Processes That Drive Evolution And ExTinction

The progression of life-forms, evolution, and extinction can be influenced or driven by many factors including impacts with space objects such as meteorites and comets, variations in the style of plate tectonics or the positions of the continents, the supercontinent cycle, and continental collisions. Plate tectonics also may cause glaciation and climate changes, which in turn influence evolution and extinction.

one of the primary mechanisms by which plate tectonics drives evolution and extinction is through tectonic-induced changes to sea level. Fluctuating sea levels cause the global climate to fluctuate between

Q Greenhouse warming, rapid climate change

Solar radiation

Plate Tectonics Evolution

Solar radiation

© Massive volcanism

© Giant impacts by meteorites

Solar radiation

Plate Tectonics Evolution
© Infobase Publishing

The main causes for mass extinctions, including (1) greenhouse warming and rapid climate change, followed by (2) massive volcanism, and (3) giant impact from space objects such as asteroids or comets warm periods when shallow seas are easily heated, and cold periods when glaciation draws the water down to place shorelines along the steep continental slopes. Many species cannot tolerate such variations in temperature and drastic changes to their shallow shelf environments, and thus become extinct. After organisms from a specific environment die off, their environmental niches are available for other species to inhabit.

Sea levels have risen and fallen dramatically in Earth history, with water covering all but 5 percent of the land surface at times, and water falling so that continents occupy 40 percent or more of the planet's surface at other times. The most important plate tectonic mechanism of changing sea level is to change the average depth of the seafloor by changing the volume of the mid-ocean ridge system. If the undersea ridges take up more space in the ocean basins, then the water will be displaced higher onto the land, much like dropping pebbles into a birdbath may cause it to overflow.

The volume of the mid-ocean ridge system can be changed through several mechanisms, all of which have the same effect. Young oceanic crust is hotter, more buoyant, and topographically higher than older crust. Thus, if the average age of the oceanic crust is decreased, then more of the crust will be at shallow depths, displacing more water onto the continents. If seafloor spreading rates are increased then the average age of oceanic crust will be decreased, the volume of the ridges will be increased, the average age of the seafloor will be decreased, and sea levels will rise. This has happened at several times in Earth history, including during the mid-Cretaceous between 110-85 Ma when sea levels were 660 feet (200 m) higher than they are today, covering much of the central United states and other low-lying continents with water. This also warmed global climates, because the sun easily warmed the abundant shallow seas. It has also been suggested that sea levels were consistently much higher in the Precambrian, when seafloor spreading rates were likely to have been generally faster.

sea levels can also rise from additional mag-matism on the seafloor. If the Earth goes through a period when seafloor volcanoes erupt more magma on the seafloor, then the space occupied by these volcanic deposits will be displaced onto the continents. The additional volcanic rocks may be erupted at hot spot volcanoes like Hawaii or along the mid-ocean ridge system; either way, the result is the same.

A third way for the mid-ocean ridge volume to increase sea level height is to simply have more ridges on the seafloor. At the present time the mid-ocean ridge system is 40,000 miles (65,000 km) long. If the Earth goes through a period where it needs to lose more heat, such as in the Precambrian, one of the ways it may do this is by increasing the length of the ridge system where magmas erupt and lose heat to the seawater. Ridge lengths were probably greater in the Precambrian, which together with faster sea floor spreading and increased magmatism may have kept sea levels high for millions of years.

Glaciation that may be induced by tectonic or astronomical causes may also change sea level. At present glaciers cover much of Antarctica, Greenland, and mountain ranges in several regions. Approximately 6 million cubic miles (25,000,000 km3) of ice is currently locked up in glaciers. If this ice were to all melt, then sea levels would rise by 230 feet (70 m), covering many coastal regions, cities, and interior farmland with shallow seas. During the last glacial maximum in the Pleistocene ice ages (20,000 years ago) sea levels were 460 feet (140 m) lower than today, with shorelines up to hundred of miles (km) seaward of their present locations, along the continental slopes.

Continental collisions and especially the formation of supercontinents can cause glaciations. When continents collide, many of the carbonate rocks deposited on continental shelves are exposed to weathering. As the carbonates and other minerals weather, the products react with atmospheric elements and tend to combine with atmospheric carbon dioxide (C02). Carbon dioxide is a greenhouse gas that keeps the climate warm, and steady reductions of Co2 in the atmosphere by weathering or other processes lowers global temperatures. Thus, times of continental collision and supercontinent formation tend to be times that draw Co2 out of the atmosphere, plunging the Earth into a cold "icehouse" period. In the cases of supercontinent formation, this icehouse may remain in effect until the supercontinent breaks up, and massive amounts of seafloor volcanism associated with new rifts and ridges add new Co2 back into the atmosphere.

The formation and dispersal of supercontinent fragments, and migrating landmasses in general, also strongly influence evolution and extinction. When supercontinents break up, a large amount of shallow continental shelf area is created. Life-forms tend to flourish in the diverse environments on the continental shelves, and many spurts in evolution have occurred in the shallow shelf areas. In contrast, when continental areas are isolated, such as Australia and Madagascar today, life-forms evolve independently on these continents. If plate tectonics brings these isolated continents into contact, the different species will compete for similar food and environments, and typically only the strongest survive.

The position of continents relative to the spin axes (or poles) of the Earth can also influence climate, evolution, and extinction. At times (like the present) when a continent is sitting on one or both of the poles, these continents tend to accumulate snow and ice, and to become heavily glaciated. This causes ocean currents to become colder, lowers global sea levels, and reflects more of the sun's radiation back to space. Together, these effects can put a large amount of stress on species, inducing or aiding extinction.

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