Jeju’s coastline is one of the most diverse in the world. From black-sand beaches to white-sand and blue-water coves, to rough and rocky basalt cliff faces and rolling black shores, this little island is a geological rainbow of phenomena.
A section of Jeju’s coastline that has caught the eye of many geologists and sightseeing adventurists is the Daepo-dong coast in Jungmun, Seogwipo City. Here you can find well-developed columnar joints formed by the spewing of lava from the Nokhajiak oreum (volcanic cone) about 140,000 to 250,000 years ago.
Columnar joints are naturally formed heptagonal, hexagonal or square vertical pillars made from rapidly cooling lava. There are two types of lava: Pahoehoe lava, which is water-like and flows quickly, and Aa lava, which is thick and viscous, and flows slowly like honey, says Dr. Jeon Yong Mun of the World Natural Heritage Management Bureau.
Pahoehoe lava is what forms lava tubes, while Aa lava is what forms columnar joints, says Dr. Jeon.
Typical Aa lava is about 10 metres thick, says Dr. Jeon. If it cools rapidly to about 900 Celsius, it contracts, or shrinks and shrivels. This contracting causes the rock to crack. Depending on the direction of the crack or split, the joints are divided into horizontal layers, called sheet joints, or vertical columnar joints.
Columnar joints are developed when the lava flow’s surface cools, specifically, from the top down, as well as the bottom up, meeting in the centre, sometimes haphazardly or only slightly irregularly, like the touching of finger tips, says Dr. Jeon. The Daepo Columnar Joint is, in general, slightly irregular. The top cooled layer of columns is about six metres thick, while the bottom cooled layer of columns is about 4 metres thick. These parallel columns are called “colonnade,” while the middle section where the columns meet is called “entablatures,” says Dr. Jeon.
However, as the columns get taller the hexagonal, heptagonal or square cracks at the surface collapse and melt into a rough surface called “clinker,” Dr. Jeon explained.
This does not explain, however, how the columns get their heptagonal, hexagonal and squared shapes. One explanation is as follows: if the lava cools evenly on a flat surface, the cracks will grow at right angles to the surface, developing cracks at 120 degrees to each other. This leads to hexagonal columns. However, this still doesn’t said how they form on angled surfaces. Dr. Jeon explains that naturally all minerals when heated or dried will crystallize, or take on crystaline shapes. This is the case of cooled lava.
Until the 18th century, it was believed that symmetrical and hexagonal basalt (lava rock) columns were crystallized only by water. By the middle of the century, however, volcanologists discovered that the crystallization of lava flows occurred as a result of cooling.
The columnar joints stretch for two kilometres along the Daepo-dong coast. There are other clusters of columnar joints along Jeju’s coast, but none as well-developed at those along Seogwipo’s coastline, says Dr. Jeon.
Legend has it that since erosion from the ocean has created natural stairs out of some of the columnar joints, it has been given the name “Chisatgae Haean,” which means “Altar of the Gods.” In the past, people held services here for the island’s 18,000 gods.
Aside from the columnar joints being a geological masterpiece, they are also aesthetically sublime and wondrous. It is as though you are looking at massive black crystals being washed continuously by the ocean’s blue, crisp water, splashing up over the columns like volcanic explosions, and trickling down vein-like through the columns.
The best time to see the columnar joints is on a rainy day or a particularly windy day because there are fewer curious tourists to navigate through, and the wind makes for larger waves.
It costs only 1,000 won to visit the site. To get there from Jeju City you must take a bus to the Jungmun bus terminal, then take a taxi from there or take a 20-minute stroll from the bus terminal, walking due east. It is located just past the International Convention Center Jeju. <Jeju Weekly>
<Sarah Warren contributor@jejuweekly.comⓒ Jeju Weekly All rights reserved>
