And then there was a volcano no one has ever heard off. It is on an island that is far off-grid and which takes a 9-hour ferry to get there. (There is a small plane, if you are happy to travel without luggage.) The volcano has long gone extinct. All that remains is this island, 15 km across with a rugged, hard-to-reach interior. The main road on the island is narrow, staying near the coast. Tracks that go into the interior are for four-wheel drive vehicles only. The interior is hilly and bumpy, never more than about 650 meters tall but with many different peaks. The dense forest makes it difficult to move around without a machete. The Java sea stretches on all sides, with not another island in sight. The sea is not deep: below it lies another world that once was land. But there are no other volcanic islands here, and it is far from the volcanoes of Java. Bawean Island is a bit of a mystery. There is a hidden history here.
Bawean island covers around 192 km2 and has some 75,000 people across 30 villages. Rice is grown but much of the island is unused. Part of the island is a protected national forest, with the main aim to protect the deer. Eco-tourism is on the agenda, with good diving and snorkeling on the coral reefs: 30% of the reefs are pristine. But the island is hard to reach and tourism remains limited. This is probably a good thing: small islands and mass tourism are a poor combination and lead to rapid degradation of the environment. The local port is too small to accommodate cruise ships: a blessing in disguise.
For those interested in military history, Bawean island was the location of one the less successful battles of the second world war, when a convoy of three ships, the USS Pope, HMS Exeter and HMS Encounter were sunk by Japanese cruisers. This happened on 1 March 1941.
The people on Bawean have a diverse origin. They are a mixture of settlers from all across the region, especially Java and Borneo. Many came from Madura (an island just off the north coast of Java), and the local language is still a variation on Madurese though with significant differences in pronunciation. The name for the island is supposed to mean ‘there is light’, referring to the sunlit peaks which could be seen from sea at sunrise. Nowadays there are strong links with Singapore, where many men go to find work. It is a tradition here that men work away from the island, without their families. So many do this that Bawean is also known as the ‘island of women’: some 77% of the population is female.
Isolated islands often have a limited but unique fauna. That is true here too. There is an endemic species of deer on the island which is highly protected but in severe danger of extinction. One wonders how it got there: presumably it has been here since the ice age when the Java sea was dry land. There are only about 200 Bawean deer left. The other endemic mammal is the Bawean warty pig, a dwarf species of which some 250 are left. In spite of being the world’s rarest pig, it is not specifically protected, perhaps because pigs are too often seen as pests! There is also a rare subspecies of serpent eagle, now down to 30 pairs. The top predator on Bawean Island is a snake: the Malayan krait, highly venomous and nocturnal. It is the only venomous snake on the island but more than makes up for that! The trees on the hills are covered with hanging mosses, sometimes so dense nothing else is visible – a sign of danger when there are venomous snakes hiding in the moist forest! The name of one of the peaks, Mount Lumut, means ‘mossy mountain’. (There is more than one ‘Mount Lumut’ in Indonesia!) Part of the island is a protected reserve, aimed to protect the deer. It benefits the pig too. Illegal logging is a problem even in the protected forest. The most recent survey found remarkably few land birds on Bawean island, attributed to the Asian cage bird trade. Nowadays, even the most isolated islands suffer from civilization.
The moss forest is an eery place. The scarcity of land birds means there is little bird song, and the moss dampens the sounds further. The silence is deafening. The most forest is found in elfin forest, which is typical for higher tropical mountains. Elfin forest consists of mixed trees some 10 meters tall creating a single canopy layer above the ground cover, with small leaves. Moss grows on the ground and lower stems but can also hang thickly from the branches. It requires a lot of moisture. Moss forest is therefore found in foggier places. This can be depressions of the crest of the mountains, as they may contain more standing water and are therefore subject to fog. The altitude of the moss forest varies with location: moss forest can form lower on small ocean islands than on the large land masses. On Bawean Island it is found as low as 550 meters, but on Java it can occur as high as 2 km. In effect, the moss follows the typical height of the cloud layer. A long dry season stops moss forest from growing.
