Plate Tectonics: Key Concepts and Evidence in Earth Science

Plate tectonics

Unifying theory explaining motion of lithospheric plates and resulting geological features

Why plate tectonics is a unifying theory

Explains earthquakes volcanoes mountain belts and rock assemblages

Lithosphere

Rigid outer shell of Earth made of crust and upper mantle about 100 km thick

Asthenosphere

Hot weak ductile layer of the upper mantle beneath the lithosphere

Source of plate motion

Mantle convection driven by Earth's internal heat

Mosaic of rigid plates

Earth's surface is divided into about a dozen moving lithospheric plates

Plate velocity formula

Velocity equals distance divided by time

Units of plate velocity

Millimeters per year

Continental drift

Idea that continents were once joined and later moved apart

Who proposed continental drift

Alfred Wegener in 1912

Evidence for continental drift

Jigsaw fit of continents matching fossils and similar rock assemblages

Mid ocean ridge

Long underwater mountain chain where new oceanic crust forms

Seafloor spreading

Process where new crust forms at mid ocean ridges and spreads outward

Who proposed seafloor spreading

Harry Hess

Magnetic anomalies

Alternating high and low magnetic intensity patterns on the seafloor

Cause of magnetic stripes

Magnetic reversals recorded in cooling basalt

Why magnetic stripes are symmetrical

New crust forms equally on both sides of mid ocean ridges

Age of oceanic crust

Increases with distance from mid ocean ridges

Oldest oceanic crust

About 180 million years

Reason oceanic crust is young

Older crust is destroyed by subduction

Divergent plate boundary

Plates move apart and new crust forms

Ocean ocean divergence

Mid ocean ridge volcanism earthquakes and rifting

Continental rifting

Stretching and thinning of continental crust forming rift valleys

Example of continental rifting

East African Rift Valley

Convergent plate boundary

Plates move toward each other and one or both are deformed

Ocean ocean convergence

Subduction creating trenches island arcs and deep earthquakes

Example of ocean ocean convergence

Mariana Trench and Mariana Islands

Ocean continent convergence

Oceanic plate subducts forming trenches volcanoes and mountains

Example of ocean continent convergence

Andes Mountains and Peru Chile Trench

Continent continent convergence

Collision causing crustal thickening folded mountains and earthquakes

Example of continent continent convergence

Himalaya Mountains and Tibetan Plateau

Transform plate boundary

Plates slide past one another horizontally

Example of transform boundary

San Andreas Fault

Earthquakes at transform boundaries

Shallow earthquakes

Mantle convection

Heat driven circulation of mantle material

Whole mantle convection

Convection extends through entire mantle to core mantle boundary

Stratified mantle convection

Upper and lower mantle convect separately

Hot spot volcanism

Volcanism caused by mantle plumes independent of plate boundaries

Example of hot spot volcanism

Hawaii

Supercontinent Pangaea

Supercontinent formed about 237 million years ago

Breakup of Pangaea

Began about 195 million years ago

Supercontinent Rodinia

Older supercontinent formed about 1.1 billion years ago

Breakup of Rodinia

Began about 750 million years ago

Grand reconstruction

Reconstruction of past continental positions through time

Main driver of plate tectonics

Earth's internal heat engine