Limonite is also called iron oxide. Mt. Aso experienced large eruptions around 300,000, 150,000 and 90,000 years ago. Due to these eruptions, a large lake, the “crater lake”, was formed in the volcanic crater.
Mineral-rich magma and organic substances like plants accumulated within the crater lake. The crater lake dried up over a long period of time, and various components in the lake infiltrated into the crater. Eventually, the components formed strata and were left lying as limonite. That is how limonite took on its present-day form.