There is a volcanic connection here. Mount Pangrango on Java has moss forest, but its twin Mount Gede does not. Gede is volcanically active whilst Pangrango has long been extinct. The sulphur emission may be part of the problem, but young volcanoes also are more free draining than old, eroded volcanoes. The moisture levels may differ. In any case, to find moss forest, avoid the most lively volcanoes. Bawean Island is extinct. It is perfect.
The island is a raised area above the bottom of sea. Under water the island extends about twice as far as above. There is a steep edge to 50 meters depth, perhaps from a time of lower sea levels. The Java sea here is about 60 meters deep.
The interior of the island has many peaks and domes, with some argument (even now!) about which peak is the highest. They reach about 650 meters. Mount Besar is considered to (just about) be the highest. Three other peaks are over 600 meters. There is also a 1-km wide lake called Kastoba which is surrounded by an elevated rim.
Landslides are common around the lake. The topsoil is a 5-meter thick layer of leave litter, sitting on a layer of clay. The topsoil can, and often does, slide. Underneath the clay is lava with a basaltic andesitic composition. Clearly, if you look below the surface everything on Bawean island is volcanic in origin: the land, the peaks and the lake.
The volcanic origin is surprising in view of the lack of visible volcanoes within the Java sea. We are far from the subduction-driven volcanic arc for which Java is famous. The thick leaf mould shows the activity is not recent. The island is extinct. But the ruggedness still reminds of the old activity. The lake is the remnant of a volcanic crater. The various peaks are plugs and domes – there are a staggering 44 of them! This volcano is very different from the stratovolcanoes further south. There were many eruptions here but they build a bumpy plateau, not a mountain.
The composition is also different. There are two different lava types on Bawean Island. The lava flows deep below the clay are andesitic basalt. To be precise, these flows are basanitic and tephritic, lowish in silica, and are enhanced in sodium and potassium. The lava flows contain leucite, where the quartz is replaced by potassium silicates. The domes and plugs are nepheline-bearing phonolites, like leucite but with more sodium. Detailed analysis shows that the Bawean lavas are high in potassium. The K2O fraction of the rocks are around 5% and the NaO fraction is up to 10%.
Dating has found that the phonolites have ages between 380,000 and 840,00 years, while the other lava flows have ages between 250,000 and 670,000 years. Often the plugs rise up through the lava flows and so are younger, but the opposite also occurs. Older lava may be present, of course. Dating shows what is at or near the surface, not what lies underneath.
The measured fractions make the island ultra-potassium-rich. These types of magma are rare in subduction environments. There are a few other potassium-rich lavas around Java among current volcanoes. The volcanic arc on Java does not have them. Away from this main line of volcanoes, Java’s ultra-potassic rocks occur in three groups of volcanoes. They are located at 20-30 km (Ringgit: East Java), 100km (Muria: central Java) and 250km (Bawean Island) from the volcanic arc. Bawean is the only one of these which is not on Java itself. The subducted plate here is deep, 250 km at Ringgit and 650 km at Bawean island. If this plate is the source of the magma, as it is in the volcanic arc, then the magma here comes from a very large depth.
The images (all from Handini et al.) show the locations of the ultra-potassic volcanoes and their compositions. The colours indicate the volcano, and the size of the dots reflects the depth of the subductin plate underneath. These are not unique. Similar magmas to Bawean island (although not identical) are found, amongst others, in the volcanic area near Rome in Italy and in the intraplate volcanics of Australia. What these places have in common is that continental crust is involved in the melt.
So is there continental crust involved? Indeed, there is. Indonesia is a continent hiding behind an island arc. The large western islands of Indonesia (Sumatra and Java) lie on the rim of a submerged continental plate known as Sundaland. It was dry land during the ice age. That 9-hour ferry to Bawean Island sails above a drowned land which people once called home.
Sundaland connected much (but not all) of Indonesia to much of Southeast Asia. There were major river valleys here, huge drainage systems flowing toward the South China Sea. The continent was used for migrations and probably for living. There are scattered populations of people in Malaysia and Indonesia with ancestry that predates the current populations. They are closer related to the indigenous populations of Papua New Guinea and Australia. Flores, a more eastern island, had a human group that was even older, which later became extinct. All must have lived on Sundaland. But as the far-away ice melted, the land became submerged. It must have been terrifying for the people who lived here. This is the ultimate Atlantis, a true continent now gone, with only scattered islands remaining. One wonders how much archaeology lies hidden below the Java sea.
