There are various hypotheses about the geological future of the Earth and what that new world will look like. The theory of supercontinental cycles explains that all the continents of the planet will be reunited again, as happened 335 million years ago, when the supercontinent Pangaea was the only emerging land mass. It is a matter of time, but we already know that the continents that we see today will collide against each other again, even though human beings may not be there to witness it.
Everything begins in the oceans, specifically in the margins that mark the limit between the ocean floor and the continents.
Wilson cycles and supercontinents
Alfred Wegener, meteorologist and geophysicist, was the first to speak, as early as 1912, of the existence of a supercontinent, called Pangaea (in Greek: “all the earth”), which housed the entire earth’s surface more than 300 million years ago (Ma). His observation led to what we know today as the tectonic plates’ theory.
50 years later, Tuzo Wilson, a Canadian geologist, showed that the Atlantic Ocean had opened and closed multiple times throughout Earth’s history. This meant that there was a cycle of creation and destruction of the great ocean basins, and from then on it is known as the Wilson cycle in his honor.
These cycles describe the geological history of an ocean basin, from its birth to its death, which occurs in three phases: (1) Opening and expansion; (2) collapse of passive margins and development of subduction zones; and (3) basin retraction and closure.
These well-differentiated phases can be seen today in places like the African Rift Valley and the Red Sea (1), the ocean trenches off the coasts of Chile and Peru (2), which formed the Andean mountain range, and also in the Himalayan mountain range (3). This last example represents the result of the closure of the ocean that existed between the tectonic plates of India and Eurasia.
So far everything seems to fit: we know that ocean basins periodically open and close at intervals of 400-500 Ma, and we are witnessing the processes that left their mark on Earth. However, it remains to be discovered how subduction zones begin on the passive margins of the oceans.
The end of the Atlantic Ocean
The oceanic lithosphere gradually cools as it separates from the oceanic ridge and becomes gravitationally unstable. When the age is greater than 10 Ma, its average density is higher than the one inmediately below, the astenoshphere, which causes the collapse and sinking of the lithosphere, thus creating subduction zones. This would be the answer we are looking for on how a passive margin becomes an active one.
The problem is that, to initiate this sinking, the oceanic lithosphere needs to be fractured at some point, which is especially difficult if we consider that the lithosphere acquires hardens over the years, while the ocean is expanding. An external force is required to fracture or weaken the solidity of the lithosphere at the passive margins, a phenomenon of such a magnitude as those we can find in other subduction zones or the collision between tectonic plates.
Some researchers have suggested that hydration of the lithosphere, or thermal erosion caused by the rise of the mantle, could help to weaken its structure or composition. But this seems unlikely, since if so, we could find subduction zones anywhere in the Atlantic.
Rather, subduction is thought to be triggered by stress transmission at plate tectonic boundaries. The ocean basins are connected to each other at some point, and if there are active subduction zones in these places, they will be susceptible to propagate towards the passive margins. This is the theory that is contemplated to explain the origin of the subduction in the Scotland Arc in the South Atlantic, and the Lesser Antilles Arc in the Caribbean Sea. This phenomenon of propagation of mechanical stress between tectonic plates is called subduction invasion.
Although the lithosphere of the passive margins is a very stable surface, subduction invasion would be the reason why we did not have records of oceanic lithosphere older than 180 Ma on the planet (Bradley, 2008). Neither current nor geological records.
The buoyancy argument
The natural tendency of the oceanic lithosphere in the passive margins is sinking, but this tendency alone is not enough to break the lithological structure of the layer: an external force or a weakening mechanism is required. If not, subduction would occur spontaneously in the passive margins due to the difference in density between this layer and the asthenosphere, as we have mentioned before.
Once this barrier is overcome, the subduction process can be irreversible and expand throughout the oceanic lithosphere. Personally, I like to use the orange example to illustrate this process:
If we try to peel an orange with our hands, it will be difficult to lift the first piece of peel at the beginning, because the outer layer forms a continuous and well-structured protection. However, the resistance of the shell is much lower when we have found a crack and we have a portion of which to pull until the rest of the casing is removed almost effortlessly. Roughly speaking, this is what happens with the lithosphere on the passive margins.
