Supercontinents of the past

The Wilson Cycle

The Supercontinent Cycle

Introversion &
Extroversion


Orthoversion

Looking Forward

References

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The Future of Supercontinents

Jeromy Sherrill

ES 767 Global Tectonics - Spring 2016
Emporia State University
Drawing of Amasia - by Jeromy Sherrill
Image drawn by Jeromy Sherrill
Introduction

Ever since Alfred Wegener proclaimed the nomadic nature of Earth's continents in 1915, much thought and research has been devoted towards understanding continental arrangements throughout the distant past. Likely the most intriguing continental arrangement is when all continents amass into a single landmass known as a supercontinent. The Earth's surface has been punctuated with multiple supercontinents throughout its geologic timeline. Within the imagination of most of the geologically curious, this timeline stretches in only one direction, the past. Despite the common association of supercontinents with ancient times, there is no reason to believe that Pangaea will be the last incarnation along Earth's supercontinent lineage. In recent years discussions of a futuristic supercontinent have begun.

Supercontients of the Past

The most recent and well known supercontinent is Pangaea, forming the landmass over which the dinosaurs lumbered. The is some evidence that suggest that Pangaea was preceded by a supercontinent called Rodina. Long predating the dinosaurs, Rodina existed one billion years ago. Enough evidence has been found to allow modern minds to assemble rough maps of this ancient supercontinent. Peering back even further into geologic history, hints of even older supercontinents (Nuna/Columbia and Kenorland) have been detected and pondered.

Due to erosion, deformation, and subduction; the evidence for even older supercontinents becomes increasingly scarce. This does not mean that supercontinents did not exist before Columbia. To the contrary, the merger of continents appears to be cyclical, as described by the Wilson cycle (Burke 2011). Going even further, a Supercontinent Cycle has also been formulated that claims that not only are continental collisions cyclical, but so is supercontinent formation. (Nance, Worsley, Moody 1988)

Map of Pangaea - provided by USGS
Image provided by the USGS
Rodinia
Image provided by the USGS

The Wilson Cycle

Named after J. Tuzo Wilson, the Wilson cycle is essentially a six-staged process that results in the cyclical opening and closing of ocean basins that also causes continents to rift apart then subsequently reassemble. In stage one of the Wilson cycle, hot spots form beneath the interior of a continent. Plumes of magma rise from these hot spots causing the above crust to thin and uplift. Rifting of the continent begins next. Dense oceanic crust fills in the voids as a narrow sea with a mid-ocean ridge forms. In stage three, seafloor spreading continues as a mature ocean basin develops. By stage four the oldest oceanic crust has thickened and become dense with age and begins to subduct beneath an adjacent continental crust. In stage five of the Wilson cycle, subduction continues and the ocean basin narrows. Lastly, the ocean basin closes completely as continental plates converge creating new mountain ranges (Burke 2011).  See image below.

The Wilson Cycle - image created by the Alfred Wegener Institute
Image courtesy of the Alfred Wegener Institute for Polar and Marine Research,
Bremerhaven, German and United States Geological Survey.


The Supercontinent Cycle

This cycle of continental rifting, ocean creation, subsequent ocean closure, and continental reunion is not a haphazard phenomenon. The supercontinent cycle hypothesis has been formulated to make sense of some unexpected regularities within this process. For instance, the Atlantic Ocean has been opened and closed numerous times all while the Pacific Ocean endures. Additionally, the interval of time between supercontinents has been consistent for the past 2.5 billion years. By age dating the mountain ranges formed by the periodic continental collisions, a surprisingly regular pattern is detected (see below image). Time between each episode of intense mountain building and an episode of rifting is approximately 400 to 500 million years (Nance, Worsley, Moody 1988).

supercontinental cycle - four stage graphic
Graphic created by Jeromy Sherrill

Introversion and Extroversion

Geoscientists largely agree upon the prior existence of supercontinents; however, where on Earth Pangaea, Rodinia, and Nuna (or Columbia) were located is highly debatable (Perkins 2012). Using paleomagnetic signatures preserved in ancient rock, the latitude of past supercontinents can be estimated with reasonable accuracy. The longitudinal coordinates however, primarily have remained regulated to the realm of speculation. 

