Plate Boundaries

Plate Tectonics Theory

6.24 Three Types of Lithospheric Plate Boundaries

  • General Types of Lithospheric Plate Boundaries:

    • Divergent Boundary:

    • Definition: Plates are pulled apart by tensional forces.

    • Result: Creation of new oceanic crust.

    • Examples:

      • Mid-ocean ridges in the world's ocean basins.

      • Continental rifting areas include:

      • Africa rift zones,

      • Red Sea basin,

      • Great Basin region in North America (Gulf of California).

    • Convergent Boundary:

    • Definition: Plates are pushed together by compressional forces.

    • Result: Collision of lithospheric plates forms mountain belts.

    • Examples:

      • Himalayas,

      • Alps,

      • Appalachian Mountains (formed during Pangaea's existence).

    • When continents collide with ocean crust:

      • Formation of subduction zones with deep ocean trenches and volcanic arcs.

      • Examples include:

      • Andes Mountains,

      • Aleutian Islands,

      • Japan,

      • Philippines,

      • Indonesia,

      • Ancient Sierra Nevada,

      • Cascades Range in northern California, Oregon, Washington.

    • Transform Boundary:

    • Definition: Plates slide past one another or are rotational.

    • Result: Creation of fault systems along plate margins.

    • Examples:

      • San Andreas Fault in California,

      • Major faults in Pakistan,

      • Turkey,

      • Jordan River/Dead Sea.

6.25 Divergent Plate Boundaries

  • Definition: Divergent plate boundaries are locations where tensional forces pull plates apart.

  • Rates of Divergent Motion: Range from 2 to 17 cm/year.

  • Spreading Centers:

    • Definition: Linear areas where new crust forms as two crustal plates move apart, often along mid-oceanic ridges.

    • Characteristics:

    • Seismically active regions,

    • Associated with frequent volcanism.

    • Youngest rocks on the ocean floor are primarily located along mid-ocean ridges.

6.26 Seafloor Spreading Formation of New Ocean Crust at Divergent Plate Boundaries

  • Features and Processes Include:

    • Involvement of oceanic crust only.

    • Tensional forces generated by gentle flow in the upper mantle pull oceanic crust apart, forming faults.

    • Volcanic activity along these faults allows magma to rise and cool, generating new seafloor/oceanic crust.

    • Formation of mid-oceanic ridges where faulting and volcanism are most active.

    • Older oceanic crust moves away from the spreading center, while new crust forms along the MOR.

    • Characteristics of new crust formation:

    • Youngest oceanic crust is found near ridges, oldest (and colder) is farther away,

    • MOR is symmetrical about the ridge with new crust accumulating equally on both sides.

    • Magnetic reversals are preserved in volcanic rocks along MORs.

    • Marine sediments thicken away from the ridge.

    • Hydrothermal Vents: Hot water vents (black smokers) are found along spreading centers, indicating volcanic activity.

Ocean Rises vs. Ridges

  • Different rates of spreading lead to variations in physical characteristics.

    • Rise: Spreads faster, less steep.

    • Ridge: Spreads slower, steeper.

  • Examples:

    • East Pacific Rise,

    • Mid-Atlantic Ridge,

    • Iceland's Mid-Atlantic Ridge segment (spreading rate: ~15 cm/yr).

6.27 Continental Rifting

  • Definition: Diverging boundaries form within continental landmasses.

    • Involves rifting (pulling apart) of continental crust only.

  • Causes: Possibly due to convection in the asthenosphere.

  • Processes:

    • Large sediment production in rifting zones,

    • Initial formation of lakes, rivers, and beaches followed by oceanic flooding.

  • Examples of Current Continental Rifts:

    • Red Sea,

    • East African Rift System.

  • Historical Context:

    • Breakup of Pangaea began with continental rifting approximately 200 million years ago, leading to the formation of the Atlantic Ocean.

6.28 Crustal Extension and Continental Rifting

  • Definition: Crustal extension develops where divergent plate boundaries occur under continental landmasses on a large scale.

  • Regions Affected:

    • Gulf of California,

    • Great Basin region (Nevada, Arizona) extending into New Mexico.

  • Driving Process:

    • As the Atlantic Ocean opened, the North American continent was pushed over a spreading center beneath Utah and Nevada.

  • Future Predictions:

    • Baja California moving northward and potentially colliding with Alaska in the future.

6.29 Convergent Plate Boundaries

  • Definition: Convergent plates move toward one another under compression.

  • Characteristics:

    • Associated with mountain building, earthquakes, and andesite volcanoes.

    • Most mountain ranges form as a result of convergence and associated crustal compression.

  • Subduction Zone:

    • Defined as a plate boundary along which one plate descends beneath another, marked by deep trenches.

    • Example: Cascadia Subduction Zone offshore of Washington, Oregon, Northern California.

  • Significance:

    • Most tsunamis result from earthquakes at subduction zones.

6.30 Three Types of Convergent Plate Boundaries

  • Types:

    1. Subduction of ocean crust (OC) beneath continental crust (CC).

    2. Subduction of ocean crust (OC) beneath ocean crust (OC).

    3. Continental collisions (continental crust CC colliding with continental crust CC).

6.31 a) Subduction of OC Beneath CC

  • Process: Denser OC is subducted beneath less dense CC.

  • Result: Formation of a continental volcanic arc due to explosive volcanism, e.g., Andes, Cascades.

  • Outcome: Produces deep and shallow focus earthquakes (with tsunami potential).

  • Rates of Subduction: Up to 15 cm/year in active Pacific margins.

6.32 b) Subduction of OC Beneath OC

  • Comparison to OC/CC: Similar features but involve older, denser, cooler OC being pushed under less dense OC.

  • Result: Formation of island volcanic arcs and a variety of seismic activity.

  • Examples: Japan, Tonga Islands, Aleutian Islands.

6.33 c) Continental Collisions

  • Characteristics:

    • When continents collide, no subduction occurs; both CC float high.

    • Formation of large mountain chains with significant seismic activity.

  • Examples of Mountain Ranges:

    • Himalayas (India/Eurasia collision),

    • Alps (Africa/Europe collision),

    • Appalachian Mountains (North America/Africa collision).

6.34 Transform Boundaries

  • Definition: Locations where one plate slides past another without producing or destroying crust.

  • Two Types:

    • Continental Transform Faults:

    • Example: San Andreas Fault in California.

    • Earthquake magnitudes can reach up to 8.5.

    • Average rates: 2-5 cm/year.

    • Structural impacts include mountain formation from compression.

    • Oceanic Transform Faults:

    • Smaller faults perpendicular to spreading centers,

    • Associated with smaller earthquakes than continental faults.

6.35 Examples of Plate Boundaries

  • Summary Table:

    • Divergent Boundaries:

    • Mid-ocean ridges separate multiple lithospheric plates (e.g., North and South American plates from African and Eurasian plates).

    • Pacific Plate: Mainly oceanic,

    • Nazca Plate: Moving eastward toward South America, creating the Andes Mountains through subduction.

  • Pacific Ring of Fire: Region associated with significant volcanic and seismic activity due to converging plate boundaries.

Important Note

  • Continental Margins vs. Plate Boundaries:

    • Continental margins may not correspond to plate boundaries.

    • Example: East Coast is part of the North American Plate, result of historical rifting and spreading processes.

    • Passive margins are not active plate boundaries but originated from ancient rifting when Pangaea formed.