A continental separation
The Korean Peninsula was not always a peninsula. Until the geological epoch known as the Miocene, it was attached to the eastern edge of the Eurasian continent. The formation of the East Sea (Sea of Japan) began between the Late Oligocene and Middle Miocene epochs, roughly 28 to 13 million years ago. This separation was not a crack, but a complex process of "back-arc rifting."
The process was driven by the Pacific Plate subducting, or diving beneath, the Eurasian Plate. This movement caused the crust behind the main volcanic arc (the Japanese islands) to stretch and thin. This extension led to the formation of several deep basins, including the Ulleung Basin and Yamato Basin, which together form the floor of the modern East Sea. The rifting phase, marked by a rapid exhumation event, peaked around 23 million years ago. Apatite mineral data from the Daegwanryeong area in the Taebaek Mountains confirms this period of rapid uplift and cooling as the continental crust stretched and broke.
Scars on the landscape
The geologic forces that tore the peninsula from the mainland left dramatic evidence across eastern Korea. The most prominent feature is the Taebaek Mountain range, which runs for approximately 500 kilometers along the eastern coast. This range is the uplifted eastern "shoulder" of the ancient rift system. Its asymmetric shape, with a steep slope to the east and a gentler slope to the west, is a feature of rift shoulders. Major peaks like Seoraksan (1,708 meters) and Odaesan (1,563 meters) are part of this uplifted block of Jurassic granite and older gneiss.
Evidence of this continental stretching is found in the deep offshore basins. The Ulleung Basin, for instance, has a sedimentary layer over 10 kilometers thick, recording the history of the initial rifting and subsequent subsidence. The rifting also triggered volcanism. The islands of Ulleungdo and Dokdo are the tops of large stratovolcanoes that rose from the sea floor. Radiometric dating shows that Dokdo formed between 4.6 and 2.5 million years ago, while the main volcanic activity on Ulleungdo occurred more recently, from about 2.7 million years ago to as recently as 5,000 years ago. These islands are composed of alkaline volcanic rocks like trachyte and andesite, products of magma generated as the crust thinned.