Definition
A sea stack is a tall, column-shaped rock that stands alone in the water just off the coast, cut off from the cliff it once belonged to by powerful wave erosion. Famous examples in Korea include Chotdae Rock (Candlestick Rock) near Dokdo, Dumujin in Baengnyeongdo, and Oryukdo Islets in Busan.
The 3-Step Magic Trick of Waves
When strong waves pound a seaside cliff for ages, even hard rock starts to break down. Waves squeeze the air trapped in cracks and then release it with a burst, and pebbles grind against the rock. Together, these forces carve a deep notch at the base and slowly form a sea cave.
On a headland (a point of land sticking out into the sea), caves can be dug from both sides. When they finally break through and meet in the middle, you get a sea arch, shaped like an elephant's trunk or a rainbow bridge. At this stage, the arch is still linked to land through its roof.
But over time, waves keep gnawing under the bridge while wind and rain weaken the top. Eventually the roof can't hold its own weight and crashes into the sea. What's left is a freestanding stone pillar in the water, fully cut off from land. That striking rock is a sea stack.
Why Does It Stay Standing as a Pillar?
If waves wore down a cliff evenly, you'd end up with nothing but flat water. So why do some spots survive as tall pillars? The secret is differential erosion, which comes from differences in rock hardness and cracking.
A rock cliff looks like one solid block, but inside, it has many cracks (called joints) formed as the rock cooled or came under stress, and sometimes softer layers are mixed in. Waves attack those weak cracks and soft layers first and wear them away much faster.
Meanwhile, parts with few cracks and especially tough rock hold up against the waves' impact to the very end. After all the weaker rock around it has been ground away and swept into the sea, the strongest section is left standing alone as a majestic stone column.
To Be Precise: A Sculpture Made of Time, Not Forever
To be more precise, a sea stack isn't a finished sculpture. It's a snapshot of rock that is still being carved away. Even alone out at sea, a stack can't escape the waves crashing against it 24 hours a day or the salty wind.
Waves keep cutting at the base of the stack, and one day the pillar loses its balance and collapses into the sea. When a stack falls and leaves only a flat rock base, or a shape that barely shows at low tide, it's called a sea stump.
In the end, a hard coastal cliff turns into a cave, then an arch, then a stack, and finally a stump that breaks down into sand. It's a huge natural drama that the sea and rock act out over a very long time.
Common misconceptions
A sea stack is a volcanic island that rose from the middle of the sea from the start.
A sea stack isn't an island born from an undersea volcanic eruption. It's an erosion landform: waves wore away a coastal cliff on land and left only the hardest part standing alone.
Where you meet it
Chotdae Rock (Candlestick Rock) at Chuam Beach in Donghae, Gangwon Province, is a classic sea stack, rising to a point like a candlestick.
Samseon Rock, rising from the shore of Ulleungdo island, is also a sea stack shaped by wave erosion.
In one sentence
When waves wear away the weak parts of a coastal cliff, the toughest column of rock left behind is a sea stack.