[BuraTamori] Kumamoto, Aso Area – How active volcano is useful to human life

The 2019, June 29th broadcast of Japanese TV show, BuraTamori featured the Aso Area of Kumamoto, Japan.

[What is BuraTamori?]

The Aso caldera, where volcanic activity continues today, is famous for its magnificent scenery.

It is natural to have a frightening image of volcanoes, but it is said that Aso’s volcano is very useful to the lives of the people of the area and also all of Japan.

The theme of this episode is,

“How is Aso the world’s most ‘useful’ volcano?”


Let’s explore the wonders of the Aso area.

Origins of the Aso Caldera

The Aso area is a huge caldera with a diameter of more than 20km and was created by a number of volcanic activities.

The large-scale eruptions have been named Aso-1 to 4, and they occured during the following period.
Aso-1: 270,000 years ago
Aso-2: 140,000 years ago
Aso-3: 120,000 years ago
Aso-4: 90,000 years ago

In particular, Aso-4 produced a massive pyroclastic flow (flow of magma and volcanic ash) that shaped most of the current terrain. The pyroclastic flow spread throughout the Kumamoto Prefecture of today, and traces of the magma flow has been found in the Yamaguchi Prefecture which is across the sea.

Today, the Aso Area, which is the crater of these volcanic activities, is inhabited by 50,000 people.
The volcanic activity was very long ago, but even so, it is a very rare place where so many people are living inside what used to be a volcanic crater.

The volcanic activity of the area still continues today, and more than 10 million tourists visit Aso every year.


Aso, an exhibition fair of volcanoes

At the center of the great Aso caldera, there is a mountain called the “central cone”.

On this mountain, there is a bowl-shaped grass field called “Kusasenri” (wide area of grass in Japanese). There is an observatory to see the whole view of Kusasenri. This was actually the volcanic crater of an eruption that occured 30,000 years ago.


The eruption at this time was a catastrophic eruption called the Plinian eruption, which is a large-scale eruption where magma and ash can reach the stratosphere, above the clouds. The Vesuvius eruption that devastated the famous Italian city, Pompeii was also a Plinian eruption.

On the other hand, looking in the other direction from the observatory of Kusasenri, you can see the hill called “komezuka” (mound of rice in Japanese), which is often shown as a symbolic scenery of Aso. Komezuka is called by this name because its beautiful shape is said to have been formed when god poured rice on the ground.


Komezuka was formed by a type of eruption called Strombolian eruption.
In a Strombolian eruption, small explosions blow the magma up in air and they form a pile of soil around the crater.

It is estimated that Komezuka was formed about 3,000 years ago. At this time, it is thought that human civilization had already formed in this area of Japan. The ancient Japanese people may have been watching the eruption.

Furthermore, from the same observatory, another mountain, Nakadake (middle mountain in Japanese) is also visible, where the eruption continues today. This volcano has live cameras attached to the crater and attracts attention from volcano researchers around the world. There are almost no volcanoes in the world where the crater is recorded with live cameras.


At the crater of Nakadake, ash rises from numerous cracks. This is the most typical type of eruption in the Aso area and it can be seen every day. When volcanic activity becomes active, stromboli eruptions can also be seen at Nakadake.

The collected video data of the crater camera is very useful for research on volcanic prediction.

With all of these types of volcanic eruptions in one place, some say that Aso is like an exhibition fair of volcanoes.

Why Aso is a “Useful Volcano”

In the northwestern part of the caldera, there is a naturally reddish brown river. This area contains a large amount of iron in the soil.

The soil taken from this area is called Aso yellow soil. The underground magma contains iron, and Aso yellow soil is formed by iron dissolving into the undergroud water due to gentle volcanic activity beneath this area.

Aso Yellow Soil

This Aso yellow soil is the material for orange crayons produced throughout Japan.

Furthermore, the most important use of Aso yellow soil is sewage treatment. In a simple experiment, Aso yellow soil pellets are put into odorous water containing hydrogen sulfide and the container is shaken a few times. Then, the odor disappears immediately.

This is because Aso yellow soil absorbs the sulfur which causes the smell.

Chemically, hydrogen sulfide and iron hydroxide contained in Aso yellow soil chemically react to turn into iron sulfide and water.

Aso yellow soil harvested in Aso is shipped to nearly half of Japan’s sewage treatment plants. As for the Tokyo area, 100% of the sewage treatment plants in Tokyo use purification pellets made from Aso soil.

People in the great city of Tokyo always benefit from the products of the Aso volcano without noticing it.

Agriculture in Aso

Aso soil has advantages such as described above, but when it comes to agriculture, the soil is acidic, so it is not suitable for farming.

The people living in the Aso area have developed their method for improving the soil from long ago. On the mountains that surrounds the giant caldera of Aso, there is something that helped improve the soil.

That is grass of the field.

There is a vast grassland in the hilltop area called Daikanmine, also known as a sightseeing spot where you can view the terrain of Aso.


Just by cutting this grass and scattering it in the soil, the acid groud is neutralized, making it suitable for agriculture. The grassy field that stretches over the outer ring mountains of the Aso crater is 16,000 hectares. This is actually the largest grassland in Japan.

This activity is the source of delicious crops such as Takana, a well-known vegetable product of Aso.

The grassland of the outer ring mountains was so important to agriculture that there were many battles among clans to compete for grassland over the history of Aso.

This can be seen from the earthen walls stretched around the grassland.
Clans that use grass from the grassland were once divided into 150 groups by these earthen walls, and the total length of the wall in the Aso area was 500 km, which is more than the distance from Tokyo to Osaka.

In addition, the grass of Aso’s grassland is not only used for agriculture, but also for thatched roofs throughout the country.

Now that there are fewer grasslands nationwide, Aso’s Susuki is used for thatched roofs on traditional Japanese houses, in areas of national cultural assets, temples, and villages.

If grassland is simply left alone, it will become a forest, so it is necessary to somehow keep the grass from growing too much. This is done by setting fire to the fields. It is an important activity that prevents forestation and promotes the growth of young grass. Today, it takes place in March every year, and it is also an Aso tradition that attracts tourists.

Investigation of the geologic stratum of Aso, the black soil indicates that grass was kept young. This layer of black soil goes down to the stratum of about 10,000 years ago, beneath the layer of ash made by the eruption of Kikai Caldera located between Kagoshima and Okinawa, which occured 7,300 years ago.

From this, some researchers say that the ancient Japanese people had been burning the grassfield to maintain young grass, for agriculture or for ease of hunting.

Australian aborigines are known to have been burning fields for easier hunting at about the same era.

Viewing the vast grassfields of Aso, we may be seeing just the same scenery that the ancient Japanese people of 10,000 years ago had been seeing.

Aso is a land where people coexist with the tough but special terrain created by great activities of the Earth.

It may be a nice and unique destination if you have already been to places like Tokyo, Osaka, or Kyoto.