Before the flood, Bawean Island was a 700-meter tall, bumpy plateau. Now it has become an island, far from anywhere, best known for catching the early morning sun while the sea is still in darkness. It is the sea that is the intruder here.
It is not the only continent here. The volcanic arc of Java is caused by subduction of the Australian plate, migrating into Indonesia at headlong speed. This plate carries continental shelf with it. This continental crust has not reached Indonesia yet. When it does, expect the mountains to rise! Already, the subduction is pushing up the edge of Sundaland. This push-up is the origin of the large islands of Sumatra and Java. They are the raised rim of the sunken land, pushed up by the approaching continent.
Sundaland is an accumulation of pieces. It grew to its final size by accretion of fragments.
200 million years ago, this was part of Pangea, the supercontinent which was split east to west by the Tethys ocean. To the south, on the far side of the Tethys, was Gondwana. The northern edge of Gondwana was a complicated place. It rifted in several places; fragments split off and were pulled in by subduction zones to the north. We have come these fragments before when discussing the fossils of Mount Everest.
One fragment came from what is now western Australia, crossed the Tethys, and merged with Eurasia. This is now known as the Billiton lineament. It finished the crossing some 100 million years ago. The southwestern part of Borneo and western Java came from this accretion. Some 90 million years ago it was followed by a second fragment; this added southeast Borneo, east Java and part of Sulawesi. This fragment originally came from northwestern Australia and Bird’s head peninsula on New Guinea. It added a different type of crust.
On the image above, the dashed lines show the various additions to Sundaland. ‘1’ shows the edge of the first accretion. ‘2’ is the limit of the second one. Later, other, more oceanic parts were added.
For the next 40 million years or so not much happened. The subduction which had pulled in the fragments had ceased. Around 45 million years ago it restarted: a new volcanic arc formed south of Sundaland and Australia began to move north. Debris from these new volcanoes covered part of Sundaland. But volcanism ceased again. The island arc moved northwards and merged with Sundaland between 5 and 10 million years ago. The merged arc formed the Southern Mountains on Java, and the impact raised the region to form the rest of the island of Java. The modern volcanoes of Java are build in the past 2 million years or so on this raised basement. The volcanoes further to the north are build on the Sundaland continental plate. It is these volcanoes that produce the potassium-rich rocks. Bawean Island is the furthest of those volcanoes. In age it belongs to the modern volcanic era – in composition it belongs to an older era.
There is something funny here. The potassium-rich volcanoes on Java are on the part that was added in the second accretion: they are absent from the area added in the earlier accretion event. Where is Bawean island? It is also on this part of the crust, close to the old suture where these two fragments joined. The secret to these volcanoes is not in the deep subduction zones. It is the crust that lies above it.
The fault and the ridge
We have come across this suture before. In an earlier post, Merapi was found to be located near the Progo Muri fault, the line along which Java was stitched together. The fault ends at Muri which, of course, is the potassium-rich volcano on the north coast of Java. This is an ancient fault, but on Java it still capable of generating substantial and damaging earthquakes. Why? The fault should longe have died, but it has been reactivated by pressure from the approaching Australian plate. There is now a bit of rotation and extension and it makes use of this ancient weakness.
The Progo-Muri fault (it has several different names) ends at Muri volcano. But in the Java sea it continues as the Muria trough, a deeper section of the Java sea, and along this trough lies the Muria fault, running towards the northeast. It seems inactive at the current time. The trough seems a very shallow one but in the rock underneath it can be traced to a depth of 4 km. It has almost completely filled with sediment. The Muria fault borders it on the east side. The Muria fault passes some 30km west and northwest of Bawean Island. On the east side of the fault is the Bawean arch, a volcanic ridge which runs from Muria volcano to Bawean island. Muria volcano and Bawean Island are somehow connected: 100 km apart, but on the same volcanic axis and active at the same time.
