The oldest geological units on Lombok are from the Oligocene-Miocene, with old volcanic units cropping out in southern parts of the island. Samalas was built up by volcanic activity before 12,000 BP. Rinjani formed between 11,940 ± 40 and 2,550 ± 50 BP, with an eruption between 5,990 ± 50 and 2,550 ± 50 BP forming the Propok Pumice with a dense rock equivalent volume of . The Rinjani Pumice, with a volume of dense rock equivalent, may have been deposited by an eruption from either Rinjani or Samalas; it is dated to 2,550 ± 50 BP, at the end of the time range during which Rinjani formed. The deposits from this eruption reached thicknesses of at distance. Additional eruptions by either Rinjani or Samalas are dated 11,980 ± 40, 11,940 ± 40, and 6,250 ± 40 BP. Eruptive activity continued until about 500 years before 1257. Most volcanic activity now occurs at the Barujari volcano with eruptions in 1884, 1904, 1906, 1909, 1915, 1966, 1994, 2004, and 2009; Rombongan was active in 1944. Volcanic activity mostly consists of explosive eruptions and ash flows.