The story begins with Japan’s iconic volcano. Mount Fuji has become the poster child of volcanism – it is what every volcano strives to be. This is the most climbed mountain in the world. The spring-time view of Mount Fuji framed by cherry blossom is the very emblem of Japan. The reflection of Fuji is printed on the backside of the 1000-yen bank note. Wedding ceremonies are held at the temple at its summit. The summit is Japan’s highest point. How did such a perfect volcano form?
Mount Fuji is where the Pacific goes to die. The western half of Japan is continental by origin, at one time part of the Asia. The eastern half is younger, with volcanic arcs adding new land and expanding the nation towards the sea. Out in the ocean is the subduction zone where the oceanic plate sinks into a trench and dives below the land. Behind Japan, the Sea of Japan has opened. It has left Japan as an island fighting trench warfare with the ocean, while protecting the continent behind it. The conflict involves earthquakes and volcanoes, whilst the multitude of hot springs are signs of the heat of the battle. This post is about a four-way battle for the heart of Japan, centred around its very own Lonely Mountain with the Five Lakes.
As the crow flies, it is 100 km from the centre of Tokyo to Mount Fuji. Driving to the slopes will take you 2 hours (do be aware that the access roads can be closed to private cars during the busy season), or you can take public transport. There are four main trails up the mountain. Each trail is divided into ten stations. The road or bus will take you as far as the 5th station, and the main climb is from there to the top. The summit, at 3776 meter, is the 10th station. The climb is on well-trodden paths and is of only moderate difficulty. For ease of access, there are separate paths for the ascent and the descent. The altitude is considerable, though, and you will find yourself short of oxygen.
The trails can be busy: a quarter of a million people climb it each year – this number has been counted at the 8th station. But the climbing season is short , and all those people are packed into the short period from July to early September. It is very much a community crowd activity, part of Japan’s social fabric. (However, until 150 years ago women were forbidden from climbing the mountain.) During the peak of the holidays, mid August, expect walking traffic jams. If you feel tempted to go for the quiet time outside of the climbing season, don’t. There will be snow and high winds, and the mountain huts (and the trails themselves) are closed. Only the climbing season is snow free. The climb up takes anywhere from 5 to 8 hours, also depending on which trail you take. The walk around the summit crater to the actual summit takes an hour. The return is 4 to 6 hours. To be on the summit at sunrise (4:30 am) requires booking a sleeping place in a hut. In fact, this is recommended for any trip to the summit. Remember to carry money (nothing is free (or cheap), including the emergency oxygen). Wooden walking sticks can be purchased at the 5th station: they make good memorabilia afterwards.
Fuji’s fame comes both from its location in Japan’s heartland and from its near-perfect shape. Hokusai’s 36 views of Mount Fuji are an excellent example of how Japan’s art is dominated by the beauty of this volcano. But beauty requires maintenance. Mount St Helens used to have the reputation of being the Fuji of America. How different it looks now, after the destructive eruption. But soon, probably within centuries, it will rebuild its cone, resurface its flanks, and re-enter the volcano beauty contest, the ultimate Miss World. Ragged volcanoes are a sign of erosion, when erosion wins out and flank collapses leave holes that do longer get filled. Age builds character but not beauty: a symmetric mountain is a sign of youthful activity. Fuji clearly must be a regular eruptor, an applicant of frequent volcanic make-up. Indeed, it has had two major eruptions in written history.
There are hidden surprises in this familiar volcano. Fuji is actually a conglomerate of five volcanoes, three of which can be recognized in the landscape. On the northern slope, 7 km from Fuji, there are outcroppings of an older volcano, called Komitake. Underneath Komitake lies a still older volcano, buried by Komitake, called Sen-Komitake (meaning ‘before Komitake’). The extinct Ashitake volcano, 20 km southeast of Fuji, forms a deeply eroded but recognizable hump in the landscape. (This is a distant member of the Fuji family and perhaps is not directly connected.) Ashitake (ojiisan) is the oldest, and was active around 400,000 years ago. Sen-Komitake started erupting around 270,000 years ago. It ended with a series of explosive eruptions 160,000 years ago. After this self destruction, the new Komitake grew until is ceased erupting about 100,000 years ago. Now another volcano took over, Ko-Fuji. This new volcano had an explosive flank collapse about 18,000 years ago. Following that collapse, modern Fuji (called Shin-Fuji) arose just west of the old summit and eventually buried its predecessor.
