Sea cave

A sea cave, also known as a littoral cave, is a geological feature formed primarily by the wave action of the sea. These caves are created through a process called erosion, where the energy of the ocean wears down rocky coastlines. Sea caves are found globally along active coastlines. Some are also found as relict caves on former coastlines. For example, some large wave-cut caves in Norway sit 100 feet above the current sea level.

These are still classified as littoral caves despite their height. In places like Thailand's Phang Nga Bay, limestone caves were originally formed by dissolution. These caves were later flooded by rising seas and are now undergoing littoral erosion. This represents a new phase of enlargement for these hybrid caves.

The formation of a sea cave requires a specific starting point in the host rock. The rock must contain a weak zone where erosion can begin more quickly. In igneous or metamorphic rocks, this weak zone is often a fault or a dike. A fault is a break in the rock structure. In sedimentary rocks, the weakness might be a bedding-plane parting. This occurs at the contact between different layers of rock.

Waves attack these specific zones of weakness with immense power. As the sea reaches into these fissures, the cracks begin to widen and deepen. This happens because of the tremendous force exerted within a confined space. This force comes from the direct action of the surf and the compression of air within the cracks.

Several physical forces work together to expand these coastal openings. The waves carry rock particles that act as an abrasive force against the walls. This is similar to how sandpaper wears down a surface. Suspended sand and small rocks help grind away the stone. This process can create different types of cave interiors. Many sea cave walls are irregular and chunky because the rock is fractured piece by piece. However, some areas may have rounded and smoothed walls. This smoothing often happens when cobbles swirl in the surf zone along the cave floor.

This motion acts like a natural polisher on the rock.

Biological and chemical factors can also influence how these caves grow. Certain living things, such as sea urchins, can assist in the enlargement process. These animals drill into the rock to feed. Over many generations, they can remove a significant amount of bedrock from floors and lower walls. Additionally, rainwater can play a role in weakening the stone. Carbonic and organic acids can leach from the soil into the rock. These acids help weaken the rock within fissures, much like the process in inland solutional caves. Small speleothems, which are mineral deposits, can sometimes develop inside these sea caves.

Sea caves can drastically change the shape of the surrounding landscape. If a cave chamber collapses, it leaves behind a feature called a littoral sinkhole. Notable examples include the Devils Punch Bowl in Oregon. Sometimes, a cave may erode all the way through a small peninsula or headland. This creates a tunnel that allows waves to attack from both sides. If the roof of such a tunnel collapses, it can leave a free-standing rock known as a sea stack. The island of Anacapa in California was likely split into three separate islets by this process.

These changes show how much power the ocean holds over the land.

Research has revealed incredible records regarding the size and scale of these features. Until 2013, the largest known sea caves were located in the United States, Hawaii, and the Shetland Islands. In 2013, the Matainaka Cave on New Zealand's South Island was announced as the world's most extensive. It reaches 1.5 km in length and has 12 separate entrances. Another large complex was surveyed at Bethells Beach in New Zealand. In terms of volume, Rikoriko Cave in the Poor Knights Islands of New Zealand is the largest known. It has a volume of 221,494 cubic meters. Many other large caves likely exist in remote or hostile sea conditions.

Various factors determine whether a sea cave will become exceptionally large. The type of host rock is a major factor in cave development. Most large caves on the Western U.S. coast and Hawaii are formed in basalt. Basalt is a very strong igneous rock. Because it is so strong, it allows caves to penetrate deep into cliffs. This is different from weaker sedimentary rocks. Interestingly, the world's longest cave, Matainaka, formed in heavily fractured Caversham sandstone. This discovery changed our understanding of which rocks can support massive sea caves. Time and sea-level changes also play a vital role in shaping these coastal wonders.