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Large waves crashing onto a shore bring a tremendous amount of energy that has a significant eroding effect, and several unique erosion features commonly form on rocky shores with strong waves.

When waves approach an irregular shore, they are slowed down to varying degrees, depending on differences in the water depth, and as they slow, they are bent or refracted. That energy is evenly spaced out in the deep water, but because of refraction, the energy of the waves, which moves perpendicular to the wave crests, is being focused on the headlands. On irregular coasts, the headlands receive much more wave energy than the intervening bays, and thus they are more strongly eroded. The result of this is coastal straightening. An irregular coast, like the west coast of Vancouver Island, will eventually become straightened, although that process will take millions of years.

The approach of waves (white lines) in the Cox Bay area of Long Beach, Vancouver Island. The red arrows represent wave energy; most of that energy is focused on the headlands of Frank Island and Cox Point.

Wave erosion is greatest in the surf zone, where the wave base is impinging strongly on the sea floor and where the waves are breaking. The result is that the substrate in the surf zone is typically eroded to a flat surface known as a wave-cut platform, or wave-cut terrace. A wave-cut platform extends across the intertidal zone.

Relatively resistant rock that does not get completely eroded during the formation of a wave-cut platform will remain behind to form a stack. Here the different layers of the sedimentary rock have different resistance to erosion. The upper part of this stack is made up of rock that resisted erosion, and that rock has protected a small pedestal of underlying softer rock. The softer rock will eventually be eroded, and the big rock will become just another boulder on the beach.

Basalt sea stack in a black lava beach under the mountain in southern Iceland.

Arches and sea caves are related to stacks because they all form as a result of the erosion of relatively non-resistant rock.

Basalt sea cave on Akun Island.


Submarine canyons are narrow and deep canyons located in the marine environment on continental shelves. They typically form at the mouths of sizeable landward river systems, both by cutting down into the continental shelf during times of low sea level and also by continual material slumping or flowing down from the mouth of the river or a delta. Underwater currents rich in sediment pass through the canyons, erode them and drain onto the ocean floor. Steep delta faces and underwater flows of sediments are released down the continental slope as underwater landslides, called turbidity flows. Erosive action of this type of flow continues to cut the canyon, and eventually, fan-shaped deposits develop on the ocean floor beyond the continental slope.



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Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Kategoria: Moje artykuły | Dodał: kolo (2019-04-09)
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