2.2 Types of plate boundaries: divergent, convergent, and transform

Plate boundaries are where tectonic plates meet, shaping Earth's surface. These zones come in three types: divergent (plates move apart), convergent (plates collide), and transform (plates slide past each other). Each type creates unique landforms and geological processes.

Understanding plate boundaries is key to grasping Earth's dynamic nature. They explain why we have mountains, volcanoes, and earthquakes. By studying these boundaries, scientists can better predict geological events and understand our planet's past and future.

Plate Boundary Types

Classification and Characteristics of Plate Boundaries

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• Plate boundaries form zones where tectonic plates interact resulting in various geological processes and landforms
• Three main types of plate boundaries exist based on relative plate motion and crust type (oceanic or continental)
  • Divergent boundaries occur where two plates move away from each other creating new crust
  • Convergent boundaries form where two plates move towards each other resulting in subduction or collision
  • Transform boundaries develop where two plates slide past each other horizontally with no creation or destruction of crust
• Plate boundary classification depends on:
  • Relative motion of the plates involved
  • Type of crust at the boundary (oceanic or continental)

Specific Plate Boundary Examples

• Divergent boundary examples:
  • Mid-Atlantic Ridge (oceanic setting)
  • East African Rift System (continental setting)
• Convergent boundary examples:
  • Mariana Trench (oceanic-oceanic subduction)
  • Himalayas (continental-continental collision)
• Transform boundary examples:
  • San Andreas Fault (continental transform)
  • Fracture zones offsetting mid-ocean ridge segments (oceanic transform)

Geological Processes at Plate Boundaries

Divergent Boundary Processes

• Characterized by seafloor spreading forming new oceanic crust
• Rift valley formation in continental settings
• Processes involved:
  • Upwelling of magma from the mantle
  • Cooling and solidification of magma to form new crust
  • Extensional forces causing thinning and rifting of existing crust
• Associated features:
  • Mid-ocean ridges (underwater mountain ranges)
  • Rift valleys (elongated depressions)
  • Volcanic activity (basaltic lava flows)

Convergent Boundary Processes

• Subduction occurs where one plate descends beneath another (typically oceanic under continental or oceanic)
• Continental collision happens when two continental plates meet
• Subduction zone processes:
  • Formation of deep oceanic trenches
  • Development of volcanic arcs (island arcs or continental volcanic belts)
  • Creation of accretionary wedges (sediment accumulation)
  • Metamorphism of subducted materials
• Continental collision processes:
  • Extensive mountain building (orogeny)
  • Crustal thickening and deformation
  • Metamorphism of rocks due to high pressure and temperature

Transform Boundary Processes

• Associated with strike-slip faulting causing horizontal displacement
• Processes and features:
  • Offset of geological features (rivers, rock formations)
  • Generation of shallow frequent earthquakes
  • Development of linear fault zones
  • Minimal vertical relief but significant horizontal movement
• Transform faults also occur as fracture zones offsetting mid-ocean ridge segments

Plate Boundary Comparisons

Forces and Motions at Boundaries

• Divergent boundaries experience extensional forces pulling plates apart
• Convergent boundaries dominated by compressional forces pushing plates together
• Transform boundaries undergo shear forces as plates slide past each other
• Plate motion rates vary among boundary types:
  • Divergent boundaries typically move slower (few cm/year)
  • Convergent and transform boundaries often move faster (up to 10 cm/year or more)

Resulting Landforms and Features

• Divergent boundaries create:
  • Rift valleys in continental settings (East African Rift)
  • Mid-ocean ridges in oceanic settings (Mid-Atlantic Ridge)
• Convergent boundaries form:
  • Mountain ranges (Andes Mountains)
  • Volcanic arcs (Ring of Fire)
  • Deep ocean trenches (Mariana Trench)
• Transform boundaries produce:
  • Linear fault zones (San Andreas Fault)
  • Offset features with little vertical relief
• Crust type influence on landforms:
  • Oceanic-oceanic convergence leads to island arcs (Japan)
  • Continental-continental convergence results in large mountain ranges (Himalayas)

Driving Forces of Plate Motion

Primary Driving Mechanisms

• Mantle convection acts as the primary driving force behind plate tectonics
  • Creates upwelling and downwelling currents moving the overlying plates
  • Convection cells transport heat from Earth's interior to the surface
• Ridge push at divergent boundaries occurs due to:
  • Elevation difference between the ridge and older cooler denser oceanic crust
  • Gravitational potential energy of the elevated ridge
• Slab pull at convergent boundaries caused by:
  • Negative buoyancy of cold dense subducting plate sinking into the mantle
  • Considered the dominant force in plate tectonics

Secondary Forces and Influences

• Gravitational sliding (trench suction) contributes to plate motion at convergent boundaries
  • Subducting plate pulls the overriding plate towards the trench
• Frictional forces at transform boundaries resist plate motion
  • Overall plate movement maintained by forces acting on other plate boundaries
• Complex patterns of global plate motions determined by:
  • Balance of driving and resisting forces
  • Variations in mantle viscosity
  • Differences in plate strength and thickness
• Other factors influencing plate motion:
  • Tidal forces (minor effect)
  • Earth's rotation (Coriolis effect on large-scale mantle flow)