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 (Child of the 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. A massive eruption of Rinjani in 1257 CE may have triggered an episode of global cooling and failed harvests.
Rinjani erupted three times on May 22, 2010 with activity continuing until early on May 23. According to the volcano's official monitoring agency, ash from Mount Barujari was reported as rising up to two km into the atmosphere and damaged crops. The volcano did not threaten villagers at that time. Lava flowed into the caldera lake, pushing its temperature up from 21°C to 35°C, while smoke spread 12 km.In February 2010 observers at the Gunung Rinjani Observation Post located 1.25 km (4000 feet) northeast of G. Rinjani saw one whitish-colored plume that rose 100 metres (328 ft) from the volcano. Dense whitish plumes (and possibly brown) rose 500 metres (1,640 ft) - 900 metres (2,953 ft) in March 2010 on 26 occasions and as high as 1,500 metres (4,921 ft) in April 2010 on 41 occasions.
Plumes seen on 1 and 2 May 2010 were "chocolate" in color and rose a maximum height of 1,600 metres (5,249 ft). From February 2010 through April 2010 seismicity decreased, although the maximum amplitude of earthquakes increased. CVGHM (Center of Volcanology and Geological Hazard Mitigation) also noted that ash eruptions and ejected incandescent material fell within Rinjani caldera, but some ash was blown out of the caldera.
Gunung Rinjani Observation Post Rinjani Sembalun is located in the village of Lawang, Sub Sembalun 12.5 km (4000 feet) northeast of G. Rinjani) in the Regency of East Lombok. Observers at this post monitor G.Rinjani, G.Barujari/G.Tenga within the Segara Anak Caldera.