New Findings from Mt. Fuji's Past Eruptions

For many people, Mount Fuji is the quintessential symbol of Japan.

However, it's also an active volcano that has erupted about 180 times in the past 5,600 years.

Mt. Fuji will certainly erupt.

We don't know when,
but it will definitely happen.

In 2021, the volcanic hazard map for Mount Fuji was revised for the first time in 17 years,

showing that the effects of an eruption may be more widespread than previously thought.

The revision was prompted by new scientific research into historic eruptions,

and a better understanding of what could happen in the next one.

This research team surveyed one crater to shed light on the major eruptions in the past.

What they discovered overturned the established theory.

We were surprised that the pyroclastic flow
reached so close to the urban area.

The reality of Mount Fuji as an active volcano is gradually becoming clear.

Today we'll look at the cutting edge of research into the eruptions of Mount Fuji.

And in today's J-Innovators corner, the keyword is also "disaster preparedness."

Today's Takumi has developed special flexible LED lighting with

a phosphorescent function that is especially useful during power outages.

Hello and welcome to Science View.

I'm Tomoko Tina Kimura.

Today, we're focusing on Mount Fuji - a symbol of Japan and also an active volcano

that has remained silent for a long time.

We'll look at how new discoveries were made about the historic eruptions of this famous mountain.

Joining us here in our Tokyo studio is Mr. David Hajime Kornhauser,

Director of Kyoto University's Office of Global Communications.

Thank you for coming over to Tokyo, Mr. Kornhauser.

It's a pleasure to be here in person.

Now, Mount Fuji is the highest mountain in Japan and has long fascinated

the Japanese people with its beautiful, well-proportioned shape.

But it's also an active volcano, isn't it?

It sure is.

Volcanoes have long activity spans of hundreds of years,

and the Japan Meteorological Agency refers to all volcanoes with a record of eruptions as

"active volcanoes" because there is the possibility that they may erupt again.

And so, Fuji is one of these.

I know that it could erupt at any time, but I'm so used to its beautiful appearance

that is there always the way it is.

The risk doesn't feel real.

I know that exactly.

Even the experts don't know if an eruption is actually imminent.

Have a look at this.

This shows the eruptions of Mount Fuji based on historical records.

In the 1,200 years from around 800 AD to the present day,

there have been 10 recorded eruptions, the most recent being in 1707.

So it has remained silent for over 300 years ever since.

Which means the next eruption could happen at anytime.

Right. We need to prepare for the possibility, and learn from past eruptions.

Of particular interest to experts is the most recent case, the Hoei eruption of 1707.

This is a drawing from that time depicting the eruption.

You can see that it's erupting from the side of the mountain rather than the top.

This was the largest eruption of Mount Fuji, lasting 16 days.

Recently, a site from this historic eruption has been discovered which shows the extent of the damage.

The Subashiri district is about 10 kilometers from the crater of the Hoei eruption.

In 2019, a major discovery relating to that eruption was made here.

This blackened and charred "wooden post" was excavated at a depth of two meters.

When the eruption occurred, houses were burned by the high-temperature ejecta,

and buried under a large amount of volcanic ash more than two meters high.

According to ancient records, 37 houses in the village were destroyed by fire,

and the remaining 39 houses all collapsed under the weight of the volcanic ash.

This is the first time this kind of evidence has been confirmed.

It was not just the area at the foot of Mount Fuji that was affected.

The eruption occurred in December, and strong westerly winds caused volcanic ash

to fall over the southern Kanto region.

It has been confirmed that several centimeters of ash were deposited

in the area where modern-day Tokyo is located, approximately 100 kilometers from the crater.

The volcanic ash devastated the farmland, making it impossible to harvest.

Records also state that many people suffered from coughs at that time.

So a huge amount of volcanic ash fell over such a wide area.

Even with today's advanced science, there's not much we could do to stop that from happening.

This doesn't mean that an eruption of that magnitude will occur every time.

But from the standpoint of disaster preparedness,

it's important to know what could happen in the worst case.

I agree.

As research into the details of the Hoei eruption progressed, new discoveries were made.

Akira Baba has been conducting field research on the Hoei eruption since 2016.

With special permission, our team accompanied him on his survey.

At an altitude of 2,380 meters, the huge Hoei crater appeared in front of us.

It has a diameter of approximately one kilometer.

The area is covered with black-colored sediment.

