Mount Rinjani

Mount Rinjani or Gunung Rinjani is an active volcano in Indonesia on the island of Lombok. Administratively the mountain is in the Regency of North Lombok, West Nusa Tenggara (Indonesian: Nusa Tenggara Barat, NTB). It rises to 3,726 metres (12,224 ft), making it the second highest volcano in Indonesia.
On the top of the volcano is a 6-by-8.5-kilometre (3.7 by 5.3 mi) caldera, which is filled partially by the crater lake known as Segara Anak or Anak Laut (Child of the Sea) due to blue color of water lake as Laut (Sea). This lake is approximately 2,000 metres (6,600 ft) above sea level and estimated to be about 200 metres (660 ft) deep; the caldera also contains hot springs. Sasak tribe and Hindu people assume the lake and the mount are sacred and some religious activities are occasionally done in the two areas. On 31 October 2015, Mount Rinjani started erupting again.

Geography
Lombok is one of the Lesser Sunda Islands, a small archipelago which, from west to east, consists of Bali, Lombok, Sumbawa, Flores, Sumba and the Timor islands; all are located at the edge of the Australian continental shelf. Volcanoes in the area are formed due to the action of oceanic crusts and the movement of the shelf itself. Rinjani is one of at least 129 active volcanoes in Indonesia, four of which belong to the volcanoes of the Sunda Arc trench system forming part of the Pacific Ring of Fire – a section of fault lines stretching from the Western Hemisphere through Japan and South East Asia. The islands of Lombok and Sumbawa lie in the central portion of the Sunda Arc. The Sunda Arc is home to some of the world's most dangerous and explosive volcanoes. The eruption of nearby Mount Tambora on Sumbawa is known for the most violent eruption in recorded history on 15 April 1815, with a scale 7 on the VEI.
The highlands are forest clad and mostly undeveloped. The lowlands are highly cultivated. Rice, soybeans, coffee, tobacco, cotton, cinnamon, cacao, cloves, cassava, corn, coconuts, copra, bananas and vanilla are the major crops grown in the fertile soils of the island. The slopes are populated by the indigenous Sasak population. There are also some basic tourist related activities established on Rinjani primarily in or about the village of Senaru.
Rinjani volcano on the island of Lombok rises to 3,726 metres (12,224 ft), second in height among Indonesian volcanoes only to Sumatra's Kerinci volcano. Rinjani has a steep-sided conical profile when viewed from the east, but the western side of the compound volcano is truncated by the 6 x 8.5 km, oval-shaped Segara Anak caldera. The western half of the caldera contains a 230-metre-deep lake whose crescentic form results from growth of the post-caldera cone Barujari at the eastern end of the caldera.

Geologic summary
On the basis of plate tectonics theory, Rinjani is one of the series of volcanoes built in the Lesser Sunda Islands due to the subduction of Indo-Australian oceanic crust beneath the Lesser Sunda Islands, and it is interpreted that the source of melted magma is about 165–200 kilometres (103–124 mi) depth.
The geology and tectonic setting of Lombok (and nearby Sumbawa) are described as being in the central portion of the Sunda Arc. The oldest exposed rocks are Miocene, suggesting that subduction and volcanism began considerably later than in Java and Sumatra to the west, where there are abundant volcanic and intrusive rocks of Late Mesozoic age. The islands are located on the eastern edge of the Sunda shelf, in a zone where crustal thickness is apparently rapidly diminishing, from west to east.
The seismic velocity structure of the crust in this region is transitional between typical oceanic and continental profiles and the Mohorovičić discontinuity (Moho) appears to lie at about 20 kilometres (12 mi) depth. These factors tend to suggest that there has been limited opportunity for crustal contamination of magmas erupted on the islands of Lombok and Sumbawa. In addition, these islands lie to the west of those parts of the eastern-most Sunda and west Banda arcs where collision with the Australian plate is apparently progressing.
The volcano of Rinjani is 165 to 190 kilometres (103–118 mi) above the Benioff Zone. There is a marked offset in the line of active volcanoes between the most easterly Sumbawa volcano (Sangeang Api) and the line of active volcanoes in Flores. This suggests that a major transcurrent fault cut across the arc between Sumbawa Island and Flores. This is considered to be a feature representing a major tectonic discontinuity between the east and west Sunda Arcs (the Sumba Fracture). Further, a marked absence of shallow and intermediate earthquake activity in the region to the south of Lombok and Sumbawa is a feature interpreted to represent a marked break in the Sunda Arc Zone. Faulting and folding caused strong deformation in the eastern part of Lombok Basin and is characterized by block faulting, shale diapirs and mud volcano.

Volcanology
The Rinjani caldera forming eruption is thought to have occurred in the 13th century. Dated to "late spring or summer of 1257," this eruption is now considered the likely source of high concentrations of sulfur found in widely dispersed ice core samples and may have been "the most powerful volcanic blast since humans learned to write." The massive eruption may have triggered an episode of global cooling and failed harvests.
Eruption rate, eruption sites, eruption type and magma composition have changed during the last 10,000 years before the caldera forming eruption. The eruptions of 1994 and 1995 have presented at Gunung Baru (or 'New Mountain' - approximately 2,300 metres (7,500 ft) above sea level) in the center of this caldera and lava flows from subsequent eruptions have entered the lake. This cone has since been renamed Gunung Barujari (or 'Gunung Baru Jari' in Indonesian).
The first historical eruption occurred in September 1847. The most recent eruption of Mount Rinjani was in May 2010 and the most recent significant eruptions occurred during a spate of activity from 1994 to 1995 which resulted in the further development of Gunung Barujari. Historical eruptions at Rinjani dating back to 1847 have been restricted to Barujari cone and the Rombongan dome (in 1944) and consist of moderate explosive activity and occasional lava flows that have entered Segara Anak lake. The eruptive history of Rinjani prior to 1847 is not available as the island of Lombok is in a location that remained very remote to the record keeping of the era.
On 3 November 1994, a cold lahar (volcanic mudflow) from the summit area of Rinjani volcano traveled down the Kokok Jenggak River killing thirty people from the village of Aikmel who were caught by surprise when collecting water from the river in the path of the flow.
In connection with the eruption of the cone Gunung Barujari the status for Gunung Rinjani has been raised from Normal (VEI Level 1) to 'be vigilant' (VEI Level 2) since May 2, 2009 . In May 2010 Gunung Rinjani was placed in the standby status by Center for Volcanology & Geological Hazard Mitigation, Indonesia with a recommendation that there be no activity within a radius of 4 kilometres (2.5 mi) from the eruption at Gunung Barujari.