Sea level has changed by hundreds of meters above and
below current levels at many times in Earth history. In fact,
sea level is constantly changing in response to a number of
different variables, many of them related to plate tectonics.
The diversity of fauna on the globe is closely related 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 were high again in the
Cretaceous.
Sea levels may 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 midocean
ridges can change dramatically, either by increasing the
total length of ridges, or by changing the rate of seafloor
spreading. The total length of ridges typically increases during
continental breakup, since continents 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
inches (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 things, 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 there were many more hot spots (such as in the
Cretaceous), and the effect may 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 favor
transgression. Regressions followed formation of the supercontinents
of Rodinia and Pangea, whereas transgressions
followed the fragmentation of Rodinia, and the Jurassic-Cretaceous
breakup of Pangea.
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