In 1257, a catastrophic eruption occurred at Samalas, a volcano on the Indonesian island of Lombok. The event had a probable Volcanic Explosivity Index of 7, making it one of the largest volcanic eruptions during the Holocene epoch. It left behind a large caldera that contains Lake Segara Anak. Later volcanic activity created more volcanic centres in the caldera, including the Barujari cone, which remains active.
The event created eruption columns reaching tens of kilometres into the atmosphere and pyroclastic flows that buried much of Lombok and crossed the sea to reach the neighbouring island of Sumbawa. The flows destroyed human habitations, including the city of Pamatan, which was the capital of a kingdom on Lombok. Ash from the eruption fell as far as 340 kilometres (210 mi) away in Java; the volcano deposited more than 10 cubic kilometres (2.4 cu mi) of rocks and ash.
The aerosols injected into the atmosphere reduced the solar radiation reaching the Earth's surface, causing a volcanic winter and cooling the atmosphere for several years. This led to famines and crop failures in Europe and elsewhere, although the exact scale of the temperature anomalies and their consequences is still debated. The eruption may have helped trigger the Little Ice Age, a centuries-long cold period during the last thousand years.
Before the site of the eruption was known, an examination of ice cores around the world had detected a large spike in sulfate deposition from around 1257 providing strong evidence of a large volcanic eruption occurring at that time. In 2013, scientists linked the historical records about Mount Samalas to these spikes. These records were written by people who witnessed the event and recorded it on the Babad Lombok, a document written on palm leaves.
Geology
Samalas (also known as Rinjani Tua) was part of what is now the Rinjani volcanic complex, on Lombok, in Indonesia. The remains of the volcano form the Segara Anak caldera, with Mount Rinjani at its eastern edge. Since the destruction of Samalas, two new volcanoes, Rombongan and Barujari, have formed in the caldera. Mount Rinjani has also been volcanically active, forming its own crater, Segara Muncar. Other volcanoes in the region include Agung, Batur, and Bratan, on the island of Bali to the west.
Lombok is one of the Lesser Sunda Islands in the Sunda Arc of Indonesia, a subduction zone where the Indo-Australian Plate subducts beneath the Eurasian Plate at a rate of 7 centimetres per year (2.8 in/year). The magmas feeding Mount Samalas and Mount Rinjani are likely derived from peridotite rocks beneath Lombok, in the mantle wedge. Before the eruption, Mount Samalas may have been as tall as 4,200 ± 100 metres (13,780 ± 330 ft), based on reconstructions that extrapolate upwards from the surviving lower slopes, and thus taller than Mount Kinabalu which is presently the highest mountain in tropical Asia; Samalas's current height is less than that of the neighbouring Mount Rinjani, which reaches 3,726 metres (12,224 ft).
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. During the early Holocene (before 5,300 BCE), there were few large explosive eruptions at Samalas and Rinjani, but activity later picked up:
- A little understood eruption (probably at Rinjani) between 5,990 ± 50 and 2,550 ± 50 BP formed the Propok Pumice with a dense rock equivalent volume of 0.1 cubic kilometres (0.024 cu mi). Samalas collapsed southward, probably during the Propok Pumice eruption, forming a hilly landslide deposit south of the volcano. This deposit covers an area of about 535 square kilometres (207 sq mi) with about 15 cubic kilometres (3.6 cu mi) of rock, making it the third-largest such deposit in Indonesia. Before the collapse, Samalas might have reached an elevation of 4,207 metres (13,802 ft).
- The Rinjani Pumice, with a volume of 0.3 cubic kilometres (0.072 cu mi) 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 6 centimetres (2.4 in) 28 kilometres (17 mi) away.
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.
The rocks of the Samalas volcano are mostly dacitic, with a SiO2 content of 62–63 percent by weight. The rocks expelled during the 1257 eruption formed from aluminum-rich basalt through fractional crystallization and other physical processes. Volcanic rocks in the Banda arc are mostly calc-alkaline ranging from basalt over andesite to dacite. The crust beneath the volcano is about 20 kilometres (12 mi) thick, and the lower extremity of the Wadati–Benioff zone is about 164 kilometres (102 mi) deep.
