The EarthScience Lesson – Formation of Sea Caves
Sea caves are created primarily through marine erosion, the process by which waves wear away rock along a coastline.
The formation of a sea cave usually begins with a natural weakness in the rock. These weaknesses may include:
fractures or joints in the rock
bedding planes between rock layers
areas where the rock is softer or more easily eroded
When waves repeatedly impact these weak areas, two main processes gradually enlarge them.
Hydraulic Action
When waves crash against the cliff, water and air are forced into cracks within the rock. The pressure from this water can widen fractures and eventually break pieces of rock away.
Abrasion
Waves also carry sand, coral fragments, and small stones. These materials grind against the rock surface, wearing it away in a process similar to sandpaper.
Why Weaknesses Matter
Sea caves rarely form randomly in solid rock. Instead, they usually develop where the rock already contains a structural weakness, such as a fracture or joint. Waves repeatedly exploit these weaknesses because fractured rock breaks apart more easily than solid rock.
Over time the opening widens as wave energy removes loose material, eventually forming a cave.
At this location the cave likely formed along a fracture or weaker section of the cliff, allowing waves to carve deeper into the rock.
Bedding Planes
Many sedimentary rocks form in layers called bedding planes. These layers develop as sediments are deposited over time and later turn into rock.
Because bedding planes are boundaries between layers, they can be weaker than the surrounding rock. Waves often exploit these weaknesses during coastal erosion.
At this location on Antigua, the visible rock layers may help guide where erosion occurs. If wave action attacks these weaker planes, it can gradually enlarge the opening and contribute to the formation of a sea cave.
Diagram – Formation of a Wave-Cut Cave
Stage 1 – Fractured Cliff
Cliff
│
│
│
~~~~~~~│~~~~~~~ Waves
│
Stage 2 – Wave Erosion
Cliff
│
( )
~~~~~~( )~~~~~~ Waves
Small cave
Stage 3 – Wave-Cut Cave
Cliff
│
( )
~~~~( )~~~ Waves
Logging Tasks
To log this EarthCache, visit the posted coordinates and carefully observe the coastal rock and sea cave.
1. Cave Shape and Wave Erosion
Observe the entrance of the sea cave.
Describe the shape of the cave opening (for example: rounded, narrow, wide, irregular).
Explain how repeated wave erosion could create this shape.
2. Location of Strongest Erosion
Look carefully at the cave and surroundingrock.
Where is the strongest erosion visible?
near the base of the cave
along the sides
near the roof
Explain why wave energy causes stronger erosion in this location.
3. Rock Layers and CaveFormation
Examine the cliff around the cave entrance.
Do you see horizontal rock layers (bedding) in the cliff?
Estimate the approximate thickness of one layer in centimeters.
Explain how these layers may influence where the sea cave formed.4. Rock Weakness and Cave Development
Sea caves often form where the rock contains a natural weakness.
Do you observe any cracks, fractures, or weaker sections of rock near the cave entrance?
Describe where they appear and explain how wave action could enlarge these weaknesses into a cave.
Optional Photo Requirement
You may include a photo of yourself or a personal item with the cave visible in the background. Please ensure no answers are revealed in the photo.
Send your answers to the cache owner before logging your find.
References
Bird, E. (2008). Coastal Geomorphology: An Introduction. Wiley.
U.S. Geological Survey (USGS) – Coastal Erosion and Sea Caves.
https://www.usgs.gov�
National Geographic Society – Coastal Landforms and Sea Cave Formation.
https://www.nationalgeographic.org