Our previous report on Ambrym (BGVN 38:05) indicated that daily degassing and an infrequent ash plume occurred through July 2013, along with frequent MODVOLC thermal alerts, consistent with the hot molten surface of the volcano's active lava lakes. The volcano, whose location was shown in the previous report in figure 27, consists of two craters, Marum and Benbow, each with a lava lake. According to the Vanuatu Geohazards Observatory (VGO), the last known eruption was in 2009. The island of Ambrym has ~7,000 inhabitants, many of whom live within the hazard zone (see figure 28 in BGVN 38:05). This report updates information through July 2014.

According to NASA's Earth Observatory, a satellite image acquired on 9 August 2013 showed steam-and-gas plumes rising from Ambrym's Benbow cone and from the active lava lake in Mbwelesu Crater (one of three active sub-craters of the Marum cone) (figure 27). Another Earth Observatory image on 7 January 2014 showed a plume from Ambrym spreading across the South Pacific Ocean (figure 28). The strong reflection of the sun off the sea surface is called "sunglint."

Figure 27. NASA Earth Observatory satellite image of Ambrym on 9 August 2013. The image shows the two active craters on the volcanic island. According to the caption, deep within Benbow (left) and Marum (right) craters, vents continuously release steam, sulfur dioxide, and other volcanic gases into the atmosphere. These gases frequently stream over the other islands in Vanuatu, along the W edge of the South Pacific. The orange spot in Marum crater is a persistent lava lake. This natural-color image was acquired by the Advanced Land Imager. Courtesy of NASA Earth Observatory (image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team, caption by Robert Simmon).

Figure 28. NASA Earth Observatory satellite photo of Ambrym (lower left) on 7 January 2014. In this natural-color image acquired by MODIS aboard the Aqua satellite, a vog plume from Ambrym spreads across the South Pacific. (Vog results when volcanic gases, primarily sulfur dioxide and other oxides of sulfur, react with oxygen and moisture in the presence of sunlight.) Some contribution to the vog may come from Gaua volcano, which the plume passes directly over but there have been no recent reports on events there. Courtesy of NASA (image courtesy Jeff Schmaltz LANCE/EOSDIS MODIS Rapid Response Team, GSFC).

As in previous years, MODVOLC thermal alerts during the reporting period were frequent, ranging from several per week to multiple times per day. This activity is consistent with the presence of active lava lakes.

According to VGO, the current alert level remains in mid-2014 at 1 (increased activity, danger near crater only).

Geological Summary. Ambrym, a large basaltic volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides Arc. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. The caldera was formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have apparently occurred almost yearly during historical time from cones within the caldera or from flank vents. However, from 1850 to 1950, reporting was mostly limited to extra-caldera eruptions that would have affected local populations.

Information Contacts: MODVOLC - HIGP, 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/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); and Vanuatu Geohazards Observatory (VGO) (URL: http://www.geohazards.gov.vu).