Eruption
The events of the 1257 eruption have been reconstructed through geological analysis of the deposits it left and by historical records. The eruption probably occurred during the northern summer in September (uncertainty of 2–3 months) that year, in light of the time it would have taken for its traces to reach the polar ice sheets and be recorded in ice cores and the pattern of tephra deposits. 1257 is the most likely year of the eruption, although a date of 1258 is also possible. It is possible that the accumulation of vapours in the magmatic system over the preceding centuries destabilized it, until the 1257 eruption commenced.
Phases
The phases of the eruption are also known as P1 (phreatic and magmatic phase), P2 (phreatomagmatic with pyroclastic flows), P3 (Plinian) and P4 (pyroclastic flows). They produced rocks with similar composition, unlike other volcanoes where different stages often feature rocks with different compositions. The duration of the P1 and P3 phases is not known individually, but the two phases combined (not including P2) lasted between 12 and 15 hours. The eruption column reached a height of 39–40 kilometres (24–25 mi) during the first stage (P1), and of 38–43 kilometres (24–27 mi) during the third stage (P3); it was high enough that SO2 in it and its sulfur isotope ratio was influenced by photolysis at high altitudes.
Event
The eruption began with a phreatic (steam explosion powered) stage that deposited 3 centimetres (1.2 in) of ash over 400 square kilometres (150 sq mi) of northwest Lombok. A magmatic stage followed, and lithic-rich pumice rained down, the fallout reaching a thickness of 8 centimetres (3.1 in) both upwind on East Lombok and on Bali. This was followed by lapilli rock as well as ash fallout, and pyroclastic flows that were partially confined within the valleys on Samalas's western flank. Some ash deposits were eroded by the pyroclastic flows, which created furrow structures in the ash. Pyroclastic flows crossed 10 kilometres (6.2 mi) of the Bali Sea, reaching the Gili Islands to the northwest of Samalas and Taliwang east of Lombok, while pumice blocks presumably covered the Alas Strait between Lombok and Sumbawa. The deposits show evidence of interaction of the lava with water, so this eruption phase was probably phreatomagmatic. It was followed by three pumice fallout episodes, with deposits over an area wider than was reached by any of the other eruption phases. These pumices fell up to 61 kilometres (38 mi) to the east, against the prevailing wind, in Sumbawa, where they are up to 7 centimetres (2.8 in) thick.
The deposition of these pumices was followed by another stage of pyroclastic flow activity, probably caused by the collapse of the eruption column that generated the flows. At this time the eruption changed from an eruption-column-generating stage to a fountain-like stage and the caldera began to form. These pyroclastic flows were deflected by the topography of Lombok, filling valleys and moving around obstacles such as older volcanoes as they expanded across the island incinerating the island's vegetation. Interaction between these flows and the air triggered the formation of additional eruption clouds and secondary pyroclastic flows. Where the flows entered the sea north and east of Lombok, steam explosions created pumice cones on the beaches and additional secondary pyroclastic flows.
Pyroclastic flows descended the northern slopes of Samalas; on the southern slopes they split into two branches that proceeded to the Alas Strait to the east and the Bali Strait to the west. Coral reefs were buried by the pyroclastic flows; some flows crossed the Alas Strait between Sumbawa and Lombok and formed deposits on Sumbawa. These pyroclastic flows reached volumes of 29 cubic kilometres (7.0 cu mi) on Lombok, and thicknesses of 35 metres (115 ft) as far as 25 kilometres (16 mi) from Samalas. The pyroclastic flows altered the geography of Lombok; they and sediments eroded from the Samalas deposits extended the shorelines of the island and buried river valleys; a new river network developed on the volcanic deposits after the eruption.
