Aso-Kuju National Park 阿蘇くじゅう国立公園

Aso-Kuju National Park was designated in 1934. Highlights of the park include Mt. Aso and its surrounding giant caldera (a depression formed by the collapse of a volcano into itself), the Kuju Mountains flanked by a range of volcanic peaks, and the widespread grasslands and marshlands that cover the open plateaus at the base of these jagged ridges.

In the south of the park, the Aso Caldera, one of the largest caldera sites in the world, measures 18km from east to west and 25 km from north to south. At the center stand the five peaks of Mt. Aso, rising up from the surrounding flat caldera floor. Viewed from a distance, these are said to resemble a sleeping Buddha. The highest peak, Mt. Takadake, is 1,592 m. Takadake is flanked by the active Mt. Nakadake, whose central crater is filled with an acidic blue pool from which steam regularly rises. The crater here can be approached on foot, by car, or by ropeway (closed at the moment), meaning different aspects of the rugged landscape can be observed close-up. The Aso area also features an almost perfectly shaped volcanic cone, Komezuka, the broad grasslands of the Kusasenri ga hama, the otherworldly barren landscapes of the Sunasenri ga hama, and the high caldera walls that surround them.

Located in the middle of the park, the Kuju Mountains offer a majestic and diverse landscape. From the volcanic peaks, including the highest point in Kyushu, Mt. Nakadake, standing at 1,791m above sea level, to the Handa, Chojabaru and Kuju plateaus, formed from ancient volcanic ash and debris, now almost completely covered in grassland plains, these areas form the contrasting landscapes that Kuju is famous for. The area’s highland marshes, Tadewara and Bogatsuru, have both been recognized as Ramsar Sites of International Importance. They are low-lying waterlogged areas in the mountain range that have evolved as unique habitats and are home to an abundance of rare plants and animals.

Mt. Tsurumi, in the north of the park, has been considered a sacred mountain since ancient times. Mt. Yufu, on the other hand, is the source of many of the natural hot springs for which Beppu and Yufuin are famous. Mt. Tsurumi has a number of small statues of deities, Buddhas, and shrines at its summit, which is accessible by ropeway or on foot. On clear days there are panoramic views of Beppu Bay, Mt. Yufu, and the Kuju Mountains.

The ecology of the park is generally classified into three categories: areas in which volcanic gases are emitted, forests that spread over the base of mountains and managed grassland. In areas of high volcanic gas emissions, very few plants and animals can survive. The forests, including both primary and secondary forests where trees have spread across areas of ancient grasslands, are home to a wide variety of plant and animal life. The grasslands of the park make up most of the area. They are a unique feature of the parkland and consist of both native, semi-natural areas of meadow, pasture, kaya reed beds and marshes, and areas where grasses and plants have been introduced and cultivated. The grasslands support over 600 species of plants, a thriving agricultural crop and livestock industry (having been adapted to specific needs by the local people). With invaluable water resources, this area is known as the ‘Reservoir of Kyushu’, with natural underground rainwater reserves in the porous volcanic ground providing pure, mineral-rich water used by no less than five million people in Kyushu.

Each spring, the grasslands are subject to controlled burnings, known as noyaki. This is done in order to keep shrubs and trees from spreading across the grasslands and to preserve their rich natural habitats. Without this human intervention, the grasslands would be taken over by forest, and diversity would be lost.

Aso-Kuju National Park showcases the contrasts typical of Japan’s volcanic landscapes. From dense forest to rugged mountains, lush marshes, hidden hot springs (onsen) and pristine waters, the park offers stunning views and a wealth of flora and fauna to enjoy in every season of the year.

Aso-Kuju National Park, Volcanoes

There has been volcanic activity in the Aso-Kuju National Park area for at least 270,000 years. The present geological structure of the area has been formed by volcanic activity during the last 90,000 years. The Aso Caldera, measuring 18 km from east to west and 25 km from north to south, was formed by four separate eruptions that took place over several thousands of years.

During the first stage, there were three periods of activity which occurred between 120,000 and 270,000 years ago, when huge pyroclastic flows formed plateaus around the active crater site. Approximately 90,000 years ago another eruption broadened the pyroclastic plateau. After this eruption, the process of caldera formation was triggered by massive landslides, cave-ins, and the eventual collapse of the crater. The caldera site then collected vast amounts of rainfall, and for a long period was a large lake. Several thousand years’ seismic activities then caused this water to drain, resulting in the current caldera structure.

The Kuju Mountains are also volcanic. This activity began approximately 150,000 years ago. Over the last 5,000 years, there have been volcanic eruptions about once every thousand years. The most recent large-scale magma eruption was that of Mt. Kurodake, about 1,700 years ago. More recently, on October 11th, 1995, Mt. Io erupted, ejecting cinders the size of a fist distance of up to 100 m from the crater. Even now, Mt. Io still emits gas fumes.

Home to two active volcanoes, Mt. Aso in the Aso range and Mt. Io in the Kuju range, the Aso-Kuju area still experiences volcanic activity, and in recent years (three times since 2010) has had a number of smaller-scale eruptions, mostly consisting of ash and rocks.

Despite the risk of having to evacuate the crater area when there is any danger of an eruption, there are many benefits to this volcanic environment, namely the rich water resources, the hot springs, and the cool climate in the summer months.