When subduction has invaded a very old ocean basin, the system will likely spread and the ocean could enter its contraction and closure phase. The only element that could prevent this contraction would be for the oceanic ridge to have a lithosphere creation rate equal to or greater than the rate of destruction in the subduction zones. Otherwise, the ridge will be dragged along with the rest of the lithosphere towards the active margins and would disappear, as it happened in the Pacific Ocean.
This process of closure favors the approximation of the continental masses, which means that America and Asia seem destined to collide. The fact that the Pacific has not yet closed seems to respond to the simple fact that it was the only ocean –known as Panthalassa– that surrounded Pangaea and, given its dimensions, the retraction phase has not finished yet.
The weak link
Applying the buoyancy argument, it seems clear that the oldest oceanic lithosphere (> 100 Ma) is the most likely area to sinking in the depths of the mantle. But you may have already noticed that in this matter nothing is so simple, because the solidity and resistance that this lithosphere acquires during its first 100 Ma requires that the energy level to start subduction is very difficult to achieve.
This opens the door to a second scenario: that the lithosphere truly susceptible to initiating subduction is much younger: between 20-100 Ma. Based on this premise, we could explain how the Scotland and the Lesser Antilles arcs initiated, and even anticipate potential subduction areas, such as the southwest (SW) of the Iberian Peninsula.
The Gibraltar Arch threatens the Atlantic Ocean
To recap, the margins of the Atlantic basin are often described as the paradigm of the passive margin. However, there are at least two regions where the oceanic lithosphere is being consumed by subduction zones: the aforementioned arcs of Scotland and the Lesser Antilles. Possibly, its origin is due to the transmission of stress generated by other subduction zones in the adjacent tectonic plates –the subduction invasion.
According to the numerical models of Moresi (2014), it is very probable that both arcs will propagate laterally towards the north, following the passive margins, where the tendency of the lithosphere to sink is greater. Once the subduction process has begun, the aged and strengthened lithosphere after more than 100 Ma will be unable to resist and will be drawn into the mantle.
The Gibraltar Arc is the third place in the Atlantic where a new subduction zone is expected to appear. Structural thrust faults on the SW margin of the Iberian Peninsula are expected to spread towards the Atlantic and favor the creation of a new active margin in the vicinity of the Portuguese coasts. Furthermore, this reactivation of the passive margin would be reinforced by the later phases of the collision between the African and Eurasian plates.
The seismicity of the region adds more solidity to this hypothesis, since it is an external agent that would contribute to weaken the oceanic lithosphere. It should be remembered that the magnitude of some historically recorded earthquakes has been very high, as we can see in the Lisbon earthquakes in 1969 and in 1755 –the latter almost destroyed the city.
Other nearby places with potential to host the start of the reactivation have also been considered, specifically off the Asturian coast in the Bay of Biscay, but geological analysis have ruled it out due to the fact that this system has remained inactive since the Burdigalian age (Miocene, 20 Ma).
Therefore, the SW of the Iberian Peninsula, together with the Scotland and the Lesser Antilles arcs, could be the origin of a new large-scale subduction system that would lead to the contraction of the ocean basin that we know, and the formation of a new supercontinent.
Supercycles and supercontinents
While Wilson cycles cover the time span from the opening of a new ocean to its closure, supercycles –or supercontinental cycles– are at a higher level, in this case we are talking about the time that elapses from the fragmentation of a supercontinent to that the great masses of land come back together to form a new one. This period was believed to be around 400-600 Ma, but in the last decade it has been recognised that they are less frequent.
Estimating its periodicity is complex, since neither the fragmentation nor the collisions between continents are instantaneous, but they last for millions of years and can even overlap each other. Furthermore, we only have numerical models to experiment and observe their behavior.
