The general mechanisms of plate tectonics – by which the earth’s crustal plates slide across the planet, coalescing, breaking apart and reforming over cycles of hundreds of millions of years – are common knowledge. We know what happens when one plate dives below another in a process known as subduction, or when two plates slide past one another or when plates move apart.
But plate tectonics is a young science, and for all our understanding of its generalities, it keeps plenty of secrets. One of the big mysteries is the question of how subduction is initiated – what exactly is the mechanism that triggers the process? Now a team of scientists believe they are a step closer to the answer with a study of a potentially developing subduction zone in the Atlantic Ocean.
Ocean Basin Evolution: The Wilson Cycle
Ocean basins come and go: they grow and contract. The process by which an ocean evolves can be (broadly) divided into three steps.
- Firstly, continental rifting is initiated (another process not fully understood).
- Secondly, after a period of maturity, subduction occurs and the period of expansion turns to contraction.
- Finally, the ocean is subsumed between continents and is lost.
The first and third stages can be seen in action on Earth today. The opening phase is clear in the Red Sea, which is splitting Africa and Arabia apart. As for closure, millions of years ago, an ocean separated the Eurasian continent and Africa; its remains lie in the fragmented mix of twisted and uplifted slivers of crust which jam the Mediterranean.
But what about the second phase, the initiation of subduction? According to Dr João Duarte, lead author of the most recent study, there’s virtually no evidence in the geological record of a passive margin becoming an active one, as scientists know must happen. “This is probably because once it happens it erases all the evidence. Everything ends up in the mantle,” he told Decoded Science.