Grasslands

The Aso Caldera is one of the largest calderas in the world. Covering a total area of 380 km², the unique landscape is characterized by mountains and grasslands. The grasslands of the Aso Caldera make up approximately 200 km² of the total area. These grasslands are a unique feature of the caldera, with native, semi-natural areas including meadows, pasture land, reed fields (for thatching), marshes, and areas where grasses and plants have been introduced and cultivated.

Due to the geographical location and climate of the park, the grasslands support a diverse range of plants and animals. The Aso grassland area is home to over 600 species of plants, successful livestock and agricultural crop industries, and an invaluable source of water. The area is known as “the thousand-year-old grassland,” but geological evidence in the soil indicates that the grasslands have been cultivated and maintained by humans for at least 10,000 years.

The Kuju Plateau, lying at the foot of the Kuju mountain range, is also known for its broad grassland landscape. The local soil contains pyroclastic sediments deposited by past volcanic activity. The soils here favor the growth of pampas and other tall grasses.

The grasslands are managed as cattle pastures, by mowing, and by controlled springtime burnings called noyaki. Without these methods, trees and shrubs would invade, turning the land into forest, and the natural wealth of the grasslands would be lost.

Waters of Aso and Kuju

The location of the Aso grasslands, at a high altitude and close to the sea, means they receive a significant amount of rainfall annually; more than double the national average for the rest of Japan. When it rains, grass covering the broad grassland areas moderates the flow of water so that it seeps more slowly into the mineral-rich volcanic ground. It then collects in natural underground reserves and feeds streams and rivers that irrigate northern Kyushu. They include six Class A rivers that provide water for living and agriculture to over five million people in the region. The area is commonly regarded as the water reservoir of Kyushu.

In certain areas, groundwater wells up to the surface under cliffs and at the base of hills, where it forms natural springs producing fresh, clean water. Over large areas of low-lying land, water collects to form a number of wetlands that support a wide range of unique plant and animal life. This has contributed to the area being awarded UNESCO Global Geopark status. There are many hot spring (onsen) towns in the Aso area. Hot springs have been an important part of the daily lives of the locals since ancient times. Many tourists visit the area to bathe in the volcanic hot spring waters, bringing significant economic benefits to the area.

Sunasenri and the Surrounding Area

At 1,506 m above sea level, Nakadake is Mt. Aso’s second highest peak after Takadake (1,592 m). Nakadake is an active volcano with seven craters on its summit. It is flanked on the west side by a crater floor covered by fine black sand of crumbled volcanic rocks, called Sunasenri ga hama. The area is a barren landscape with high volcanic gas levels in which few plants can grow.

Of the seven craters of Nakadake, Crater No. 1 is unique in that visitors can approach the crater and observe the acidic blue pond that has formed there. The color is produced by dissolved volcanic minerals. The temperature naturally remains at around 60°C, increasing during periods of volcanic activity. Crater No. 1 constantly discharges volcanic gases and is under continuous surveillance. The gas is mostly composed of water vapor. However, levels of hazardous substances, such as sulfur dioxide, can change from minute to minute and, on particularly active days, can reach dangerous levels without warning. For this reason, access is limited to ensure visitors’ safety. It is recommended that people with asthma, or cardiovascular or heart disease, do not approach the Nakadake area. Visitors who do approach the crater area must stay away from restricted zones, pay close attention to announcements regarding volcanic gas emissions, and follow all instructions by crater monitors during an emergency.

The remaining craters are inactive and made up of volcanic rock: pumice, scoria (a pumice-like rock created by the hardening of gaseous basaltic magma) and welded tuff (a hardened rock created from the heat and pressure of a pyroclastic flow). Much like Sunasenri, the area is covered in volcanic ash and debris.

Features of Sunasenri

Sunasenri ga hama is a crater floor covered with volcanic ash. The area is dotted with volcanic ash deposits and masses of hardened magma called “volcanic bombs.” The size of these volcanic bombs can vary from several centimeters to tens of centimeters, some even measuring more than 1 m in diameter, and can be seen distributed across the rugged terrain of the Sunasenri ga hama.

The terrain of the Sunasenri ga hama is made up of volcanic rock: pumice, scoria (a pumice-like rock created by the hardening of gaseous basaltic magma) and welded tuff (a hardened rock created from the heat and pressure of a pyroclastic flow). Volcanic ash and pumice fragments discharged from the nearby active crater are distributed over Sunasenri ga hama, creating an ever-changing landscape. During an eruption, ash fall can dramatically decrease visibility and obscure the view.

Sunasenri ga hama is a barren landscape. The area is subject to high steam levels and volcanic gases, such as hydrogen sulfide and carbon dioxide. These gases are constantly emitted from the Nakadake crater, fumaroles or vents, and from hot spring sources. The hazardous gases create an environment in which few plants can grow. Visitors are strongly advised to stay on the wooden walkway.

Crater No.5

The Nakadake peak of Mt. Aso was formed by a series of multiple eruptions. Today, Nakadake has seven craters located within a 1 km area, running north to south. Crater No.5, in contrast to craters number 1 and 2, has not erupted in recent years. However, the entire crater area is constantly under surveillance to ensure the safety of visitors, and to enable large-scale volcanic research into disaster prevention measures.

The terrain around Crater No.5 is barren and made up of volcanic rock: pumice, scoria (a pumice-like rock created by the hardening of gaseous basaltic magma), and welded tuff (a hardened rock created by the heat and pressure of a pyroclastic flow).

Even though Crater No.5 is inactive, the level of sulfuric acid in the air from the remaining active crater areas is still high. Please do not step beyond the “No Entry” barriers.