Moderate ash-laden emissions from Crater 1 of Naka-dake were observed on almost every day in April (some daily ash accumulations are listed in table 3). Volcanic flame had been observed rising 20-40 m above [the pit in] the floor of Crater 1 on 25 March, and the remainder of the water pool in the crater disappeared on 16 April. Average tremor amplitude remained at around 0.3 µm in April.

Table 3. April-May 1985 daily ash accumulation at the JMA's Aso-san Weather Station, 1.2 km SW of Crater no. 1 of Naka-dake. [Ash was measured each day at 0900 and represents the amount deposited the previous 24 hours. There was no accumulation on other days in April and May.]

On 6 May, a small eruption was observed at a new pit about 10 m E of the one that had formed on 1 March. Rocks several tens of centimeters in diameter rose to a height of 50 m above the floor of Crater 1 and ash-laden emission was almost continuous from the 1 March pit. It was not certain if juvenile material was included in the 6 May tephra.

Geological Summary. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.