Seafloor Spreading and Marine Tectonics
Along the planet-girdling mid-ocean ridge system, molten rock rises to forge new oceanic crust that spreads outward, recording Earth's magnetic history and driving the plate tectonic cycle of ocean basins.
Definition
Seafloor spreading is the process by which new oceanic crust forms at mid-ocean ridges and moves away on either side; marine tectonics is the broader study of the plate-tectonic creation, motion, and destruction of the ocean floor.
Scope
This topic covers the generation of oceanic crust at mid-ocean ridges, the process of seafloor spreading and its rates, the magnetic anomaly record that confirmed it, the cooling and subsidence of crust as it ages, and the subduction of old ocean floor at trenches that completes the tectonic cycle.
Core questions
- How is new oceanic crust created at mid-ocean ridges?
- What evidence from magnetic anomalies established seafloor spreading?
- Why does the seafloor deepen and cool as it ages away from the ridge?
- How is old oceanic crust destroyed, and what completes the tectonic cycle?
Key theories
- Seafloor spreading hypothesis
- Hess proposed that ocean floor is continually created at ridges and consumed at trenches, with the seafloor acting as a conveyor belt that carries continents apart.
- Magnetic stripes and reversals
- Symmetric magnetic anomaly stripes parallel to ridges record alternating polarities of Earth's field frozen into cooling crust, providing the decisive proof of spreading and a precise dating tool.
Mechanisms
Beneath mid-ocean ridges, decompression melting of the mantle produces magma that erupts and intrudes to form new oceanic crust; as the crust spreads outward it cools, contracts, and subsides, recording reversals of Earth's magnetic field as symmetric stripes. Eventually the dense, aged crust descends into the mantle at subduction zones, completing the cycle.
Clinical relevance
Marine tectonics governs the distribution of earthquakes, volcanism, and tsunami sources along plate boundaries, hosts hydrothermal mineral deposits at ridges, and provides the magnetic timescale used throughout the Earth sciences.
History
Hess advanced the seafloor spreading concept in 1962; Vine and Matthews interpreted ridge magnetic anomalies as a record of spreading and field reversals in 1963, and Wilson's transform-fault concept completed the framework, together forming the observational core of the plate tectonic revolution.
Key figures
- Harry Hess
- Frederick Vine
- Drummond Matthews
- Tuzo Wilson
Related topics
Seminal works
- hess1962
- vineMatthews1963
Frequently asked questions
- What are magnetic stripes on the seafloor?
- They are bands of oceanic crust magnetized in alternating directions, recording reversals of Earth's magnetic field as the crust formed and cooled, and their symmetry about ridges proved seafloor spreading.
- Why is the deep seafloor deeper farther from a ridge?
- As newly formed crust moves away from the ridge it cools and contracts, becoming denser and sinking, so seafloor depth increases in a predictable way with the age of the crust.