Bromo Volcano (Tengger Caldera)

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. 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 due to the impact of ballistics from 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.

Post-caldera vents of Tengger Caldera - Batok in center, Bromo to its left. Nearby Semeru volcano is smoking in background.

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 to that of Krakatau in 1883. The final stage of activity has involved the emplacement of 6 intra-caldera vents.

	1. Segarawedi Kidul (2661m) 2. Segarawedi Lor (I and II) (2650m) 3. Mount Kursi (2581m) 4. Mount Bromo (2392m) 5. Mount Batok (2470m)
Fig.1 Sketch Map of Tengger (Sand Sea) Caldera

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. 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 Bromo is usually short-lived, usually 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 damaged property and must thus have thrown rocks several km. The 08.06.04 eruption also shows that Bromo can be dangerous, especially as it is frequently climbed by tourists and eruptions can come without warning. In 2004, the main eruptive event came without any obvious precursor and caught a number of tourists by surprise as they were on the staircase leading up to the crater. Two tourists were killed in the event and several more injured. The photos accompanying this text were taken weeks after this eruption and show the amount of ballistics that were thrown from the crater during the eruption and some of the damage they caused. High concentrations of volcanic gases in the crater (especially Hydrogen Sulphide) may also pose a danger, and have accounted for fatalities in the past according to van Gerven and Pichler (1995).

View into Bromo Crater, July 2004	View into Bromo Crater, July 2004	View into Bromo Crater, July 2004

Building on Flank of Bromo with Roof Destroyed by 2004 Eruption	Vegetation Damaged by Impact of Bombs from 2004 Eruption	Fresh Lapilli from 2004 Eruption by Staircase

Volcanic Bomb from 2004 Eruption (Banknote for Scale)	Volcanic Bomb from 2004 Eruption	Recent Ash Deposits on Flank of Bromo

Morning View from Mt. Penanjakan Viewpoint	Expansive Morning View from Mt. Penanjakan Viewpoint

Erosion Valleys on N Flank of Bromo	Evening View of Bromo from Batok

View from Bromo Flank to Batok	Multiple Small Impact Craters from 2004 Eruption (Located in Foreground of Picture to Left)	View from Base of Caldera to Bromo. Note Impact Damage to Roof of Toilet

Whilst most visits to Bromo occur without incident, those wishing to avoid any risk may look over the Tengger Caldera from the highly frequented viewpoint on Mt. Penanjakan (2770m) which represents a highpoint on the northern rim of the caldera. The viewpoint is particularly popular at sunrise and provides expansive views of the caldera with nearby Semeru volcano in the background.

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