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What is a Sea Stack?

Mary McMahon
Updated Mar 05, 2024
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A sea stack is a pillar of rock found in the water close to a coastline. Sea stacks are common along many of the world's coastlines, and some have even become quite famous. Like other features found along shorelines, sea stacks are also in a constant state of flux, with new stacks emerging all the time while old ones disappear. Some sea stacks have been known to erode into very unusual and striking formations, making them popular subjects for photographers and painters.

A sea stack is caused by the natural erosion of headlands along the coastline. Typically, the ocean wears a hole through the headlands first, creating an arch which slowly expands over time as it erodes. Ultimately, the arch collapses, leaving a sea stack on one side and the headland on the other. Separated from the shoreline, the sea stack will slowly start to erode, ultimately melting away into the water or collapsing.

Essentially, a sea stack is like a very small island, and in some cases, sea stacks have actually started out as islands which have been worn away. Many migratory birds use sea stacks for nesting and shelter, appreciating their isolation and relative safety. Sea stacks are also popular among rock climbers, since many such stacks pose interesting and fun climbing challenges.

The distribution of sea stacks in an area varies, depending on the kind of rock the headlands are formed from, ambient weather conditions, and the prevailing currents of the water. In some cases, an area may be littered with sea stacks made from very hard stone, while in other instances a shoreline has only a few stacks composed of soft, crumbly materials like limestone and sandstone. Since many headlands are formed from the former ocean floor, some sea stacks also reveal interesting fossil remains as they erode.

Some caution is advised when one is around sea stacks. It is possible for stacks to collapse unexpectedly, to the detriment of anyone who happens to be standing or boating nearby. When climbing sea stacks, it is a good idea to watch out for soft, crumbly rock which could give way under the weight of a climber, and to avoid especially narrow, spindly sea stacks as they can be very fragile. In addition, because sea stacks are used as nesting areas by birds, access to stacks may be restricted by conservation agencies in the interest of protecting the birds.

Frequently Asked Questions

What is a sea stack and how does it form?

A sea stack is a vertical column of rock in the sea, formed by the erosion of cliffs by waves. Over time, the relentless pounding of the ocean carves out caves in a headland; these caves eventually become arches when they break through to the other side. When the arch collapses, it leaves a stack isolated from the shore. This geological process can take thousands of years.

Can sea stacks be found all over the world?

Yes, sea stacks are found on coastlines around the world. They are particularly common along areas of coastline with strong waves and where the rock type is prone to erosion, such as sedimentary or volcanic rock. Famous examples include the Twelve Apostles in Australia and Old Harry Rocks in England.

Are sea stacks important for local ecosystems?

Sea stacks play a crucial role in local ecosystems. They often serve as nesting grounds for seabirds and can act as safe havens for marine life, providing shelter from strong currents and predators. Their unique structure can also influence local biodiversity, supporting a range of plant and animal species.

How long do sea stacks last?

The lifespan of a sea stack varies greatly and depends on the material it's made of and the environmental conditions it's exposed to. While some may last only a few years, others can endure for centuries. However, all sea stacks are temporary features on a geological timescale, as erosion will eventually cause them to collapse.

What are some of the largest sea stacks in the world?

Some of the largest sea stacks in the world include the Old Man of Hoy in Scotland, which stands at about 450 feet tall, and the Kicker Rock off the coast of the Galápagos Islands, which rises approximately 500 feet above the ocean surface. These impressive natural monuments are testaments to the power of erosive forces over time.

Can sea stacks be dangerous to navigate around?

Navigating near sea stacks can be dangerous due to the rough waters and sharp rocks. They are often surrounded by treacherous waves and strong currents that can pose a risk to boats. Additionally, the base of sea stacks can be unstable due to ongoing erosion, making it hazardous for climbers or explorers on foot.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon286431 — On Aug 21, 2012

What are some characteristics of all stacks? I would really like to know.

By Chappie — On Mar 20, 2012

Thanks for the info! I was delighted to find it, as I was working on a poem I wrote some 12-14 years ago when I lived in WA state, near the coast. I'd watched a rain storm moving in from the ocean, and mentioned "the stacks" -- but for the life of me, I could not remember what stacks were.

By PelesTears — On Aug 21, 2010

@ Aplenty- I have been to the Sonoma Coast and I agree that it is beautiful. The article stated that people like to climb sea stacks, but the ones in this area are quite dangerous to climb. The sea arches and sea stacks in the area are all made of shale, making them very unstable to climb. The park service employees warned us about swimming and climbing the rocks, saying they have to rescue numerous people every year. The ranger said they rescue people who get pulled out to sea, beachcombers attacked by seals (sea lions?), and climbers stuck on rocks when the tide comes in a few times a year.

By aplenty — On Aug 21, 2010

I used to vacation In Bodega Bay in California as a kid, and we would often go to shell beach and Goat Rock Park. Goat Rock is nice in the morning fog, and I assume it qualifies as a sea stack. There are also many other sea stacks, sea arches, caves, and sea cliffs in the area.

It is an awesome place to explore, but I remember swimming to be particularly dangerous. The water is riddled with White sharks, seals, sea lions, rip currents, and kelp beds. The water is also dark and cold (not very inviting to a young child).

Beach combing in the area is fun though. There are plenty of tide pools to explore, shells galore, and tons of washed up seaweed to scare a little sister. I plan on taking my daughter to the same beaches when she is a little older.

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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