Pangaea Pangaea was a supercontinent that existed
during the late Paleozoic and early Mesozoic eras, forming approximately 300 million years
ago. It began to break apart around 100 million years after it formed. The single global ocean
which surrounded Pangaea is accordingly named Panthalassa.
The name Pangaea is derived from Ancient Greek pan (πᾶν) meaning "entire", and Gaia (Γαῖα)
meaning "Mother Earth". The name was coined during a 1927 symposium discussing Alfred
Wegener's theory of continental drift. In his book The Origin of Continents and Oceans
(Die Entstehung der Kontinente und Ozeane), first published in 1915, he postulated that
prior to breaking up and drifting to their present locations, all the continents had
at one time formed a single supercontinent which he called the "Urkontinent". The name
occurs in the 1920 and 1922 editions of Die Entstehung der Kontinente und Ozeane, but
only once, when Wegener refers to the ancient supercontinent as "the Pangaea of the Carboniferous".
Formation of Pangaea The forming of supercontinents and their breaking
up appears to have been cyclical through Earth's history. There may have been many others before
Pangaea. The fourth-last supercontinent, called Columbia or Nuna, appears to have assembled
in the period 2.0–1.8 Ga. Columbia/Nuna broke up and the next supercontinent, Rodinia,
formed from the accretion and assembly of its fragments. Rodinia lasted from about 1.1
billion years ago (Ga) until about 750 million years ago, but its exact configuration and
geodynamic history are not nearly as well understood as those of the later supercontinents,
Pannotia and Pangaea. When Rodinia broke up, it split into three
pieces: the supercontinent of Proto-Laurasia, the supercontinent of Proto-Gondwana, and
the smaller Congo craton. Proto-Laurasia and Proto-Gondwana were separated by the Proto-Tethys
Ocean. Next Proto-Laurasia itself split apart to form the continents of Laurentia, Siberia
and Baltica. Baltica moved to the east of Laurentia, and Siberia moved northeast of
Laurentia. The splitting also created two new oceans, the Iapetus Ocean and Paleoasian
Ocean. Most of the above masses coalesced again to form the relatively short-lived supercontinent
of Pannotia. This supercontinent included large amounts of land near the poles and,
near the equator, only a relatively small strip connecting the polar masses. Pannotia
lasted until 540 Ma, near the beginning of the Cambrian period and then broke up, giving
rise to the continents of Laurentia, Baltica, and the southern supercontinent of Gondwana.
In the Cambrian period, the continent of Laurentia, which would later become North America, sat
on the equator, with three bordering oceans: the Panthalassic Ocean to the north and west,
the Iapetus Ocean to the south and the Khanty Ocean to the east. In the Earliest Ordovician,
around 480 Ma, the microcontinent of Avalonia – a landmass incorporating fragments of
what would become eastern Newfoundland, the southern British Isles, and parts of Belgium,
northern France, Nova Scotia, New England, Iberia and northwest Africa – broke free
from Gondwana and began its journey to Laurentia. Baltica, Laurentia, and Avalonia all came
together by the end of the Ordovician to form a minor supercontinent called Euramerica or
Laurussia, closing the Iapetus Ocean. The collision also resulted in the formation of
the northern Appalachians. Siberia sat near Euramerica, with the Khanty Ocean between
the two continents. While all this was happening, Gondwana drifted slowly towards the South
Pole. This was the first step of the formation of Pangaea.
The second step in the formation of Pangaea was the collision of Gondwana with Euramerica.
By Silurian time, 440 Ma, Baltica had already collided with Laurentia, forming Euramerica.
Avalonia had not yet collided with Laurentia, but as Avalonia inched towards Laurentia,
the seaway between them, a remnant of the Iapetus Ocean, was slowly shrinking. Meanwhile,
southern Europe broke off from Gondwana and began to move towards Euramerica across the
newly formed Rheic Ocean. It collided with southern Baltica in the Devonian, though this
microcontinent was an underwater plate. The Iapetus Ocean's sister ocean, the Khanty Ocean,
shrank as an island arc from Siberia collided with eastern Baltica (now part of Euramerica).
