The story of how continents break apart is proving to be more complex than previously understood. New research, published in by Scientific American, and further detailed in studies from Tulane University and elsewhere, reveals that failed rifting events – attempts by continents to split that ultimately didn’t result in new ocean basins – can actually strengthen the Earth’s crust, making future breakups more difficult. This challenges long-held assumptions about continental drift and the processes governing plate tectonics.
The Unexpected Strength of Failed Rifts
For decades, geologists believed that stretching and thinning of the Earth’s crust were precursors to continental breakup. The East African Rift, a visible example of this process, has been a key area of study. However, recent investigations focusing on the Turkana Depression, located between Kenya and Ethiopia, have uncovered a surprising phenomenon. Researchers found that a section of the African tectonic plate that had been previously stretched and thinned was now resisting further deformation. This is the opposite of what conventional wisdom predicted.
The key to this unexpected resilience, according to the Tulane University-led study, lies in a heating event that occurred approximately 80 million years ago. This event dehydrated the plate, removing water and carbon dioxide from its deep layers. The removal of these volatile compounds resulted in a stronger, more rigid plate structure. “The team brought a wide range of skills and data sets to visualize the plate structure and its properties, and our modeling systematically eliminated the possible factors controlling where plate rifting initiates,” explains Cynthia Ebinger, a professor in the earth and environmental sciences department at Tulane University’s School of Science and Engineering.
Continental Fragments in Unexpected Places
The complexities of continental breakup are further illustrated by the presence of continental crust fragments found in the middle of oceans. These “geological misfits,” as described in Scientific American, have puzzled scientists for years, even being cited as arguments against the theory of plate tectonics. However, the recent research suggests these fragments are not anomalies, but rather a natural consequence of how supercontinents break apart.
When a continent begins to split, narrow fault zones can isolate small chunks of continental crust, effectively marooning them on newly formed oceanic crust. This process occurs at mid-ocean spreading centers, where magma rises to create new oceanic crust and drive continents apart. The contrast between the thin, dense basalt of the oceanic crust and the thicker, more buoyant granite-based continental crust is stark, making these isolated continental fragments readily identifiable.
Failed Rifts and the Importance of Driving Forces
Not all rifting events lead to complete continental breakup. Research published in Nature highlights the role of driving forces and lithospheric strength in determining whether a rift will ultimately succeed or fail. The study demonstrates that the timing, duration, and magnitude of reductions in boundary traction – the forces acting on the edges of the plate – significantly influence the outcome.
Specifically, the research indicates that later initiation of traction reduction and slower reduction rates are more likely to promote continental breakup. A 25% reduction in boundary traction can still lead to a split under optimal conditions, but a 50% reduction often results in a “failed” rift, similar to the Mid-Continent Rift in North America or the West and Central African rift system. The models used in the study also showed that extension velocities can exhibit non-monotonic behavior, even temporarily increasing during periods of force reduction, due to the dynamic interplay of forces.
Implications for Understanding Earth’s Evolution
These findings have significant implications for understanding the long-term evolution of Earth’s continents. The realization that past rifting events can strengthen the crust suggests that the history of continental breakup is not a simple linear process. Instead, it’s a complex interplay of tectonic forces, volatile content, and lithospheric properties. The ability of rifts to accelerate towards breakup even when currently extending slowly, as demonstrated by the Nature study, highlights the importance of considering temporally varying driving forces.
The research underscores the need for a more nuanced understanding of continental rifting, moving beyond simplistic models of stretching and thinning. By incorporating factors such as dehydration, the timing of force reductions, and the dynamic balance of tectonic forces, scientists can develop more accurate predictions about where and when continents are likely to break apart in the future. This knowledge is crucial for understanding the distribution of earthquakes, volcanoes, and other geological hazards, as well as for unraveling the history of Earth’s continents and oceans.
The study of continental rifts, as outlined in ScienceDirect, reveals that these zones are characterized by faults, sedimentary basins, earthquakes, and volcanism. Understanding these features is key to deciphering the complex processes at play during continental breakup.
