Plate Tectonics and Boundaries

Tectonic Plates and Plate Boundaries

What Are Tectonic Plates?

• Tectonic plates, also known as lithospheric plates, are massive, irregularly shaped slabs of solid rock.
• They generally comprise both continental and oceanic lithosphere.
• Faults, which are breaks in the Earth’s crust where rocks have slipped past each other, form along these boundaries.
• Earthquakes frequently occur at these plate boundaries.

Plate Boundaries

• Earth's outer surface is composed of approximately 20 tectonic plates, fitting together like a jigsaw puzzle.
• Plate boundaries are the meeting points of these plates.
• There are three primary types of plate boundaries:
  • Convergent
  • Divergent
  • Transform

Convergent Plate Boundary

• Plates move towards each other.
• Occur at:
  • Oceanic-oceanic crust
  • Continental-oceanic crust
  • Continental-continental crust

Oceanic-Oceanic Convergence

• One oceanic plate subducts under another.
• The older, denser plate sinks into the mantle.
• Landforms:
  • Deep trenches are formed at the subduction zone.
  • Island arcs (volcanoes) form beyond the trench due to the release of trapped water in the mantle.
  • Earthquakes result from stress buildup as the plate descends, potentially causing tsunamis.
  • Example: The Pacific Plate subducting under the Philippine Plate, creating the Marianas Trench.

Oceanic-Continental Convergence

• Denser oceanic crust subducts beneath buoyant continental crust.
• Landforms:
  • A trench forms at the subduction zone.
  • Volcanic arcs (coastal mountains) form on the continent's coast as water is released from the mantle.
  • Subducting plate releases stress through earthquakes and potentially tsunamis.
  • Example: The Andes Mountains along the coast of Chile and the Peru-Chile trench.

Continental-Continental Convergence

• Two plates carrying continental crust collide, causing their edges to crumple and fold upwards.
• This forms mountains because continental crust is too buoyant to subduct.
• Landforms:
  • Mountains
  • Example: The Himalaya Mountains, formed by the collision of the African and European Plates.

Divergent Plate Boundary

• Plates move away from one another.
• Occur:
  • On land
  • In the ocean

Divergent Boundaries on Land

• Plates separate due to spreading centers caused by convection currents in the mantle.
• New crust is pushed to the surface.
• Landforms:
  • Chain of volcanoes and rift valleys.
  • Example: East African Rift Valley (Ethiopia, Kenya, Tanzania, and Uganda).

Divergent Boundaries in the Ocean

• As oceanic plates move apart, large ridges form.
• New crust is pushed upward, displacing older crust.
• Landforms:
  • Large cracks in the Earth’s crust.
  • Ocean ridges.
  • Volcanoes.
  • Example: Mid-Atlantic Ridge in the Atlantic Ocean.

Transform Plate Boundary

• Plates slide past one another.
• Occur at any type of crust.
• Plate edges are not smooth, leading to pressure buildup.
• Eventually, plates slip, releasing energy as an earthquake.
• Landforms:
  • Fault lines (cracks in the Earth’s crust).
  • Tsunamis if the boundary is underwater.
  • Example: San Andreas Fault.

Types of Plate Boundaries: Summary

Plate Tectonics Theory

• 1912: Alfred Wegener proposes the idea of “Continental Drift”.
• 1929: Arthur Holmes stated that the continents are carried by flow of the mantle.
• 1950s: Discovery of Mid-ocean ridges and the process of seafloor spreading.
• 1960s: Mid-ocean ridges and ocean floor are explored.
• 1970s: Development of seismic tomography reveals more of Earth’s interior.