LR

Plate Tectonics Vocabulary — Lecture 4

Major Tectonic Plates

  • Eurasia plate
  • Indo-Australia plate
  • North America plate
  • Juan de Fuca plate
  • Cocos plate
  • Pacific plate
  • Nazca plate
  • Caribbean plate
  • Arabia plate
  • Indian plate
  • Africa plate
  • Australian plate
  • Scotia plate
  • Boundary types: convergent, divergent, transform

What Drives Plate Motion

  • Plate motion is driven by mantle convection and gravity.
  • Convection: hot, less dense mantle material rises, interacts with the lithosphere, cools, and sinks, driving motion.

Mantle Convection

  • Hot, less dense mantle material slowly rises from the outer core.
  • As it rises, it hits lithosphere, cools, and sinks again.
  • Convection = motion induced by density changes in a fluid or mobile solid.

What Drives Plate Motion? (Convection Details)

  • This process, including gravity, helps push and pull plates across Earth’s surface.
  • Plates are being pulled down under gravity at subduction zones and pushed at ridges.

Isostasy and Crust Thickness

  • Isostasy: a state of balance where lithosphere is supported by the plastic asthenosphere.
  • Oceanic lithosphere (dense): thickness up to 140\ \text{km}; crust ~ 7\ \text{km}; Fe-rich silicate.
  • Continental lithosphere (less dense): thickness up to 280\ \text{km}; crust ~ 40\ \text{km}; Si, Al-rich.
  • Isostatic balance explains why oceanic crust rests below sea level and continental crust sits higher.

Crust Types

  • Continental Crust: granitic, Al/Si/O-rich; thicker and less dense.
  • Oceanic Crust: basaltic, Fe-rich; thinner and more dense.
  • Granite = continental crust; Basalt = oceanic crust.

Continental vs Oceanic Crust (Composition & Thickness)

  • Continental crust: thicker, less dense, granitic composition.
  • Oceanic crust: thinner, more dense, basaltic composition.

Plate Tectonics: Fundamentals

  • Plates are made of continental crust, oceanic crust, or both.
  • Gray areas on maps often indicate continental crust.

Boundary Classifications

  • Boundary Classification: Ocean–Ocean; Continent–Ocean; Continent–Continental
  • Relative Motion Classification: Convergent; Divergent; Transform
  • Crust Type Classification: Oceanic–Oceanic; Continental–Oceanic; Continental–Continental

Plate Boundary Combinations

  1. Continent–Continental Divergent
  2. Ocean–Ocean Divergent
  3. Continent–Ocean Convergent
  4. Ocean–Ocean Convergent
  5. Continent–Continental Convergent
  6. Ocean–Ocean Transform
  7. Continent–Continental Transform

Divergent Plate Boundaries

  • Divergent boundaries include Continental–Continental and Oceanic–Oceanic types.

Divergent Boundary: Rifting

  • Divergent forces stretch continental crust.
  • Continental crust thins and a rift valley forms.
  • Rift valleys are bounded by faults that experience quakes.
  • Lithosphere melts from the mantle as rifting proceeds toward ocean formation.

New Ocean Crust – Red Sea (Divergence Example)

  • Continued rifting of a continent can create a new ocean basin.
  • Resulting boundary has continental crust on both sides with new oceanic crust forming between.
  • Triple junctions can occur where multiple plate boundaries meet.

Divergence & Magma: Lithosphere Melting

  • As rifting continues, upper mantle melts via decompression melting.
  • Magma is buoyant and rises through fractures.
  • Resulting crust is basaltic and forms oceanic crust from Fe-rich mantle.

Decompression Melting

  • Hot rocks under high pressure melt when pressure decreases.
  • Phase change: solid to liquid as pressure drops, forming magma.

Origin of Oceanic vs Continental Crust

  • Oceanic crust originates from Fe-rich mantle melting to form Fe-rich basalt.
  • Continental crust is Fe-poor and forms granite.

Mid-Atlantic Ridge (Divergence Case)

  • Location: Mid-Atlantic Ridge between North America and Europe; Africa–South America boundary
  • Features: shallow earthquakes; volcanic activity; Iceland as an example of surface volcanism at a boundary
  • Evidence: spreading ridge and basaltic crust with age/magnetic stripe patterns

Summary: Divergent Boundary Characteristics

  • Broad, elongate volcanic ridge with a central rift valley.
  • Cracking/rifting generates new crust.
  • Shallow earthquakes occur along the boundary (<20\ \text{km}).
  • Basalt volcanism due to decompression melting of the mantle.
  • Source of new oceanic crust (Fe-rich basalt).
  • Magnetic stripes and age progression observed in oceanic crust.