What is a Sea Stack and How is It Formed?

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What is a Sea Stack and How is It Formed?

A sea stack is one of the several beautiful and awe-inspiring creations of nature. In this ScienceStruck article, we shall tell you what it is and how it is formed.


Risin og Kellingin, which means the ‘giant and the witch’, as a matter of fact, are two sea stacks located just off the northern coast of the island of Eysturoy in the Faroe Islands. Legend has it that they were actually a giant and his wife (witch), who got turned into stone on exposure to the sun as they were trying to bring the Faroe Islands to Iceland. They have stood there ever since, longingly looking across the ocean towards Iceland.

Go to a beach and you will realize how small we truly are. And it’s not just the vast and endless expanse of the sea or ocean that will inspire awe, sometimes you might come across some natural structures that will simply seem other-worldly. One such structure that you are likely to find in many of the famous beaches around the world is a sea stack.

What is a Sea Stack?

By definition, a sea stack is a naturally occurring pillar made up of stacked rocks, that projects up towards the sky. It is usually found surrounded by water, especially during high tide, isolated and separate from the mainland mass at a small distance away. During low tide, the water recedes far enough so that some sea stacks become accessible via the beach.

Standing at the foot of a sea stack, looking up at that magnificent, yet oddly placed rock formation, one is likely to wonder how exactly did it get there? While the possibility of a divine hand at work might seem alluring, the truth is actually a little more interesting. These grand structures of rocks are actually created by the erosive powers of waves and winds.

Sea Stack Formation

How sea stack is formed?

Sea stacks are formed from headlands. A headland is a coastal land-form that is quite high, and has a sheer drop that extends out into the sea or ocean. Parts of the headland that jut out into the water slowly get eroded over time by the mechanical energy of winds and waves. Essentially, the softer and weaker part of the rocks get eroded and break away, leaving behind the harder and stronger rock.

The formation process usually begins when water starts smashing against the portion of the headland rock that is jutting out, causing it to slowly erode. Coastal winds too assist in this process. Both, waves and winds chisel away at the rocks continuously for many years, until finally, a cave is formed.

Cave-formation is the first step towards the development of a sea stack. It acts like a chink in the headland’s armor – a weak spot which is most likely to give way and break. So, the waves and winds continue to smash into the cave, cracking and breaking away its rocky interior. During storms especially, the combined force of winds and waves pounds heavily at it, until eventually, they drill a hole through the back of the cave. What remains of the cave then is just the overhead arched section. At this point, the second step in the formation of the sea stack is complete.

In the third and final step, waves and winds erode and break down the rocky arch as well, and finally, an isolated vertical stack or pillar of rocks, known as the sea stack, is all that is left.

It is interesting to note that, the destructive powers that create a sea stack are also the ones that eventually destroy it. The waves and winds continue to erode the remaining hard rock that comprise the surface and structure of the sea stack. It is a process that may take many years. But eventually, the outcome is that the structure becomes too weak, and the sea stack collapses under its own weight.

Significance of Sea Stacks

Apart from being a sight to behold, sea stacks also serve many important purposes. They are basically like small islands, isolated and separated from the mainland. As such, due their relative safety, many migratory birds use them for nesting and as a shelter.

The number and distribution of sea stacks in a given coastal area is an indicator of the geology and weather conditions there. Headlands made up of soft and crumbly rocks like limestone and sandstone, tend to have only a few (if any) sea stacks. On the other hand, headlands mainly comprising hard rocks may be littered with several, usually closely spaced sea stacks.

Many headlands were formally ocean floors. So when they get eroded and sea stacks are formed, they reveal interesting sea-life fossil remains.

Sea stacks also are an important attraction for tourists, bird watchers, as well as for rock climbers looking for challenging climbs. However, since they are constantly in a state of flux, and are sure to collapse someday, caution should be exercised while climbing or getting close to one.

Sea Stack Images and Examples

Here are some spectacular sea stacks found in different locations around the world.

1) Risin og Kellingin, Eysturoy Island, Faroe Islands

2) Bako Sea Stack, Borneo, Malaysia

3) Sail Rock, The Black Sea, Russia

4) Old Man of Hoy, Hoy Island, Scotland

5) Koh Poda Rock, Krabi, Thailand

6) Ball’s Pyramid, near Lord Howe Island, Australia

7) Sea stack at Torre Sant’Andrea, Torre Sant’Andrea, Italy 

So we can conclude that sea stacks are nothing but pillars of rocks that are found in many coastal areas around the world. They are formed due to rock erosion caused by waves and winds. They have importance in their natural surroundings, and thanks to their scenic appeal, most of them have become popular tourist attractions. 

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