Researchers' detailed information about Mt. Bromo
Researchers' view at Mt. Bromo most likely differs completely from yours. Researchers are used to write very detailed about Mt. Bromo and the other craters nearby. Their information broadens your view and changes your feelings about this moonlike feature and any other volcano you might climb in Indonesia. That Mt. Bromo is an active volcano is realy exciting. Moreover, for most of you it isn't a daily opportunity to get that close to an active volcano.
To inspire and inform you, we simplified researchers' text. Two researchers who helped doing this are Mr. Manfred van Bergen from Utrecht University and Adriano Mazzini who is working at the University of Oslo.
Take a look at Mt. Bromo's detailed ancient and contemporary history before or after visiting the active volcano. Remarkable information reveiled...
The real stuff. Mt. Bromo's story according to researchers.
The Bromo-Tengger-Semeru National Park is quiet a complex area compared to its name. To make it a little more understandable; we have the Bromo-Tengger caldera which can be divided into 5 smaller calderas, one of them is the Sandsea caldera of Mt. Bromo. In between Mt. Bromo and Mt. Semeru, one can find the Jambangan caldera.
The most recent caldera is indeed the 9 ×10 km-wide Sandsea caldera, which formed incrementally during the late Pleistocene and early Holocene, between 33.000 and 45.000 years ago. An overlapping cluster of postcaldera cones was constructed on the floor of the ‘Sandsea’ caldera within the past several thousand years. The youngest of these is the Bromo tuff cone. Its eruptive activity resumed on 26th November 2010. More than 50 mild-to-moderate explosive eruptions have occurred since 1804, as recorded in the Global Volcanic Network (GVN) data base.
The Bromo-Tengger caldera - how it all started
The Bromo-Tengger caldera is 8 km across at its widest point and the inner caldera floor is around 2100 m a.s.l. It contains a cluster of post-caldera constructs within its walls. The presently active vent Mt. Bromo is just 100 meters higher than the caldera floor. Just northeast of the Bromo-Tengger caldera, there is an older, overlapping caldera structure, the Ngadisari caldera (Carn, S.A. 1999).
Activity began with construction of the Ngadisari volcano approximately 820 thousand years ago. A caldera forming eruption that formed the Ngadisari caldera took place approximately 150 thousand years ago. This was followed about 5 thousand years later by the formation of the Bromo-Tengger caldera, although the Tengger volcano grew about 265 thousand years ago. Post-caldera activity is still present. The oldest vent is believed to be the Segarawedi Kidul, a vent just south of Bromo. It has an ejecta volume of 4.5 km3.
A striking attribute of the Bromo-Tengger caldera is its linear northeastern, or Cemorolawang, wall. The stratigraphy of the Cemorolawang wall is markedly different from that of the northern and southern walls of the Bromo-Tengger caldera. The pyroclastic flow deposits in the Cemorolawang wall may be products of later eruptions from within the Bromo-Tengger caldera, which were channelled along the Sapikerep valley (the valley northeast of Mt. Bromo, the darker part on the picture) and subsequently exhumed by further incremental caldera collapse. The evolution of the Ngadisari Caldera is less clear.
The Jambangan caldera
Very little is known about the history of the Jambangan caldera, which is situated north of Mt. Semeru or south of Mt. Bromo. The western wall of the Jambangan caldera, extending about 6 km in the north direction. Its external morphology appears very similar to that of the Bromo-Tengger caldera and consists of two or more overlapping structures, namely the Ayek-ayek caldera and the Jambangan caldera.
You can find several intra-caldera vents, including the crater lake of Ranu Kumbolo (no 2 on picture 2). This deep lake may indicate a third explosive eruption from the complex. In the Jombangan caldera one can find material, which may originate from Semeru and Mahameru.
Semeru volcano in the Bromo-Tengger-Semeru National Park
Gunung Semeru is Java’s highest peak with 3676 meters and has been persistently active since recording started in 1818, with virtually continuous unrest since 1967. Semeru volcano is a stratovolcano and one of the most active volcanoes in Indonesia, although the eruption is mild (Carn, S.A. 1999).
Its activity consists of vulcanian and phreatomagmatic eruptions, which produce short-lived eruption columns several times a day. Strombolian eruptions also occur based on the presence of scoriae and bombs on the upper slopes. Extrusion of lava domes take place during more intense eruptive episodes that occur every 5–7 years. Pyroclastic flows occur once every five years on average, mostly as block-and-ash flows. Block-and-ash flows triggered bydome collapse have travelled as far as 12 km to the southeast and south directions, following drainage networks. Lava flows from flank fissures or vents - e.g. in 1895 and 1941 - have also been recognized as far as 9 km from the summit on the southeast flank (Solikhin, S. et al., 2012).
The northern summit of the mountain is known as Mahameru, with Semeru’s active crater farther southeast. Since 1967 cycles of lava-dome growth have occurred in the crater, feeding pyroclastic flows in the southeastern quadrant that have travelled up to 11 km downslope.
