According to the expert reconstruction from many sources, it is predicted that an old volcanic Rinjani, which situated in the western side of the present Mt. Rinjani had existed with the height of 5000 m above mean sea level.
In the pre-quarter time (>1.6 my), Rinjani area consists of sedimentary rocks, then when volcanic activities accored in the Pleistocene time (<1.6 my), the mountain underwent volcano-tectonic activities. As a result of intense activity in the form of huge and strong explosions (paroxysmal explosion) followed by the collapse of the mountain body, the mountain lost most of its body and the rest was left in the form of Segara Anak calderas. This was followed by the forming of Mt. Barujari, Mt. Rombongan, and the emergence of Anak Barujari hill.
Mt. Barujari is t 170×200 m in size, with the height of 2296 – 2376 m, or appears t 600 m over the calderas base. The latest eruption of this mount was on June 1994, which resulted in ash rain, lahar, lake water surface decline of t 4 m and an increase in the water temperature from 18°C to 40°C during 3 months. As a result of heavy rain over its top part, the pile-up of ash and sand caused a lahar flood along the Tanggik River and approximately t 31 people died, 7 people badly injured, agriculture fiels and dam destroyed, and irrigation ways silted up.
There were eight eruptions since 1847, all of them in Rinjani (Segara Anak) calderas. Based on its historical record, Mt. Rinjani has never erupted, so Mt. Rinjani belongs to the B-type of active volcanos. The types of eruption in Rinjani calderas were effusive (lava flows) and explosive (pyroclastic falls) which formed stratotype volcano. Eruptions were recorded 8 times, i.e in 1847; 1884; 1901; 1905; 1915; 1944; 1966; and 1994.
There were only three explosions that were recorded, Ie. in 1944, 1966, and 1994. The first explosion in 1944 formed the new cone of Mt. Rombongan, which emerged from the caldera’s lake situated in the northwest part of Mt. Barujari feet which became wider to the nort and west. The second explosion in 1966 produced lava, which came out from the eastern part of Mt. Barujari sloApe and reached the lake in the north and south. The last explosion in 1994 also came from Mt. Barujari, which produced lava in the west through the lake and caused the emergence of the Anak Barujari mount and also resulted in the increasing of Mt. Barujari activities.