Mount Tambora, or Tomboro, is an active stratovolcano in West Nusa Tenggara, Indonesia. Located on Sumbawa in the Lesser Sunda Islands, volcanism is the result of subduction zones. The 1815 eruption was the largest in recorded history, erupting up to 150 cubic kilometers of volcanic material, making it a VEI-7 on the Volcanic Explosivity Index. This caused the summer of 1816 to become known as the "Year Without a Summer" due to global cooling from the eruption.
Geographical setting
Mount Tambora is situated in the northern part of Sumbawa island, part of the Lesser Sunda Islands. It is a segment of the Sunda Arc, a chain of volcanic islands that make up the southern chain of the Indonesian archipelago. Tambora forms its own peninsula on Sumbawa, known as the Sanggar peninsula. To the north of the peninsula is the Flores Sea and to the south is the 86 kilometres (53 mi) long and 36 kilometres (22 mi) wide Saleh Bay. At the mouth of Saleh Bay there is an islet called Mojo. Besides the seismologists and vulcanologists who monitor the mountain's activity, Mount Tambora is an area of interest to archaeologists and biologists. The mountain also attracts tourists for hiking and wildlife activities, though in small numbers. The two nearest cities are Dompu and Bima. There are three concentrations of villages around the mountain slope. At the east is Sanggar village, to the northwest are Doro Peti and Pesanggrahan villages, and to the west is Calabai village. There are two routes of ascent to the caldera. The first begins at Doro Mboha village on the southeast of the mountain and follows a paved road through a cashew plantation to an elevation of 1,150 metres (3,770 ft). The road terminates at the southern part of the caldera, which at 1,950 metres (6,400 ft) is reachable only by hiking. This location is only one hour from the caldera, and usually serves as a base camp from which volcanic activity can be monitored. The second route starts from Pancasila village at the northwest of the mountain and is only accessible on foot. The 16-kilometre (10 mi) hike from Pancasila at 740 metres (2,430 ft) elevation to the caldera of the volcano takes approximately 14 hours with several stops (pos) en route to the top. The trail leads through dense jungle with wildlife such as Elaeocarpus, Asian water monitor, reticulated python, hawks, orange-footed scrubfowl, pale-shouldered cicadabird (Coracina dohertyi), brown and scaly-crowned honeyeater, yellow-crested cockatoo, yellow-ringed white-eye, helmeted friarbird, wild boar, Javan rusa and crab-eating macaques.
History
Geological history
Formation
Tambora lies 340 kilometres (210 mi) north of the Java Trench system and 180 to 190 kilometres (110 to 120 mi) above the upper surface of the active north-dipping subduction zone. Sumbawa Island is flanked to the north and south by oceanic crust. The convergence rate of the Australian Plate beneath the Sunda Plate is 7.8 centimetres (3.1 in) per year. Estimates for the onset of the volcanism at Mount Tambora range from 57 to 43 ka. The latter estimate published in 2012 is based on argon dating of the first pre-caldera lava flows. The formation of Tambora drained a large magma chamber pre-existing under the mountain. The Mojo islet was formed as part of this process in which Saleh Bay first appeared as a sea basin about 25,000 years ago. A high volcanic cone with a single central vent formed before the 1815 eruption, which follows a stratovolcano shape. The diameter at the base is 60 kilometres (37 mi). The volcano frequently erupted lava, which descended over steep slopes. Tambora has produced trachybasalt and trachyandesite rocks which are rich in potassium. The volcanics contain phenocrysts of apatite, biotite, clinopyroxene, leucite, magnetite, olivine and plagioclase, with the exact composition of the phenocrysts varying between different rock types. Orthopyroxene is absent in the trachyandesites of Tambora. Olivine is most present in the rocks with less than 53 percent SiO2, while it is absent in the more silica-rich volcanics, characterised by the presence of biotite phenocrysts. The mafic series also contain titanium magnetite and the trachybasalts are dominated by anorthosite-rich plagioclase. Rubidium, strontium and phosphorus pentoxide are especially rich in the lavas from Tambora, more than the comparable ones from Mount Rinjani. The lavas of Tambora are slightly enriched in zircon compared with those of Rinjani. The magma involved in the 1815 eruption originated in the mantle and was further modified by melts derived from subducted sediments, fluids derived from the subducted crust and crystallization processes in magma chambers. 87Sr86Sr ratios of Mount Tambora are similar to those of Mount Rinjani, but lower than those measured at Sangeang Api. Potassium levels of Tambora volcanics exceed 3 weight percent, placing them in the shoshonite range for alkaline series. Since the 1815 eruption, the lowermost portion contains deposits of interlayered sequences of lava and pyroclastic materials. Approximately 40% of the layers are represented in the 1-to-4 m-thick (3.3-to-13.1 ft) lava flows. Thick scoria beds were produced by the fragmentation of lava flows. Within the upper section, the lava is interbedded with scoria, tuffs, pyroclastic flows and pyroclastic falls. Tambora has at least 20 parasitic cones and lava domes, including Doro Afi Toi, Kadiendi Nae, Molo and Tahe. The main product of these parasitic vents is basaltic lava flows.
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