Previous theories regarding the precise location of past supercontinents involve choosing between introversion and extroversion models. The introversion model suggests that each supercontinent has been centered around the same positon on the planet. With introversion, the center of each supercontinent is located approximately 0 degrees from center-mass of the previous supercontinent. Conversely, the extroversion model suggests that center-mass of a supercontinent is located on the opposite side of the planet (180 degrees) from the center of its predecessor (Gershon 2012). In a nutshell, these models state that all supercontinents throughout history have been centered on the same coordinates, or have alternated between two opposing coordinates. In 2012 however, a third model has emerged. (Gershon 2012)

Orthoversion

In 2012 a team of geologist at Yale University led by Ross Mitchell proposed the orthoversion model. This new model positions the center of each supercontinent 90 degrees from the center-mass of the previous supercontinent. As with earlier models, the Orthoversion model calculates the paleolatitude of past supercontinents by analyzing the magnetism preserved within ancient continental rocks. However, going one step further, the Yale team estimated the paleolongitude by accounting for locational changes of Earth’s magnetic pole. Combining the estimates of paleolatitude and paleolongitude the model concluded that the center of Pangaea was located 90 degrees from the center of Rodinia. Additionally, it was determined that center-mass of Rodinia was positioned 90 degrees from the center of Nuna that existed 2 billion years ago. (Perkins 2012)

This pattern of 90-degree offsets is not a coincidence. The 90-degree offset of a supercontinent from the center of its predecessor corresponds to the rifted supercontinent’s former edge. As suggested by the Wilson cycle, continental plates eventually collide, or re-collide, once all the dense oceanic crust is subducted beneath the adjacent continent, effectively closing the ocean and merging the two continental plates. In the orthoversion model, this former subduction zone later becomes the center of the next supercontinent. (Perkins 2012)


Looking Forward

The orthoversion model is not limited to determining positions of ancient supercontinents. The Yale team has turned their model towards the future. They predict that the next supercontinent, known as Amasia, will not be huddled around the equator as other models suggests. Based on the results of their model, the Yale team predicts that Earth’s next supercontinent will be center 90 degrees from Pangaea, thus placing Amasia over the artic. (Perkins 2012)

No definitive timetable has been drawn, but sometime over the next several hundred million years the Caribbean Sea and Atlantic Ocean will close with the merger of western South America and eastern North America. Also, Australia will collide with Asia and the massive continental duo will join with the Americas completing a supercontinent that dominates the Northern Hemisphere. (Gershon 2012)


Amasia drawing - drawing by Jeromy Sherrill

References

Burke, K. (2011). Plate Tectonics, the Wilson Cycle, and Mantle Plumes: Geodynamics from the Top. Annu. Rev. Earth Planet. Sci. Annual Review of Earth and Planetary Sciences, 39(1), 1-29. doi:10.1146/annurev-earth-040809-152521

Nance, R. D., Worsley, T. R., & Moody, J. B. (1988). The Supercontinent Cycle. Sci Am Scientific American, 259(1), 72-79. doi:10.1038/scientificamerican0788-72

Gershon, E. (2012, February 8). As next supercontinent forms, Arctic Ocean, Caribbean will vanish first. Retrieved April 15, 2016, from http://news.yale.edu/2012/02/08/next-supercontinent-forms-arctic-ocean-caribbean-will-vanish-first

Perkins, S. (2012, February 08). Meet 'Amasia,' the Next Supercontinent. Retrieved April 16, 2016, from http://www.sciencemag.org/news/2012/02/meet-amasia-next-supercontinent