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  • Writer's pictureTom McAndrew

Coastal Erosional Features of Jeju Island

Updated: Mar 19

Suitable for A Level, IB, IGCSE, GCSE students and teachers of Geography

The Oedologae Sea Stack is a great example of a coastal erosional feature. It is

approximately 20 metres high and the pillar circumference is 7-10 metres. The

trachyandesite rocks are dense but a very porous, unlike basalt. Eruptions some 120,000

years ago led to the formation of this part of Jeju Island’s coastline. This grey coloured

volcanic rock is so porous that rainwater will seep through the surface leaving barely any

trace of surface rivers. Much of this freshwater pours and seeps into the sea through the

surrounding porous rock. This baseflow no doubt weakens joints and bedding planes along the coast and forms an interface of river hydrological erosion and coastal erosion.

The coastline around the Oedologae Sea Stack is a plunging cliff coastline type. The stack itself has been formed as waves refracted around a headland area in a lava zone. An inlet has been created by the erosion processes of hydraulic action, wave pounding under storm conditions and abrasion. Joints got widened into geos and widened further into caves. Two caves were eventually hollowed out to the point of meeting, forming an arch. The further erosion of the arch and widening of the space led to the unsupported rock falling into the sea. This material was further eroded and carried away by coastal transport processes.

Other processes are at work towards the top of Oedologae. Sub-aerial weathering, being

exposed to the wind and some frost during the winter weakens and breaks down material at the top of the stack. Salt crystalisation, especially during the summer as the salt crystals expand more with the increased temperatures, helps to further disintegrate the rock to create a thin soil layer. Once this has been colonized by grass and small shrubs, bio mechanical weathering occurs as roots weaken small cracks and widen them. Over time this is lowering the height of the stack. A large stump nearby is testament to a former stack that has been eroded to the point of collapse at its base.



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