The great tectonic plates of Asia and the Pacific Rim shaped and formed the landscape of the Far East millions of years ago. The Indo-Australian Plate pushed beneath the Eurasian Plate to lift the massive Himalayan Mountains skyward. The twisting movement of the Philippine Plate caused the landscape of eastern Asia to sink, creating the shallow Yellow Sea and the East China Sea. The Pacific Plate and the great North American Plate pressed north and west along the Pacific Rim and forced widespread areas of eastern Asia to tilt westward. Beyond the Asian continental shelf the Japanese archipelago emerged from the waters of the Pacific, sitting apart from the Eurasian landmass like a mirror image of the British Isles. North of the Japanese Islands, near the eastern extent of continental Asia, the earth's crust shifted and buckled, thrusting its granite foundation upward to create the Korean peninsula, a jagged land mass that stretches nearly 966 kilometers (595 miles) southward from the Asian continent.
Situated within the cordilleran belt that stretches across the northern Pacific Rim, the entire peninsula rests on a granite foundation. Until about 245 million years ago most of the Korean Peninsula was little more than a weathered and eroded lowland at the eastern extent of the Asian continent. As geologic forces began twisting and tilting the land, the ground heaved skyward in the east to form a series of ragged mountain ranges. In the west and southwest, the landscape actually sank, leaving the western and southern landscape dominated by low, hilly terrain with gradual slopes, open plains, winding river basins, and jagged, irregular coastlines. Over time, many of the smaller mountains in this region disappeared beneath the waters of the Yellow Sea leaving the thousands of small islands that now grace Korea's south and west coasts. Most of the 3,479 islands that surround the peninsula are actually the tops of ancient ridges and mountains.
There is very little flat terrain on the Korean Peninsula and few places are totally out of sight of mountains. Nearly 80% of the country is characterized by hills and rugged mountains, many of which were born from volcanic activity. The Kaema Plateau lords over Korea's steep and rugged northern highlands, rising to an average height of 1,500 meters. The beautiful 2,744 meter granite peak Paektu-san dominates the geography of the Kaema Plateau like a formidable guardian of the northeastern approaches to the peninsula. The geologic upheavals that gave birth to the peninsula formed two principal mountain ranges rarely exceeding a height of 1,200 meters: the craggy, steep terrain of the Nangnim Sanmaek range in the north and the T'aebaek Sanmaek range in the south. Taken together, this spectacular panorama of granite pinnacles and deep narrow canyons with many waterfalls and rapids creates a ragged spine running from north to south that forms Korea's geological backbone and constitutes the drainage divide between the western and the eastern slopes
. A number of smaller mountain ranges originating in the Nangnim and the T'aebaek ranges run parallel to each other in a generally northeast-to-southwest direction. These two mountain ranges have been a great barrier to communication between eastern and western Korea since early times.
The terrain is a spectacular panorama of sculpted granite and rocky pinnacles that pierce the sky. Deep, narrow canyons with nearly vertical walls are common, carved by fast running streams with numerous waterfalls and rapids. Those streams that flow eastward from the T'aebaek Divide are short, straight and fast, cascading rapidly toward the ocean. Sand spits and sand bars enclose the coastal lagoons frequently found at the mouth of these streams, a situation that produced many fine harbors and broad, clean beaches. Korea's eastern sea coast, where the T'aebaek Sanmaek mountain range rises from the East Sea, is a nearly unbroken shoreline of precipitous cliffs and rocky outcroppings.
Limestone deposits are quite common in Korea, and a number of spectacular caves can be found on the peninsula, all with stalagmites and stalactites, ponds, and streams. The most famous of these caves is T'ongnyonggul, near Yongbyon on the southern side of the Chongchon River in North Korea. This massive cave is about 5 kilometers long and several of its chambers are nearly 150 m wide and 50 m high. Among the most famous limestone caves in South Korea are Kossigul, Kosudonggul, Songryugul, and Hwangsonggul.
Only about 15% of Korea is not considered mountainous. The southern and western coastal regions, where most of the country's farmland and population are located, are comprised of rolling plains, low hills, and wide river basins. Once or twice each year, during the summer and early autumn, heavy rains and the occasional typhoon swell rivers and streams, triggering disastrous floods that inundate low-lying valley plains. During the dry seasons, many of these same streams shrink in size exposing much of the stream bed.
Five major rivers and a relatively large number of minor streams drain the Korean Peninsula, most of which flow westward to the Yellow Sea and south to the Korea Strait after draining the gentler western and southern slopes of the peninsula. There are no significant rivers or plains along Korea's east coast, where high mountains and steep terrain are close to the sea. Korea's mightiest river is the Yalu River (Amnok-gang). Born on the snowy slopes of Paektu-san, the Yalu flows westward for nearly 790 kilometers (482 miles) to the northern coast, where it empties into Korea Bay. The snow melt of Paektu-san also feeds the Tumen River (Tumen-gang), a 521 kilometer (318 mile) cascade that empties eastward into the East Sea at the northeastern-most point of the Korean Peninsula.
South Korea's four largest waterways, the Han, Kum, Naktong, and Somjin Rivers follow a gradual descent to the west and south. The Han River flows westward from the foothills of the T'aebaek Divide for nearly 514 kilometers (314 miles) before reaching its mouth in the Yellow Sea. The Kum River (Kum-gang), meanders nearly 401 kilometers (245 miles) as it drains the southwestern peninsula, first flowing north from the low country of southern Korea, then turning west and finally south to the Yellow Sea. The Naktong River (Naktong-gang) winds for 521 kilometers (318 miles) from the southern end of the T'aebaek mountains and empties into the East Sea at the southeastern corner of the peninsula. The gradual descent to the west and south of Korea's major rivers and streams makes for wider waterways and slower currents, permitting inland river navigation for long distances, which is very important for internal commerce.