A Tectonic Collision Zone
The Izu Peninsula is the result of a slow-motion tectonic collision. This landmass was not part of Japan's main island, Honshu. Around 20 million years ago, it began as a collection of submarine volcanoes far to the south, sitting on the Philippine Sea Plate. As this plate moved northwards, it carried the volcanic islands with it, on a collision course with Honshu. The initial impact occurred around 15 million years ago, and the main collision that formed the peninsula as we see it today happened about 600,000 years ago. This process is unique; the Izu Peninsula is the only place on Earth where two active volcanic island arcs are colliding.
This geological crash is not a simple subduction, where one plate slides neatly under another. The Izu volcanic arc is too buoyant to be easily forced down. As it slams into Honshu, pieces of the oceanic plate—including ancient submarine volcanoes—are scraped off and thrust upwards, becoming part of the land. This process stranded sections of the deep ocean floor on the Japanese mainland. The evidence is visible in the rocks. Exposures of pillow lava, which only form when basaltic lava erupts underwater, can be found on the peninsula, such as at Ishiki in Nishi-Izu Town. These rounded, pillow-shaped rocks are a direct record of the area's submarine volcanic past.
An Ophiolite on Display
The collection of rocks exposed by this collision forms what geologists call an ophiolite. An ophiolite is a sequence of rocks representing a complete slice of oceanic crust and the upper part of the Earth's mantle. Usually, this sequence is hidden deep beneath the oceans. On Izu, however, portions of this ophiolitic sequence are accessible on land. This includes the pillow lavas that formed the surface of the ancient seafloor, but also the underlying structures like magmatic dikes—the vertical channels that once fed magma to the underwater eruptions.
The collision continues today. The Philippine Sea Plate pushes northwestward, causing the Izu block to press into Honshu. This constant pressure creates significant geological activity. The region is defined by features like the Tanna Fault, a major left-lateral strike-slip fault that runs along the peninsula's northeast side. This fault was responsible for the magnitude 7.0 North Izu earthquake in 1930. Geological studies show that over its history, the two sides of the Tanna Fault have shifted by about 1 kilometer relative to each other. This ongoing tectonic activity, with its earthquakes and hot springs, is caused by the immense forces that first brought submarine lavas to the surface.