During August 2021, reports came in about an eruption in the ocean south of Japan. The volcano was new to me: Fukuto okanoba. It sounded like something from star wars. It was a friendly explosion, meaning no one was inconvenienced by it. The submarine explosion had produced a tephra raft 60 km long, plus an ash cloud reaching 15 km high. Afterwards, it turned out the eruption had already been in progress for a few days before the explosion and had created new land – an island 1 kilometer across. The final explosion has created a large crater in this new island, and had reduced it to a rim. Perhaps this shouldn’t have been listed as a submarine explosion, even though there had been nothing but sea here just a week earlier. After the explosion, there was no further activity.
The remnant island produced some nice images. This is the best one I have found: a Planetlab image taken on August 17 showing the island and the beautiful circular crater with a diameter of 630 meter. Something is bubbling up in the precise centre, as if to say ‘oops, sorry’. On the right, wave erosion is already carrying bits of the island away into the ocean, causing a brown discolouration, while the central spout is turning the water green.
The image allows us to estimate the size of the explosion. The crater is a decent size but does not appear to be very deep. Assuming an optimistic average depth of 100 meter, and assuming that the island itself was created just days before from volcanic ejecta, i.e. quite low density and unconsolidated, it gives a tephra volume of 0.03 km3. If instead the crater is a bowl (a half sphere), this becomes 0.06 km3. This estimate puts the explosion at a VEI 3 level. The activity in the preceding days which had created the island would have been a bit bigger, and perhaps have reached 0.1km3 – a high-end VEI 3. The extensive area of floating debris may have even been created in part by this previous activity.
We learned that this was in fact a known volcano. The Japanese name, Fukutoku-Oka-No-Ba, was not its original name: an older name existed for the same submarine volcano, Shin-Iwo-Jima. This volcano had history. The old name meant ‘New Iwo Jima’ or new sulphur island. Iwo Jima is the (old) name of the well-known nearby volcanic island, so this name just meant ‘new island near Iwo Jima’. The old name show that this volcano had already created an island once before. When that island turned out to be ephemeral, the old name lost its meaning and the undersea mountain received its new name.
There have been several eruptions here in the past century. The ‘new’ island appeared after an eruption in 1905. Other confirmed eruptions occurred in 1914, 1973, 1974, 1986, 1987, 1992, 2005 and 2010. They were mostly small, at VEI 0-2. The 1904 and 1914 eruptions are classified as VEI 3, the same as the recent explosion. The 1986 eruption also created an island: the current incarnation is the third such island in 120 years. The islands do not last long: they tend to disappear under the waves within a month or so. The Pacific ocean, and in particular its cargo of typhoons, is not friendly to intruders. There are more reports of eruptions but they are unconfirmed, and they may be fumarole activity discolouring the sea water. This was seen several times in the decade after the 2010 eruption.
The culprit lies underneath the waters, save from the Pacific fury. Fukutoku-Oka-No-Ba is a highly active undersea volcano, which has a peak some 15 to 30 meters under water. The depth varies with each eruption, but it will always stabilize at the depth to which wave erosion can reach. Over time, the waves form a plateau at some 30 meters depth. The eruptions build on this plateau – but only temporary, until the sea reclaims its property.
Taking the bathymetry and overlaying the new island gives the following result. I tried to get the scale correct, but the precise position is not as certain. In fact I shifted the coordinates by 30 meters to avoid the new island going over the edge, and put the vent in the centre of the plateau. The precise PlanetScope coordinates would put the vent 30 meters further to the north-northeast, along the apparent rift. The steepness of the slope outside of the -50 contours makes it more difficult to create the new island at that location. The deep hole visible in the bathymetry near the western edge of the plateau now nicely coincides with the indentation of the island. A 30 meter error in the latitude coordinates of the bathymetry map is perhaps acceptable. However, this does not mean that my positioning is correct! Feel free to move the island around.
This is clearly an impressive volcano, and a very active one. It is not huge. The ocean here is deep, 2 kilometers or more, but the volcano is sitting on a larger plateau which rises far above the deep sea floor. The base of the volcano is around 400 to 500 meter deep, and around 5 km in diameter. We are talking about a 5 km wide, 500 meter tall volcano with a 1-km flat top and a 500-meter rift running from the centre. Without wave erosion, we can estimate that the top would have been about 100 meter above sea level.
It is not the only volcano here. Just 4 kilometers away is Minami iwojima, a steep extinct cone reaching a height of 900 meters above sea. It dwarfs our exploder.
