Plate tectonics

Plate tectonics. In 1967, McKenzie and Parker, and separately Morgan, brought the ideas together. A tectonic (lithospheric) plate is a huge, irregular slab of solid rock, of continental and/or oceanic lithosphere, that moves horizontally over the asthenosphere as a rigid unit. A plate is called continental or oceanic by what covers most of it: the Pacific plate is oceanic; the Eurasian plate continental.

Seven major plates: Antarctica and surrounding ocean; North American; South American (the two separated along the Caribbean islands); Pacific; India-Australia-New Zealand; Africa with the eastern Atlantic floor; Eurasia and adjacent ocean. Young fold mountains, trenches and faults surround them.

Some minor plates: Cocos (between Central America and the Pacific plate), Nazca (between South America and the Pacific plate), Arabian (mostly the Arabian landmass), Philippine (between the Asiatic and Pacific plates) and Caroline (north of New Guinea).

• It is the plate that moves, not the continent alone, as Wegener thought. All plates have moved throughout the earth's history and will keep moving. Pangaea itself was formed when continental masses on different plates came together.
• Palaeomagnetic data show where each landmass was in the past; India's past positions were traced from rocks near Nagpur.

Three kinds of plate boundary:

• Divergent: plates pull apart and new crust forms at spreading sites; the best-known is the Mid-Atlantic Ridge.
• Convergent: one plate dives under another and crust is destroyed, at a subduction zone. It can be ocean-continent, ocean-ocean or continent-continent.
• Transform: plates slide past each other; crust is neither made nor destroyed. Transform faults usually run perpendicular to the ridges.

Rates of movement come from the strips of normal and reverse magnetism parallel to the ridges. The Arctic Ridge is the slowest (under 2.5 cm a year); the East Pacific Rise near Easter Island is the fastest (over 15 cm a year).

The driving force. Hot, softened mantle below the plates moves in a cycle: heated material rises, spreads, cools and sinks again, forming convection cells. The heat comes from radioactive decay and residual heat. Holmes's idea of the 1930s later shaped Hess's thinking.

Example (new): at 5 cm a year, two plates would move 5 × 1,000,000 = 50,00,000 cm, that is 50 km, apart in a million years: slow in a lifetime, huge over geological time.