Mount Fuji climbing season has arrived and many people are excited to hike Mount Fuji! Despite its beauty, it is important to remember that Mount Fuji is an active volcano. As such, it presents the ever-present danger of an eruption for both the surrounding towns and the Greater Kanto Region. Volcanologists believe it is due to blow. When? They aren’t sure.

What is Volcanology?

Unlike in the movies, scientists cannot accurately predict when or how a volcano will erupt. Volcanology is still fairly new as a branch of science and so modern data records are limited. Most eruption statistics are limited to historical records. 

Record-keepers in the past simply observed volcanic eruptions and wrote down what they saw. While this data is helpful in identifying eruption patterns, it is not particularly conclusive. Observations were less detailed and reports often missed critical information, such as small earthquakes, that occurred before an eruption.

These reports were also disjointed. One report might mention a plume of smoke in the sky, but not where it comes from. Another report might mention a tsunami, but not the cause.

Today, volcanology uses a variety of monitoring techniques to help keep data as accurate as possible. Seismographs are usually deployed near volcanic areas to measure seismic activity. Harmonic tremors are continuous rhythmic earthquakes that often indicate magma movement within the volcano. Scientists try to predict eruptions based on these tremors.

There are also a variety of surface monitoring methods, including GPS, topological mapping, and pressuremeters. Volcanologists use these techniques to look for changes such as swelling (which would indicate pressure building) or sinking (which would indicate draining magma chambers). While these technologies are always improving, they are by no means perfect.

The sheer number of things that must be monitored makes it easy for scientists to miss a small sign. Combine this with unknowns, such as the type of gas in the magma and the magma composition, and there are simply too many variables for scientists today to make accurate predictions.

For more information on why scientists can’t forecast volcanic eruptions, check out this video:

Volcanoes in Japan

Japan is one of many countries that are along the Pacific Ring of Fire. This area has significant tectonic plate movement, which causes earthquakes and volcano formation as the plates grind together. Nearly all record-breaking earthquakes and volcanic eruptions happened along the Ring.

Magma is also closer to the surface in some of these areas. These are called hotspots and they can make island chains like Hawaii or thermally active areas like Yellowstone. 

This is the reason Japan is home to at least 111 active volcanoes. The Japan Meteorological Agency issues warnings for volcanic activity on a Level 1 to Level 5 scale. Level 1 indicates no activity and is merely an indicator that the volcano is active. Level 5 indicates that everyone must evacuate the area. There are current warnings (above Level 1) on 11 volcanoes in Japan. 

At the time of this article, there are only three volcanoes with a Level 3 warning that indicates that no one should approach the volcano. Two of these are underwater in the Ogasawara Island chain. Sakurajima, located in Kyushu, is the third. It ejected a large plume of ash and debris on June 16th, 2018.

For more information about currently active volcanoes in Japan, please visit the JMA website.

Mount Fuji is a bit unique in terms of its formation. It is located on a triple trench junction of three tectonic plates. The Amurian, the Okhotsk, and the Filipino plates all converge under Mount Fuji. The Afar Triple Junction, located between the Red Sea and the Gulf of Aden, is another example of this kind of tectonic junction.

Mount Fuji as it is today formed over a small group of overlapping volcanoes. This is why the main summit of Mount Fuji is no longer believed to be active. Instead, flank vents are the primary source of eruptions and are smaller volcanoes within the mountain.

History of Mount Fuji’s Eruptions

Mount Fuji’s iconic cone formed between roughly 5600 and 3700 years ago. Explosive eruptions from the summit crater occurred approximately 3500 to 2300 years ago. These were the last eruptions from the main cone and all subsequent eruptions occurred from smaller craters on Mount Fuji’s flanks.

Over the next 1000 years or so, scientists believe Mount Fuji had relatively frequent eruptions. Written records start around the year 700. These note that Mount Fuji likely erupted every 30 or so years until around the year 1100. Scientists believe there was additional volcanic activity between 1420 and 1511.

On October 28th, 1707, an 8.6 magnitude earthquake struck off the coast of Japan in the Nankai Trough region (roughly between Wakayama and Kochi). This was Japan’s strongest quake on record until the Tohoku earthquake in 2011. After this quake, there was a noticeable increase in seismic activity around Mount Fuji.

These tremors grew in frequency to near-constant rumbling starting in early December 1707. Modern scientists believe that this was likely caused by magma moving under the surface. On the morning of December 16th, 1707, a final earthquake occurred. It was so large it was felt in Tokyo.

Later that day, an explosive eruption occurred on the southeastern flank of Mount Fuji. There was no magma released in this eruption, but there was a huge amount of ash. The debris was then carried by air currents into the Kawasaki and Yokohama areas. In some areas, it was measured to be up to 10 centimeters deep.

The total death toll of the 1707 eruption was not recorded, although it was noted that many people starved as a result.

For more information on the history of Mount Fuji’s eruptions, please check out the Smithsonian Global Volcanism Program.

Powerful Earthquakes and Mount Fuji

As mentioned before, the last eruption of Mount Fuji was likely triggered by a powerful earthquake in the Nankai Trough region. This region is known for its particularly powerful earthquakes and their subsequent tsunami.

Many scientists believe that there will be a correlation between the next Mount Fuji eruption and a powerful earthquake in the Tokai region of the Nankai Trough. This region is located roughly around Shizuoka Prefecture. The Japanese government often refers to a “Big One” predicted to strike Japan and this forecasted earthquake usually refers to a Tokai quake. 

However, it is important to keep in mind that not all volcanic eruptions are foreshadowed by an earthquake.