"Scoria" is a porous volcanic rock, which can be found all over Mount Fuji.

Baba believes that the key to understanding what occurred during the Hoei eruption is Mount Hoei,

the mountain that formed on the rim of the Hoei crater.

Mt. Hoei is also depicted in an illustration from the time, and was created during the eruption.

Until recently, it was thought to have been formed by the up-thrusting of underground magma,

or uplift, associated with volcanic eruptions.

The theory was based on this reddish-brown layer of rock, found only around Mt. Hoei's summit.

It was thought that older strata had been exposed here.

However, through repeated field surveys, Baba came up with a different hypothesis.

It's reasonable to assume that Mt. Hoei itself
was created by ejecta from the Hoei eruption,

and that westerly winds caused
the volcanic ash to fall in the east.

Baba decided to take a new approach to analyze the rocks that make up Mount Hoei.

This rock may not seem to indicate anything,

but by measuring its paleomagnetic direction,
we can work out if it was created in 1707.

Paleomagnetism is the study of the Earth's magnetic field in rocks and sediments.

Over time, the magnetic field is constantly changing in its direction and strength.

When magma erupts onto the Earth's surface, then cools and solidifies into rock,

it records the direction of the magnetic field at that time.

In other words, by measuring the paleomagnetic field of the rocks and

comparing it to a reference database, it is possible to estimate the date at which the rock was created.

This is a graph of temporal changes
in the geomagnetic field direction.

Plotting the data collected at Mt. Hoei's summit

reveals that it overlaps with
the field direction around 1700.

And by also confirming the characteristics
of the rock composition and layering of the strata,

we found that Mt. Hoei is not an uplift,
but rather a buildup of ejecta.

Baba also found important clues to understand the progression of the volcanic eruption.

In 2020, at a construction site in the Subashiri district, he found layers of

rocks that had preserved the ejecta from the Hoei eruption.

If you cut through to the slope and look at the layers…

From this white layer up to the surface
is all ejecta from the Hoei eruption.

Ejecta from the 16-day eruption fell and built up about 1.5 meters at this location.

About 50 cm of ejecta was deposited in Subashiri
in the first 24 hours of the eruption.

It has been confirmed that one-third of the ejecta was produced in the first day of the eruption.

Baba speculates that the Hoei eruption progressed as follows.

The eruption began around 10 am.

It started with an eruption of whitish smoke.

The plume was carried by the prevailing westerly winds,

causing pumice to fall on the eastern side of the volcano.

After midday, the plume turned blackish.

At night, the eruptions became more intense, ejecting volcanic bombs.

Large pieces of scoria piled up near the crater, gradually forming Mount Hoei.

When the 16-day eruption finally ended, Mount Hoei had been created,

an estimated two to three hundred meters high.

It's amazing to see a chronological timeline of how exactly Mount Hoei was formed.

It really is!

The Hoei eruption was an explosive eruption that ejected cinders and volcanic ash,

causing extensive damage.

But these are not the only phenomena associated with volcanic activity.

Do you know of any others?

Well, there could be thick lava flows, and I know that pyroclastic flows are also very dangerous.

Yes.

There are various other phenomena, such as the release of volcanic gases and the collapse of the volcanic edifice.

Records of Mount Fuji's previous 180 eruptions show that almost all of these types have occurred there.

Which means that any of those phenomena could happen again in the next eruption…

That's right.

Is it difficult to predict exactly what kind of volcanic phenomena will happen in the next eruption?

Unfortunately yes.

One of the issues is that the location of the crater itself is hard to predict.

In fact, there hasn't actually been an eruption at the summit of Mount Fuji for about 2,300 years.

Since then, craters have randomly opened at other locations within a radius of

about 13 kilometers from the summit.

I see.

Since we cannot anticipate the location of the crater, it's even more difficult to

predict what happens after that.

How should we prepare for Mount Fuji's next eruption?

As Baba continued his research, he made a discovery that completely overturned previous assumptions.

In 2018, something was found in a Self-Defense Forces training area about

two kilometers from the urban area.

This is a pyroclastic flow deposit
that came from Mt. Fuji.

It's made up of various large and small stones
which are deposited like this.

A pyroclastic flow is a phenomenon in which volcanic ash and rocks of various sizes,

along with hot volcanic gases, flow down the mountain slope at a speed of about 100 kilometers per hour.

These are the traces of a pyroclastic flow found at the foot of the mountain.

