Coastal sea caves form where the ocean hollows out the land. Waves have incalculable force, and fling themselves ashore in a tireless pounding rhythm. They carry grit and pebbles that slowly scour away solid rock. The ocean finds entry along bands of soft sandstone or in limestone made of layered shells of sea creatures. Caves in marble, metamorphosed limestone, build more slowly, but last longer. Sea caves in granite or basalt are extremely durable, and can grow huge.
Striking sea caves
The California coast hides many coastal sea caves, notably near the city of Santa Cruz, in Big Sur and along the cliffs of La Jolla. The California Channel Islands are heavily faulted and highly susceptible to erosion. Several of the islands are ringed with caves, and some are touched with arches. Painted Cave on Santa Cruz Island is colored with rock and algae. It is over a quarter mile long, and its entry is the site of a waterfall in winter and early spring.
The Shetland Islands are riddled with fascinating caves, but Fingal's Cave is to the south, in the Inner Hebrides of Scotland. It formed in jointed basalt rock, and stands on the uninhabited island of Staffa. The waves echo against its high-arched ceiling ceaselessly, the sound changing pitch and tempo with wind and tide.
The Isle of Capri has been a holiday destination for centuries. Visitors tour its Blue Grotto to see the astounding blue of the water and the silver shadows of the cave. The light in the grotto has traveled far under the water, which has absorbed all the light wavelengths longer than blue. On the nearby island of Ischia is the green Magician's Grotto.
The Hawaiian island of Kauai holds ancient volcanic cliffs, the Na Pali. Centuries of waves have carved a chain of caves where the cliffs meet the sea. They are inaccessible except from the water. Waterfalls ornament the caves, and some hold skylights open to the sun. Colorful fish, seabirds and sharks shelter in some. These grottoes can only be visited when the weather is right and only by small boat, making a visit a rare and precious experience.
The formation of sea caves
Some sea caves occur where rock is faulted; others form between layers of different types of rock. Still other seeming sea caves are really lava tubes. Lava flows crusted over as they ran down hill and cooled, but the interiors of the flows stayed hot and kept running toward the sea.
Littoral caves are sea caves at sea level. If their roofs collapse, they may become blowholes, where ocean water escapes into the air. If the sea level changes, they may become relict caves, set on a raised sea terrace and cut off from their source.
Underwater caves can be visited only by divers. They are beautiful, dangerous and seldom seen. Coastal sea caves can be dangerous, too, if the wind or weather changes, not to mention the tide. Yet they are some of the most breathtaking places on earth.