Selasa, 21 Juni 2011

DEFINITION OF PLATE TECTONICS, SUPERCONTINENTS, AND LIFE

Plate tectonic motions, especially the supercontinent cycle,

profoundly affect the distribution and evolution of life on

Earth. Plate tectonic activity such as rifting, continental collision,

and drifting continents affects the distribution of lifeforms,

the formation and destruction of ecological niches,

and radiation and extinction blooms. Plate tectonic effects

also can induce sea-level changes, initiate periods of global

glaciation, change the global climate from hothouse to icehouse

conditions, and affect seawater salinity and nutrient

supply. All of these consequences of plate tectonics have profound

influences on life on Earth.

Changes in latitude brought on by continental drift bring

land areas into latitudes with better or worse climate conditions.

This has different consequences for different organisms,

depending on their temperature tolerance, as well as

food availability in their environment. Biological diversity

generally increases toward the equator, so, in general, as continents

drift poleward more organisms tend to go extinct, and

as they drift equatorward, diversification may increase.

Tectonics and supercontinent dispersal breaks apart and

separates faunal provinces, which then evolve separately.

Continental collisions and supercontinent amalgamation

build barriers to migration but eventually bring isolated

fauna together. One of the biggest mass extinctions (at the

end of Permian) occurred with the formation of a supercontinent

(Pangea), sea-level regression, evaporite formation, and

global warming. At the boundary between the Permian and

Triassic periods and between the Paleozoic and Mesozoic

periods (245 million years ago), 96 percent of all species

became extinct. Lost were the rugose corals, trilobites, many

types of brachiopods, and marine organisms including many

foraminifer species.

The Siberian flood basalts cover a large area of the Central

Siberian Plateau northwest of Lake Baikal. They are more

than half a mile thick over an area of 210,000 square miles

(547,000 km2) but have been significantly eroded from an

estimated volume of 1,240,000 cubic miles (3,3133,000 km3).

They were erupted over a period of less than one million years

(remarkably short!) 250 million years ago, at the end of the

Permian at the Permian-Triassic boundary. They are remarkably

coincident in time with the major Permian-Triassic

extinction, implying a causal link. The Permian-Triassic

boundary at 250 million years ago marks the greatest extinction

in Earth history, where 90 percent of marine species and

70 percent of terrestrial vertebrates became extinct. It has

been postulated that the rapid volcanism and degassing

released enough sulfur dioxide to cause a rapid global cooling,

inducing a short ice age with associated rapid fall of sea level.

Soon after the ice age took hold, the effects of the carbon

dioxide took over and the atmosphere heated, resulting in a

global warming. The rapidly fluctuating climate postulated to

have been caused by the volcanic gases is thought to have

killed off many organisms, which were simply unable to cope

with the wildly fluctuating climate extremes.

Continental breakup may cause physical isolation of

species that cannot swim or fly between the diverging continents.

Physical isolation (via tectonics) produces adaptive

radiation-continental dispersal and thus increases biotic

diversity. For example, mammals had an explosive radiation

(in 10–20 million years) in the Paleocene-Eocene, right after

breakup of Pangea.

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