On the other hand, there is some debate when defining what a supercontinent is –as is the case with the concept of a continent–. Generally, we refer to the grouping of all or most of the emerged lands in a large continental mass. For example, there is broad agreement to define Pangaea (250 Ma ago) and Rodinia (1.1 Ga) as such, but some authors also consider Gondwana (600 Ma) to be a supercontinent, despite not meeting the requirement of bring together the vast majority of emerging continental masses that existed at that time –Laurentia, Siberia and Baltic were not part of it–. As an alternative, it is proposed that Gondwana be classified as a megacontient.
The outcome: Aurica
A classic discussion among geologists focuses on concluding which ocean will be the next to close: the Atlantic or the Pacific. Depending on the answer given, the tectonic plates will be subjected to different forces and will move accordingly to end up forming the next supercontinent.
The first supercontinent proposal derives from the closure of the Pacific Ocean at the expense of the increasing expansion of the Atlantic, which would become the immense ocean that surrounded the entire supercontinent, called Novopangaea (Roy Livermore at the BBC, and Nield, 2007). Australia and Antarctica would move north to end up joined between the Americas and Eurasia. Europe would remain as the northernmost point on the globe.
In the following hypothesis, the opposite is argued: it maintains that the Atlantic would be the ocean that closes while the Pacific expands again, until reaching a version similar to that of Pangaea 250 Ma ago, but this time leaving the Indian Ocean as an inland sea . This proposal is called Pangaea Proxima, Neopangaea, or Pangaea II (Scotese, 2007).
A third hypothesis predicts that the North American and Eurasian plates migrate north to meet at the North Pole, followed by the rest of the continents except Antarctica, which would remain at the South Pole. Again, the Pacific Ocean would contract until it disappeared while the Atlantic would become the great surrounding ocean. The name given to this supercontinent is Amasia (Hoffman, 1997).
All three models assume that either the Pacific, the Atlantic, or both will continue to grow for about 100-400 Ma, leaving behind an oceanic lithosphere of up to 600 Ma, and as we have seen previously, this seems unlikely.
But, adjusting to the information provided so far on the longevity of the oceanic lithosphere, we can consider an alternative option in which both oceans could be closed simultaneously, resulting in a new proposal for the configuration of the next supercontinent: Aurica.
How Aurica will be shaped?
Naturally, this scenario is only possible if we consider the expansion of one or more ocean basins, such as the Indian Ocean, a future inter-African ocean in the Rift Valley, or a new oceanic ridge that develops in the vicinity of the Antarctica. Eurasia would also need to be divided by a rift, as is happening in Africa. This situation could be generated by the gravitational collapse of the Tibetan plateau and its propagation through the existing tectonic depressions in Asia –as it already happened at the birth of the Atlantic–, until it connected with the mid-arctic ridge, known as the Gakkel ridge.
In about 20 Ma, South America will separate from its northern half to rotate slightly and shift north along with Australia. Meanwhile, the subduction zones, which will have spread to a greater extent on the west margin of the Atlantic, will cause an asymmetry in the closure of the ocean and, therefore, the new Eurafrica will be dragged by this movement. On the other hand, the subduction invasion would extend from the Scotia arc to Antarctica, which will be forced to rotate north until it collides with the western edge of South America.
Within 150 Ma, the Pacific Ocean will have become a considerably smaller inland sea, and Eurafrica will have completely separated from Asia. The Indian and Antarctic Oceans will expand to become a new super-ocean.
Taking into account the average speed of the tectonic plates in the present, the new supercontinent could be fully formed in about 300 Ma. The definitive picture would leave Australia and America at the center of continental reunification, and that is why the authors of the article on which this text is based (Duarte et. al., 2018) have proposed that the name of the future supercontinent be Aurica.
Duarte, Joao & Schellart, Wouter & Rosas, Filipe. (2018). The future of Earth’s oceans: consequences of subduction initiation in the Atlantic and implications for supercontinent formation. Geological Magazine. 155. 10.1017/S0016756816000716.