DEFINITION OF PANGEA

The supercontinent that formed in the Late Paleozoic,

lasting from about 300–200 million years ago, and

included most of the planet’s continental masses. The former

existence of Pangea, meaning all land, was first postulated by

Alfred Wegener in 1924, when he added the Australian and

Antarctic landmasses to an 1885 supercontinent reconstruction

of Gondwana by Eduard Suess that included Africa,

India, Madagascar, and South America. He used the fit of the

shapes of the coastlines of the now dispersed continental

fragments, together with features such as mineral belts, faunal

and floral belts, mountain ranges, and paleoclimate zones

that matched across his reconstructed Pangean landmass to

support the hypothesis that the continents were formerly

together. Wegener proposed that the supercontinent broke up

first into two large fragments including Laurasia in the north,

and Gondwana in the south, and then continued breaking

up, leading to the present distribution of continents and

oceans. Wegener’s ideas were not generally accepted at first

but since the discovery of seafloor magnetic anomalies and

the plate tectonic revolution, the general framework of his

Pangea model has become recognized as generally valid.

The Pangean supercontinent began amalgamating from

different continental fragments with the collision of Gondwana

and Laurentia and Baltica in the Middle Carboniferous,

resulting in the Alleghenian, Mauritanide, and Variscan

orogenies. Final assembly of Pangea involved the collision of

the South China and Cimmerian blocks with the Paleo-

Tethyan margin, resulting in the early Yenshanian and

Indonesian orogenies in the Middle to Late Triassic.

The formation of Pangea is associated with global climate

change and rapid biological evolution. The numerous

collisions caused an overall thickening of the continental

crust that decreased continental land area and resulted in a

lowering of sea level. The uplift and rapid erosion of many

carbonate rocks that had been deposited on trailing or passive

margins caused a decrease in the carbonate 87Sr/86Sr

ratios in the ocean. During the final stages of the coalescence

of Pangea, drainage systems were largely internal, erosion

rates were high, and the climate, with large parts of the

supercontinent lying between 15° and 30° latitude, became

arid, with widespread red-bed deposition. Soon, however, the

effects of the erosion and burial of large amounts of carbonate

and the associated drawdown of atmospheric CO2 caused

climates to rapidly cool, resulting in high-latitude glaciations.

The main glaciations of Pangea started in the late Devonian

and early Carboniferous, began escalating in intensity

by 333 million years ago, peaked in the Late Carboniferous

by 292 million years ago, and ended in the early Permian by

272 million years ago. These glaciations resulted in major

global regressions as the continental ice sheets used much of

the water on the planet. Wegener, and many geologists since,

used the distribution of Pangean glacial deposits as one of the

main lines of reasoning to support the idea of continental

drift. If the glacial deposits of similar age are plotted on a

map of the present distribution of the continents, the ice flow

patterns indicate that the oceans too must have been covered.

However, there is not enough water on the planet to make ice

sheets so large that they can cover the entire area required if

the continents have not moved. If the glacial deposits are

plotted on a map of Pangea however, they cover a much

smaller area, the ice flow directions are seen to be radially

outward from depocenters, and the total volume of ice is able

to be accommodated by the amount of water on Earth.

Pangea began rifting and breaking apart about 230 million

years ago, with numerous continental rifts, flood basalts,

and mafic dikes intruding into the continental crust. Major

breakup and seafloor spreading began about 175 million

years ago in the central Atlantic, when North and South

America broke away from Pangea, 165 million years ago off

Somalia, and 160 million years ago off the coast of northwest

Australia. Sea levels began to rise with breakup because of

the increase in volume of the mid-ocean ridges that displaced

seawater onto the continents, forming marine transgressions.

Episodic transgressions and evaporation of seawater from

restricted basins led to the deposition of thick salts in parts of

the Atlantic, with some salt deposits reaching 1.2 miles (2

km) thick off the east coast of North America, Spain, and

northwest Africa. Several rifts along these margins have several

to 10 kilometers of non-marine sandstones, shales, redbeds,

and volcanics, associated with breakup of Pangea.

Many are very fossiliferous, including plants, mudcracks, and

even dinosaur footprints attesting to the shallow water and

subaerial nature of these deposits.

Breakup of the supercontinent was also associated with a

dramatic climate change and high sea levels. The increased

volcanism at the oceanic ridges released many gases to the

atmosphere, inducing global warming, leading to a global

greenhouse, ideal for carbonate production on passive or

trailing continental margins.

See also PALEOZOIC; SUPERCONTINENT CYCLE.

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