6.2 Plate tectonics & tectonic settings

From Continental Drift to Plate Tectonics

Alfred Wegener proposed continental drift in 1912, citing the jigsaw fit of the continents, matching fossils such as Mesosaurus and Glossopteris across oceans, aligned mountain belts, and ancient glacial deposits on now-tropical land. Lacking a workable mechanism, his idea languished until mid-twentieth-century seafloor research supplied one. Plate tectonics now unifies these observations: Earth's rigid lithosphere (crust plus uppermost mantle) is broken into plates that move over the weaker, ductile asthenosphere, propelled by mantle convection, ridge push, and slab pull.

Earth has about seven major plates — the Pacific, North American, South American, Eurasian, African, Indo-Australian, and Antarctic — plus many smaller ones. Almost all earthquakes, volcanoes, and mountain-building occur at their boundaries rather than their interiors, which is why a map of global seismicity outlines the plates almost perfectly. Modern GPS geodesy now measures these motions directly, confirming rates of a few centimeters per year.

Two Kinds of Crust

The two crust types behave very differently at plate boundaries. Oceanic crust is thin (about 7 kilometers), dense, and basaltic, whereas continental crust is thick (roughly 25 to 70 kilometers), buoyant, and granitic. This density contrast is why oceanic lithosphere readily subducts while continental lithosphere resists it, and why the oldest ocean floor is geologically young even though some continental rocks approach four billion years in age. Because buoyant continental crust floats higher on the mantle, the principle of isostasy keeps thick crust standing as tall mountains with deep, low-density roots, balanced much like an iceberg in water.

Continental margins are described as active where a plate boundary runs along them — the earthquake- and volcano-prone Pacific coast is the classic example — or passive where they lie within a plate far from any boundary, like the tectonically quiet, sediment-draped Atlantic seaboard. Where three plates meet, the boundary is a triple junction, and the worldwide network of ridges, trenches, and transforms links into a single moving mosaic.

Lines of Evidence

• Seafloor spreading (Harry Hess): new oceanic crust forms at mid-ocean ridges and moves symmetrically away, so the ocean floor is youngest at ridges, oldest near trenches, and nowhere older than about 200 million years.
• Paleomagnetism: as basalt cools past its Curie point, magnetite locks in the ambient magnetic field. Symmetric magnetic stripes of normal and reversed polarity mirror each ridge (the Vine-Matthews-Morley hypothesis), directly recording spreading. Apparent polar wander paths further show that the continents, not the poles, moved.
• Narrow belts of earthquakes and volcanoes, systematic heat-flow patterns, and matched geology across rifted margins all corroborate the model.

The Three Plate Boundaries

Divergent boundaries pull plates apart. At mid-ocean ridges, magma rises, cools, and builds new oceanic lithosphere, producing shallow earthquakes, basaltic volcanism, and high heat flow. On land, divergence begins as a rift valley such as the East African Rift.

Convergent boundaries bring plates together, and the outcome depends on the crust involved. In ocean-continent collision, the denser oceanic plate subducts beneath the continent, creating a deep trench, a Wadati-Benioff zone of earthquakes that deepen away from the trench, and a continental volcanic arc like the Andes. In ocean-ocean convergence, one plate subducts to build a volcanic island arc such as Japan or the Aleutians. In continent-continent collision, neither plate subducts easily, so the crust crumples into great orogenic belts like the Himalaya. Subduction zones host the planet's deepest earthquakes and most explosive volcanoes.

Transform boundaries slide plates horizontally past one another along strike-slip faults, so lithosphere is neither created nor destroyed. Most offset mid-ocean-ridge segments on the seafloor; a few, like the San Andreas Fault, cut through continents and generate large, shallow earthquakes.

Hot Spots and the Wilson Cycle

Hot spots are long-lived plumes of hot mantle largely independent of plate boundaries. As a plate drifts over a fixed plume it leaves a chain of progressively older volcanoes — the Hawaiian-Emperor seamount chain, whose age progression and sharp bend record changes in plate motion. Yellowstone is a continental hot spot sitting above such a plume.

The Wilson cycle describes the life of an ocean basin: (1) a continent rifts; (2) a young sea opens, like the Red Sea; (3) a mature ocean widens with passive margins, like the Atlantic; (4) subduction begins and the ocean starts to close; and (5) continents collide and are sutured into mountains like the Himalaya, eventually reassembling into a supercontinent such as Pangaea. The cycle knits divergent, convergent, and transform processes into one continuous narrative.

Driving Forces

• Slab pull — dense subducting slabs sink and drag their plates along; now considered the dominant force.
• Ridge push — elevated, hot ridges gravitationally shove plates away from the crest.
• Mantle convection and basal drag — large-scale flow in the mantle beneath the plates.

Together these forces move plates only a few centimeters per year, roughly the rate fingernails grow, yet over geologic time they open entire oceans and raise the highest mountain ranges.