RAS question
What does the term 'subduction' mean in plate tectonics?
Correct answer: (A) One plate sliding under another at a convergent boundary.
In plate tectonics, subduction is the process at a convergent boundary where one tectonic plate slides beneath another and sinks into the mantle.
Explanation
Subduction means underthrusting at a convergent plate boundary: one tectonic plate moves below another, rather than the two plates separating or sliding side by side. The USGS plate-tectonics page identifies subduction zones as places where one plate is sliding beneath another and marks the overriding plate in such zones. The denser, usually oceanic, plate sinks below the less dense, usually continental, plate into the mantle. That movement is important because it is associated with trenches and volcanic arcs. Hence option A is the precise meaning of subduction in plate tectonics.
Why the other options are wrong
- (B) Plates moving apart describe a divergent boundary, unlike subduction at a convergent boundary.
- (C) Horizontal sliding is a transform or strike-slip setting; the U.S. Geological Survey, Volcanoes: Plate-Tectonics Theory separately describes it as plates sliding horizontally past one another, not one plate going beneath another.
- (D) A mid-ocean ridge forms where plates move apart at a divergent boundary, so it is linked to spreading rather than subduction.
Concept
This tests plate-boundary processes, especially the distinction between convergent, divergent and transform margins. RAS repeats this idea because tectonic setting explains landforms such as trenches, volcanic arcs and mid-ocean ridges.