Plate Boundaries: Processes & Landforms
Convergent Plate Boundaries
• Plates move toward each other; density controls which plate dives.
• Oceanic–Continental: denser oceanic plate subducts under continental plate → forms trench at meeting line, mantle melting produces magma, rises as mantle plume → surface volcanic arc on continent; frequent earthquakes; under-water displacement may trigger .
• Oceanic–Oceanic: older/denser oceanic plate subducts → trench + magma → chain of volcanoes that rise above sea level as volcanic island arc; earthquakes and possible tsunamis accompany movement.
• Continental–Continental: similar densities → no subduction. Plates crumple, uplifting wide mountain ranges; produces shallow earthquakes, but neither volcanism nor tsunamis.
Divergent Plate Boundaries
• Plates move apart, creating tension zone; mostly under oceans.
• Vertical gap develops into rift valley; mantle material (magma) rises to fill space, cools, and solidifies, forming new crust.
• Continuous supply and sideways push of new material cause seafloor spreading and build long underwater mountain chains (oceanic ridges).
• Movement produces shallow earthquakes.
Transform Fault Boundaries
• Plates slide horizontally past each other (e.g., San Andreas, West Valley Fault).
• Movement fractures crust, generating nearly vertical cracks called faults.
• Sudden release of built-up stress yields strong, abrupt earthquakes; no subduction, volcanism, or tsunami generation directly tied to this boundary type.
Quick Recall
• Subduction, trenches, magma generation, volcanic arcs/island arcs, tsunamis → only where at least one oceanic plate converges.
• Mountain ranges without volcanism → continental–continental convergence.
• Rift valley + oceanic ridge + new seafloor → divergence.
• Faults + powerful quakes → transform boundaries.
• Earthquakes occur at all boundary types; depth/intensity varies with mechanism.