Sea Level Changes Supercontinents And Life

sea level has changed by about a thousand feet (hundreds of meters) above and below current levels many times in Earth history. In fact, sea level constantly changes in response to a number of different variables, many of them related to plate tectonics. The diversity of fauna on the globe closely relates to sea levels, with greater diversity during sea level high stands, and lower diversity during sea level lows. For instance, sea level was 1,970 feet (600 m) higher than now during the Ordovician, and the sea level high stand was associated with a biotic explosion. sea levels reached a low stand at the end of the Permian, and this low was associated with a great mass extinction. sea levels rose again in the Cretaceous.

sea levels change at different rates and amounts in response to changes in several other Earth systems. Local tectonic effects may mimic sea level changes through regional subsidence or uplift, and these effects must be taken into account and filtered out when trying to deduce ancient, global (eustatic) sea level changes. The global volume of the mid-ocean ridges can change dramatically, either by increasing the total length of ridges or changing the rate of sea-floor spreading. The total length of ridges typically increases during continental breakup, since conti-

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Sea level has risen and fallen dramatically through Earth history. Global sea level rise and fall must be separated from local subsidence and uplift events along individual coastlines, by correlating events between different continents. Global eustatic sea level curves (Vail curves) also show the height of the world's oceans after local effects have been removed.

nents are being rifted apart and some continental rifts can evolve into mid-ocean ridges. Additionally, if seafloor spreading rates are increased, the amount of young, topographically elevated ridges is increased relative to the slower, older, topographically lower ridges that occupy a smaller volume. If the volume of the ridges increases by either mechanism, then a volume of water equal to the increased ridge volume is displaced and sea level rises, inundating the continents. Changes in ridge volume are able to change sea levels positively or negatively by about 985 feet (300 m) from present values, at rates of about 0.4 inch (1 cm) every 1,000 years.

Continent-continent collisions, such as those associated with supercontinent formation, can lower sea levels by reducing the area of the continents. When continents collide, mountains and plateaus are uplifted, and the amount of material that is taken from below sea level to higher elevations no longer displaces seawater, causing sea levels to drop. The contemporaneous India-Asia collision has caused sea levels to drop by 33 feet (10 m).

Other factors such as mid-plate volcanism can also change sea levels. The Hawaiian Islands are hot-spot style, mid-plate volcanoes that have been erupted onto the seafloor, displacing an amount of water equal to their volume. Although this effect is not large at present, at some periods in Earth history many more hot spots existed (such as in the Cretaceous), and the effect would have been larger.

The effects of the supercontinent cycle on sea level may be summarized as follows: Continent assembly favors regression, whereas continental fragmentation and dispersal favors transgression. Regressions followed formation of the supercontinents of Rodinia and Pangaea, whereas transgressions followed the fragmentation of Rodinia and the Jurassic-Cretaceous breakup of Pangaea.

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Responses

  • Fiori
    Did the water level reduce when the continents split?
    2 years ago

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