Mount Rinjani deep dive
Mount Rinjani's vast caldera was formed in 1257 by the eruption of the original Samalas volcano, an event with a Volcanic Explosivity Index of 7 — one of the largest eruptions of the past 7,000 years and global enough to have triggered measurable cooling in medieval Europe. The current Rinjani peak (3,726m) is the surviving remnant on the caldera rim, and Mount Barujari, the small active cone inside the caldera lake, has erupted multiple times in the past two decades.
# Mount Rinjani Volcanic History: From the Samalas Cataclysm to Today's Smoking Cone
Most trekkers reach the crater rim of Mount Rinjani, look down at Segara Anak lake and the small smoking cone of Barujari, and assume they are looking at the volcano. They are not. They are looking at the wound left behind when a much larger volcano — Samalas — destroyed itself in the year 1257 in one of the most violent eruptions of the past several thousand years. Rinjani's summit is what survived. The caldera below is the empty space where the original mountain used to be.
This guide unpacks the geological story for the trekker who wants to understand what they are walking on. It is written from the perspective of a volcanologist who has worked on the Rinjani complex through Indonesia's Center for Volcanology and Geological Hazard Mitigation (PVMBG).
Lombok sits on the Sunda volcanic arc, the chain of volcanoes that runs from Sumatra through Java, Bali, Lombok, Sumbawa and onward to the Banda Sea. The arc exists because the Indo-Australian tectonic plate is subducting beneath the Eurasian plate at roughly 7 centimeters per year, dragging seawater and oceanic crust into the mantle, where they melt and rise as magma. Every major peak you can see from a Lombok beach is a product of this subduction.
The Rinjani volcanic complex is one of the largest single-edifice systems in the arc. The original mountain — what geologists call proto-Rinjani or Samalas — likely stood somewhere between 4,200 and 4,600 meters, making it taller than any peak in modern Indonesia outside of Papua. It was a stratovolcano, the classic conical shape built by alternating layers of lava and ash over hundreds of thousands of years.
For decades, climate scientists were puzzled by a mysterious cooling event in the late 1250s. Tree-ring records from Europe and North America showed unusually cold summers in 1258 and 1259. Medieval chronicles in England and France described a "year without a summer" with crop failures and famine. Ice cores from Greenland and Antarctica showed a sulfate spike in 1258 — chemical evidence of a massive volcanic eruption — but for years nobody knew which volcano was responsible. The "mystery eruption of 1258" was one of the most-studied unsolved problems in paleoclimate.
In 2013, a research team led by French volcanologist Franck Lavigne published evidence that conclusively identified the source: Mount Samalas on Lombok. They matched the chemistry of glass shards in the ice cores with pumice from the Samalas deposits, dated the eruption using radiocarbon and historical Sasak palm-leaf manuscripts (the Babad Lombok), and reconstructed the event in unprecedented detail.
The Samalas eruption was a Volcanic Explosivity Index 7 event — the same scale as Tambora 1815 and significantly larger than Krakatoa 1883. It ejected an estimated 40 cubic kilometers of pyroclastic material into the atmosphere, sent a plume above 40 kilometers into the stratosphere, and lofted enough sulfur dioxide to circle the planet and cool global temperatures by an estimated 0.7 to 1.0 degrees Celsius for several years.
The eruption destroyed the original mountain. The summit collapsed in on itself as the magma chamber emptied, creating the 8 by 6 kilometer caldera that you now see from the crater rim. Pyroclastic flows traveled tens of kilometers down the flanks of the volcano. The Sasak kingdom centered at Pamatan, then the political center of Lombok, was buried entirely. The Babad Lombok manuscript describes this destruction in language that maps almost perfectly onto modern volcanological understanding: "Mount Samalas collapsed, and the country was destroyed and buried."
This is one of the most dramatic geological events in human historical memory anywhere on Earth. And it happened on this island.
After the eruption, the new caldera floor sat at roughly 2,000 meters elevation — high enough that rainwater accumulated faster than it could evaporate or drain. Over decades to centuries, Segara Anak ("child of the sea") lake formed, filling the eastern half of the caldera with deep, mineral-rich water. The lake currently covers about 11 square kilometers and reaches depths of around 230 meters in places.
