For a brief period this was the nation’s capital. During the war of independence (called a ‘police action’ by the Dutch, in an older echo of some current events), Yogyakarta served as the seat of the Republic between 1946 (the fall of Jakarta to the Dutch) until 1948. Indonesia later rewarded the city by granting it a special status. It remains the only city in the country ruled by a monarchy. It is governed by sultan Hamengkubawono X (yes, there have been nine before him) since 1988. In a break with tradition, he has chosen his daughter as his heir. The sultan lives in the Kraton (really!), which here means a walled palace. In this case it is more like a walled city, containing a variety of buildings.
Yogyakarta, on the island of Java, is an ancient city. In the 16th and 17th century, it was the seat of a kingdom which ruled much of Java. Long before that, the 8th and 9th century the Mataram kingdom build two magnificent temples here. Nowadays, Lonely Planet calls Yogyakarta ‘the soul of Indonesia’ because of its art and history. The metallic music of the gamelan can be heard: a sacred version is played once a year by the royal family. Gamelan music is among the recordings on board the Voyagers, on their way to the stars: Yogyakarta is where this type of music was first written down.
The two famous temples are Borobudur and Prambanan. Tours can be booked to visit these from Yogyakarta. And so can another famous attraction, often combined in the same tour because it is not far to Java’s most active active volcano. Merapi lies 30 kilometers to the north.
At 2910 meters tall, Gunung Merapi is a little smaller than the volcano just to its north, Gunung Merbabu (3140 meters). (Gunung means, not surprisingly, mountain.) The two volcanoes are only 10 kilometers apart, and are connected by a 1500-meters high saddle. Merbabu is considered dormant: there have only been two minor (VEI-2) eruptions in the past 5 centuries. The name is perhaps surprising, in view of this lack of activity: it translates as ‘Mountain of Ash’.
Gunung Merapi is quite a contrast to sleepy Merbabu. The name already gives a hint to its nature: ‘Mountain of Fire’. This is not an uncommon name: there are two other volcanoes in Indonesia with the same name (another Merapi is found at Ijen (also Java) and a volcano called Marapi is an active volcano on Sumatra.) It has minor eruptions every few years, larger ones perhaps once a decade, significant ones perhaps 100 years apart, and a very large eruption 1000 years ago. The lava it produces is basaltic to basaltic andesite.
Merapi is an impressive stratovolcano for tourism, but an uncomfortable neighbour to a major city. It is even more uncomfortable to the many people living closer to it. The land here is fertile and that attracts people. Over a million people live on Merapi’s flanks. During the 2006 eruptions, 200,000 people were evacuated, and in 2010 it was over 400,000 people. In the latter eruption over 350 people were killed when people refused to leave. Merapi was in the original list of decadal volcanoes exactly because it presents a danger to so many people.
In spite of the danger, or maybe because of it, the sultanate is considered have a close spiritual connection to Merapi. The Kraton lies midway between the mountain and the sea: in the local folklore it guards the straight path between the court of the spirits underneath Merapi and the court of the Queen of the South Sea at Parangtritis beach. One side of the main pavilion of the Kraton looks straight north at Gunung Merapi; the other side looks straight south at the Indian ocean. It keeps the balance between the two.
Eruptions follow a typical pattern. A lava dome grows up at or near the summit. After a few years it becomes too big and collapses under gravity. This gives an explosion. The explosion causes pyroclastic flows which raise down the slopes through the drainage channels, often reaching a distance of a few kilometers. Larger explosions reach further. The large eruptions of 2010, 1872 and 1822 were strongly explosive, and the flows went far enough to reach populated areas. The explosions remove the rebuild tip of the mountain. Over the next few year this rebuilds as a new lava dome, and the sequence repeats. Each dome grows in a slightly different location, but alwqys near the summit.
The eruptions also produce lava flows, from the same region as the domes, and typically traveling a few kilometers. Some have reached 10 km from the summit. The pyroclastic flows extend a bit further, with some deposits found as far as 20 kilometers distance. Lahar deposits are found in the valleys: these come from heavy rain, and carry with them the pyroclastic debris. This volcano has history.
Merapi is fed by subduction of the Australia plate. The oceanic plate is still relatively young (120 million years at this location). On top of this subducting plate is a thick layer of sediment. The crust here is continental (Sundaland, part of Eurasia) but is young. This makes the magma more oceanic, basaltic and andesite, compared to the more continental magmas of Sumatra where the crust is older and the eruptions more rhyolitic.
Merapi is famous for its pyroclastic flows, arising from explosions and from the regular dome collapses. An old description for pyroclastic flows, coined in 1933, is indeed ‘Merapi-type glowing clouds’. (In the Indonesian language (a form of Malay) it is ‘awan panas guguran’ (‘hot cloud fall’): googling this term shows that it is often appears in combination with ‘Merapi’!