The map shows the underwater view. There is a 25-km long, bendy ridge with several distinct peaks. Minami iwojima is the dominant one, but there are three other named peaks, all of which reach to just below sea level. Wave erosion has gotten the better of all of them. Only Fututoku okanoba appears active. There are a few other, unnamed peaks, and to the west is a larger flat-topped ridge which is 700 meters below sea level: the depth suggests this ridge has suffered subsidence.
Remember that rift in the summit of Fukutoku okanoba? Extend it the other way, and it points directly at the extinct cone of Minami iwojima. Our volcano is either a satellite of Minami iwojima, or its structure has a fault caused by it.
The arc of the Volcano Islands
The two islands are part of a larger volcanic region. These are the well-named ‘Volcano Islands’. There are four – now, temporarily, five – islands forming a 500-km long but sparsely populated chain. The chain runs from 800 km to 1300 km south of Tokyo.
The five islands are Minami iwojima, Fukutoku okanoba, Iwo Jima, Kita iwojima and Nishinoshima. Iwo Jima (renamed to Ioto in Japan) is 60 km north of Fukutoku okanoba, Kita iwojima lies 80 km further north, and finally Nishinoshima is 200 km further – a bit further but it seems to belong to the group. There are also more submarine volcanoes, about 500 meters below sea level: Minami hiyoshi, Nikko (about 50 and 100 km south of Minami iwojima), and three near Nishinoshima: Doyo seamount 50 km to its north, and Kikata and Kaitokyu seamounts 50 and 100 km to its south. Finally, there are some submarine vents near Kita iwojima. Together with the Volcano Islands, they form the central, active part of a 2000-km long series of volcanoes (many of them submarine) stretching from Japan to the Mariana Islands, with volcanoes typically 50-60 km apart.
There is a second string of islands 100 km to the east, with as largest one Chichijima. These are called the Bonin islands (meaning ‘uninhabited’ which is no longer fully correct). The term Osagawara islands is also used for these islands.
Of the Volcano Islands, Minami iwojima and Kita iwojima are quite similar to each other. Both are steep cones of similar height (913 and 792 meters) and size (2 km across), have sea cliffs of several hundred meters tall, lack any sign of current activity (no fumaroles or hot springs) or recent eruptions. Both consist of tholeitic basalt as expected of oceanics arcs: they are extinct tholeitic stratovolcanoes. The ages are not known, but may be as much as a few hundred thousand years. If they are old, they may have originally formed as submarine volcanoes and reached their current height by uplift. However, this is speculative.
The other three islands are lower, lie just below or above sea level, are flat-topped, highly active with vents and frequent eruptions, and have alkaline lavas which are indistinguishable between Iwo Jima and Fukutoku okanoba. Nishinoshima also erupts alkaline lava but of a slightly different composition. Nishinoshima had a long lasting eruption from 2013-2020, 40 years after the previous eruption, which made the island much larger. The original island has been above sea level since at least the 16th century. Iwo Jima shows phreatic eruptions with active fumaroles, and has been inflating for hundreds of years at typical rates of 2 meters per decade. It had a significant eruption perhaps 2500 years ago. Fukutoku okanoba erupts every few decades, at times forming an ephemeral island. The new volcanoes are very different from the previous cones. There has been a change of the guard, out with the old and in with the new.
What is going on? And why are these volcanoes here, in the middle of the ocean? For that, we need to get to the bottom.
The Battle for the Pacific
The Pacific Ocean has a violent temper. Above, there are the frequent typhoons which are probably the main reason that the Volcanic Islands find it so hard to grow up. Underneath, it is fighting for supremacy with smaller plates all around the ocean. One of those battles is taking place near the Volcanic Islands. And this battle is not going well for the Pacific plate.
The Pacific plate covers much of the Pacific ocean. It is an old plate which has ruled its kingdom for a long time and does not take kindly to competition. But new plates are growing, younger and hotter and more nimble. The Pacific plate is being challenged. In this particular region, the challenger is the Philippine sea plate. It is still young(ish) but already heavily scarred by the conflict. From west to east, we find the Palau Kyushu Ridge (known, obviously, for Palau), a wide basin called the Parece Vela basin in the south and the Shikoku Basin in the north, the West Mariana Ridge which becomes the Izu Bonin Arc further north, another basin, the Mariana Trough, which is in the south but with a similar smaller basin in the north called the Ogasawara Trough, the Mariana Ridge (home of Guam), and finally a deep trench, called the Mariana Trench in the south and the Izu Bonin (or Izu Osagawara) Trench in the north. This trench is the boundary between the Philippine and the Pacific plate. This is where the Pacific plate is subdued by the challenger.
Subduction and roll-back
In this region the ocean floor of the Pacific plate is around 150 million year old. The Philippine plate has a variety of ages, but in this area is typically 50 million years old. The Pacific plate is much older and therefore colder and denser, and in the collision zone it is the one to subduct. Old crust, once subducting, sinks steeply. This causes all kinds of problems at the surface.
