Plate Tectonics: Subduction, Convergence, Transform Boundaries, and Hotspots

Detection of Subduction Zones

• Earthquakes: The primary method to detect subduction.

  • Earthquakes occur deep within the subducting plate, far deeper than expected in the flowing asthenosphere, indicating that the rigid lithosphere is still present and undergoing brittle fracture.

  • The trajectory of these deep earthquakes can be traced upwards to the flexing oceanic plate at the surface.

  • Earthquakes also occur in the overriding plate due to friction and stress where the plates are forced together.

• Deep Sea Trenches: Visible surface feature indicating subduction.

  • Form where the oceanic plate flexes and dips downward, creating very low spots on the seafloor.

  • Trenches can be $2$ to $3$ times deeper than the typical ocean basin.

  • Contrary to diagrams, the drop-off into a trench is gradual, not sudden, reflecting the gentle bending of a large slab of rock.

Volcanic Activity at Convergent Boundaries: Arc Volcanoes

• Process: As the subducting oceanic plate descends into the mantle, it melts, and this magma rises to the surface, forming volcanoes.

• Arc Shape: Volcanoes at subduction zones typically form a curved pattern (an