Mount Rinjani (; ) is an active stratovolcano situated in regencial North Lombok of West Nusa Tenggara province on the Indonesian island of Lombok. It reaches an elevation of , making it the second-highest volcano in Indonesia and the highest point in the province of West Nusa Tenggara.
Adjacent to the volcano is a caldera measuring approximately , which contains the crater lake Sagara Anak ( ) — named for its striking blue coloration reminiscent of the ocean. The lake lies at an elevation of around above sea level and is estimated to be about deep. The caldera also features several hot springs.
Mount Rinjani and its crater lake hold significant spiritual importance for the indigenous Sasak people and certain folk religious communities, serving as sites for various religious ceremonies. In April 2018, the United Nations Educational, Scientific and Cultural Organization (UNESCO) recognized the Mount Rinjani Caldera as part of the Global Geoparks Network. Notably, the volcano's eruption in 1257 is considered one of the most powerful global volcanic events of the last 2,000 years.
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 , 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 the 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 depth.Hamilton, W.B., 1979, Tectonic Map of the Indonesian region: USGS Map I‑875‑D, 1:5,000,000.Hamilton, W.B., 1979, Tectonics of the Indonesian region: USGS Prof. Paper 1078, 345 p.+ map.
The geology and tectonic setting of Lombok (and nearby Sumbawa) are described as being in the central portion of the Sunda Arc.(qf. Foden and Varne 1981b) 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.(Curray et al, 1977)
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 depth.(Curray et al. 1977) 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 easternmost Sunda and west Banda arcs where collision with the Australian plate is apparently progressing.(qf. Foden and Varne 1981b).
The volcano of Rinjani is above the Benioff Zone.(Hamilton 1979) 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).Hedervari (1978) and Ritsema (1954) 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.(Prasetyo 1992). Abbot and Chamalaun (1978)Wikibooks:The Geology of Indonesia/The lesser Sunda Islands
Volcanology
The Rinjani caldera-forming eruption is thought to have occurred in the 13th century. Dated to "late spring or summer of 1257," this 1257 Samalas 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."Siegfried, Tom (2012). "Mystery Volcano's Name Revealed Despite Gag Order", Science News, v. 182 no. 1, 14 July 2012, p. 2. The massive eruption may have triggered an episode of global cooling and failed harvests. Before this eruption, the Segara Anak caldera was a volcanic mountain named Samalas, which was higher than Rinjani.
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 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 was raised from Normal (VEI Level 1) to 'be vigilant' (VEI Level 2) since 2 May 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 from the eruption at Gunung Barujari.
Volcanic composition
In Lombok, Rinjani volcano lies approximately north of the Sunda Trench (also known as Java trenchSunda Trench (4°30' S 11°10' S 100°00' E 119°00' Accredited by: SCGN (Apr. 1987) The trench was studied in some detail in the 1920s-1930s by Dutch geodesist F.A. Vening Meinesz, who made classic pendulum gravity measurements in a Dutch submarine. Shown as Java Trench in ACUF (Advisory Committee on Undersea Features Gazetteer). see also: http://www.gebco.net/) and is situated about above the active north dipping Benioff zone.(Hamilton 1979) cited from The Geology of Indonesia/The lesser Sunda Islands-IV. Volcanic Activity and Composition, accessed 2010-05-10 Based on the composition of andesites which have very low Ni concentrations and low Mg/Mg+Fe It is suggested that the Rinjani suite is of mantle origin, but that all the andesites and dacites as well as many of the basalts have probably been modified by fractional crystallization processes.Foden and Varne (1981) cited from The Geology of Indonesia/The lesser Sunda Islands-IV. Volcanic Activity and Composition, accessed 2010-05-10 It is concluded that the Rinjani calc-alkaline suite, which in many respects is typical of many suites erupted by circum-pacific volcanoes, probably originated by partial melting of the peridotite mantle-wedge overlying the active Benioff Zone beneath Lombok Island.Hamilton (1979) cited from The Geology of Indonesia/The lesser Sunda Islands-IV. Volcanic Activity and Composition The Pleistocene-Recent calcalkaline suite from the active volcano, Rinjani is composed of a diverse range of lavas. These include: ankaramite, high-Al basalt, andesite, high-K andesite and dacite. Sr-isotopic and geochemical constraints suggest that this suite was derived from the sub-arc mantle. Geochemical models suggest that fractional crystallization is an important process in the suite's differentiation, although the series: ankaramite-high-Al basalt-andesite-dacite does not represent a continuously evolving spectrum of liquids.
In September 1995 an aviation report was issued concerning an unconfirmed ash cloud from Rinjani. A NOTAM about volcanic activity from Rinjani was issued by the Bali Flight Information Region on the morning of 12 September. An ash cloud was reportedly drifting to the south west with the cloud top around altitude.
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. One person remained missing as of 9 November 1994. No damage to the village was reported. Local volcanologists noted that additional lahars could be triggered by heavy rainfall.
During 4 June 1994 – January 1995 the DVGHM (Directorate of Volcanology and Geological Hazard Mitigation) noted that explosions occurred on Rinjani. Those explosions came from the Barujari volcano.
Between 3 June 1994 and 21 November 1994 records of Rinjani's eruptive history indicate activity accorded Volcanic Explosivity Index (VEI) with a rating of 3(?) with the area of activity described as Gunung Barujari. Eruptive characteristics documented for the events of that time are described as, central vent eruption with an explosive eruption, with pyroclastic flow(s), lava flow(s), fatalities and mudflow(s) (lahars).
In May 1994 a glow was noticed on the crater floor of Barujari cone, which at this time had undergone no significant activity since August 1966. A portable seismograph (PS-2) and telemetry seismograph (Teledyne) were put into operation on 27 May and 9 June, respectively. One volcanic earthquake event/day was recorded on 27, 28, 30, and 31 May. After 4 June, however, volcanic tremor with a maximum amplitude of 35 mm was recorded, presumably associated with the upward movement of magma.