What surprised Baba was its scale.

Here, the pyroclastic flow deposit reaches a height of 15 meters.

And there are also clues to determine the timing of the eruption.

Here you can actually see
carbonized pieces of wood.

This is the charred remains of vegetation that once grew at the foot of the mountain

and was caught in a pyroclastic flow with a temperature of nearly 400 degrees Celcius.

Dating of the carbon in the wood chips, and paleomagnetic measurements of the rocks

indicate that it was a pyroclastic flow from the 600s.

Baba's investigations confirmed that the pyroclastic flow deposit is spread

over an area 700 meters in width and 3.6 kilometers in length.

The estimated amount of deposit is 12.4 million cubic meters.

That's about 5 times the amount previously thought to be the maximum.

It's probably the largest pyroclastic
flow deposit currently known.

We were also surprised that the pyroclastic flow
came so close to the urban area.

Based on this new scientific knowledge, in 2021 the Mount Fuji Eruption Hazard Map

was revised for the first time in 17 years.

The volume of a possible pyroclastic flow has been revised from 2.4 million cubic meters

to 10 million cubic meters, based on Baba and his team's research.

The topographic data is reflected more precisely,

and it's estimated that it could spread further to the northeast and southwest.

The lava flow area was also reviewed.

A lava flow is a phenomenon in which extremely hot magma from a crater flows

slowly over the Earth's surface.

12 cities and towns in Yamanashi, Shizuoka, and Kanagawa Prefectures are now included,

so we must be alert for lava flows in a larger area.

Researchers are also concerned about ash fall from explosive eruptions.

There is a possibility that more than 2 centimeters of ash could fall even in the Tokyo metropolitan area.

2 cm may not seem like much,

but in Kagoshima, which is home to Sakurajima,
one of the world's most active volcanoes,

ash falls frequently yet rarely exceeds 1 cm.

The last time we exceeded 1 cm,
it was the amount for the whole year in 1985.

We should think about the danger of volcanic ash
in a completely different way.

Mannen points out that if volcanic ash accumulates in the Tokyo metropolitan area,

railways would be greatly affected.

Railways send out various safety-related signals
through the rails.

If volcanic ash built up on the rails,
those signals could not be transmitted.

There's also the effect of volcanic ash on the water supply to consider.

Volcanic ash contains fluorine and
if volcanic ash gets into the water,

the concentration of fluorine can easily
exceed the drinking water standard.

Water is an essential lifeline.

So, this would be a serious problem.

The new hazard map shows the specific areas which could be affected

by different aspects of a volcanic eruption.

Yes, it does.

The important thing is for people to look at the hazard map not as someone else's problem, but as their own.

Even if you live far away from Mount Fuji, think about what you would do if you lived there.

That will help in finding things that are relevant to all disaster preparedness.

The most important thing is to be ready.

That's right.

Coming up next is our J-Innovators corner.

Here, too, we will look at technologies related to disaster preparedness.

Electric lighting is an essential part of everyday life.

We've all experienced the inconvenience of the lights going out during a blackout.

In that situation, phosphorescent substances can be useful.

When exposed to light, they will absorb the light and then re-emit it, even after the light source is gone.

Phosphorescent substances are used in emergency signs, glow-in-the-dark tape, and watch dials.

However, when the light exposure is inconsistent, perhaps due to changes in the weather,

the luminescence can be uneven, or fade within a few hours.

In a sudden power outage, wouldn't it be reassuring to have a phosphorescent product

that could emit steady, continuous light for 10 hours at a time?

One Takumi or innovator has developed just such a revolutionary product for use in emergencies.

We visited a lighting equipment manufacturer with a factory in Fukuchiyama City, Kyoto Prefecture.

The head of the development department is today's Takumi, Yoshida Kazuki.

This flexible light has a phosphorescent function,
which can help in a disaster.

A flexible LED board is covered with a soft silicone cover,

allowing it to bend freely and be installed on curved or uneven surfaces.

This silicone tubing contains a phosphorescent pigment which provides a long-lasting glow.

We were shown how this light is actually used.

Here it's used as normal lighting for a staircase.
Now, I'll turn off the electricity.

As soon as the electricity is cut off, the LED light is replaced by a greenish glow.

It's designed to last for about 10 hours, more than enough time for an emergency evacuation.

Its key features are its shape and flexibility, which mean it can be used in

places where ordinary lighting is difficult to install.

