Plate Tectonics

Notes

Plate tectonics was proposed by McKenzie, Parker and Morgan (1967-68), building on continental drift (Wegener) and seafloor spreading (Harry Hess, 1960-62). The lithosphere is broken into rigid plates that float over the plastic asthenosphere and move relative to each other.

Plates

Major (7): Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic plates.
Minor: Cocos, Nazca, Arabian, Philippine, Caribbean, Juan de Fuca, Scotia, Caroline, Fiji.

Types of plate boundaries

Divergent (constructive): plates move apart, new crust forms. Examples: Mid-Atlantic Ridge, East African Rift. Associated with seafloor spreading, volcanoes and shallow earthquakes.
Convergent (destructive): plates collide; one is subducted (or crumples).
- Oceanic-continental: the oceanic plate subducts under the continental plate, forming a trench and volcanic fold mountains (Andes).
- Oceanic-oceanic: an island arc forms (Japan, Mariana Islands).
- Continental-continental: collision, no subduction; forms fold mountains (Himalayas, Alps).
Transform (conservative): plates slide past each other; no crust is created or destroyed. Example: San Andreas Fault.

Driving mechanisms: mantle convection currents (Arthur Holmes, 1929), slab pull and ridge push. Plates move at about 2 to 15 cm per year. The Indian plate moved north and collided with the Eurasian plate about 50 million years ago, raising the Himalayas.

Evidence: the fit of continents, ocean-floor topography and age (youngest at the ridge), paleomagnetic stripes (Vine, Matthews and Morley, 1963), distribution of earthquakes and volcanoes, and hotspots (e.g., Hawaii; the Deccan Traps linked to the Reunion hotspot).

Key Points

Plate tectonics: McKenzie, Parker and Morgan (1967); seafloor spreading: Harry Hess.
Seven major plates; boundaries: divergent, convergent, transform.
Continental-continental collision forms the Himalayas, with no subduction.
Transform boundary: San Andreas Fault.
Driven by convection currents, slab pull and ridge push.

MCQs

Q1. The Mid-Atlantic Ridge is an example of a:

(a) Convergent boundary
(b) Divergent boundary
(c) Transform boundary
(d) Collision zone
Ans: (b).

Q2. The San Andreas Fault is an example of a:

(a) Divergent boundary
(b) Convergent boundary
(c) Transform boundary
(d) Hotspot
Ans: (c).

Q3. The Himalayas were formed by the collision of:

(a) Oceanic and continental plates
(b) Two oceanic plates
(c) Two continental plates
(d) Transform plates
Ans: (c). Indian and Eurasian plates.