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 Wikipedia
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 200km3.
Aso caldera has often been called the largest in the world, but actually Indonesia’s Danau Toba, measuring100km by 30 km, is the world’s largest. Furthermore, in Japan, Kussharo caldera, 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.