Young and Old Merapi
On the southern flank of Merapi there are two steep, eroded hills, Plawangan and Turgo. They are islands of basaltic rock, rising 30 meters above the pyroclastic surroundings. The solid rock are used to advantage, by building the local volcano observatory on Plawangan. It was stable and provided an excellent view of Merapi. There was one drawback: it was a bit too close for comfort. The site was abandoned in the 1990’s because of the danger of the pyroclastic flows. Volcano observatories prefer to observe volcanoes from a safe distance, and preferably from the city it is trying to protect – and its night life.
Both hill date from the earliest know phase of Merapi. They have been dated to approximately 130,000 years. A similar outcrop on the northern flank of Merapi is Bibi, which is slightly younger. These are the remnants of the first volcanic activity known here. It is now know as proto-Merapi.
Around 30,000 years ago a large volcano developed here, probably resembling the current mountain. It is called Old Merapi. It was active volcano. Deposits show that it regularly produced VEI-3 and 4 subplinian eruptions. Very different from the current dome-building-and-collapse events. But the mountain did not survive. Several flank collapses destroyed the mountain. The last one may have been as recent as 5,000 years ago. It left a rim on the north and east side of the current mountain, which is known as Somma Merapi.
The current mountain grew after this final collapse and is therefore known as Young Merapi. Some 6,700 year ago a thick layer of lava was erupted. Around 2,000 years ago the lava composition became more potassium-rich. Since that time, the mountain has been almost continuously active. 90% of all 14C radiocarbon dates obtained for Merapi are from the last 2000 years!
What changed 2000 years ago? The change from explosive eruptions to dome building can come from degassing. Gas-rich magma causes explosions. Gas-poor magma instead forms a dome, and the (milder) explosion comes when this dome collapses. If the magma ascent is slow, gas can escape. If the ascent is fast, it doesn’t. A slow-down 2000 years ago may have led to the current phase. As long as this continues, the mountain is not as unsafe as it could be – something that does put the bar rather low.
Around 1200 years ago there may have been a partial flank collapse of Young Merapi. The size of this is strongly disputed (and the exact date of 1006 you’ll find on wikipedia is wildly optimistic). It damaged the temples in the region and perhaps contributed to the local leadership relocating to a place further east around 928 AD. However, this is far from established.
Dating of the deposits has shown in the past 2000 years there have been two periods of low activity, a short hiatus between 1300 and 1450 AD and a long hiatus from around 600 to 900 AD. After the interruption, the lavas became more evolved (andesitic basalt): during the hiatus, the magma chamber had slightly aged and received little new inflow. The magma chamber is likely small and quite dependent on the amount of inflow.
Progo Muri fault
A look at the map of Java shows that the central part of the island is much thinner than the easter and western part. It looks like two islands connected by a bridge, and with a thin extension to the east. The four parts are different. West Java has a mountainous volcanic area, the thin centre has two lines of volcanoes running northwest to southeast, East Java has a few isolated volcanoes and the thin extension has the large calderas.
This area is the southeastern edge of Sundaland, the continental block that extends across southeast Asia, Borneo, Sumatra and Java. East Java is part of a separate block, the East Java Terrane, which joined Sundaland a long time ago, in the late Cretaceous, the last days of the dinosaurs.
Along the southern edge is the subducting Australian plate. The subduction is throwing up the volcanic arc running east-west. The plate collision is beginning to affect the region. 10 million years ago. East Java originally was oriented northwest-southeast. The collision and compression caused it to rotate by about 30 degrees, to its current (approximately) east-west orientation.
Merapi lies where the central Java connects to the East Java, where the thin meets the wide. The parts are stitched together with a fault, called the Progo Muri fault. (Muri is the volcano that lies isolated at the northwestern top of East Java.) This fault line can be followed from coast to coast, running to theeast of Yogyakarta. The southern part is also known as the Opak river fault: it caused an M6 earthquake in 2006. West of the fault lies the low, fertile basin of Yogyakarta. This basin is caused by extension. It is bordered to the north by the line of volcanoes of which Merapi is the southernmost and youngest member. Merapi lies close to Progo Muri fault line, and has grown up within this extension basin.
A look at how Java is moving under the stress from the approaching Australian plate shows what is going on. The subduction zone around Indonesia is quite strongly curved. It runs south in the Indian ocean, southeast around Sumatra and turns east around Java. This curve puts a torque on the Sunda plate and torques cause rotation.
The forces are pushing Java to the right (east). This is accommodated by a fault running roughly east-west along the north side of Java. But there is also rotation with east Java moving more southeast and east Java northeast. East Java is still rotating, just like it did 10 million years ago, pushed by the Australian bulldozer. This rotation has activated the ancient connection between the old Sunda plate and the recent (100 million year) addition: the Muria Progo fault. It is a left lateral fault, as expected from the different ways the two sides are moving, but it also has extension as east Java is rotating away from the south side of central Java. This extension has formed a graben, in which Yogyakarta is located.
