Last Christmas, my family flew back to the midwest in a Boeing 737. Getting an airplane to stay up in the air requires a tremendous amount of design, but most of the flight delivers all the excitement of a long bus ride. The action usually occurs during the takeoff (and to some extent, the landing).
New geophysical discoveries show similar action for the tectonic history of Earth. Plate tectonics requires three essential processes to operate. First, rigid continental and oceanic plates must float and move around on the more fluid upper mantle below. Second, hot magma from the mantle must well-up through cracks that form as the plates move apart. Third, where the plates collide with one another, one of the plates must move below the other, back into the mantle, in a process called subduction. Scientists largely understand the mechanisms of the first two steps as well as how subduction functions. However, the initiation of subduction remains more enigmatic.
As described in Science, recent research demonstrates that Earth’s tectonic activity fluctuated significantly in the past because the subduction zones shut down (as a result of continental plates coming into contact). In particular, the time period when the Rodinia “supercontinent” formed corresponds to a dramatic decrease in subduction. (On a side note, when subduction reinitiated leading to the break-up of Rodinia, Earth experienced a glaciation which covered almost the entire globe.) Most of the current subduction zones reside in the Pacific Ocean. Consequently, the closing of the Pacific basin in roughly 350 million years will bring another period of tectonic inactivity.
As continental plates collide, subduction inevitably terminates. However, such a process does not necessarily lead to subduction initiating somewhere else. For example, the Indian and African continents collided with Eurasia 35-50 million years ago, shutting down a subduction zone (this collision caused the formation and continued growth of the Himalayas—home to Mount Everest). Yet since the collision, no new subduction zones have formed in the region nor does it appear that any will.
Most scientists assumed that plate tectonics operated slowly and continuously over the bulk of Earth’s history. But new scientific results argue for an “on-again/off-again” tectonic past. Because of the importance of subduction initiation for plate tectonics activity, RTB’s creation model predicts that scientists will find substantial fine-tuning as they better understand this critical process.
(As an added bonus, a more sporadic nature of Earth’s plate tectonic activity also solves another long-standing geological problem. Scientists measure the tectonic activity and the rate at which heat escapes from Earth today. Extrapolating those two quantities just two billion years in the past leads to an unacceptably high interior temperature for Earth. However, if tectonic activity experienced periods of stagnation, heat would escape more slowly in the past compared to extrapolations from the measured values assuming continuous plate tectonics. Extrapolations incorporating intermittent tectonic activity give reasonable temperatures all the way back to Earth’s formation.)
Going back to the plane analogy, sustaining flight necessitates a high degree of fine-tuning on the part of the pilots and the engineers of the aircraft. Air travel requires an even greater amount of fine-tuning because planes must take off and land—without a successful takeoff, no one can go home for the holidays. Similarly, without subduction initiation the life-essential process of plate tectonics would never take off on Earth.