Supercontinents and Climate
The motion of the tectonic plates periodically causes most of the continental landmasses of the planet to collide with each other, forming giant continents known as supercontinents. For much of the past several billion years, these supercontinents have alternately formed and broken up in a process called the supercontinent cycle. The last supercontinent was known as Pangaea, which broke up about 160 million years ago to form the present-day plates on the planet. Before that the previous supercontinent was Gondwana, which formed about 600-500 million years ago, and the one before that was Rodinia, formed around a billion years ago.
The distribution of landmasses and formation and breakup of supercontinents has dramatically influenced global and local climate on timescales of 100 million years, with cycles repeating for the past few billion years of Earth's history. The supercontinent cycle predicts that the planet should have periods of global warming associated with supercontinent breakup, and global cooling associated with supercontinent formation. The supercontinent cycle affects sea-level changes, initiates periods of global glaciation, changes the global climate from hothouse to icehouse conditions, and influences seawater salinity and nutrient supply. All of these consequences of plate tectonics have profound influences on life on Earth.
Sea level has changed by thousands of feet (hundreds of meters) above and below current levels many times during Earth's history. In fact, sea level is constantly changing in response to a number of different variables, many of them related to plate tectonics, the supercontinent cycle, and climate. sea level was 1,970 feet (600 m) higher than now during the ordovician and reached a low stand at the end of the Permian. sea levels were high again in the Cretaceous during the breakup of the supercontinent of Pangaea.
Pangaea Supercontinent Formation
Late Carboniferous 300 Ma Global icehouse; low sea level; continental collisions
Pangaea Supercontinent Breakup
Maps of continental positions during cold and warm climates showing the relationship between climate and tectonics
Sea levels may change at different rates and amounts in response to different phases of the supercontinent cycle, and the sea level changes are closely related to climate. The global volume of the midocean ridges can change dramatically, either by increasing the total length of ridges or changing the rate of seafloor spreading. Either process produces more volcanism; increases the volume of volcanoes on the seafloor, raising sea levels; and puts a lot of extra Co2 into the atmosphere, raising global temperatures. The total length of ridges typically increases during continental breakup, since continents are being rifted apart and some continental rifts can evolve into midocean 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 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). Times when supercontinents amalgamate are associated with times when seas drop to low levels.
Other factors, such as midplate volcanism, can also change sea levels. The Hawaiian islands are hot-spot-style midplate 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's 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 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.
Continue reading here: Climate and Seasonality
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