The next surprise is that this perfectly symmetric volcano actually has a severe case of acne. The flanks are covered with 100 parasitic cones and craters. Beauty rarely survives a look with a magnifying glass.
The final surprise lies in the magma. Eruptions in and around Mount Fuji have mainly produced tholeitic basalt. Basalt is not common in Japan, and there is clearly something different about Fuji. Ashitake was andesitic, a common lava type of Japan. But each of the later volcanoes of Fuji started with effusive basaltic eruptions, over time evolving towards explosive andesite/dacite eruptions after which the reset button was pressed and a new volcano took over with basaltic output.
The current volcano, Shin-Fuji, started with voluminous basaltic eruptions between 17,000 and 8,000 years ago. After a quiet interlude with less activity, a mix of large explosive (pyroclastic) and effusive eruptions took place between 5,500 and 3,500 years ago, both from the summit and the flanks. By the end of this phase, the mountain had reached its current height. 2,900 years ago there was a major flank collapse with a land slide volume of 1 km3. It is not known whether this was volcanic or was triggered by an earthquake. Now a series of summit eruptions followed, the last one of which was 2,200 years ago. Since that time, Fuji has been less active, with two major eruptions and 8 small eruptions, all on the flanks.
The two major eruptions of the past 2000 years were a large effusive eruption in 864 and a plinian explosion in December 1707. The 1707 eruption was triggered by an earthquake, not just any earthquake, but this was the Great Hoei earthquake. It had an estimated magnitude of M8.6 and came only a few years after the ‘minor’ M8.2 Genroku earthquake on 31 December 1703. These huge earthquakes are another aspect of Japan, and they can apparently affect Fuji’s magma chambers.
The 1707 eruption came 7 weeks after the major earthquake. Volcanic rumblings had started already in February 1704, suggesting the first trigger was the 1703 Genroku earthquake. Mount Fuji is thought to have two main magma chambers, a dacitic one at 8 km depth and a basaltic chamber at 20 km. Models suggest that the deeper chamber was affected by the quakes, and that after the first event magma may have risen from there to the shallower chamber. This pressurised the upper chamber and after the second jolt it exploded. Between 16 December 1797 and 1 January 1708, there were three main explosions. Together they amounted to a VEI-5 eruption of about 1.6 km3 of ash and tephra. The ejecta covered a wide area which included modern Tokyo, severely affecting the local agriculture. The eruption left three considerable dents in Fuji’s flank, at altitudes of 2100 to 3000 m. The largest of the craters is 1 km across. The eruption produced no lava, but magma accumulation under the surface created a notable bump which nowadays is called Mount Hoei. (Hoei is the name of the era from 1704 to 1711. Genroku is the name of the preceding era.)
The other major historical eruption started in June 864. This was a mainly effusive eruption which produced about 1.4 km3 of lava. It erupted at lower altitude on the northwestern flank, from a 6-km long fissure. The eruption lasted two years. The lava buried an existing lake, and the interaction with the lake water formed some pillow lava and a phreatic explosion. The area is now deeply forested and is in part inaccessible due to the terrain and the trees!
The smaller eruptions were also flank events, in some cases on both the northern and southern flank simultaneously. Eruptions in the years 800 and 1083 were mainly explosive. Eruptions in 937 and 1033 produced significant lava flows, with lengths of 17 km and 7 km.
The average eruption rate of Shin-Fuji over its existence amounts to 5 km3 of magma per 1000 years. It may already be in a declining phase. The older eruptions were mainly effusive whilst explosive eruptions are a more recent phenomenon. The rate is very much higher than that of other volcanoes of the Japan arc. There is something about Fuji.