The cartoon illustrates the process. The red bar is the Pacific plate, and grey plays the part of the Philippine sea plate. T1, T2 and T3 are three different times. Over time, more of the Pacific plate begins to subduct, and the location where the sinking begins is moving backward. This leaves a gap, and the Philippine has to move in to fill the gap. Near the boundary between the plates a ridge develops: an arc. In front of the arc is the trench formed by the sinking plate. Behind the arc, the plate is stretching and thinning in order to be able to fill the extra space. This forms a basin.
Why the backward movement of the trench? This is because the sinking plate is down the creek with a paddle. Initially, the subduction is fairly shallow. But over time it becomes steeper. The subducted slab is now moving downward and backward, like a paddle moving through the upper mantle. The mantle underneath the Philippine plate has to move forward to fill the vacuum created by the paddling plate, and it pulls the Philippine plate with it: it is sucked forward. The trench begins to move backward.
This steepening is very notable along the Izu Bonin Trench, where the angle has reached 55 degrees, and even more so along the Mariana Trench where the Pacific plate descends almost vertically.
The paddling causes the trench to retreat. The Philippine plate now has to fill more space. That is fine if there is a spreading centre creating more crust, but when there isn’t one, the extra space is filled instead by thinning of the crust. The thinning crust sinks and a basin forms. Below the thin crust, the magma can circulate up and a weak spreading centre may form.
Basins and Ridges
This happened in the Philippine sea plate 30 milion years ago: it formed the Parece Vela Basin in the south and a little latter the Shikoku Basin in the north. But the basins eventually ran out of magma and spreading here ceased 15 million years ago. So the process repeated, and around 7 million years ago the Mariana basin began to form, closer to the front. At the moment this basin is 1000 km long and 200 km wide; it is widening by 2 to 4 cm per year. Further north no new basin has formed (so far, at least). The Philippine sea plate has a bit of clockwise rotation, and it is pulling away a bit from the Pacific plate in the southern region. This has made the whole process faster in the south. The Mariana Trough has become so pronounced that it is sometimes considered as its own micro plate, and the adjoining trench, the world famous Mariana trench, now contains the deepest surface on Earth. As Bond would say, straight down, with a twist.
The Philippine sea plate is a mosaic of ridges and basins. The ridges are (or were) volcanic arcs. They formed in the usual way: once the subducted plate reaches a certain depth it will begin to melt. The melt percolates up and forms a chain of volcanic islands, typically 50-200 km from the plate boundary. The basin forms adjacent to the trench, in between the volcanic arc and the plate boundary. Every time a basin forms, the volcanic chain begins to move away from the plate boundary and the volcanoes become extinct. This has happened to the Kyushu-Palau Ridge which became extinct 25 million years ago, and to the West Mariana Ridge 7 million years ago. A new volcanic arc will form, at first widening the existing arc and later as a separate arc. The spreading centre moves away from the subduction front as the crust stretches, and once it has moved far enough that the volcanic islands form between the spreading centre and the front, they can grow in one location. This can take a few million years. The Mariana Ridge has formed such a new volcanic arc. The Izu Bonin Ridge has not yet done so.
There is another process to consider. A ridge can also build up adjacent to the trench, pushed up by the collision with the subducting plate. Behind it, the descending plate can pull the overriding plate down, causing a trough in front of the volcanic arc. The pushed-up ridge is the forearc, and the trough is known as the forearc basin. There is one such basin in our area: the Ogasawara Trough. The Volcano Islands are one the west side of this trough, and they form the volcanic arc. The Bonin Islands are on the other side, on the pushed-up ridge next to the trench. The Bonin Islands are not volcanic, although like all ocean floor, they are created from lava.
When looking are the mosaic of ridge across the Philippine sea plate, it is worth remembering that originally there was just one ridge, but every time a basin formed in front of it, eventually a new volcanic ridge had to form.
The subducted plate consists of normal oceanic crust. When it melts during the subduction, it forms thoelitic basalt (a mafic mantle melt). This is the type of basalt found on the two extinct stratovolcanoes in the Volcano Island chain, and widely elsewhere along the volcanic arc. But why does Fukutoku okanoba have a different lava, even though it is next to the tholeitic (and extinct) Minami iwojima? And why is this same lava found on Iwo Jima – and nowhere else?
The lavas of Fukutoku okanoba are alkaline (not mafic) with an SiO content of 60%, high potassium and sodium (9% together), and low calcium and magnesium. They are so-called ‘alkaline shoshonitic lavas’, and the area where they are found is called the ‘alkalic volcano province’ – it coincides with the Volcanic Islands.
