Bromo is the currently active vent of the Tengger Caldera on Java, Indonesia. It has been pretty active with over 50 short eruptions since 1804. Most of the eruptive products are basalt and andesite driven.
The most recent activity was prior to Dec. 17, 2015. The most recent Darwin Volcanic Ash Advisory warns of ash surface to FL120 (approximately 3 km). ftp://ftp.bom.gov.au/anon/gen/vaac/2015/IDD41305.201512171529.txt
The area around Bromo and Tengger is intensely volcanic, mostly driven by basalt and andesitic eruptions out of a spreading back arc subduction region.
Population within 30 km of the volcano and caldera is over 1.4 million. There are nearly 23 million Indonesians within 100 km of the volcanoes.
Bromo / Tengger is 855 km east of Krakatoa. It is 389 km west of Rinjani.
Bromo is a mostly tephra cone that shares Tengger Caldera with five older vents. It is currently the only active vent. The Tengger Caldera in turn, sits some 50 km west of the Lamongan volcanic field which consists of over 61 basaltic cinder / spatter cones, a minimum of 29 maars and a central volcano with three vents. Lamongan dates from some 40,000 years ago while Tengger is much older. The Iyang Argapura volcanic complex is some 80 km to the west of Bromo. It is heavily eroded and the last known (unverified) eruption was 1597. The active and dangerous Raung volcanic complex and caldera is located some 135 km to the west of Bromo / Tengger. This part of Java is clearly a very active volcanic area. http://www.sciencedirect.com/science/article/pii/S0377027399001146
Bromo’s eruptions have varied significantly over the years. There was a reported lava lake for a time in the mid-19th Century. More recently, activity has been Strombolian in nature in the VEI 1 – 2 range with the occasional departure to a VEI 3. The last VEI3 was in 2010.
Explosiveness of eruptions is driven by interaction of basalt with a mostly saturated ground due to 1700 – 1800 mm / year average rainfall . This rainfall in turn charges a substantial water table. Most of the surrounding countryside has porous pyroclastic flow emplacements which tend to allow water to pass downward pretty easily and contact magma on the move, hence the prevalence of maars in relatively close proximity to Bromo / Tengger. Because of all this, there is a significant phreatomagmatic component to many of the larger explosions.
Eruptions from Bromo are typically short-lived, lasting three months or less. They are usually VEI 1 eruptions with weak explosive activity, steam and ash venting. The 2010 – 2011 eruption sequence was at times very ashy, though the ash plume was not particularly high in the atmosphere, probably for the same reason Eyjafjallajökull was an ashy eruption – the interaction of fresh basalt with water. Unlike Iceland, there is little ice in Indonesia. On the other hand there is plenty of water above and below the surface.
One of the more dangerous things about Bromo is like Hekla in Iceland, it gives relatively little warning before erupting. It is also poorly monitored. Photovolcanica claims it has a single seismometer in place.
The locals make regular religious sacrifices to the volcano during their annual Hindu Yadnya Kasada ceremony. The ceremony involves torch bearers, altars, climbing up and down Bromo.
Indonesia created the Bromo Tengger Semeru National Park in 1982. The park encompasses some 500 km2 of caldera, volcanoes, lakes and rivers. It also gets a lot of public traffic into and out of the caldera and up into Bromo. This has led to some visitor deaths as Bromo occasionally cuts loose and hits visitors with volcanic bombs launched out of the crater. http://www.indonesia.travel/sites/site/319/bromo-tengger-semeru-national-park
Due to significant rainfall coupled with soft volcanic debris making up Bromo’s cone, it along with the other older cones in the Tengger Caldera are fluted, making them very picturesque.
The Tengger Caldera measures some 16 km across. Activity there can be measured back 820,000 years. The caldera is at the northern end of a volcanic massif extending to the Semeru volcano, another active stratovolcano. There are remains of five stratovolcanoes topped with calderas in the massif.
The original stratovolcano is thought to have grown to the vicinity of 4,500 m high, making it one of the tallest volcanoes in Java. It had a smaller parasitic volcano on its flank, Ngadisari which went caldera around 150,000 years ago creating the Ngadisari caldera. Tengger itself went caldera some 45,000 years ago. Eruption size is thought to be similar to that of Krakatau in 1883. The caldera floor is also known as the Sandsea Caldera due to its rather uniform fill of pyroclastic and more recent debris. In some writings, Sandsea is treated as a separate caldera. To me it appears to be the incremental fill of the Tengger Caldera by successive eruptions out of the newer intracaldera vents and rain-driven erosion off those same vents.
Since Tengger went caldera, there were six vents built inside the caldera. The most recent of these and currently active is Mount Bromo, topping out at some 2,300 m. The top of the cone is some 300 m above the floor of the Sandsea Caldera. Neighboring cones are twice as high above the caldera floor.
The entire Tengger Volcanic Complex stretches all the way to Mount Semeru some 15 km to the south. Remains of stratovolcanoes and their calderas comprise the area between the two volcanoes. The newer Lamongan volcanic field lies some 50 km to the east. Both fields overlap one another about halfway between the two principal volcanoes.
Like we saw in Rinjani Tectonics post, the overall magma supply into the region comes courtesy of the subduction of the Australian Plate under the Eurasian Plate. The subduction takes place at the Java Trench, some 300 km south of Bromo. Crust thickness under Bromo / Tengger is some 25 km. https://volcanohotspot.wordpress.com/2015/11/13/what-are-the-tectonics-behind-rinjani/
One of the things going on is a north – south extension of crust under this part of Java. This has led to what is thought to be rapid ascent of primitive magmas into the more recent Lamongan Volcanic Field. These magmas appear to be the least evolved magmas in the region. One paper believes they are ancestral to what is currently fueling Bromo. The important thing to remember is the prevalence of basalts and andesites among the eruptive products of Bromo / Tengger and Lamongan to its east as they indicate a relatively robust pathway for magma to reach the surface. http://petrology.oxfordjournals.org/content/42/9/1643.full
Bromo / Tengger is an active volcanic complex. Bromo in the caldera is one of the more active Indonesian volcanoes over the last 200 years. The volcanic complex is littered with calderas. Magma supply appears to be substantial and relatively primitive in chemical composition. Primitive rising magmas interact with substantial water in the ground, leading to explosive eruptions. I would expect this region to remain highly active and dangerous for the foreseeable future.