So the suture where an ocean disappeared is still there. It separates the stable Sundaland continental crust from the squashed and broken crust (‘melange’) to the east where the later block joined. The volcanic ridge follows this line, and Muria and Bawean Island are the two end points of the this arch. Everything is connected and the dots line up.
The making of an island
Bawean Island is a funny place. It is a reluctant volcano where none should exist, producing a lava type that shouldn’t be there. It all stems from a collision 90 million years ago when a lost fragment of northwestern Australia traveled across an ocean that no longer exists to find safety in the north. The potassium-rich Javanese volcanoes are all on this fragment. The safe home has lasted all this time, while Gondwana split and Australia separated into isolation.
But now all of Australia is following the route of this ancient fragment. Land is rising in anticipation and Java has started to rotate under the pressure. Exactly what happened is unclear, but the trough suggests there may have been a bit of extension in the Java sea. The old suture began to leak. Deep underneath, the subducting plate is producing a bit of melt. Somehow this found its way up through the leaky fault, in the process melting and mixing with the local crust. The melt fraction is probably quite small. For some time, perhaps a million years, lava was produced and build up the arch, stretching from Muria to Bawean Island. The latter formed at the northeastern end of the arc, not as a stratovolcano but as a series of close vents and domes. Muria formed at the southwestern end, also originally as an island that only very recently joined Java. Like Bawean Island, Muria has lava domes although it did build a stratovolcano. Muria began to form around 1 million years ago, probably at a similar time as Bawean island. They also went extinct at similar times.
The time scale makes it unlikely that any hot spot was involved. The arch is a bit more than 100 km long. The subducting plate is moving at 6-7 cm per year. (In fact rather slower than that because here it is sharply bending down, so mainly moving vertically into the abyss.) To move 100 km at that speed takes 1.5 million years. The ages of Muri and Bawean Island are much closer than that – they are similar. Therefore, it is perhaps more likely that the cause is in the fault. Under pressure from Australia, Java started to slowly rotate: we have seen this in the Merapi area. This rotation caused a bit of extension in the old suture, and that this allowed the two volcanoes to form, as well as the connecting arch.
This is speculation. But it is nice the think that this isolated island in the middle of the Java sea exists because of that isolated continent to the south. Bawean Island is part of Australia’s history.
70 years ago nations fought battles near Bawean island. They did so at a place where people from many different places settled and mixed, and which formed from mixing continents. It is an echo of the ever-changing kaleidoscope that became the world we know today. There is history everywhere.
Albert, November 2022
Postscript. There is one other thing which I was unsure whether it is related or not. There is a line of earthquakes that runs across the Java Sea. The image below is taken from https://earthquake.usgs.gov and shows earthquake larger than 4.5 over the last 100 years. The largest events are M6.6. The line has two clusters, which are near the two largest islands in the Java Sea. These are extraordinarily deep, 540 to 640 km. This is where the subducting plate hits the mantle barrier and sinks through. Is this line related to the Bawean island? With a bit of roll-back, in the past it may have been directly underneath the island. I do not know.
Leterrier et al., Journal of Southeast Asian Earth Sciences, Volume 4, 1990, Pages 171-187. Potassic volcanism in Central Java and South Sulawesi, Indonesia
Handini et al 2022, IOP Conf. Ser.: Earth Environ. Sci. 1071, 012013. Geochemistry of arc alkaline magmatism of Java Island, Sunda Arc: a statistical review
Susilohadi and Soeprapto, Berita Sedimento, 2015. Plio-Pleistocene Seismic Stratigraphy of the Java Sea between Bawean Island and East Java
Lunt, P. 2019, Marine and Petroleum Geology Volume 105, Pages 17-31. The origin of the East Java Sea basins deduced from sequence stratigraphy
Karimah et al., Jurnal Penelitian Pendidikan IPA 8(3):1495-1502 (2022). 3D Modelling of Geoelectrical Resistivity Data to Determine the Direction of Landslides in Kastoba Lake, Bawean Island, Indonesia
Albab et al. 2017, Bulletin of Marie Geology, 32. Seismic Facies of Pleistocene–Holocene Channel-fill Deposits in Bawean Island and Adjacent Waters, Southeast Java Sea