The Supercontinent Cycle

Have you ever wondered how the earth’s plates move and why? Here’s an explanation.

Plate tectonics is the theory that the earth is split into plates of varying sizes, that move atop a semi-molten mantle. This theory has evolved over many years and there are many questions related to this theory that are still debated in scientific literature today. The plate tectonic system is thought to have developed very early into the Earth’s formation. Therefore, it can offer up many insights on the changing nature of the Earth’s surface and system through a deep geological timescale.

Many scientists have hypothesised questions relating to plate tectonics over the years, without fully understanding the processes that were occurring. For example, Abraham Ortelius, a sixteenth-century cartographer questioned the concept of moving continents when he observed that the coastlines of America, Europe and Africa potentially matched up when put together on a map. Further development in mapmaking and reliable maps led to more discussion of the inter-fitting nature of the continents, with many acknowledging that it could not be a coincidence. This in turn led to evolving theories on how these continents could have been once connected.

A major development in the theory of plate tectonics was made by Alfred Wegener in 1912, he built on Ortelius’ earlier observations in relation to interlocking continental coastlines. Wegener was a meteorologist, who was intrigued by the concept of moving continents. He developed the theory of continental drift to explain the movement of the continents. Furthermore, he collected a wide variety of fossil and rock evidence to back up the idea that the continents had once all been joined together into one supercontinent, which he named Pangea. However, as he had no model to show how the continents had moved, his ideas were largely denounced at the time.

It was only in the 1960’s that Wegener’s theory of continental drift was revived. This revival only occurred due to the discovery of new evidence, in relation to geophysics and oceanography. These discoveries included the use of magnetic surveys on the ocean floor to discover the edges of the plates across the globe. This was discovered with the use of machines used to monitor nuclear testing called seismic listening networks. Another aspect that indicated seafloor spreading on the ocean floor and hence the theory of plate tectonics was the presence of alternating patterns of magnetic anomalies. Furthermore, there was also evidence of magnetic anomalies in rocks across varying parts of the world’s landmasses to show that the continents had moved apart.

A British geologist Arthur Holmes was one of the first scientists to attempt to give a reason for how the plates moved at this time, he put forth his convection current hypothesis which suggested that continents would drift apart as they were carried along the backs of the laterally-moving convection currents. Although his theory was not entirely correct, it was a hypotheses that was certainly on the right track. It was this period that advanced the theory of plate tectonics greatly, as major questions relating to how the plates formed, how the continents were pulled apart and drifted back together were being studied and discussed throughout prominent scientific literature.

Fast-forward to modern day and the theory of plate tectonics has been widely established as the correct one. There are still however, many questions to be answered in and around the area of plate tectonics. One of these questions is when did the plate tectonic system emerge? Some researchers suggest that plate tectonics stopped and started sporadically for billions of years before falling into a continuous cycle. Evidence of chemical alterations in rocks indicative of the process of subduction have been found in rocks from the Archean eon, which some scientists suggest can be linked to the emergence of plate tectonics. This era was roughly 3.8 billion to 2.5 billion years ago.

Many scientists believe that the plate tectonic system had its roots in the Hadean Archaean transition, which occurred 4.2-4.0 billion years ago. However, there is much debate surrounding this topic, with others suggesting the plate tectonic process began during the Meso to Neo-Proterozoic times, which occurred 2.0-0.8 billion years ago. This debate also extends to the cyclical nature of plate tectonic processes and when did this start occurring. This cyclical nature is referred to as the supercontinent cycle in the literature discussing this process.

The supercontinent cycle is described as an outcome of plate tectonics as a self-organizing system. A supercontinent cycle consists of the landmasses assembling together and breaking up again. There have been three evident supercontinent cycles:

Pangea,

Rodina and

Columbia.

However, some argue there have been more that have occurred over the past 2 billion years. This suggests that the Earth seems to run a cycle from supercontinent to supercontinent lasting roughly 600 million years. Before the two billion years ago mark, the occurrence of an older supercontinent is uncertain, and possibly only smaller and separated landmasses existed.

Another major question in this topic area and in geology itself is how did the Earth's tectonic plates form? The subsequent movement of the plates has erased much of the evidence of their origin. It is believed that the plates, which we now know are interlocking slabs of crust that float on Earth's viscous upper mantle, were created by a process similar to the subduction seen today. Subduction is when one plate dives below another, usually the heavier subducting under the lighter.

A geophysicist from Yale, David Bercovici, produced a model to help explain this process. Bercovici's model suggests that early subduction created weak spots in Earth's crust, he stated that these weak spots then formed the plate boundaries between the plates. Plate tectonics is defined by the idea that strong plates are separated by weak boundaries, and that the action at those boundaries creates geological phenomena such as volcanoes, mountains and earthquakes.

The theory of plate tectonics and the science surrounding it are still incredibly relevant to study in the modern day, especially as the theory is relatively new and still expanding. Plate tectonics and supercontinent cycles are paramount to understanding the earth’s systems and processes. These questions and the plate tectonics theory itself form the cornerstone to many modern geoscience discoveries.

References:

https://pubmed.ncbi.nlm.nih.gov/24717430/

https://www.scientificamerican.com/article/alfred-wegener-and-the-hypothesis-o/

https://researchportal.helsinki.fi/en/publications/the-supercontinent-cycle