A wide area of the southern Izu Peninsula, including Nishi-Izu and Minami-Izu, formed from submarine volcanoes and volcanic islands that erupted before Izu collided with the main island of Honshu. Senganmon, which lies at the end of a walking path, and Mt. Eboshi, seen to the right of Senganmon, are parts of a volcanic neck—the exposed "magma conduits" that once lay beneath submarine volcanoes.

At Senganmon, a tunnel carved by the waves cuts through the center of the rock, forming a huge gate-like opening. Locals once likened this to the gate of Kumomi Sengen Shrine at the summit of Mt. Eboshi and called it Sengenmon, but it later became known as Senganmon, meaning it is worth a thousand kanme (an old measure of value), or "a gate worth a thousand coins."