Question 8 of 9: Plate Tectonics and the Rock Cycle

Question 8: Plate Tectonics and the Rock Cycle (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.

Plate tectonics supplies the driving heat, pressure, and material recycling that powers every arm of the rock cycle. At divergent boundaries (mid-ocean ridges), mantle upwelling and decompression melting generate basaltic magma that cools to form new oceanic igneous crust; at convergent boundaries, subduction of a cold oceanic plate carries hydrated crust and sediment to depth, where dewatering lowers the mantle wedge's melting point and produces the andesitic–granitic magmatism of volcanic arcs (igneous rock), while the intense heat and directed stress in the subduction zone and continental collision belt drive regional metamorphism (subduction-zone blueschist/eclogite; collision-belt schist/gneiss). Mountain building at convergent margins also uplifts crust into the reach of surface weathering and erosion, generating the sediment that is transported, deposited, buried, and lithified into sedimentary rock in adjacent basins – sediment that itself may later be subducted or deeply buried and metamorphosed, or melted, continuing the cycle. Transform boundaries contribute comparatively little magmatism but generate dynamic (cataclastic) metamorphism along the fault zone itself.

Crucially, the rock cycle has no fixed starting point and no single direction: a metamorphic rock can be weathered into sediment; sediment can be buried deep enough to metamorphose without ever melting; an igneous rock can be uplifted, eroded, and redeposited as sediment derived from itself. Plate tectonics is the engine that supplies the three ingredients – heat, pressure, and the vertical/lateral motion that exposes deep rock to surface weathering or buries surface rock to depth – that keep the cycle running continuously over geologic time.