The lovely smell might change for a more sulfurous one – at least if recent forecasts of volcanic gas emissions or even an eruption of Banda Api come true…
The name of the Indonesian Banda Islands in Central Maluku is strongly connected with the world’s history of the last 500 years. When people found that nutmeg was good for countless diseases and even an aphrodisiac, they begun to pay gold for spices. European explorers sailed the world’s oceans and discovered entire continents on their quest for the “Spice Islands”. In 1512 the first Portuguese expedition arrived at the archipelago. This was the beginning of an infamous plantation-style colonialism with enormous implications for the rest of the world, not to mention the Bandanese themselves, most of whom were massacred or enslaved by the Dutch by the 1620s.
Thanks to the diligent recordings of the time, earliest eruption reports of Banda Api are frequent from 1586 on. In 1615 Pieter van den Broecke saw “Gonnapi (i.e. Gunung Api or ‘volcano’) Banda” throwing out thick ash clouds and glowing stones. Twenty years later a Johann Sigmund Wurffbain even climbed the volcano and described the summit: From an 80 m large opening in the crater thick clouds of smoke rose, occasionally accompanied by flames. An enormous outburst is mentioned in 1694 of “Gounapi near Banda”.
The GVP has listed 23 confirmed eruptions, but several more might have gone unnoticed by the outside world. Most of Banda Api’s historic outbreaks were of a very effusive Strombolian type, producing VEI 3 eruptions up to the end of the 17th century. The later ones turned out as VEI 2 or 1. However, the last eruption, in 1988, classified as an VEI 3 again.
Looking at a map of the Banda islands, a similarity with Anak Krakatau comes to mind – one younger volcano surrounded by the remains of its forbears. Indeed, the development of Banda Api can be compared to that of the Krakatau volcano. The two neighboring islands Banda Neira (Naira) and Banda Besar (Lontho[i]r) are thought to be the remains of two largely submarine calderas that preceded the construction of the Banda Api stratovolcano. The outer caldera has a diameter of 7 km, the nested inner caldera is 3 km wide. Banda Api has grown within the inner caldera from a depth of 4.5 km to 640 m a.s.l. It was at least the fourth generation of ancient volcanics in the area.
During the formation of the ancient Lonthor volcano a change of magma (in chemical composition) took place from more mafic (tholeiitic basalts/andesite) to more felsic rocks. With that more explosive mixture in the magma reservoir, Lonthor had those powerful eruptions which produced its calderas. The lavas from modern Banda Api are almost exclusively dacite, i.e. they contain a high percentage (59-67%) of SiO2 minerals (there are different rocks mentioned for Banda Api in other sources, I took this from Vroon, 1992).
There have been considerations that all of the Banda Islands could be remains of one humongous caldera of some 50 km in diameter, but this had to be dismissed as impossible.
THE 1988 ERUPTION
No wonder many islanders still remember this eruption – “which fried the fish in the sea” when the lava entered the water. It came not entirely unexpected as earthquakes have been felt for about a week in advance. From 8 May, they were felt every few minutes at the nearby village of Neira. Banda Api erupted on 9 May after 86 years of slumber.
Luckily, this was one of the eruptions that have been predicted. The only people to lose their lives were four who had disregarded evacuation orders. A total of more than 10 000 evacuations have been carried out, from Banda Api and Naira to safer places. From the second day of eruption on, VSI operated an observation post at Neira city, equipped with a radio and a seismometer.
In the morning of May 9, two simultaneous black eruption columns of gas and tephra that included large incandescent blocks rose (apparently*) 3-5 km into the sky. Several lava streams began flowing down and explosions originated from five craters on the N and S flanks. Eyewitnesses report that at one time “the entire eruption column bent sideways to the south”, thereby blasting or spraying tephra over a wide area of the western tip of Lonthor Island. This was probably caused by slope failure above the vent. Material slumped into the vent and temporarily deflected the eruption column. The duration of this directed blast was less than a minute.
Two large lava flows emerged from two fissure craters ~400 m apart and at different elevations on the N and NW flanks. They reached the sea just 4-5 hours after the start of the eruption – overrunning two villages in their path. A graben had developed between the two N-flank vents, causing the two craters to coalesce later that day. Small fissures on the shore produced phreatic explosions.
Explosions and lava flows went on for a week; all craters were quiet by 18 May. But it took the volcano another year, until July 1989, to reach its former background level of activity.
Later surveys revealed that the activity had been concentrated at two vents on the N side and at numerous vents on the S side, along a fissure oriented NNE-SSW. That crack had opened up along the entire south flank, cut across the summit and down the northern flank to an elevation of 200m – thus forming a 3-km-long arc. Most of the 9.4 x 106 m³ of lava was erupted during the first 12-18 hours. The first few minutes of the eruption had also produced a pumice lapilli tephra of light gray to beige color with prominent dark euhedral (well-defined) phenocrysts (single embedded crystals) of pyroxene. Material erupted later was more scoriaceous and darker in color.
