emerge from the mountains. When the rivers are large enough to extend their courses to the sea, low deltaic plains develop in front of the fans; this occurs most frequently where the rivers empty into shallow and sheltered bays, as in the deltas of Kantō (Kwanto), Nōbi, and Ōsaka. In most places, however, fan surfaces plunge directly into the sea and are separated by low, sandy beach ridges.

Dissected plains are common. Intense disturbances have caused many former alluvial fans, deltas, and sea bottoms to be substantially uplifted to form flat-topped uplands such as those found in the Kantō Plain. Frequently the uplands have been overlain with volcanic ash, as in the Kantō and Tokachi plains.

Geologic framework

Fuji, MountEarl and Nazima Kowall/CorbisJapan is one of the world’s most geologically unstable(broad, gently sloping volcanic cones) are rare, and extensive lava plateaus are lacking. One of the characteristics of the volcanic areas is the prevalence of calderas (large, circular, basin-shaped volcanic depressions), especially in the northeast and southwest, many of which are filled with water, such as Lakes Kutcharo, Towada, and Ashi.

The cause of this instability—indeed, the reason for Japan’s existence—is the tectonic movement of several of the Earth’s major crustal plates in the vicinity of the archipelago. Most important is the subduction (sinking) of the Pacific Plate (in the north) and the Philippine Plate (in the south) beneath the Eurasian Plate, upon which Japan lies. The movements of these plates have formed six mountain arcs off the northeastern coast of Asia: from northeast to southwest, the Chishima Range