The name may need some explaining. Shoshonite is a type of rock: a potassium rich, basaltic trachyandesite. Its name comes from the Shoshone river in Wyoming, after this type of rock was identified in Yellowstone. Trachyandesite is a rock with about 60% SiO2 and about 9% K2O+Na2O, low in magnesium and iron. ‘Basaltic’ means it was erupted effusively (normally with low viscosity) and cooled rapidly at the surface; basalt has a bit lower SiO2 (less than 53%) so this is ‘basaltic’ rather than ‘basalt’. Shoshonite is present at Fukutoku okanoba and at Iwo Jima. Nishinoshima has similar lava but it has less potassium and a bit more magnesium and iron: it is an intermediate between typical arc lavas and the stuff found at Fukutoku okanoba.
It is still a bit of a mystery why this particular region has this peculiar lava. It is a recent change, because the extinct stratovolcanoes here show normal lavas. Something has changed.
Shoshonite is found at the Hiyoshi submarine volcano at 23 degrees north, at Fukutoku okanoba, and at Iwo Jima at 24.8 degrees, with a transition zone to Nishinoshima further north. This is precisely the region north of the Mariana Trough. The oldest shoshonites are found in the south, adjacent to the Mariana Trough, dated to 3-4 million years ago, while the youngest are obviously from the two currently active volcanoes further north. It is tempting to link this to the northward spreading of the Mariana Trough. The trough is a few million years older than the oldest shoshonites. Is this the delay between spreading and the formation of a new volcanic arc? Is the region of the Volcanic Islands getting ready to join the Mariana Trough?
A change of melt
It is not immediately clear why basin formation would change the composition of the magma. Neither is it clear why the volcanoes are at sea level (with possible uplift) if a basin would be forming. Is there an alternative explanation?
If a small spreading centre is forming, the mantle heat may recently have increased, and caused some uplift. Volcanic arcs form from melt of the subducted plate. If there is more heat, there may now be a partial melt of the wedge between the subducted plate and the Philippine plate. The alkaline lava suggests a low melt fraction.
Here the forearc trough comes into view. The Volcanic Islands are along this basin: can this be part of the problem? The forearc basin is pulled down by friction with the subducting plate. Erosion can occur: material is pulled off the bottom of the Philippine plate and carried backward by the descending plate, enriching the mantle material in this wedge. This could explains some chemical peculiarities. The change of melt source must have occurred recently, over a 200-km long arc. This makes an origin in the Osagawara forearc basin plausible. But the true source of the shoshonitic lavas remains an unsolved problem, somehow related to the battle of the plates.
Shoshonitic lava can form when oceanic magma crystallizes at higher than usual pressure. Either olivine or orthopyroxene can crystallize, depending on how much water there is in the rock. If wet, olivine forms which increases the potassium and silicate in the remaining melt. Shoshonitic lavas are found in different locations. Some are associated with a continental margin, related to crustal thickening. This is seen for instance in the Andes, Kamchatka, or the Eolian arc north of Etna. The high pressure at the base of the thickened crust causes the shoshonite to form. The second location is in island arcs which are becoming unstable, for instance when subduction is ceasing or changing direction. They appear to be a late phase in the evolution of an island arc, perhaps related to a deepening of the subduction melt.
Perhaps the paddling movement of the subducting plate is making it deeper, with the melt now forming deeper and at higher pressure. A second possibility is that spreading is indeed taking place, move the current volcanic islands further from the subduction zone, and therefore higher above the subducting plate. Perhaps the forearc basin is beginning to become a spreading basin taking up the retreat of the subduction front and the Volcanic Islands will have to move east, until they pass the new spreading centre and find a new stable location – forming a new volcanic arc, with the current arc becoming one more scar on the Philippine plate.
Speculating further, why have Fukutuko okanoba and Iwo Jima become so active? The removal of olivine from the melt may make it more buoyant, causing the residual melt to rapidly rise into much shallower magma chambers. The amount of magma may not be as high, especially if the arc is beginning to be starved of magma, but what is there would reach the surface easily and regularly giving frequent but smaller eruptions. As happened this month.
If any of this speculation is correct, changes are afoot in the Volcanic Islands, perhaps as soon as in the next hundred thousand years. Keep posted.
VC has long been fascinated by Iwo Jima (Ioto) and its extreme inflation. Now we have seen its sibling erupt. These volcanoes may or may not be tracing the beginning of a rift, which will eventually extend the Mariana Trough. Either way, it is all part of the battle for Pacific supremacy between the ancient Pacific plate, ruler of all, and the upstart (now middle aged) in the west. Our volcanoes are driven by events outside of their control, and their eruptions are collateral damage in a titanic battle. We might call that friendly fire.