A repeat of the 1707 Hoei eruption has the potential to cause major disruption. If the ash were to be blown towards Tokyo, after two weeks a few centimeters of ash would have fallen in the city area. If it rained, this would be enough to make roads impassable for all but four-wheel drive vehicles, stop trains from running and close the airports. Power outages are likely even at 0.3cm of ash and rain and mobile phone masts may also cease to work. Food supplies would be disrupted. This is a worst case scenario, and a two-week event would give time to keep on top of the ash. We have seen the impact of ash in the La Palma and Etna eruptions, but never in a city on the scale of Tokyo.
At the moment there are no indications of any imminent eruption. But Fuji is active. There is a steady rate of low-frequency earthquakes, located a few km northeast of the summit, at a depth of 15 km. This is suggested to be a dike that is receiving new magma, plausibly from the main magma chamber at 20 km. This is probably normal activity for Mount Fuji and does not presage an eruption. Shallower magma intrusions are likely to cause notable earthquake activity, as was the case in 1704. Any resumption of eruptions is therefore likely to give us years of warning. The quarter of a million annual visitors are safe from the magma. Just be cautious with the weather.
There is something about Fuji. The picture shows three volcanoes in the region: Fuki, Izu-Oshima and Miyakejima. This figure is taken from Aoki et al, 2019, Earth-Science Reviews, 194, 264. The first two volcanoes are elliptical in shape while the third one is round. The ellipticity indicates that Fuji and Izu-Oshima are affected by the local stress in the rocks. They are children of plate tectonics. Mount Fuji is also unusual in showing evidence that its eruptions can be triggered by earthquakes. To really understand the mountain, we need to look at its faults. And Japan has an abundance of those.
Faults of Japan
Japan is riddled with over 2000 active faults. The two most significant in-land faults are the Itoigawa-Shizuoka line, which runs north-south across the country just west of Tokyo and the Median Tectonic line which follows the axis of Japan from Tokyo southwards. The latter is a strike-slip fault. The destructive 1995 Kobe earthquake happened on a side shoot of this fault. Mount Fuji is located around 50 km from the intersection of two main faults, although it is not associated directly with either.
The faults are related to the battling plates. Did I mention the four-way battle for the heart of Japan? That was the simple version. The picture below gives the actual situation. Japan is sitting on two separate plates. The northern half of Japan belongs to the Okhotsk plate, a mini-plate which also contains Kamchatka and the Kuril Islands. At one time the Okhotsk plate was thought to be part of the North American plate. Now it is seen as a remnant of the extinct Kula plate, one of the plates of the Pacific ocean. This remnant carried an ancient oceanic flood basalt and this helped it to avoid subduction. The southwestern half of the Japan belongs to the Amur plate, a break-away part of the Eurasian plate which runs from Lake Baikal to Japan. On the eastern side of Japan is the old Pacific plate, and to the south is the younger Philippine Sea Plate. Finally, between the Philippine plate and the Yangtze plate (part of Eurasia) is a microplate called the Okinawa plate. It owns the tip of Okinawa, the southernmost of the main islands of Japan. So Japan lies on three plates, and borders two more. No wonder it is so earthquake prone. (In fact one sliver of Japan, the Izu peninsula, sits on the Philippine plate, so that makes Japan worth four plates. Of all the continents, only Eurasia has more.) It really is the Battle of the Five Plates. And it is centred on the Lonely Mountain. Mount Fuji.
(The precise location of the boundary between the Okhotsk and Amur plates remains under discussion. GPS measurements indicate that the two move independently and should be considered as separate plates but it is not easy to trace the precise line of separation.)
I have left out one more detail. Honshu, Japan’s main island, originally started out as a double. It split off from Eurasia as two separate fragments, one on the Okhotsk plate and one on the Amur plate. Only later did the two meet and join forces. The suture of the two runs near the Itoigawa-Shizuoka tectonic line. The rocks on either side are very different, with granite half a billion year old on the southwest side and young sediment on the north side.