Mount Ontake, located on the border of Nagano and Gifu Prefectures, erupted with no warning on September 27th, 2014. The eruption occurred on a Saturday with great weather and autumn foliage in full color. This meant that the mountain was very popular for hikers. Unfortunately, several hundred people were on the slopes at the time of the eruption, resulting in 63 deaths and over 70 injuries.

I personally knew someone who was hiking on Ontake that day. Luckily, she had completed her climb and was nearly down the mountain at the time of the eruption.

Relatives of the deceased climbers filed a lawsuit against the Japanese Meteorological Agency for failing to predict the earthquake. While their frustration is understandable, eruptions like Mount Ontake are virtually impossible to predict with current technologies.

There were no large earthquakes. Volcanic activity seemed within normal levels. The 2014 Mount Ontake eruption was phreatic. These eruptions are caused by water getting trapped underground and being heated by the volcano. This usually happens very quickly, which causes gas pressure to build and the result is an explosion.

The 1980 eruption of Mount St. Helens was also a phreatic eruption. This video does a good job explaining how phreatic eruptions work.

Current monitoring techniques are simply unable to detect pressure build up that happens so quickly, making phreatic eruptions unpredictable. Scientists only noticed readings were a bit off about seven minutes before the eruption.

Unlike Mount Ontake, Mount Fuji tends to have Plinian eruptions. These are characterized by incredible explosive power. All that power takes time to build. As long as nothing significant changes within the mountain, it should erupt with a reasonable degree of warning.

Impact of an Eruption

Nearly 700,000 people live in the villages surrounding the base of Mount Fuji today. Widen that to a 100 km radius and that number swells to over 25,000,000 people.

The Japanese government still hasn’t formed an official contingency plan should Mount Fuji erupt. However, they are in the process of forming a committee that will include a variety of scientists and experts. The committee will then draft an emergency plan.

In 2002, the Cabinet Office estimated that ashfall from an eruption similar to the 1707 event could cause up to 2.5 trillion yen of damage. However, this number may no longer be accurate as aviation, other means of transportation, and secondary infrastructure were not taken into account, which would raise the total damage estimate.

Potential damage ranges from mechanical problems in cars, adverse health effects, mass power outages, and water filtration system failure. The Greater Kanto Area has nearly 40 million people. An eruption of Mount Fuji would have a devastating impact on Japan if the Kanto area is affected.

What to do if Mount Fuji Erupts?

If Mount Fuji erupts while you’re nearby (hiking the mountain, visiting the area, or living in the surrounding area), avoid areas downwind and protect your nose and mouth. Use a damp cloth if you do not have a mask. Remove any contact lenses as soon as possible because the ash can be especially damaging to your eyes.

If you’re climbing the mountain, turn around immediately and follow the mountain hut workers’ instructions. If you packed a hard hat, as suggested by the Japanese government, put it on to protect your head from falling rocks. Check to see if the mountain huts have extras if you do not have one. 

If you encounter people who are injured, check to see if it’s safe for you to approach. A lot of would-be saviors end up perishing because they did not assess danger before stopping to help. Let the authorities know, if possible. And if it’s safe and you’re trained, administer first aid until the rescue teams arrive.

Avoid any bodies of water because flash floods can occur. If you can, change into a long-sleeved shirt and long pants. Try to cover your neck as well. The ash is very irritating to the skin.

If you want to know more about what to do if you’re outside during an eruption, please check out this link and look at our risk references.

If you live nearby

Make sure you have an emergency kit ready. It should include protective eye goggles, masks rated against volcanic ash, a flashlight, a battery-operated radio, a first-aid kit, protective clothing (long-sleeved shirts, long pants, neck protection), and rations of both food and water. Have your evacuation route planned and consider having a map on hand.

Always try to wait for instructions from the local authorities, because there may be hazards (such as falling debris) that you may not be aware of. If you haven’t been ordered to evacuate, seal all windows, doors, and vents as thoroughly as possible. You can use plastic and tape or damp towels.

Ashfall can put a lot of extra weight on your roof and it may need to be removed to prevent collapse. If you do not have training, be as careful as possible. The ash can make rooftops more slippery than usual.

If Mount Fuji erupts while you’re in Tokyo

Listen to the local authorities. The ash cloud of a volcanic eruption can make it difficult to breath. It can also cause irritation to eyes, nose, throat, and skin.

Do your best to keep your face covered and consider getting an industry-certified mask. Japan has many masks, but most do not meet the required specifications to filter out fine volcanic ash. If the mask filters PM2.5 or smaller, it should be fine. Make sure it fits your face well so that it filters the air properly

Stay indoors unless instructed otherwise. Try to seal any doors, windows, or vents that lead outdoors. You can use tape and plastic or rolled up, damp towels. If you can, set up a single entry/exit point for the building. Do not use any exhaust fans or other appliances (like AC). Clean up settled ash with a damp cloth. Do not vacuum the ash!

If you must stay indoors for several days, try to keep the air as clean as possible. Avoid using the stove and do not smoke. 

If you’re in any area that is affected by ash, avoid driving. Volcanic ash is incredibly pervasive. It will get into your car’s ventilation system and it will damage most of your car’s components. If you must drive, do so slowly and do not use your headlights. It can make it more difficult to see due to the particulates in the air.

Once you are safe

Always try to contact friends and family as soon as possible. Do not call them! Send text messages or use social media instead, to keep the phone systems free for emergency workers. Otherwise, you may be reported as missing, which can cost manpower in search & rescue operations.

In Closing

Mount Fuji is arguably one of the most closely monitored volcanoes in the world. It is naïve to assume it will never erupt. The best method of survival is to prepare yourself as thoroughly as you can and do your research.

See our article: Is Mount Fuji Safe to Climb?