Rock and ash
Volcanic rocks ejected by the eruption covered Bali and Lombok and parts of Sumbawa. Tephra in the form of layers of fine ash from the eruption fell as far away as Java, forming part of the Muntilan Tephra, which was found on the slopes of other volcanoes of Java, but could not be linked to eruptions in these volcanic systems. This tephra is now considered to be a product of the 1257 eruption and is thus also known as the Samalas Tephra. It reaches thicknesses of 2–3 centimetres (0.79–1.18 in) on Mount Merapi, 15 centimetres (5.9 in) on Mount Bromo, 22 centimetres (8.7 in) at Ijen and 12–17 centimetres (4.7–6.7 in) on Bali's Agung volcano. In Lake Logung 340 kilometres (210 mi) away from Samalas on Java it is 3 centimetres (1.2 in) thick. Most of the tephra was deposited west-southwest of Samalas. Considering the thickness of Samalas Tephra found at Mount Merapi, the total volume may have reached 32–39 cubic kilometres (7.7–9.4 cu mi). The dispersal index (the surface area covered by an ash or tephra fall) of the eruption reached 7,500 square kilometres (2,900 sq mi) during the first stage and 110,500 square kilometres (42,700 sq mi) during the third stage, implying that these were a Plinian eruption and an Ultraplinian eruption respectively.
Pumice falls with a fine graining and creamy colour from the Samalas eruption have been used as a tephrochronological marker on Bali. Tephra from the volcano was found in Colombian sediments, and in ice cores as far as 13,500 kilometres (8,400 mi) away, and a tephra layer sampled at Dongdao island in the South China Sea has been tentatively linked to Samalas. Ash and aerosols might have impacted humans and corals at large distances from the eruption.
There are several estimates of the volumes expelled during the various stages of the Samalas eruption. The first stage reached a volume of 12.6–13.4 cubic kilometres (3.0–3.2 cu mi). The phreatomagmatic phase has been estimated to have had a volume of 0.9–3.5 cubic kilometres (0.22–0.84 cu mi). The total dense rock equivalent volume of the whole eruption was at least 40 cubic kilometres (9.6 cu mi). On Lombok, it forms the Kalibabak Formation, a geological rock formation. The magma erupted was trachydacitic and contained amphibole, apatite, clinopyroxene, iron sulfide, orthopyroxene, plagioclase, and titanomagnetite. It formed out of basaltic magma by fractional crystallization and had a temperature of about 1,000 °C (1,830 °F). Its eruption may have been triggered either by the entry of new magma into the magma chamber or the effects of gas bubble buoyancy.
Intensity
The eruption had a Volcanic Explosivity Index of 7, making it one of the largest eruptions of the current epoch, the Holocene. Eruptions of comparable intensity include the Kurile lake eruption (in Kamchatka, Russia) in the 7th millennium BC, the Mount Mazama (United States, Oregon) eruption in the 6th millennium BC, the Cerro Blanco (Argentina) eruption about 4,200 years ago, the Minoan eruption (in Santorini, Greece) between 1627 and 1600 BC, the Tierra Blanca Joven eruption of Lake Ilopango (El Salvador) in the 5th century, and Mt. Tambora in 1815. Such large volcanic eruptions can result in catastrophic impacts on humans and widespread loss of life both close to the volcano and at greater distances.
Caldera
The eruption created the 6–7 kilometres (3.7–4.3 mi) wide Segara Anak caldera where the Samalas mountain was formerly located; within its 700–2,800 metres (2,300–9,200 ft) high walls, a 200 metres (660 ft) deep crater lake formed called Lake Segara Anak. The Barujari cone rises 320 metres (1,050 ft) above the water of the lake and has erupted 15 times since 1847. A crater lake may have existed on Samalas before the eruption and supplied its phreatomagmatic phase with 0.1–0.3 cubic kilometres (0.024–0.072 cu mi) of water. Alternatively, the water could have been supplied by aquifers. Approximately 2.1–2.9 cubic kilometres (0.50–0.70 cu mi) of rock from Rinjani fell into the caldera, a collapse that was witnessed by humans and left a collapse structure that cuts into Rinjani's slopes facing the Samalas caldera.