Behind this island arc was a new ocean, the Ural Ocean.
By late Silurian time, North and South China split from Gondwana and started to head northward,
shrinking the Proto-Tethys Ocean in their path and opening the new Paleo-Tethys Ocean
to their south. In the Devonian Period, Gondwana itself headed towards Euramerica, causing
the Rheic Ocean to shrink. In the Early Carboniferous, northwest Africa had touched the southeastern
coast of Euramerica, creating the southern portion of the Appalachian Mountains, and
the Meseta Mountains. South America moved northward to southern Euramerica, while the
eastern portion of Gondwana (India, Antarctica and Australia) headed toward the South Pole
from the equator. North and South China were on independent continents. The Kazakhstania
microcontinent had collided with Siberia. (Siberia had been a separate continent for
millions of years since the deformation of the supercontinent Pannotia in the Middle
Carboniferous.) Western Kazakhstania collided with Baltica
in the Late Carboniferous, closing the Ural Ocean between them and the western Proto-Tethys
in them (Uralian orogeny), causing the formation of not only the Ural Mountains but also the
supercontinent of Laurasia. This was the last step of the formation of Pangaea. Meanwhile,
South America had collided with southern Laurentia, closing the Rheic Ocean and forming the southernmost
part of the Appalachians and Ouachita Mountains. By this time, Gondwana was positioned near
the South Pole and glaciers were forming in Antarctica, India, Australia, southern Africa
and South America. The North China block collided with Siberia by Late Carboniferous time, completely
closing the Proto-Tethys Ocean. By Early Permian time, the Cimmerian plate
split from Gondwana and headed towards Laurasia, thus closing the Paleo-Tethys Ocean, but forming
a new ocean, the Tethys Ocean, in its southern end. Most of the landmasses were all in one.
By the Triassic Period, Pangaea rotated a little and the Cimmerian plate was still travelling
across the shrinking Paleo-Tethys, until the Middle Jurassic time. The Paleo-Tethys had
closed from west to east, creating the Cimmerian Orogeny. Pangaea, which looked like a C, with
the new Tethys Ocean inside the C, had rifted by the Middle Jurassic, and its deformation
is explained below. Evidence of existence
Fossil evidence for Pangaea includes the presence of similar and identical species on continents
that are now great distances apart. For example, fossils of the therapsid Lystrosaurus have
been found in South Africa, India and Australia, alongside members of the Glossopteris flora,
whose distribution would have ranged from the polar circle to the equator if the continents
had been in their present position; similarly, the freshwater reptile Mesosaurus has been
found in only localized regions of the coasts of Brazil and West Africa.
Additional evidence for Pangaea is found in the geology of adjacent continents, including
matching geological trends between the eastern coast of South America and the western coast
of Africa. The polar ice cap of the Carboniferous Period covered the southern end of Pangaea.
Glacial deposits, specifically till, of the same age and structure are found on many separate
continents which would have been together in the continent of Pangaea.
Paleomagnetic study of apparent polar wandering paths also support the theory of a supercontinent.
Geologists can determine the movement of continental plates by examining the orientation of magnetic
minerals in rocks; when rocks are formed, they take on the magnetic properties of the
Earth and indicate in which direction the poles lie relative to the rock. Since the
magnetic poles drift about the rotational pole with a period of only a few thousand
years, measurements from numerous lavas spanning several thousand years are averaged to give
an apparent mean polar position. Samples of sedimentary rock and intrusive igneous rock
have magnetic orientations that are typically an average of these "secular variations" in
the orientation of Magnetic North because their magnetic fields were not formed in an
instant, as is the case in a cooling lava. Magnetic differences between sample groups
whose age varies by millions of years is due to a combination of true polar wander and
the drifting of continents. The true polar wander component is identical for all samples,
and can be removed, leaving geologists with the portion of this motion that shows continental
drift and can be used to help reconstruct earlier continental positions.
