The Wilson Cycle: Intro

“If the continents have moved, then they have drifted like rafts and formed the ocean floors in their wake. It is to this wake that we should look first.”
~ John Tuzo Wilson

Canadian geophysicist and geologist, John Tuzo Wilson, posed the question (and the title of his article in 1966) - ‘Did the Atlantic close and then re-open?’ Spoiler alert - yes. That, and his many contributions to plate tectonics, including the concept of hotspots and transform faults, led to the Wilson Cycle (also known as the Supercontinent Cycle) being named after him.

The Wilson Cycle (WC) refers to the process of continent break-up and ocean-opening followed by subduction, collision, ocean-closing, and continent formation (see the diagram below). This can take tens to hundreds of millons of years (very deep time) to complete.

This quick video provides animation of the WC: https://youtu.be/I_q3sAcuzIY

Step 1 of the WC starts with a tectonically stable continent/craton, eroded down and perhaps scarred by earlier collisions. Rifting (or faulting), crustal thinning, and thermal uplift caused by tectonic stretching of the continent allows the upper mantle (plume) to rise up and fill in. This can lead to earthquakes and volcanic flows. Sometime the plume can die out leading to a failed rift, but when rifting continues, things get quite interesting.

Step 2 The fractures are deep and oriented perpendicular to the extensional direction. As the continent breaks apart, the plume develops convection cells that further the rifting and deepen the basin allowing water in. The mantle material exposed by the rifting is made of much denser (or mafic) material than continental crust (or felsic) and sinks, cools and hardens/crystallizes forming oceanic crust. A new ocean basin is created.

Step 3 The two new continents continue to drift apart; the rift becomes a young spreading ridge, and the new ocean crust sinks further into the mantle as it cools and becomes denser. Sediment is now collecting on the new ocean floors.

Step 4 The Mid Ocean Ridge (MOR) continues to create new ocean crust and the new ocean deepens as the oceanic crust matures and continues to sink into the mantle. The mature oceanic crust is much heavier than the bordering continental crust, and cracks can develop causing the oceanic crust to flex downwards forming a young subduction zone. Part of the oceanic crust is dragged deep into trench and the water-laden oceanic crust melts due to the higher temperatures of the mantle. Volcanic Island arcs are created. The rifted continental crust is now well below the surface of the ocean.

Step 5 Divergence ceases, and convergence begins. The MOR is eventually subducted, or consumed at the ocean basin margin. Associated volcanism and subduction continues, along with collision, narrowing the ocean and causing mountains to form. This is now part of the ocean-closing cycle.

Step 6 As the continents/cratons continue to collide, folding, faulting, and earthquakes occur creating new mountains (think of the Himalayas), while catching up bits of volcanic rock, oceanic crust and sedimentary rock. A new continent is formed.

Step 7 The continent matures and erodes. Rinse and repeat with continents colliding, eventually forming supercontinents, and dispersing again in a much longer and even deeper time.

The Wilson Cycle is somewhat simplified and doesn’t go into all the sorts of tectonic variations of rift zones and diversity of plate tectonics, but it was a landmark starting point and a sign of Wilson’s genius. Later geologists and geophysicists stand on the shoulders of this giant. We’ll dive further into the Wilson Cycle in future posts, and have a look at the rock types created and how geologists piece together the Wilson Cycle in real rocks. It’ll be fun she said :)

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