Mount Bromo

Mount Bromo and Mount Semeru in One of National Park, Mount Bromo (2392m) is a small pyroclastic cone which rises 133m above the surrounding floor of the massive (9x10km) Tengger Caldera. This caldera, also referred to as the sand-sea caldera due to its flat sandy floor, is the result of the collapse of the top half of a probably nearly 4500m high pre-caldera stratovolcano over 250000 years ago. Mount Bromo has been frequently active since historical records began in 1804, with a total of over 50 eruptions being recorded up to the present date. Due to its location on the base of a steep-walled caldera, it generally only poses a local threat to life during explosive eruptions. These may occur with little warning. Such an eruption occurred on 08.06.2004 and accounted for 2 fatalities in the vicinity of the crater due to the impact of ballistics. Damage to vegetation may occur over a much wider radius, as could be witnessed during e.g. the 2010-2011 eruptive phase.

Several phases can be defined in the evolution of the Tengger Caldera volcanic complex (summarized in Gerven and Pichler, 1995. J. Southeast Asian Earth Sci. 11(2), p.125-133). The original Tengger stratovolcano was built up by a succession of pyroclastic deposits and lava flows to a height of about 4500m. This would make it the highest mountain in Java in the present day. A further slightly smaller volcano, Ngadisari, adjoined it in the NE side. Ngadisari collapsed after a massive eruption around 150000 years ago, forming a caldera. At a later time, eruptions from vents in the Ngadisiri Caldera resulted in the emplacement of significant amounts of ignimbrites (pyroclastic flow deposits) and also some lava flows. The products of these eruptions built up what is now the NE wall of the Tengger caldera, also referred to as Cemoro Lawang. The Tengger Caldera itself formed over 45000 years ago following what must have been a huge eruption, possibly similar in magnitude to that of Krakatau in 1883. The final stage of activity has involved the emplacement of 6 intra-caldera vents.

The six vents created by the Post-Caldera phase of activity are numbered in order of appearance in Fig.1. These mainly consist of ash fall and lapilli units, with only the eruption of Segarawedi Kidul being associated with the emplacement of pyroclastic flow units. Batok appears to be the youngest, yet only Bromo is presently active.
Numerous eruptions of Bromo have now been documented. Recent activity appears to have been largely phreatic, involving water that accumulates in porous pyroclastic material in the caldera as a result of the high amount of rainfall experienced in the area. This has led to the eruption of ash, lapilli and, in the more powerful stages of the eruptions, volcanic bombs. The eruptions are difficult to predict, although Abidin et al., 2004 (J. Glob. Pos. Sys. 3(1-2), p.16-24) show that the volcano inflates gradually by a few cm pre-eruption and deflates rapidly afterwards. This fluctuation could be detected by GPS surveying methodology. The pressure source appears to lie in the center of the post-caldera unit. Tremor and shock sources under Bromo have also been studied during a phase of high activity in 1995 (Gottschämmer and Surono, 2000. J. Volc. Geotherm. Res. 101, p.199-209). Both sources were found to be located under the NW part of Bromo in the vicinity of the then active vent. The authors concluded that the tremor signals were largely due to flow of gases and steam through an irregularly shaped conduit. The shock signals were attributed to fissuring of the conduit wall due to the high pressure inside. As of 2011, the regular monitoring network remains rudimentary with only a single seismometer in place. Hence, no inflation data is generally available, nor is it possible to localize tremors.
Activity at Bromo has taken on different forms in historic times and Bromo apparently contained a lava lake from 1836-1841 and again briefly in 1842. From its beginnings, activity at the Tengger Caldera has been dominated by eruption of basaltic to andesitic rocks. The basaltic rocks are mainly found in lava flows from the pre-caldera phase of activity. All recent lava flows of the post-caldera unit are andesitic in nature, as are most of the recent eruptates of Bromo (van Gerven and Pichler, 1995).

Historical activity at Mount Bromo National Park is usually short-lived, generally lasting for three months or less, with the major events taking place over a shorter period. The eruptions usually involve weak explosive activity (V.E.I 1) and periods of steam and ash venting. However, slightly more powerful eruptions have been witnessed in 1915-1916 and 1948, reaching an explosivity index of 3. These apparently led to property damage by direct impact of ballistics.

2010-2011 Eruptive Phase
An increase in volcanic tremor was detected at Bromo at the beginning of November 2010. By the 22nd, tremor was almost continuous and increased degassing was evident, leading authorities to raise the alert level to 4 on the 23rd. First significant ash emissions were noted on 26 Nov. marking the onset of the eruption. Output of ash was high at several stages in the eruption and led to significnt damage to vegetation in downwind areas. Flights have been disrupted at nearby Malang City airport, but also as far away (400km) as Bali, where numerous flights were cancelled on 28.01.11 when an up to 5500m high ash cloud extended from Bromo to the island.
Activity in mid-March 2011 as documented below could be distinguished into largely strombolian phases accompanied by little ash and variable levels of degassing, and phases during which significant amounts of ash were released. Strombolian phases were sometimes accompanied by powerful shockwaves, often after short periods (10-30 secs) of complete calm, presumably signifying blockage of the vent resulting in pressure buildup.

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