Secondary hazards suchs as lahars and debris flows are the salient dangers in the Semeru region. They have claimed 689 victims since 1885. The city of Lumajang, 33 km to the southeast, was destroyed by a debris flow in 1909. These flows are controlled primarily by topography and are initiated during heavy rains (Carn, S.A. 1999).
A scar is visible on the western flank of Semeru (see picture 1), and existing topographic maps show a similar breach on the eastern side of the volcano extending eastward. This feature demarcates the boundary between Mahameru and Semeru and could promote a southward sector collapse of the volcano in the event of a major eruption. The 1941 flank eruption fissure and neighbouring flank vents are on the extension of this structure, indicating that it is a deep-seated fracture (Carn, S.A. 1999).
Semeru has produced several notable flank eruptions, most recently in 1941–1942. This flank eruption followed a 28-year period of quiescence. A 1.3-km-long fissure on the east-southeast flank, aligned northwest, 3X106 m3 of blocky andesitic lava was effused over 1 month. The flow had reached
a length of 6.9 km by February 1942 (Carn, S.A. 1999).
The most effective lahar producer on Earth
Semeru is known as one of the most effective lahar producers on Earth. Large-scale lahars exceeding 5 million m3 in volume have occurred at least five times since 1884. During the rainy season, Semeru produces small to medium-scale lahars (≤0.1 million m3) every week, which are triggered by the remobilization of pyroclastic debris. Daily explosions deposit pyroclastic debris as far as 3 km from the vent area (Solikhin, S. et al., 2012).
Pyroclastic flow results from the collapse of the lava tongue. Lahar which flows to the south can reach the southern coastal area, passing several villages. However, there have been no victims because the villages have been relocated before the lahar buries them. The valleys are deep and wide and can accommodate the lahar deposit. On several occasions, disastrous lahar flows took place in the southeast sector. Fortunately, along the Semut River, the eastern slope has escaped destruction, since the upper part of the river is naturally protected by an earlier lava flow, the product of flank eruption in 1941 (Siswowidjoyo, S. et al., 1997).
Mitigation efforts
For early warnings the Volcano Observatory was established in 1953. The main observatory at the eastern slope is supported by two small observatories. To mitigate against secondary dangers dikes, chek darns and-sabo dams along the rivers where the flows may occur were constructed. Mitigation structures were created after the 1909 debris flow that reached the city Lumajang. The so called 'vluchtheuvel' that then was built is a pile of earth and rock; a site of about 2-3 m higher than the surrounding areas. These vluchthevels were improved after 1981.
Most of the area in the eastern part of the Semeru should not be used for dwellings, including Lumajang city, with a population of about 200,000 people. There is no detailed description of the danger which may come from the volcano. In the 1992 revised hazard map, Lumajang is still included as a lahar prone area. The type of danger is also not explained in detail (Siswowidjoyo, S. et al., 1997).
The persistent eruptive activity of Semeru that began in 1967
started with continuous emission of gas from a vent in the southern
part of the Jonggring-Seloko crater. This was followed by lava extrusion
which formed a stream-like lava flow that moved towards the
south slope. Eruptive activity continued until 1974 and was characterized
by extrusion of lava, which created a lava dome that grew in
the west and south area of the summit. The extrusion later shifted
to the SE but remained in the summit area. This eruptive activity
was accompanied by ash-producing explosions, lava avalanches, and
block-and-ash flows that widened the SE-trending channel. Since
1979, the rate of dome growth decreased, but the activity continued
with lava avalanches from the dome, occurrence of dome collapsedriven
block-and-ash flows, and explosions.
6. The most recent 2002–2003 (vei 2–3) eruption
Semeru is one of the few persistently active composite volcanoes
on Earth erupting on a daily basis. Based on chronicles, Thouret et
al. (2007) identified three eruptive styles for Semeru since 1818:
(1) persistent vulcanian and phreatomagmatic regime of short-lived
eruption columns, which occur several times a day; (2) increased activity
levels every five to seven years to produce pyroclastic density
currents (block-and-ash flows, and associated surges) from domes
growing in the Jonggring-Seloko vent; and (3) flank lava flows of andesitic
composition and aa type, stem from parasitic lava cones and
flank eruptive vents and reached 5 to 8 km in length on the S and SE
flanks (Fig. 5). The two most recent lava flows of Semeru were
emplaced on the SE flank: the 1895 lava flow reached c.10.5 km
from the Jonggring Seloko vent area, covering an area of ~7 km2,
while the 1941–1942 lava flow from a parasitic lava cone reached
c.6.5 km and covered an area of ~2.5 km2.
Seven to eight years after the two 1994 and 1995 explosive episodes
the eruptive activity moved towards the second eruptive
style, characterized by a high number of explosions and pyroclastic
flows at the end of December 2002 (GVN, 2002). The peak of the
eruptive episode occurred on 29th December 2002 when block-andash
flows with a total volume of 3.5 to 5 million m3 traveled
~11 km from the summit down the Besuk Bang Valley. About 500
people were evacuated from the hamlet of Supit to the district capital
Pronojiwo for about one week but no casualties were recorded.