The Wilson Cycle — Ocean Basin Opening and Closing: 18-Geol-A1 Mineralogy and Petrology, Undated paper, Q8

Question 8: The Wilson Cycle — Ocean Basin Opening and Closing (Part 2 – 10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The Wilson Cycle is the plate-tectonic model describing the repeated opening and closing of ocean basins through a full cycle of continental rifting, seafloor spreading, subduction and continental collision — a cycle that can take an entire supercontinent from breakup back to a new collisional suture over roughly 300–500 Myr. Each stage has a modern or ancient type example.

The six idealized Wilson Cycle stages, from continental rifting through a mature passive-margin ocean to subduction, closure and continental collision.

The six stages

Continental rifting — mantle upwelling/thermal doming stretches and thins continental lithosphere, producing normal faulting and a rift valley (e.g. East African Rift).
Young (embryonic) ocean basin — rifting succeeds, seafloor spreading begins along a narrow, young mid-ocean ridge (e.g. Red Sea).
Mature ocean basin — continued spreading widens the basin; both margins are tectonically quiet, passive margins with thick sediment wedges (e.g. modern Atlantic Ocean).
Subduction initiation — the oceanic lithosphere, now old, cold and dense, begins to subduct at one margin, converting a passive margin into an active, volcanic-arc margin (e.g. Andean-type margin, western South America).
Closing ocean basin — continued subduction consumes oceanic lithosphere faster than spreading creates it, shrinking the basin while arc/continental-margin magmatism continues (e.g. the former Tethys Ocean prior to India–Asia collision).
Continental collision and suturing — once all intervening oceanic lithosphere is consumed, the two continental masses collide, thickening crust into a mountain belt and preserving slivers of the former ocean floor as obducted ophiolite along the suture (e.g. Himalaya–Tibet, recording closure of Tethys).

Topic. The Wilson Cycle: the full rift-to-collision plate-tectonic cycle of ocean basin opening and closing.

Key relations. Opening (rift → young ocean → mature ocean) is driven by extension and seafloor spreading; closing (subduction initiation → basin shrinkage → collision) is driven by convergence once old, dense oceanic lithosphere begins to subduct; the cycle completes with a sutured mountain belt, often marked by obducted ophiolite.

Why this works. Oceanic lithosphere becomes progressively denser as it ages and cools (thermal subsidence), which is why every ocean basin eventually becomes gravitationally unstable enough to subduct somewhere along its margin regardless of how it opened — the Wilson Cycle is therefore not a single unusual event but the expected long-term fate of any rifted ocean basin.

Common pitfall. Treating the Wilson Cycle as strictly linear/one-directional at a single location; different segments of the same original rift system can be at different stages simultaneously (e.g. the modern Red Sea is still opening while the Mediterranean, part of the same former Tethyan system, is already closing).

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §1–2, 12–13 (plate tectonics framework, tectonic settings of magmatism, ophiolites).

Question 8 of 12: The Wilson Cycle — Ocean Basin Opening and Closing

Question 8: The Wilson Cycle — Ocean Basin Opening and Closing (Part 2 – 10 marks)

The six stages