Earth Science: Tides, Coasts, and Waves
The continental coastline of the Earth is about 250,000 miles (440,000 km) long. But the area of transition between the ocean and land is constantly changing due to the tides and surge.
Coastal landscapes are shaped over time by the destructive forces of the seas that slowly but steadily eat away at rocky shores. These forces, in combination with weathering and surface-level erosion, cause shorelines to retreat further inland. Waves and currents move large amounts of sand along shallow and sandy shorelines, thereby creating sandbanks that constantly change position.
In other areas, rivers carry sediment from the mainland into the sea, where it is deposited in sediment plumes by the mouth of the river. Coral reefs can also expand from coastal areas into the ocean and thus transform the shoreline. The fluctuations of the sea level, or rather the rise and lowering of the mainland, are of particular importance. The so-called isostatic adjustment of areas that were once covered by large ice sheets is still occurring.
Due to the persistent rise in the sea level, beaches and cliffs continue to be shifted inland while in other regions landmasses are sinking over time. The ocean penetrates valleys creating irregular coasts, or floods glacial troughs in the instance of fjord coastlines. Most coastlines are still very young due to the sea-level rise of approximately 51 inches (130 cm) over the past 18,000 years.
The gravitational pull of the moon together with the centrifugal force of the Earth create a tidal bulge on the ocean surface facing the moon. The same occurrence happens on the opposite side of the Earth, except that there, the Earth’s centrifugal force is the only cause of bulging water. A depression forms between the two tidal bulges and due to the Earth’s rotation, the sea levels rise and fall twice a day as described.
The ocean’s retreat to its lowest point is called the ebb, and the period of rise in sea level is called a flood. The tides’ amplitudes vary from region to region. For example, the narrow bays on the east coast of Canada act together as a funnel, so that the tidal range between high and low tides in the Bay of Fundy is 49 feet (15 m). By contrast, the tidal range along the North Sea coast only reaches a maximum of 11.5 feet (3.5 m).
WAVES
Most waves are created by wind Waves often travel across the open sea in the form of small swells until they break on the shore. Most waves reach a height of about ten feet (3 m). in exceptional cases, they can be over 98 feet (30 m) high.
Waves are measured from the top, or crest, of the wave to the base of the trough. The length is the distance between wave crests.