The Dōzen Islands have an unusual landscape. In the middle is a tall mountain surrounded by a ring of islands. This landscape was formed by a large-scale volcanic eruption from the central mountain followed by a collapse of the land. This kind of volcanic depression is called a caldera. In Japan, Mt. Aso in Kyushu is one of the most famous calderas.
The Dōzen Caldera was formed around five million years ago. As one of the oldest calderas in Japan it has been selected as one of the Top 100 Geological Landscapes.
It was likely formed by the following process.
- A large volcano was formed by volcanic activity.
- As stores of magma within the volcano erupt, it becomes hollow and the center collapses.
- Another eruption occurs from the center of this collapsed land and the central pyroclastic cone is formed.
- Following the eruption, changes in the earth’s crust cause the bottom half of the volcano to sink under the sea. Erosion occurs forming the current landscape.
Lifestyle on the Dōzen Islands has been shaped by this landscape and geology. The sunken land of the caldera formed a sea that is protected by the surrounding island. This inner sea provides a safe port for the Dōzen Islands. The towering central mountain (central pyroclastic cone) also has the function of a lighthouse, lighting up the seas at night.
There are two explanations for the formation of the inner sea. One claims that it was largely volcanic activity that caused the caldera to become a sea, whilst the other claims that erosion played the larger part.