Segara Anak is more than a scenic feature. It is a geochemical indicator. Volcanologists monitor the lake temperature and dissolved gas levels because changes can presage renewed activity. The lake's hot springs (Aik Kalak) on the northern shore are surface evidence of continued geothermal heat below.
The Samalas eruption was not the end of volcanism in the caldera. A new vent began building a cone on the southwestern shore of Segara Anak, probably starting within decades of the 1257 collapse. This new cone is Mount Barujari (sometimes called Gunung Baru, "new mountain"), which now rises about 320 meters above the lake to an elevation of around 2,376 meters.
Barujari is one of Indonesia's most active small volcanoes. Documented eruptions include events in 1944, 1966, 1994, 2004, 2009, 2015, and 2016. Most are Strombolian-style, producing ash plumes a few kilometers high and lava flows that extend into the lake. The 2015 eruption sent ash plumes high enough to disrupt flights at Lombok International Airport for several days and forced evacuation of trekkers from Plawangan Sembalun camp.
Barujari is monitored continuously by PVMBG with seismometers and thermal sensors on the caldera rim. The activity status is published in real time on the agency's website. Trekkers should always check the current status before booking, especially during periods of regional seismic activity.
What you climb on summit day is the surviving fragment of the original Samalas edifice. The peak at 3,726 meters sits on the southeastern rim of the caldera, with the cliff falling more than 1,700 meters on its inner face down to Segara Anak. The volcanic scree you grind through on the final summit push is the eroded remnant of stratified lava and ash layers that built up over the original mountain's life.
Look closely at the rocks under your headlamp during the 2am summit push. The black scoria with gas vesicles is from later, more fluid lava flows. The lighter pumice is from explosive phases — possibly even from the 1257 eruption itself, which left pumice deposits up to 40 meters thick on the upper flanks. You are walking through stratigraphy that records the entire eruptive history of the volcano.
Plawangan Sembalun (2,639m) is a high-elevation vegetated bench on the eastern caldera rim. It marks where the ancient outer flank of Samalas eroded back toward the caldera. The flat camping area here is unusual on the otherwise steep complex.
Plawangan Senaru (2,641m) is the equivalent on the northern rim. The two camps offer mirror views of the caldera from opposite sides.
Aik Kalak hot springs are geothermal vents along the northern shore of Segara Anak. The water emerges at around 40 to 50 degrees Celsius and is mildly acidic. Local Sasak tradition holds that bathing in the springs has healing properties.
The Senaru waterfalls (Sendang Gile and Tiu Kelep) on the northern slopes are eroded into the lava flows that pre-date the 1257 collapse. The basalt columns visible at Tiu Kelep are textbook examples of slow-cooling volcanic flows.
The Sasak people of Lombok have an oral and written tradition that almost certainly preserves memory of the Samalas eruption. The Babad Lombok manuscript describes the destruction of Pamatan kingdom and the burial of villages. Modern archaeological work on the southern slopes of Rinjani has located buried settlements with pottery and tools from the 13th century, sealed under several meters of ash — direct physical evidence corroborating the chronicles.
This makes the Samalas eruption one of the only major volcanic events of the past millennium where cultural memory, written record, geological evidence, and global climate data all align. It is a case study taught in volcanology programs worldwide.
You are walking on, sleeping on, and taking photos of an active geological system that has destroyed itself within human memory and is rebuilding itself one Strombolian eruption at a time. The 1257 event would not be survivable today if it recurred — Lombok's million-plus residents would have nowhere to evacuate to. But the probability of another VEI-7 event from this complex is low on human timescales; the magma system needed centuries to recharge before Samalas, and it would need similar centuries again.
Smaller eruptions from Barujari are likely within the next decade. They will close the trek temporarily when they happen, but they pose minimal risk to anyone not standing on the caldera rim during the event. Always check current PVMBG status before booking. The mountain is dynamic, not dormant.
Stand on the summit at first light, look down at Barujari steaming in the caldera below, and you are looking at an active volcanic system rebuilding itself in real time. There are not many trekking destinations in the world where the geology is this immediate.