The line of volcanoes to which Merapi belongs began with Ungaran, followed by Telemoyo, Merbabu and now Merapi. The activity has moved to the south, more into the graben. The extension explains why Merapi can be so active. The old fault line may have helped creating a bit of weakness. Indonesian volcanoes do not need much help, but Merapi is different in that the magma supply to the edifice is continuous, rather than intermittent. An unobstructed conduit is helpful.
The two magnificent temples, Borobudur and Prambanan, stand strangely isolated. Their abandonment forms part of an old story which talks about an eruption which changed the whole area around Yogyakarta into a wilderness and nearly completed buried Borobudur under ash. The story has been dated to 1006 AD, which is where the data in Wikipedia comes from. However, there is only one deposit around Merapi which could correspond, but it seen mainly in one valley and it is a bit older, by about a century. Oral traditions need to be taken with some caution. The most recent larger collapse is at least 1500 years old.
The temples in Central Java were build in the period 720 to 928AD. The dates are fairly well known because of stone inscriptions. After 850 AD only smaller temples were build but the building itself continued. At the end of the period, the local culture (called Mataram) suddenly relocated to east Java, palace and all. Was this because of Merapi? Some temples were abandoned, whilst other remained in some kind of use until a few centuries later.
Borobudur is still the world’s largest Buddhist temple; Prambanan remains the largest Hindu temple in Java. Construction at Borobudur began around 760 AD. In 780 AD Buddhism arrived and the construction changed character. Later Hinduism arrived but this had little effect on the building which continued until around 850. After this, major building ceased in the area. Smaller temples were quickly (and basically) finished. In 928 the ruling family moved the east Java. Some of the local temples were maintained until around 1260 when volcanic ash began to collect inside the Sambisari and Kedulan temples. This is very close to the massive (VEI-7) Rinjani eruption, and one wonders whether the aftermath of this eruption depopulated the area.
Borobudur was lost after this time. When Europeans arrived, it was hidden in deep jungle. The temple is build on a small hill. It appears that the surrounding area became a lake somewhere around 1200 AD. The most likely cause was river damming by a debris flow or lahar. The lake remained for some time and may be the reason the site was forgotten. It now stands as an impressive memory of an ancient civilization, proving the basic error of colonization, that the locals were ‘savages’ who need civilized oversight. In reality, their buildings were on par with the rest of the early medieval world, whether Europe, America or the forgotten empires of dark Africa. Perhaps the best comparison is young Angkor Wat, a larger area but smaller structure than Borobudur. They show us worlds of which the very existence was denied for far too long.
Prambanan was build along the Opak river. That caused two problems. The river could bring lahars: to secure the temple, the river was shifted further away. The second problem was that this river follows a fault line. Prambanan was eventually destroyed by an earthquake. But by that time it had long been abandoned, when the rulers left in 928 AD. Exactly why they left is still a matter of speculation. A major eruption from Merapi did not really happen and would not have been enough for such a drastic decision. But the region had perhaps become impoverished as suggested by the limited building work. Did the ruling family move because they could no longer afford the maintenance? Without them, people kept at least some of the temples clean and operating, but not forever. In the 15th century Islam took over, but by that time the temples had already been forgotten.
Building your major temples so close to Merapi was always going to be a problem. Often the pyroclastic flows would run down only a few kilometers from the summit. But every now and then they would reach much further, leaving thick lahar deposits even near Yogyakarta. Temples build too close were buried and eventually abandoned. Even majestic Borobudur would succumb to the mountain. Now the temples of Borobudur and Prambanan are visible again. But Merapi is still there and the danger remains.
Digging down, there are two ash layers in the Borobudur region which exceed anything modern Merapi has done. The layers are more than ten meters thick, and are dated to 120,000 and 30,000 years ago. The younger layer may come from proto-Merapi. Clearly, such an eruption would destroy both the temple and Yogyakarta. No major eruption has occurred in the past 10,000 years: the change in eruption style as helped. But what happened twice will happen again, although hopefully not for a very long time.
And the volcanoes threaten in different ways. In the large 2010 eruption, Borobudur was covered in ash. Prambana escaped, but was covered four years later by ash from Kelud. The acid in the ash is damaging to the stone and erodes the intricate stone work. During the 2020 eruption of Merapi, the statues (stupas) were covered in plastic in order to safeguard against the acidic ash.
Earthquakes remain a danger. Borobudur contains more than 500 Buddha statues. Of those, 200 were headless, lost during earthquakes. The neck is a weakness in these statues.
I wonder though whether the biggest danger is not from Merapi. Indonesia has had two VEI-7 eruptions in the past millennium, from Rinjani and Tambora. The Rinjani eruption appears to be at the time Borobudur was finally abandoned. Perhaps we are safeguarding these monuments against the wrong volcanoes. And it is not just the monuments. Cities like Yogyakarta with its living culture and ancient history are treasures. We should not lose them. Not even to a volcano.
Albert, October 2022