In between the Itoigawa-Shizokua and the Takankura tectonic lines lies the Fossa Magna: the central valley stretching from the Pacific to the Sea of Japan. This is a wide area which includes the Kanto plain of Tokyo. It contains up to 6 km depth of sediment from the sea which separated the two halves of Japan. The Fossa Magna is known as the rift valley of Japan.
Most of the major earthquakes of Japan come from the two main subduction zones, which form two deep trenches along the east (Pacific) coast. The Japan Trench is where the Pacific plate dives down underneath the Okhotsk plate. This trench follows the Japanese coast (at some distance) until Tokyo: south of Tokyo the trench bends further away from the land. From here on, the Nankai Trough takes over: it comes from the subduction of the Philippine plate, and it follows the coast rather closer to land than the Pacific plate did. There is a short boundary between the Pacific and Philippine plate which is called the Sagami Trough: it runs east-west, southeast of Tokyo.
Japan is famous for its earthquakes. 20% of the world’s largest earthquakes (magnitude 6 or higher) occur in the country. The 2011 Tohoku earthquake is well remembered: it left 20,000 dead and 360 billion dollars of damage. Surviving the earthquake itself was only half the story: the devastation came from the tsunami afterwards. The Great Kanto earthquake of 1 September 1923 (next year this will be a century ago) with magnitude M7.9 destroyed more than half of all buildings in Tokyo and caused the death of 105,000 people, mostly from the fires after the quake. The M6.9 Kobe earthquake of 1995 hit in an unexpected place, killed 6,000 people and caused immense damage. The Great Hoei and Genroku earthquakes of 1703 and 1707 were already mentioned. In 1792, a comparably small earthquake (small for Japan, which suffers an M7 on average once a year and an M8 once a decade) with magnitude M6.4 caused a flank collapse of Mount Unzen, an active volcano. The landslide swept up a tsunami that killed 15,000 people. In 1891, an M8 struck Gifu, in-land. Nothing was left standing in the city. And there are many more historical examples of major earthquakes, often accompanied by towering tsunamis.
All of the plate boundaries can produce major earthquakes. The 2011 Tohoku earthquake ruptured much of the Japan Trench. (What actually fails in such a megathrust earthquake is the contact layer between the subducting plate and the overlying plate. The two lock, the subducting plate pulls the overlying plate with it and in the process pushes it forward. When the lock fails, the overlying plate shoots back in a large quake.) The Japan Trench tends to fail in single, huge events, perhaps 500-1000 years apart, as it did in 2011. But in between there are plenty of smaller (M7 or M8) events as well. The 1707 megathrust earthquake ruptured 700 km of the Nankai Trough south of Edo (near Tokyo). The M8 earthquakes in 1944 and 1946 too were on this fault. The Nankai Trough often fails in doublets, producing two major earthquakes every 100-150 years. The 1703 Genroku earthquake ruptured a large section of the Sagami Trough. The 1923 Great Kanto earthquake also was on the Sagami Trough, but closer to Tokyo.
And the other side of Japan is not safe either. An M7.6 earthquake in 1833, possibly originating on the boundary between the Amur and Okhotsk plates, hit the northwest coast of Japan with a large tsunami. The M7.7 Hokkaido earthquake in 1993 also came from this region. And as mentioned, there are also large earthquakes on the other faults, especially the Median Tectonic Line. Japan has shaky foundations.
Plates of Tokyo
Let’s have a closer look at the Tokyo and Mount Fuji region. The plate boundaries are complex here, and the precise location is not always known, especially on land. The dashed lines drawn below indicate such uncertain locations. The arrows show the approximate directions of the plate motions. Tokyo and the Tokyo Bay are on the Okhotsk Plate. But the Philippine Sea Plate is not far away, and underneath the Pacific Plate is heading for Tokyo.
The Philippine and Pacific plate are both subducting. You may imagine some competition here, about who can subduct better. The Pacific plate is older and colder and therefore is the densest. The Philippine plate is younger and warmer and is not as dense. Thus, the order of priority is set: the Pacific plate subducts under the Philippine plate which subducts under the Amur plate. There is a layer cake under southern Japan, where the Amur, Philippine and Pacific plate are stacked. The situation further north is easier as the Pacific plate subducts directly under the Okhotsk plate, meaning there is one less layer to worry about.
