4: Plate Tectonics

Plate Tectonics

The unifying theory that Earth's lithosphere is divided into rigid plates that move over the asthenosphere, explaining the formation of continents, oceans, mountains, volcanoes, and earthquakes.

Alfred Wegener

German meteorologist and geophysicist who proposed the Continental Drift Theory in 1915.

Continental Drift Theory

The hypothesis that continents were once joined in a supercontinent (Pangea) and have since drifted apart.

Pangea

The supercontinent that existed about 300 million years ago, comprising all of Earth's landmasses.

Evidence for Continental Drift

Jigsaw fit of continents, matching geological formations and rock types, distribution of identical fossils across oceans, and paleoclimate indicators (e.g., glacial deposits in tropics).

Limitations of Early Continental Drift Theory

Lack of a known mechanism for moving continents, imperfect continent fits, and rejection by the geological establishment.

Contractionism

An early 20th-century theory that Earth's mountains formed as the planet cooled and contracted.

Permanentism

The belief that continents and oceans have always been in their current positions, explained by land bridges and geosynclines.

Paleomagnetism

The study of Earth's past magnetic field recorded in rocks; key evidence proving continental movement via Apparent Polar Wandering Paths (APWP).

Sea-Floor Spreading

The process by which new oceanic crust forms at mid-ocean ridges and spreads outward, proposed by Harry Hess (1960).

Harry Hess

American geologist who proposed the sea-floor spreading hypothesis, a cornerstone of plate tectonic theory.

Vine-Matthews-Morley (VMM) Hypothesis

Explains the symmetrical pattern of magnetic stripes on the ocean floor as recording reversals of Earth's magnetic field during sea-floor spreading.

J. Tuzo Wilson

Canadian geophysicist who contributed the concepts of hotspots, transform faults, and the Wilson Cycle, and helped unify plate tectonic theory.

Hotspot

A stationary plume of hot mantle material that creates volcanoes on an overlying moving plate (e.g., Hawaiian Island chain).

Transform Fault

A plate boundary where plates slide horizontally past each other, connecting segments of divergent boundaries; earthquakes occur along the active segment.

Wilson Cycle

The cyclical process of the opening and closing of ocean basins via rifting, sea-floor spreading, subduction, and continental collision.

Lithosphere

The rigid outer layer of Earth, comprising the crust and the uppermost solid mantle; broken into tectonic plates.

Asthenosphere

The ductile, partially molten layer of the upper mantle below the lithosphere, allowing for plate movement.

Divergent Boundary

A plate boundary where plates move apart, creating new crust (e.g., mid-ocean ridges, continental rift valleys).

Convergent Boundary

A plate boundary where plates move toward each other, leading to subduction or collision.

Transform Boundary

A plate boundary where plates slide past each other horizontally (e.g., San Andreas Fault).

Ocean-Ocean Convergence

A convergent boundary where one oceanic plate subducts beneath another, forming a trench and a volcanic island arc.

Ocean-Continent Convergence

A convergent boundary where an oceanic plate subducts beneath a continental plate, forming a trench and a volcanic mountain chain (e.g., Andes).

Continent-Continent Convergence

A convergent boundary where two continental plates collide, forming a major mountain range (e.g., Himalayas).

Subduction

The process where one tectonic plate moves under another and sinks into the mantle.

Accretionary Wedge

A mass of sediment and oceanic rock scraped off a subducting plate and accumulated at a convergent margin.

Flux Melting

Melting triggered at subduction zones when water released from the subducting plate lowers the melting point of the overlying mantle.

Pillow Lava

Bulbous shapes of basalt formed when lava erupts underwater and cools rapidly.

Sheeted Dike Complex

A series of vertical, parallel dikes formed as magma fills cracks in the crust at divergent boundaries.

Gabbro

The coarse-grained, intrusive equivalent of basalt, forming the lower part of oceanic crust.

Mid-Atlantic Ridge

A divergent plate boundary in the Atlantic Ocean where the Eurasian and North American plates are moving apart.

San Andreas Fault

A major transform fault in California where the Pacific Plate slides past the North American Plate.

Ring of Fire

A major area around the Pacific Ocean basin where many earthquakes and volcanic eruptions occur due to plate boundaries.

Ridge Push

The force driving plate motion caused by the elevated magma at a mid-ocean ridge pushing the lithosphere outward.

Slab Pull

The force driving plate motion caused by the gravitational pull of a cold, dense subducting plate sinking into the mantle.

Mantle Convection

The slow, creeping motion of Earth's solid mantle caused by heat from the core, contributing to plate movement.

Seismic Amplification

The phenomenon where seismic shaking is intensified in soft, unconsolidated sediments compared to bedrock.

Engineering Implication of Ground Type

Soft soils amplify seismic waves, increasing damage during earthquakes; a critical factor in seismic design and hazard assessment.