MOUNT SANGIANG TREKKING

Sangeang Api volcano, one of the most active in the Lesser Sunda Islands, forms a small 13-km-wide island off the NE coast of Sumbawa Island. Two large trachybasaltic-to-tranchyandesitic volcanic cones, 1949-m-high Doro Api and 1795-m-high Doro Mantoi, were constructed in the center and on the eastern rim, respectively, of an older, largely obscured caldera. Flank vents occur on the south side of Doro Mantoi and near the northern coast. Intermittent historical eruptions have been recorded since 1512, most of them during in the 20th century.

It erupted in 1988 and the island's inhabitants were evacuated. Between its first recorded eruption in 1512 and 1989 it erupted 17 times.
 

The island of Sangeang is part of the Lesser Sunda Islands. It is located northeast of Sumbawa in the Flores Sea, and is 13 kilometers wide with an area of 153 km2.

Sangeang Api (Gunung Api or Gunung Sangeang) is an active complex volcano on the island of Sangeang in Indonesia. It consists of two volcanic cones, 1,949 m (6,394 ft) Doro Api and 1,795 m (5,889 ft) Doro Mantoi. Sangeang Api is one of the most active volcanoes in the Lesser Sunda Islands. It erupted in 1988 and the island's inhabitants were evacuated.

Sangeang Api Volcano consists of 2 cones. Doro Api (1949 m) is the active cone and Doro Mantoi (1795 m).
Activity at the volcano usually begins with Vulcanian eruptions, followed by Strombolian explosions and lava flows.

A large earthquake (mag 6.3) hit 18 km west of the volcano on 1st December 2006.

Sangeang Api is an active alkaline volcano in the Indonesian Sunda Arc with an eruptive history of less than 1 Ma. It is composed of oxidised, potassic, chlorine and volatile-enriched, silica-undersaturated basaltic and more fractionated lavas with abundant clinopyroxene-rich mafic and ultramafic xenoliths. There is very good evidence that suites of lavas and xenoliths are co-magmatic and that the xenoliths are accumulative rocks in equilibrium with the Sangeang Api melts.and that the crystallisation of their CPX-, olivine-, ±plagioclase and magnetite assemblages drove this differentiation at uppermost mantle and crustal depths. The xenoliths however exhibit complex histories indicating fluctuating crystallisation conditions that periodically precipitated or partially melted “postcumulate” amphibole. The xenolith minerals contain numerous melt inclusions many of which also contain fluid or vapour phases.
 

Laser ICPMS analyses were undertaken on minerals and glasses . The pargasitic amphiboles are distinguished by their remarkably high Ba contents and high Ba/La and Ba/Th ratios. These Ba –spikes are not observed in the CPX. The amphibole also has much higher Nb and Ta contents than the CPX and has much higher Nb/Ta ratios (~23). The CPX has relatively high Th/U and Glass/CPX distribution coefficient ratio of DU/ DTh = 0.4 implies crystallisation at < 1Gpa.

Recently acquired U-Th-Ra isotope data (Turner, Foden et al., 2003) indicate that although the melts have limited 238U/230Th disequilibrium, spanning a range from slight Th- to slight U-excess, they also show extreme enrichment of 226Ra relative to 230Th. These results were interpreted to indicate extremely rapid transit of melts from the lower wedge. And yet, although the excess 226Ra might result from fractionation during dehydration and melting on the slab, the limited U-Th disequilibrium tends to discount this. The extreme Ra-Th fractionation might otherwise record subsequent differentiation processes involving the production of the xenolith assemblages from the Sangeang melts as they rise through the upper plate. The extreme Ra/Th fractionation implied by the 226Ra disequilibrium might imply that any batch of melt has “seen” a very large mass of crystals, mimicing the impact of the dynamic melting process (Beattie, 1993; Wood et al., 1999). Physically this might suggest that the interaction between melts and cumulates was one of percolation flow during which repeated melting and precipitation of intergranular amphibole might have occurred. As the CPX precipitation took place under conditions where DU/DTh << 1 CPX growth may enrich Th and create sites for 226Ra growth subsequently diffused and partitioned to the adjacent intergranular melt film and the amphibole this periodically precipitates (cf Feineman , DePaolo and Ryerson, 2002). The extreme Ba –enrichment of the amphibole can be taken as evidence of the importance of this phase in the 226Ra budget. Although this process of percolation flow leads to the buffering of the major element composition of the melts, these liquids will experience extreme enrichments of incompatible elements and volatiles and relative depletions of compatible trace elements (Cr, Ni, V

The observation of very alkalic melts produced by the incongruent melting of amphibole also provide a source of a high K end member to subduction magmas. In this model wedge melts with calc alkaline or arc tholeiite affinities may be contaminated by upper plate alkalic liquids like those described here. This process is supported by the high Nb/Ta ratio of the amphiboles in the Sangeang suites . Note that as the origin of the “secondary” amphibole could be from a range of precursor melts and will have Nd-, Sr- and Pb-isotopic compositions that reflect these, this model will not be easily identified by conventional isotope mixing arguments.
 

Trekking Itinery to Mount Sangeang Api Island

Depart from Bima at 08.00 Am and directly transfer to Wera Sangeang village, by local boat across to the island and prepare for the trek.

Around 10 Pm, Start your trek to Summit of Mount Sangeang Api and arrive at Summit around 06.30 Am. Enjoy the great view of the mountai and return to your base camp.

By boat return to Wera sangeang village and transfer back to Bima.