Plate Tectonics and Continental Drift
The Earth's lithosphere is fragmented into several major and minor tectonic plates. These plates, composed of both continental and oceanic crust, float on the semi-molten asthenosphere, enabling their relative displacement.
Driving Mechanisms of Plate Movement
Mantle Convection
Heat from the Earth's core drives convection currents within the mantle. Hot, less dense material rises, while cooler, denser material sinks. These currents exert a drag force on the overlying plates.
Ridge Push
At mid-ocean ridges, new lithosphere is formed through volcanism. The newly formed, hot lithosphere is elevated relative to older, colder lithosphere further away from the ridge. This elevation creates a gravitational force, pushing the plate away from the ridge crest.
Slab Pull
At subduction zones, older, denser oceanic lithosphere sinks back into the mantle. As the slab descends, it pulls the rest of the plate along with it. Slab pull is considered a primary driver of plate motion.
Plate Boundary Types and Interactions
Divergent Boundaries
Where plates move apart, typically at mid-ocean ridges. This results in seafloor spreading and the creation of new oceanic crust.
Convergent Boundaries
Where plates collide. The outcome depends on the types of crust involved: oceanic-oceanic, oceanic-continental, or continental-continental. Subduction, volcanic arc formation, and mountain building are common results.
Transform Boundaries
Where plates slide past each other horizontally. These boundaries are characterized by faults and earthquakes.
Evidence for Plate Displacement
- Fit of the Continents: The shapes of continents, like South America and Africa, appear to fit together like puzzle pieces.
- Fossil Distribution: Identical fossil species found on widely separated continents suggest they were once joined.
- Geological Similarities: Matching rock formations and mountain ranges across different continents provide evidence of past connections.
- Paleomagnetism: The study of the Earth's magnetic field preserved in rocks shows that continents have moved relative to the magnetic poles.
- Seafloor Spreading: Magnetic anomalies in the oceanic crust provide evidence for the creation of new crust at mid-ocean ridges.
- GPS and Satellite Measurements: Modern technology allows for precise measurement of plate velocities in real-time.
Rates of Displacement
Plates typically shift at rates of a few centimeters per year, comparable to the rate at which fingernails grow. The rate varies between different plates.