Mount Tambora

http://upload.wikimedia.org/wikipedia/commons/thumb/b/bd/Caldera_Mt_Tambora_Sumbawa_Indonesia.jpg/640px-Caldera_Mt_Tambora_Sumbawa_Indonesia.jpg

Mount Tambora (or Tamboro) is an active stratovolcano, also known as a composite volcano, on the island of Sumbawa, Indonesia. Sumbawa is flanked both to the north and south by oceanic crust, and Tambora was formed by the active subduction zone beneath it. This raised Mount Tambora as high as 4,300 m (14,100 ft),[5] making it formerly one of the tallest peaks in the Indonesian archipelago. After a large magma chamber inside the mountain filled over the course of several decades, volcanic activity reached a historic climax in the super-colossal eruption of April 10, 1815. The 1815 eruption was approximately VEI 7, the only eruption of that size since the Lake Taupo eruption in about 180 CE.

With an estimated ejecta volume of 160 km3 (38 cu mi), Tambora's 1815 outburst was the largest volcanic eruption in recorded history. The explosion was heard on Sumatra island more than 2,000 km (1,200 mi) away. Heavy volcanic ash falls were observed as far away as Borneo, Sulawesi, Java and Maluku islands. Most deaths from the eruption were from starvation and disease, as the eruptive fallout ruined agricultural productivity in the local region. The death toll was at least 71,000 people (the deadliest eruption in recorded history), of whom 11,000–12,000 were killed directly by the eruption; the often-cited figure of 92,000 people killed is believed to be overestimated.

The eruption caused global climate anomalies that included the phenomenon known as "volcanic winter": 1816 became known as the "Year Without a Summer" because of the effect on North American and European weather. Agricultural crops failed and livestock died in much of the Northern Hemisphere, resulting in the worst famine of the 19th century.

During an excavation in 2004, a team of archaeologists discovered cultural remains buried by the 1815 eruption. They were kept intact beneath the 3 m (9.8 ft) deep pyroclastic deposits. At the site, dubbed the Pompeii of the East, the artifacts were preserved in the positions they had occupied in 1815.

Geographical setting

Mount Tambora is located on Sumbawa Island, part of the Lesser Sunda Islands. It is a segment of the Sunda Arc, a string of volcanic islands that forms the southern chain of the Indonesian archipelago. Tambora forms its own peninsula on Sumbawa, known as the Sanggar peninsula. At the north of the peninsula is the Flores Sea, and at the south is Saleh Bay, 86 km (53 mi) long and 36 km (22 mi) wide. At the mouth of Saleh Bay there is an 30,000 hectares islet called Moyo (Indonesian: Pulau Moyo) which has a guest shelter or luxurius resort where celebrities such as Princess Diana once stayed.

Besides its interest for seismologists and volcanologists, who monitor the mountain's activity, Mount Tambora is an area of scientific studies for archaeologists and biologists. The mountain also attracts tourists for hiking and wildlife activities. 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 three ascent routes to reach the caldera. The first route starts from Doro Mboha village south of the mountain. This route follows a paved road through a cashew plantation until it reaches 1,150 m (3,770 ft) above sea level. The end of this route is the southern part of the caldera at 1,950 m (6,400 ft), reachable by a hiking track. This location is usually used as a base camp to monitor the volcanic activity, because it only takes one hour to reach the caldera. The second route is located in southwest of the mountain start from Doro Peti village, the Tambora volcanic monitoring station is located in this village. The third route starts from Pancasila village northwest of the mountain. This route passing through a coffee plantation. Using the third route, the caldera is accessible only by foot. The highest point of Tambora located on a hill near the westen rim of the caldera.

In August 2011, the alert level for the volcano was raised from Level I to Level II after increasing activity was reported in the caldera, including earthquakes and smoke emissions.In September 2011 the alert level was raised to Level III after further increases in activity.

Geological history

Formation

Tambora lies 340 km (210 mi) north of the Java Trench system and 180–190 km (110–120 mi) above the upper surface of the active north-dipping subduction zone. Sumbawa island is flanked to both the north and south by the oceanic crust.The convergence rate is 7.8 cm (3.1 in) per year.[18] Tambora is estimated to have formed around 57,000 years ago.Depositing its strata has drained off a large magma chamber inside the mountain. The Mojo islet was formed as part of this geological process in which Saleh Bay, collapsing into the caldera of the drained magma chamber, first appeared as a sea basin, about 25,000 years ago.

According to a geological survey, prior to the 1815 eruption Tambora had the shape of a typical stratovolcano, with a high symmetrical volcanic cone and a single central vent. The diameter at the base is 60 km (37 mi).The central vent emitted lava frequently, which cascaded down a steep slope.

Since the 1815 eruption, the lowermost portion contains deposits of interlayered sequences of lava and pyroclastic materials. The 1–4 m (3.3–13 ft) thick lava flows constitute approximately 40% of the layers' thickness. Thick scoria beds were produced by the fragmentation of lava flows. Within the upper section, the lava is interbedded with scoria, tuffs and pyroclastic flows and falls. There are at least twenty subsidiary or parasitic cones. Some of them have names: Tahe, 844 m (2,769 ft); Molo, 602 m (1,975 ft); Kadiendinae; Kubah, 1,648 m (5,407 ft); and Doro Api Toi. Most of these parasitic cones have produced basaltic lavas.

Eruptive history

Use of the radiocarbon dating technique has established the dates of three of Mount Tambora's eruptions prior to the 1815 eruption. The magnitudes of these eruptions are unknown. The estimated dates are 3910 BCE ± 200 years, 3050 BCE and 740 CE ± 150 years. They were all explosive central vent eruptions with similar characteristics, except the lattermost eruption had no pyroclastic flows.

In 1812, Mount Tambora entered a period of high activity, with its climactic eruption being the catastrophic explosive event of April 1815.The VEI 7 eruption had a total tephra ejecta volume of 160 km3 (38 cu mi). It was an explosive central vent eruption with pyroclastic flows and a caldera collapse, causing tsunamis and extensive land and property damage. It had a long-term effect on global climate. This activity ceased on 15 July 1815. Follow-up activity was recorded in August 1819 consisting of a small eruption (VEI = 2) with flames and rumbling aftershocks, and was considered to be part of the 1815 eruption sequence.Around 1880 ± 30 years, Tambora went into eruption again, but only inside the caldera.Small lava flows and lava dome extrusions were formed. This eruption (VEI = 2) created the Doro Api Toi parasitic cone inside the caldera.

Mount Tambora is still active. Minor lava domes and flows have been extruded on the caldera floor during the 19th and 20th centuries.[1] The last eruption was recorded in 1967.However, it was a very small, non-explosive eruption (VEI = 0).