Plate Tectonics
Plate Tectonics Theory
- Earth’s lithosphere is broken into moving plates.
- Plates drift on semi-fluid asthenosphere at \approx 1{-}2\ \text{in/yr}.
Types of Plates
- Oceanic: thin, dense basaltic crust beneath oceans.
- Continental: thick, light granitic crust beneath continents.
Driving Forces
- Mantle convection: hot material rises, cools, sinks → drags plates.
- Gravity: slab pull & ridge push augment motion.
Plate Boundary Types
- Divergent – apart
- Convergent – collide
- Transform – slide past
Divergent Boundaries
- “Constructive”; rifting causes seafloor spreading and new crust.
- Features: mid-ocean ridge, rift valley, fissure volcanoes.
- Example: Mid-Atlantic Ridge through Iceland (continental rifting).
Convergent Boundaries ("Destructive")
- Collisions yield strong quakes, subduction, volcanism.
Oceanic–Continental
- Oceanic plate subducts → trench + continental volcanic arc.
- Example: Andes Mountains.
Oceanic–Oceanic
- Older/denser slab subducts → deep trench + island arc.
- Example: Mariana Trench, Aleutian Islands.
Continental–Continental
- Neither plate subducts; crust crumples into high mountains.
- Example: Himalayas from India–Asia collision.
- Plates grind laterally; crust conserved.
- Produce shallow, powerful earthquakes; create fault valleys.
- Example: San Andreas Fault (California).