Stages of Formation Stacks

Sea stacks are formed through erosion

Understanding the world around you, including geological formations such as sea stacks, can make the world a more interesting and meaningful place. Sea stacks are formed through a series of erosional process, where the power of the sea tears away at the rock on the coastline. These geological features occur in specific circumstances when the shape of the coastline means that waves are refracted around the headland.

The Sea Erodes the Cliffs

The first stage leading to the formation of a sea stack is the erosion of a headland, which projects into the sea in a sharp point. The power of the waves and the sediment it picks up, including sand and rocks, tears away at the cliffs like sandpaper does against wood. This makes part of the rock fall away from the cliff into the ocean.

Caves are Formed

The erosion of the headland by the ocean begins to form a hollow in the rock. It is where there are weaknesses in the rock of the structure of the cliff that caves are more likely to appear. The power of the waves and the sediment they throw at the headland forms a hollow, called a sea cave or littoral cave.

Caves Are Eroded Into a Sea Arch

Sea arches may occur where the headland projects into the sea in a sharp point. A cave forms on one side of the headland. As the waves and the rocks carried by the water continue to batter the walls of the cave, it becomes larger. Eventually, the back wall of the cave breaks through to the other side of the headland, creating a tunnel in the rock from one side of the headland to the other, which looks like an arch. This is called a sea arch and the rock that it makes it up is stronger and more structurally sound than the stone that was eroded away to form the cave, which is why it is left standing.

The Sea Arch Collapses

A sea arch is made of a pillar of rock in the sea, connected to the headland at the top by a piece of rock. Eventually, the piece of rock that links the pillar to the headland gives way, leaving just the pillar out at sea. This tower of rock is known as a sea stack and, as it too erodes, it may shorten and become known as a sea stump.

References

• University of California: Faults in Basalts: Sea Caves and Sea Stacks at Santa Cruz Island
• University of Wisconsin: Erosional Coastal Landforms
• University of Nevada, Las Vegas: Coastal Processes