If we focus a bit more on the Tokyo, area, we find more complications. There is a large bend in the Philippine plate, creating an extension into the area of the Izu peninsula, between Suruga and Sagami Bay. This is the result of a volcanic arc, a line of volcanic islands stretching into the Philippine Sea called the Izu-Bonin arc. As the plates moved, this arc collided with Japan. Volcanic islands do not subduct willingly: instead they tend to join the land they collide with. The arc also brought a larger volcanic block with it which became the Izu peninsula. This is how Japan grows.
The Izu peninsula, land that was plastered unto Honshu, contains several eroded (extinct) volcanoes. Hot springs also tell of a volcanic history. It is a mountainous land with a spectacular coast line. If climbing Mount Fuji was a step too far, a visit to the Izu peninsula may help you regain your sense of wonder and your breath. And the volcanoes here are much lower, thanks to age and erosion. (The area is also full of golf courses, if that is a sport you enjoy and can afford. The old calderas should make for an easy hole-in-one. Not to be attempted during an eruption.)
Remember the layer cake? It is slightly more complex than I said. Underneath Okhotsk you should only find the Pacific Plate layer. But the Okhotsk Plate is itself moving southwestward – not fast, but measurable. So it is overrunning places where the Philippine plate used to be, with the Pacific plate underneath it. Because of that overshoot, the Philippine/Pacific layer cake does extend underneath the Okhotsk plate. In addition, as recent as 3 million years ago the Philippine Sea Plate was moving north, before it changed to northwest, so this added to the complex layering. Below the surface, the Philippine Plate extends further than you might expect.
The double layering has been detected underneath the metropolitan area of Tokyo. The subducting Pacific plate here lies at a depth of 80-90 km. But there is also a transition zone at 20 km depth, and this is thought to be the top of the Philippine Sea Plate. It becomes even shallower further south, and lies at around 10 km deep underneath the entrance to Tokyo Bay. Follow this 10 km contour, and you’ll find that both the location of the 1923 and 1703 earthquakes are on this line. These were shallow megathrust earthquakes close to Tokyo. The location and depth explains the damage they did.
It provides a warning. An exact repeat of the 1923 earthquake may be a century or more away, as the quake has resolved the stress here in the only way it could. But further east the boundary may have remained locked for 300 years. The Tohoku 2011 earthquake which ruptured the boundary between the Pacific and Okhotsk plates left the Philippine plate untouched and therefore also did not resolve the stress here. A large earthquake in Tokyo in the next 30 years is far from certain, but the danger should not be neglected. The probability of an earthquake causing ground shaking larger than 0.9g in the Tokyo metropolitan area within the next 30 years has been estimated at around 30%.
Fuji and the Arc
Close to the Izu peninsula stands another volcano, one that is often ignored. Mount Hakone is famous for its large and scenic crater lake, but is perhaps best known for its beautiful views of Mount Fuji. The caldera is 10 km wide (the lake is located along part of the caldera wall). It overlaps with a second one, almost as large. Twice, a large stratovolcano blew up here; the last time this happened was 50,000 years ago. Since that time, smaller cones have grown inside the caldera. The last eruption of one of these cones was 3000 years ago. There are traces of a few smaller, phreatic explosions. That was dramatically confirmed in 2015 when there was such a phreatic eruption. Luckily, this eruption gave sufficient warning and the site had been closed to tourism.
In its time, Hakone produced lava ranging from rhyolitic to basaltic, the entire suite of possibilities. The mountain may not have been much smaller than Fuji is now, produced a similar (but not identical) range of lava, and it is only 30 km away. It seems to be a member of the Fuji family. However, it is the black sheep of the family as it twice explosively self-destructed. The other family members have had large eruptions and flank collapses, but they have avoided VEI-7 type explosions.