Because it's used as normal lighting,

the light from the LED consistently shines on the phosphorescent pigment.

Therefore, the phosphorescent glow is steady and even.

LED lighting for everyday use, and phosphorescent lighting in case of emergency.

However, balancing the two functions was not an easy task.

It was difficult to find the correct ratio for the phosphorescent pigment.

There are various phosphorescent pigments, and the color and duration of the glow

will vary depending on various factors.

Here are two silicone sheets, mixed with a phosphorescent pigment that glows green.

The one on the left was mixed with more phosphorescent pigment, and the one on the right with less.

As a cover for an LED light, the one with more pigment causes the light to turn green,

and is not bright enough.

On the other hand, with less pigment the light glows white and is bright enough to be used as normal lighting.

However, when the electricity is turned off, the situation is reversed.

The one with more phosphorescent pigment would be sufficient for emergency lighting,

but the one with less would not.

In order to make it compatible with conventional LED lighting,

Yoshida tested various pigments, adjusting the balance in increments of 0.1%,

in pursuit of the optimum mixing ratio.

There were lots of things to test.

It took about a year to find
the optimum mixing ratio.

Yoshida's company originally focused on everyday lighting equipment,

rather than products related to disaster preparedness.

The impetus for a change in direction was unexpected.

In July 2018, a typhoon caused massive flooding in western Japan.

Relatives of company president Kawata were affected by the disaster,

and he heard that they were very anxious, especially when the power went out in the middle of the night.

As a lighting manufacturer, it's no good
if our products cannot provide light at critical times.

I became aware of the issue,
and understood the challenge.

Two months later, during a sales pitch for his lighting equipment, Kawata blurted out the following to a customer.

I told him we were developing emergency lighting,
which would come on even if the power goes out.

He was interested and thought it'd be a good idea,
so I immediately called Yoshida.

I told him to mix in some phosphorescent pigment
and make an emergency light.

Honestly speaking, I felt like,
'You've got to be kidding me!'

That's completely unrealistic.

It was the president's request, so Yoshida got to work right away.

Kawata wanted a light that could be used as an ordinary light in normal circumstances,

and also as an emergency guidance light in the event of a blackout.

If it's to illuminate the way in an emergency, it must not get in the way, and fit in any location.

So, in a few days, Yoshida created a sample by mixing a phosphorescent pigment

into some silicone tubing, and placing a strip of LED board inside.

However, there was a problem with this design.

When the tube was bent, the LED board moved about inside, resulting in uneven illumination.

So they next developed a special extrusion molding machine, the only one of its kind in Japan,

to integrate the silicone tubing and the LED board, based on equipment

that was used to make electric wires for lighting.

This machine uses pressure to mold the silicone around the LED board,

integrating the two components inside the machine, and pushing the finished product out the other end.

However, another problem arose.

When the silicone was molded under pressure, the force was concentrated on the exit point,

and the LED board was repeatedly damaged.

Through a process of trial and error, Yoshida and his team designed a mold that

can fix and integrate the LED board inside the silicone tubing, at the optimum pressure.

This mold worked well and produced a stable finished product.

This is a cross-section.

Unlike the earlier version that was simply placed inside the tube,

the LED board is now firmly fixed to the silicone.

No matter how much it is bent, the LED board will not move, so the lighting will not become uneven.

Yoshida and his team created flexible LED lighting that can provide steady,

continuous light in normal and emergency situations.

Development is the creation
of something out of nothing.

The pain and joy of such creation is a
real pleasure that's unlike anything else.

I think this is a brilliant product.

I can imagine it being used not only in regular households

but also offices and stairwells in commercial facilities.

Yes, exactly.

After seeing this, I was really thinking I'd like to have this in my own home.

Today, we learned about the history and the latest research on the eruptions of

Mount Fuji and the next eruption is sure to come someday.

Mr. Kornhauser, when that time comes, will there be warning signs?

It's generally believed that when magma rises from under the ground, the surface itself swells.

Mount Fuji is currently being monitored 24 hours a day by the Japan Meteorological Agency

using a variety of observation instruments.

They hope to be able to detect the swelling of the mountain body due to rising magma,

and accompanying earthquakes, as signs of an imminent eruption.

That is good to know.

It's really important to be prepared for the worst.

Yes, you really have to be prepared for anything.

Mr. Kornhauser, thank you very much for joining us today in Tokyo.

You're very welcome.