Plate Tectonics: Boundaries and Lithospheric Plates

Plate Boundaries

• These are the areas between where two plates have separated or diverged from one another.

Lithospheric Plates

• Also known as tectonic plates
• Can be large or small in size
• Formed, deformed, and destroyed over roughly 400,000 years (Wilson Cycle).
• Important examples include the Pacific, North American, and Eurasian plates.
• Most dynamic geological activity occurs at plate boundaries
  • Examples include volcanic and seismic (earthquake) activity.

Types of Plate Boundaries

• Divergent/Constructive: Plates move apart.
• Convergent/Destructive: Plates come together.
• Transform/Conservative: Plates slide past one another.

Transform Boundaries

• Plates slide past each other
• Lithosphere is neither lost nor added
• Earthquakes are common, but volcanic activity is not
• Famous examples include the San Andreas Fault and the Alpine Fault.

Divergent Boundaries

• Plates move apart
• New lithosphere is created (constructive)
• Site of some of the youngest rocks on Earth
• Magma rises to fill the gap as plates separate
• Associated with volcanism (eruptions, hot springs) and some seismic activity
• Typically submarine, with exceptions like Iceland
• Example: South America moving away from Africa.

Convergent Boundaries

• Plates come together, resulting in crust consumption.
• Three types: oceanic-continental, oceanic-oceanic, and continental-continental.

Oceanic-Continental Convergence

• The denser oceanic plate subducts beneath the continental plate at a subduction zone.
• Creates orogenic activity (mountain building)
• Results in earthquakes and volcanoes

Oceanic-Oceanic Convergence

• One oceanic plate subducts beneath another, based on temperature (denser, cooler plate sinks).
• Forms a subduction zone
• Can result in earthquakes and volcanoes

Continental-Continental Convergence

• Plates of relatively equal densities collide and squeeze upwards.
• Compression leads to tall mountains and seismic activity
• The area where they meet is called the suture zone
• Example: Himalayas
  • Fossil evidence at the top of Mt. Everest

Plate Movement Mechanisms

• Three mechanisms contribute to plate movement: convection current, ridge push, and slab pull.

Convection Current

• Warmer rock/magma rises, cools, and sinks, dragging the lithosphere along.
• Heat source: radioactive element decay.

Ridge Push

• Gravity causes elevated lithosphere at the ridge axis to push lithosphere further away from the axis.
• Occurs at divergent boundaries, such as mid-ocean ridges
• Operates due to gravity.

Slab Pull

• Occurs at convergent boundaries (oceanic subduction zones).
• Older, colder, and therefore denser plates sink, pulling the rest of the plate downwards.
• Operates due to gravity.
• This appears to be the more dominant force in plate movement out of the two (ridge push and slab pull).

Explore plate boundaries: divergent, convergent, and transform. Learn about lithospheric plates, their formation, and the geological activities at their boundaries such as earthquakes and volcanoes. Examples include the San Andreas and Alpine Faults.