Large areas of limestone exist throughout the central Sobaek mountains and eastwards to the T'aeback hills and coast. The deeper caves are in the Gongwean province. Jeju or Jejudo Island (known as Cheju until 2000 when a new style of transliteration was adopted) covers an area of 1845 sq km. It is unusual in Korea in that it is entirely volcanic. It first emerged from the North Pacific Ocean a little over a million years ago; the central volcano, Hallasan (Mount Halla, 1950 m) formed between 300 and 100 thousand years ago and the most recent volcanic activity was about 25,000 years ago, though Halla may have erupted as recently as 1570 AD. The lavas are mainly basaltic with some andesite, trachyte, tuff and, more recently, some sedimentary rocks and calcareous aeolian sands. The latter overlie many of the lava tubes which give rise to the karst speleothems.
The combination of the form of eruptions, the nature of the lava and the gentle slopes of the island have been conductive to the formation of lava tubes - some 84 having been so far documented by the Jeju Island Cave Research Institute (Son 2001). Jeju has a much wider range of volcanic features, ranging from the huge shield volcano, Halla, with its encircling parasitic cones or oreums, to spectacular columnar basalt and tuff sea-cliffs, cinder cones, volcanic plugs, craters, including maar or explosion crater, lava bombs and tree moulds.
Text by Tony Oldham (2004). With kind permission.