Topography and Geology of the Seto Inland Sea
The Seto Inland Sea is surrounded by Japan’s main islands of Honshu, Shikoku, and Kyushu. It spans approximately 450 kilometers east to west, connecting the Sea of Japan with the Pacific Ocean through four straits (seto).
From land to sea
The topography of the Seto Inland Sea developed over millions of years through volcanic activity, tectonic movement, erosion, and changes in sea levels. During the last ice age, around 20,000 years ago, sea levels were around 130 meters lower than they are now. Fossils, including those of mammoths and deer, indicate that the area once supported terrestrial life. The sea developed as we know it today around 10,000 years ago when the ice melted, and sea levels rose.
Tidal currents and coastal habitats
The sea is relatively shallow, with an average depth of 38 meters and a maximum depth of 105 meters. Its interconnected basins and narrow straits amplify tidal currents, causing pronounced differences between high and low tides. This varied geography supports diverse coastal habitats, including mudflats and sandy beaches.
White sands
There are approximately 1,000 islands in the Seto Inland Sea, many of which are composed of granite. Erosion of this granite contributes to the white sands that define the coastlines of the region.