The long-lasting eruption at Batu Tara exhibited frequent eruptive activity, both Strombolian and Vulcanian styles, previously reported through May 2014 (BGVN 39:01). This report describes events from May 2014 to June 2016. The material here was compiled largely from events reported by the Darwin Volcano Ash Advisory Center (VAAC) and expeditions by Volcano Discovery. The activity observed by visitors on 26 November 2015 corresponds to the end of frequent thermal anomalies. There were no further reports of ash plumes or regular thermal anomalies through October 2016.

Nearly daily to weekly thermal alerts were measured by MODIS thermal sensors from Aqua and Terra satellites during January 2007 to 27 October 2013. The nearly daily to weekly alerts began again on 11 January 2014 and continued until 25 November 2015; no thermal alerts were observed afterward through mid-June 2016. MIROVA noted low-to-moderate radiative power from MODIS sensors from mid-May to late November 2015 (figure 18), then infrequent low to moderate thermal anomalies (1-2 per month) through approximately 25 May 2016; no alerts after that were detected, at least through 21 June 2016.

Figure 18. Graph of the log radiative power of thermal anomalies at Batu Tara identified in MODIS data by MIROVA during September 2015-August 2016. Courtesy of MIROVA.

Ash plumes were frequently reported by the Darwin VAAC from 22 March 2014 to mid-June 2016 (table 7). The shortest air route between Jakarta, Indonesia, and Sydney, Australia, is directly over Batu Tara, whose frequent ash plumes are to be avoided. Ash plumes were reported by the VAAC at least once a month during March-June 2014, with several reports in June. Only one ash emission was reported four months later, on 1 November. Ash plumes were identified in satellite data during April and May 2015, and again during September and October.

Table 7. Ash plumes from Batu Tara reported from March 2014 to October 2015. Courtesy of Darwin VAAC.

Visitor observations, July and November 2015. Volcano Discovery made a video during an expedition on 3 July 2015 that showed a pyroclastic flow generated by an explosion travelling down a scarp and out over the ocean for ~150 m (figure 18). The report stated that a "strong Strombolian explosion from Batu Tara volcano occurred on the evening of 2 July 2015. The eruption is preceded by a gradual build-up with increasing degassing and rockfalls as a result of inflation of the crater. Immediately before the explosion, glow becomes visible as the magma is about to break through the crust sealing the vent." Photos by Tom Pfeifer on 3 July showed an explosion followed by a pyroclastic flow that traveled down a scarp towards the ocean.

Volcano Discovery further reported that during another visit over 23-26 November 2015 the activity "was characterized by mild, ash-rich Strombolian eruptions at intervals between 10-30 minutes alternated/interrupted by much more powerful Vulcanian-type explosions that produced shock waves and ejected dense columns of tephra (ash, blocks and incandescent lava bombs) in a single, cannon-shot like explosion." The explosions "sent ballistics to several hundred meters of height all over the upper half of the volcano. During the first two observation days, these explosions occurred at intervals between 3-10 hours, but became the dominant type of activity during the last 24 hours of our stay, when they occurred at shorter intervals of 1-2 hours typically. Ash plumes from some of these explosions rose more than 1 km from the crater." They noted that the activity was at similar levels as during previous visits between 2012 and June 2015.

Geological Summary. The small isolated island of Batu Tara in the Flores Sea about 50 km N of Lembata (fomerly Lomblen) Island contains a scarp on the eastern side similar to the Sciara del Fuoco of Italy's Stromboli volcano. Vegetation covers the flanks to within 50 m of the summit. Batu Tara lies north of the main volcanic arc and is noted for its potassic leucite-bearing basanitic and tephritic rocks. The first historical eruption, during 1847-52, produced explosions and a lava flow.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA, a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/, http://modis.higp.hawaii.edu/); Tom Pfeiffer, Volcano Discovery (URL: https://www.volcanodiscovery.com/photos/batu-tara/nov2015expedition.html, https://www.volcanodiscovery.com/batu-tara-videos.html).