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Sea caves are found all over the world in many types of rock. Unlike inland caves, which are often formed by chemical solution processes, sea caves form by the mechanical erosion of the ocean. Waves can hit shorelines with blows averaging several hundred pounds per square inch, and where there are faults or seams in the rock that create a weakness, a cave can eventually develop. Since sea caves are created by the actions of the ocean, they can only extend inland as far as the water reaches. As a result, sea caves are often not very long, generally less than 50m, though some caves have exceeded 300m.
The existence of sea caves is not dependent on the kind of rock. However, it helps if the rock is weaker and less resistant to erosion, like sandstone. Typically sea caves are formed along a weakness in the rock, like faults, different sediments or weaker layers. Faults in the rock sometimes produce chains of caves everywhere the fault reaches the sea shore.
Pre-existing caves are opened by coastal erosion, and the water starts to widen the cave. Typically those caves are karst caves. Many karst areas at the coast have caves opening to the sea, where the entrance is widened by the waves. They are often called sea caves, which is only partly true.
Sandstone caves, like those at Thunder Cove, took millions of years to form. What is now dry land used to be covered with water. Sand on the bottom of the sea became pressed together and pushed upward. Water from the sea flowed away as this happened, causing caves to form in the cliffs. Over the years, moving water carried the loosened sandstone away, forming the large holes or caves inside the cliffs.
Sometimes faults, existing caves or weaknesses in the rock produce a small hole to the surface. The water swashing into the sea cave builds up a high pressure inside the cave, which forces water and air out of the small hole. This is called a blowhole. They are found all over the world along the coasts.
Thunder Cove doesn't contain a blowhole, but an excellent example can be found in Atlantic Canada at "The Ovens" near Lunenberg, Nova Scotia.
Several factors contribute to the development of relatively large sea caves. The nature of the zone of weakness itself is surely a factor, although difficult to quantify. A more readily observed factor is the situation of the cave’s entrance relative to prevailing sea conditions. At Santa Cruz Island, the largest caves face into the prevailing northwest swell conditions—a factor that also makes them more difficult to survey. Caves in well-protected bays sheltered from prevailing seas and winds tend to be smaller, as are caves in areas where the seas tend to be calmer. Thunder Cove's sea cave is not protected, facing the North Atlantic at an angle, so it can be expected enlarge with time as long as the surrounding coastal and cliff erosion is slower.
The type of host rock is important as well. All of the largest sea caves are in basalt, a relatively strong host rock compared to sedimentary rock. Basaltic caves can penetrate far into cliffs where most of the surface erodes relatively slowly. In weaker rock, erosion along a relative zone of weakness may not greatly outstrip that of the cliff face.
Time is another factor. The active littoral zone changes throughout geological time by an interplay between sea-level change and regional uplift. Recurrent ice ages during the Pleistocene have changed sea levels within a vertical range of some 200 meters. Significant sea caves have formed in the California Channel Islands that are now totally submerged by the rise in sea levels over the last 12 000 years. In regions of steady uplift, continual littoral erosion may produce sea caves of great height — Painted Cave is almost 40 m high at its entrance.
Finally, caves that are larger tend to be more complex. By far the majority of sea caves consist of a single passage or chamber. Those formed on faults tend to have canyon-like or angled passages that are very straight. In Seal Canyon Cave on Santa Cruz Island, entrance light is still visible from the back of the cave 189 m from the entrance. By contrast, caves formed along horizontal bedding planes tend to be wider with lower ceiling heights. In some areas, sea caves may have dry upper levels, lifted above the active littoral zone by regional uplift.
Physically, Thunder Cove's sea cave is non-complex, relatively small and shallow at this stage in its development. However, its sea-alignment and rock composition make it ideal for expansion over the coming millennia.
NOTE:
This earthcache must be accessed from the beach, near the parking coordinates. It is NOT accessible at high tide, so you must consult the local tide tables to plan your visit.
While visiting the sea cave, you should also visit Reference Points 1 ("Erosional Arch") and 2 ("Teapot")[also a Virtual Cache - "I'm a Little Teapot" (GC8A2Z)] to see two excellent examples of oceanic sandstone erosion.
To claim this Earthcache:
E-mail me the answers to the following questions:
1. Estimate the height and width of the opening of the cave.
2. What is the estimated height of the cave’s cliff?
3. What is the predominant rock the cliff is made of?
Do NOT post your answers on your log, encrypted or otherwise.
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