Izu Peninsula: Where Three Plates Collide to Create Hot Spring Paradise
The Izu Peninsula (伊豆半島), which extends southeastward from Tokyo into Sagami Bay and Suruga Bay, is a geological accident that has created one of Japan's best tourist attractions. Tectonically, the peninsula formed when the Philippine Sea Plate collided with the Okhotsk Plate near the Nankai Trough, with the Philippine Sea Plate serving as its foundation. The Philippine Sea Plate, Amurian Plate, and Okhotsk Plate merge at Mount Fuji, resulting in extraordinary geological complexity.
The peninsula's geology is divided into various regions, indicating its complex genesis. The southern portion is mostly composed of breccia (angular rock fragments cemented together), whilst the centre and northern regions are made up of multiple extensively eroded volcanoes. The Amagi Mountain Range dominates the peninsula's centre, with Mount Amagi at 1,406 meters (4,613 feet), Mount Atami at 773 meters (2,536 feet) to the east, and Mount Daruma at 982 meters (3,222 feet) to the west. These mountain ranges stretch underwater into neighbouring bays, confirming their undersea origins.
The Kano River in the north of the peninsula flows through a graben valley generated by plate tectonics, which is a depression formed as the earth's crust expands and lowers. This tectonic activity significantly influenced the peninsula's geological properties and continues to shape its current landscape. Because of its underlying geology, the peninsula is still prone to regular earthquakes and tsunamis, and residents live in a state of permanent geological instability.
Paradoxically, the same tectonic upheaval developed Japan's most opulent tourism asset: the peninsula is rich in hot springs (onsen). Japan is located on four different tectonic plates, with the Pacific and Philippine plates colliding east of Tokyo to form the 1,600-kilometer Izu-Bonin Mariana Arc. This impact creates enormous pressure, and along this arc, four undersea volcanoes exist every 100 kilometres. The ensuing thermal activity generates conditions for hot springs around the Izu Peninsula, changing a geological hazard into a source of relaxation and wellbeing for millions of people each year seeking therapeutic bathing experiences.
