Aso caldera is a geographical feature of Kumamoto Prefecture, Japan. It stretches 25 kilometers north to south and 18 kilometers east to west. The central core lays at the center of five major mountains in the area. Aso valley runs along the northern base of Mount Aso and Nango valley along the south. According to research of caldera sediment, lakes used to exist in these valleys. The dried up lake areas have come to be called Old Aso Lake, Kugino Lake, and Aso Valley Lake.
Aso caldera formed from four major pyroclastic flow events which occurred between 90,000 and 270,000 years ago. The largest of these was the fourth, which reached as far as nearby Yamaguchi Prefecture 160 kilometers away. The fourth eruption left a massive pyroclastic plateau which is what remains even today. In 1985 it was discovered that volcanic ash from the fourth eruption covered much of the Japanese islands. The extent of the fourth eruption is estimate to be approximately 200 km³.
Main eruption age and eruption volume:
- Aso1: About 266,000 years ago, erupted 32 DRE km 3  .
- Aso2: About 141,000 years ago, erupted 32 DRE km 3.
- Aso3: About 130,000 years ago, erupted 96 DRE km 3.
- Aso4: About 90,000 years ago, erupted 384 DRE km 3.
Note: DRE (Dense-rock equivalent) is the equivalent magma eruption volume . The volume of erupted sediments is much higher.
Aso caldera has often been called the largest in the world, but actually Indonesia's Danau Toba, measuring 100 km by 30 km, is the world's largest. Furthermore, in Japan, the caldera forming Lake Kussharo, which measures 26 km by 20 km, is larger than Aso caldera leaving it the second largest in Japan. It's not rare to see calderas of this scale; however, to see calderas with an interior stable enough to cultivate land, build highways and lay railroads is quite rare.
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