gas-and-tephra plumes that rose 500-700 m high.

Source: Instituto Nicaragüense de Estudios Territoriales (INETER)

IG reported that on 17 May new fumaroles on Tungurahua's W flank, about 1 km below the crater, were observed from the Tungurahua Observatory (OVT) in Guadalupe (11 km N). Explosions during 17-23 May produced gas-and-steam plumes that rose to altitudes of 6-9 km (19,700-29,500 ft) a.s.l and mostly drifted SW, W, and NW. Ashfall was reported on most days in areas within 8 km SW, W, NW, N, and NNE, but reached as far as Guadalupe (11 km N), Ambato (31 km NW), and Penipe (15 km SW). On 17 May incandescence emanated from the crater. Blocks were also ejected from the crater, and rolled down the flanks, on 17, 19, and 21 May. Sounds resembling "cannon shots" were occasionally reported. Large windows vibrated on 21 May. Ash-and-gas plumes rose from the crater on 24 May and likely drifted W.

Source: Instituto Geofísico-Escuela Politécnica Nacional (IG-EPN)

RVO reported that during 19-22 May gray-to-brown ash plumes rose above Ulawun and fine ash fell on the NW and W flanks. Weak, fluctuating incandescence was observed on 22 May.

Source: Rabaul Volcano Observatory (RVO)

Based on information from JMA, the Tokyo VAAC reported that during 18-22 May explosions from Sakura-jima produced plumes that rose to altitudes of 1.2-3 km (4,000-10,000 ft) a.s.l. Plumes drifted in multiple directions. On 21 May, a pilot observed an ash plume that rose to an altitude of 3 km (10,000 ft) a.s.l. and drifted N.

Source: Tokyo Volcanic Ash Advisory Center (VAAC)

Based on analyses of satellite imagery, the Darwin VAAC reported that on 21 May an ash plume from Bagana rose to an altitude of 2.1 km (7,000 ft) a.s.l. and drifted 65 km NW.

Source: Darwin Volcanic Ash Advisory Centre (VAAC)

Based on analyses of satellite imagery, the Darwin VAAC reported that during 17 and 19-22 May ash plumes from Batu Tara rose to altitudes of 1.8-2.4 km (6,000-8,000 ft) a.s.l. and drifted 75-110 km W, NW, and N.

Source: Darwin Volcanic Ash Advisory Centre (VAAC)

SERNAGEOMIN reported that during 2011 seismic activity from Chaitén remained low and that no growth of the lava-dome complex was detected. Visual observations indicated low emissions. The Alert Level was lowered to Green, Level 2, on a three-color scale.

Source: Servicio Nacional de Geología y Minería (SERNAGEOMIN)

Based on analyses of satellite imagery, the Darwin VAAC reported that on 19 May an ash plume from Dukono rose to an altitude of 3 km (10,000 ft) a.s.l. and drifted 110 km NE.

Source: Darwin Volcanic Ash Advisory Centre (VAAC)

KVERT reported moderate seismic activity at Karymsky during 13-20 May. Seismic data indicated that possible ash plumes rose to an altitude of 2.5 km (8,200 ft) a.s.l. A thermal anomaly was detected in satellite imagery during 16-17 and 19 May. The Aviation Color Code remained at Orange. Based on information from KVERT, the Tokyo VAAC reported that on 21 May an eruption produced a plume that rose to an altitude of 6.7 km (22,000 ft) a.s.l.

Sources: Tokyo Volcanic Ash Advisory Center (VAAC), Kamchatkan Volcanic Eruption Response Team (KVERT)

HVO reported that two lava lakes at Kīlauea were active during 18-24 May. The level of the summit lava lake remained mostly stable deep in the vent inset within the E wall of Halemaʻumaʻu Crater. Lava from a vent above the south side cascaded down into the lake. A gas plume from the vent drifted SW and likely deposited very small amounts of ash nearby. At Puʻu ʻŌʻō crater, lava mostly from a vent near the W edge of the perched lava lake in the center of the crater floor continued to fill the lake. The lake level fluctuated and occasionally overflowed the edges or flowed through rim breaches, sending lava onto the Puʻu ʻŌʻō crater floor. The rim of the perched lava lake was elevated several meters higher than the surrounding crater floor, which was 52 m below the E crater rim on 11 May. On 20 May a small lobe of lava visible in the web camera appeared on the W edge of the crater floor. The (preliminary) sulfur dioxide emission rate from all east rift zone sources was 800 tonnes/day on 20 May; the emission rates were slowly increasing.

Source: US Geological Survey Hawaiian Volcano Observatory (HVO)

KVERT reported that during 13-20 May seismicity from Kizimen was above background levels, and indicated that possible ash plumes rose to an altitude of 5.9 km (19,400 ft) a.s.l. and 4.9 km (16,100 ft) a.s.l. on 14 and 17 May, respectively. Satellite images showed a large bright thermal anomaly daily on the volcano and ash plumes that drifted 24 km S and SE during 14-15 May. The Aviation Color Code remained at Orange.

Source: Kamchatkan Volcanic Eruption Response Team (KVERT)

CENAPRED reported that on 19 May steam-and-gas emissions from Popocatépetl occasionally contained small amounts of ash. The Washington VAAC noted that satellite imagery on 22 May showed a rapidly dissipating area of ash about 650 km SW.

Sources: Washington Volcanic Ash Advisory Center (VAAC), Centro Nacional de Prevencion de Desastres (CENAPRED)

KVERT reported that during 15-16 May seismic data at Shiveluch indicated that possible ash plumes rose to an altitude of 3.4 km (11,200 ft) a.s.l. According to ground-based observations ash plumes rose to an altitude of 6 km (19,700 ft) a.s.l. on 15 May and to 4 km (13,100 ft) a.s.l. on 16 May. Satellite imagery showed ash plumes drifting 58 km SE on 15 May and a thermal anomaly over the lava dome during 15-16 and 19 May. The Aviation Color Code remained at Orange.

Source: Kamchatkan Volcanic Eruption Response Team (KVERT)

MVO reported that during 13-20 May activity at the Soufrière Hills lava dome was at a low level. On 15 May a pyroclastic flow that occurred in the February 2010 collapse scar caused light ashfall in inhabited areas. The Hazard Level remained at 3.

Source: Montserrat Volcano Observatory (MVO)