Hakone lies along the line of the accreting Izu volcanic arc and seems to belong to it. It also lies close to Mount Fuji. Does this indicate that there could be a relation between the volcanism of Fuji and that of the approaching arc? The behaviour of Mount Fuji is a result of many things: this is an extraordinary complex region. But Fuji is clearly affected by the accretion of the Izu arc. First, the ellipticity of the mountain is along the direction of the Izu arc, and is the same as that of Izu-Oshima, the largest and closest island of the Izu chain of volcanoes, located in Sagami Bay. Second, the lava composition points at a relation to the Izu-Bonin arc. The arc shows a mix of tholeitic basalts and rhyolite, very different from the andesite of northern Japan. Mount Hakone shows a similar range. Fuji shares the basalt but it is not documented as having produced rhyolite: instead it provides us with occasional dacite and andesite. You could see Mount Fuji as a mix of Izu-Bonin arc and more traditional Japanese volcanoes while the intermediate Mount Hakone is more similar to the Izu-Bonin volcanism.
The source of the Izu-Bonin magmas is the melt of the subducting Pacific plate, with added melt from the Philippine plate crust. For Mount Fuji, it has been suggested that most of the magma also originates from the Pacific plate, but moves through the overlying layer of the Philippine plate without this adding much to the melt. This forms the magma chamber at 20 km. From there the magma moves up to the 8-km reservoir where it evolves to the dacite/andesite composition – if given enough time.
A look at the maps above shows that it is not so clear which plate Mount Fuji is on! It does not help that the volcano family has obliterated all surface rocks. At depth Fuji belongs to the Philippine plate but all three plates put in bids for this real estate at the surface. (They may have missed their chance. Some 20 years ago the government returned the title to the summit and surrounding area to the original owners. It is now private land.) Mount Fuji is located not far from the triple point between these plates. However, triple points are not normally the location of major volcanoes unless they are caused by a hot spot – which Fuji is not.
The precise location with respect to the triple point is probably not too important. The main aspect is that it is at the end of the accreting volcanic island arc and that is mixing behaviour from the arc and from other Japan volcanoes.
The high magma production rate, 100 times that of other Japanese volcanoes on Honshu, is not so easily explained. Izu-Oshima, roughly 100 km away, is the nearest frequently erupting volcano on the Izu-Bonin arc. It erupts every 30-40 years, and has a subsidiary volcano just below sea level which also has left deposits on the neighbouring islands. But it’s eruption volume is dwarfed by Fuji. In between lies Hakone but this erupts rather little, apart from the occasional VEI-7. Otherwise the region in between Izu-Oshima and Fuji lacks active volcanism. Either Mount Fuji is collecting magma from a large area, or it has a separate cause of melt.
The melt may be affected by the Fossa Magna in which it is located. This has the appearance of a rift valley, where the low stress allows for easy magma melt and accumulation. Imagine a volcanic arc entering a rift zone! And perhaps from this mix of cultures, Mount Fuji grew and became the unexpected icon of Japan. Japan is not known for encouraging cultural melt, but volcanoes make their own rules.
One can speculate further. Three million years ago, the Philippine plate changed direction from north to northwest. Before that time it would not have affected the region of Mount Fuji. Perhaps the plate has only recently reached this region, pushing down the Pacific plate in the process. Could this have triggered the melt and initiated the volcanism, 400,000 years ago?
There is much we don’t know about Mount Fuji. On the outside: it is the most beautiful, recognizable and touristic volcano on Earth. On the inside, it remains a bit of a mystery. Eruptions are rare enough that we are likely never to see one in our life time. But they are not that rare, with perhaps a 10% chance in the next 30 years, and when it does erupt there may be impacts as far as Tokyo.
But that is not the main message. Mount Fuji is a tectonic sign post where five of the most important faults of Japan come together. Only the Japan Trench keeps its distance. Fuji is not just a poster child for volcanoes. It stands as a reminder of the other powers of nature to which few nations are as exposed as Japan. The reverence for Mount Fuji shows a deep respect for powers we cannot control.
Albert, 1 September 2022