*Analysis of imagery from Japan’s GMS satellite later suggested that the eruption clouds reached much higher altitudes than indicated by ground observations. Measurements of ‘coldest surface temperatures of an eruption cloud’ corresponded to altitudes of >16 km, rather than the reported <5km.
PVMBG reported that seismicity at Banda Api had been increasing since the beginning of March; during 1-4 April seismic patterns were similar to those recorded before the eruption in 1988. A weak white steam plume was observed from the main crater with a max. height of 25 m from the summit.
The increased number of volcanic earthquakes, indicating the process of fracturing rocks due to the movement of magma (gas, liquid and solid rock), suggests that this activity is significant enough to anticipate the possibility of an eruption.
The Alert Level was raised to 2 (on a scale of 1-4) on 5 April; visitors and residents were warned not to approach Banda Api within a 1-km radius of the summit area. Authorities are now preparing to evacuate over 770 people living on the slopes of the volcano. Even if it does not erupt, the emission of toxic fumes is very likely. Images from latest PVMBG report:
G. Banda Api’s magmas have risen from the Benioff zone at 130 km depth and through 15 km oceanic crust.
The Banda Arc sits in a complex subduction setting and its origin has been a subject of continuing controversy and proposals. Together with the eastern end of the Sunda Arc it represents a continent-arc collision zone. Situated at the centre of three converging and colliding major tectonic plates (the Indo-Australian, Eurasian and Pacific plates) the Banda Arc comprises young oceanic crust enclosed by a volcanic inner arc, outer arc islands and a trough parallel to the Australian continental margin. The inner, or intra-oceanic arc encloses the Banda Sea and forms an almost 180° curve. It was formed about 1.5 million years ago.
At the Banda arc, the Indo-Australian plate is subducting NNE-wards underneath the Southeast-Asian plate. This is also influenced by westward plate motions of the Pacific plate, situated to the east. Present day tectonics around the Eastern end of the Sunda – Banda arc are characterised by the collision between leading portions of the passive Australian continental margin and the island arc that borders the Flores and Banda Seas.
Volcanism in the Sunda – Banda arc ranges from an intra-oceanic arc environment (NE Banda) to arc volcanism dominated by a crustal input (Sunda). The Banda Sea crust is essentially oceanic in nature but different models have been proposed as to its age and origin. The Banda arc itself is probably constructed on oceanic crust covered by 1–2 km of sediments. (after O. Nebel et al., 2010).
A more detailed post on the Banda Arc tectonics has been written by KarenZ on the former VC.
The Banda Islands have been a great diving spot before the 1988 eruption, which must have changed for a couple of years afterwards. The good news is, the diving has never been as good as it is NOW: “The 1988 volcanic eruption of Gunung Api provided a unique opportunity to study the rate at which a reef-building coral community develops on a fresh lava flow. Five years after the eruption, the sheltered lava flow under investigation supported a diverse coral community. Higher average coral diversity, coral abundance and cover were recorded on the lava flow than on an adjacent carbonate reef not covered by the lava.” Today, almost 30 years later, the formerly completely destroyed coral reefs around the volcano are some of the most diverse and best thriving in the area.
Another good news is that eruption-affected corals can even be of service to volcanology: This paper looks into the possibility of determining past volcanic events by investigation of corals. As their calcite structures re-grow after an eruption they incorporate any volcanic ash covering the dead surfaces. This would produce sort of layers or banding which could be analysed. In the case of the 1988 eruption, X-radiographs of coral skeletons confirm that the timing of formation of both the coral death surfaces and the orange banding is coincident with the 1988 eruption.
One of the attraction for visitors is the Dutch colonial building style that still prevails on the islands, and Fort Belgica on Banda Neira is THE place to visit for history and volcano views. This lovely combination, together with the outstanding role the islands played in Indonesian history, has been appreciated by displaying this image on the reverse of the brand-new 1000-Rupiah notes by Bank of Indonesia:
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– Banda Api, GVP
– The Geology of Indonesia/Banda Arc (Wikibooks)
– Rolling open Earth’s deepest forearc basin (2016, paywalled)
– Neumann v. Padang: History of Volcanology in […] (1983)
– Subduction of continental material in the Banda Arc, […] (1992, PDF)
– Latest PVMBG report on G. Banda Api
– G. Banda Api, description by PVMBG
– Corals as Proxy Recorders of Volcanic Activity […] (1996, paywalled)
– Book review: Nathaniel’s Nutmeg
– Bottles-Up (Travel agents)
– Resources about the Banda Islands’ history