Sea stacks are natural rock towers that have been formed along the shoreline by erosion.
A sea stack is a rock pillar found in the water near the shoreline. Sea stacks are common along many of the world’s coastlines, and some have even become quite famous. Like other features found along the coast, sea stacks are also in a constant state of change, with new stacks appearing all the time while old ones disappear. Some sea pilings have been known to erode into very unusual and conspicuous formations, making them popular subjects for photographers and painters.
Some sea stacks started out as small islands that were worn away.
A sea stack is caused by natural erosion of headlands along the coast. Typically, the ocean first bores a hole through the headlands, creating an arch that slowly expands over time as it erodes. Eventually the arch collapses, leaving a mound of sea on one side and the promontory on the other. Cut off from the shoreline, the sea stack will begin to slowly erode, melt into the water, or collapse.
Essentially, a sea stack is like a very small island, and in some cases sea stacks actually started out as islands that were worn away. Many migratory birds use sea pilings for nesting and shelter, valuing their isolation and relative safety. Sea stacks are also popular with rock climbers, as many of these stacks present interesting and fun climbing challenges.
The distribution of sea stacks in an area varies depending on the type of rock from which the promontories are formed, the ambient climatic conditions, and the prevailing water currents. In some cases, an area may be filled with sea mounds made of very hard rock, while in other cases the shoreline has only a few mounds made of soft, brittle materials like limestone and sandstone. Since many promontories form from the ancient ocean floor, some sea stacks also reveal interesting fossil remains as they erode.
Some caution is advised when near sea stacks. Batteries may collapse unexpectedly, to the detriment of anyone standing or boating nearby. When climbing sea pilings, it’s a good idea to watch out for soft, brittle rocks that can sink under a climber’s weight, and to avoid particularly narrow and skinny sea pilings, as they can be very brittle. Also, since seamounts are used as nesting areas for birds, conservation agencies may restrict access to the mounds in order to protect the birds.