The continuity of mountain chains provides further evidence for Pangaea. One example
of this is the Appalachian Mountains chain which extends from the southeastern United
States to the Caledonides of Ireland, Britain, Greenland, and Scandinavia.
Rifting and break-up There were three major phases in the break-up
of Pangaea. The first phase began in the Early-Middle Jurassic (about 175 Ma), when Pangaea began
to rift from the Tethys Ocean in the east to the Pacific in the west, ultimately giving
rise to the supercontinents Laurasia and Gondwana. The rifting that took place between North
America and Africa produced multiple failed rifts. One rift resulted in a new ocean, the
North Atlantic Ocean. The Atlantic Ocean did not open uniformly;
rifting began in the north-central Atlantic. The South Atlantic did not open until the
Cretaceous when Laurasia started to rotate clockwise and moved northward with North America
to the north, and Eurasia to the south. The clockwise motion of Laurasia led to the closing
of the Tethys Ocean. Meanwhile, on the other side of Africa and along the adjacent margins
of east Africa, Antarctica and Madagascar, new rifts were forming that would not only
lead to the formation of the southwestern Indian Ocean but also open up in the Cretaceous.
The second major phase in the break-up of Pangaea began in the Early Cretaceous (150–140 Ma),
when the minor supercontinent of Gondwana separated into multiple continents (Africa,
South America, India, Antarctica, and Australia). About 200 Ma, the continent of Cimmeria,
as mentioned above (see "Formation of Pangaea"), collided with Eurasia. However, a subduction
zone was forming, as soon as Cimmeria collided. This subduction zone was called the Tethyan
Trench. This trench might have subducted what is called the Tethyan mid-ocean ridge, a ridge
responsible for the Tethys Ocean's expansion. It probably caused Africa, India and Australia
to move northward. In the Early Cretaceous, Atlantica, today's South America and Africa,
finally separated from eastern Gondwana (Antarctica, India and Australia), causing the opening
of a "South Indian Ocean". In the Middle Cretaceous, Gondwana fragmented to open up the South Atlantic
Ocean as South America started to move westward away from Africa. The South Atlantic did not
develop uniformly; rather, it rifted from south to north.
Also, at the same time, Madagascar and India began to separate from Antarctica and moved
northward, opening up the Indian Ocean. Madagascar and India separated from each other 100–90 Ma
in the Late Cretaceous. India continued to move northward toward Eurasia at 15 centimeters
(6 in) a year (a plate tectonic record), closing the Tethys Ocean, while Madagascar
stopped and became locked to the African Plate. New Zealand, New Caledonia and the rest of
Zealandia began to separate from Australia, moving eastward toward the Pacific and opening
the Coral Sea and Tasman Sea. The third major and final phase of the break-up
of Pangaea occurred in the early Cenozoic (Paleocene to Oligocene). Laurasia split when
North America/Greenland (also called Laurentia) broke free from Eurasia, opening the Norwegian
Sea about 60–55 Ma. The Atlantic and Indian Oceans continued to expand, closing the Tethys
Ocean. Meanwhile, Australia split from Antarctica
and moved rapidly northward, just as India had done more than 40 million years before.
It is currently on a collision course with eastern Asia. Both Australia and India are
currently moving northeast at 5–6 centimeters (2–3 in) a year. Antarctica has been near
or at the South Pole since the formation of Pangaea about 280 Ma. India started to collide
with Asia beginning about 35 Ma, forming the Himalayan orogeny, and also finally closing
the Tethys Seaway; this collision continues today. The African Plate started to change
directions, from west to northwest toward Europe, and South America began to move in
a northward direction, separating it from Antarctica and allowing complete oceanic circulation
around Antarctica for the first time. This motion, together with decreasing atmospheric
carbon dioxide concentrations, caused a rapid cooling of Antarctica and allowed glaciers
to form. This glaciation eventually coalesced into the kilometers-thick ice sheets seen
today. Other major events took place during the Cenozoic, including the opening of the
Gulf of California, the uplift of the Alps, and the opening of the Sea of Japan. The break-up
of Pangaea continues today in the Red Sea Rift and East African Rift.