4.2 Seafloor spreading process

Seafloor spreading is a key process in plate tectonics. It happens at mid-ocean ridges where new oceanic crust forms as plates move apart. This process creates and expands ocean basins, driving the movement of tectonic plates.

The rate of seafloor spreading varies globally, affecting the shape of plate boundaries. Fast-spreading ridges create smoother seafloors, while slow-spreading ones produce rugged terrain. This process is balanced by subduction at convergent boundaries.

Seafloor Spreading and Plate Tectonics

Fundamental Concepts

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• Seafloor spreading creates new oceanic crust at mid-ocean ridges and spreads outward, pushing older crust away from the ridge
• Drives movement of lithospheric plates and contributes to the Wilson Cycle
• Occurs at divergent plate boundaries where tectonic plates move apart
• Involves upwelling of magma from mantle, cooling and solidifying to form new oceanic crust
• Pushes older crust away from ridge, leading to expansion of ocean basins
• Balanced by subduction at convergent plate boundaries where oceanic crust recycles into mantle

Global Variations and Rates

• Seafloor spreading rates vary worldwide, ranging from a few millimeters to several centimeters per year
• Influences geometry and evolution of plate boundaries
• Fast-spreading ridges (East Pacific Rise) produce smoother topography
• Slow-spreading ridges (Mid-Atlantic Ridge) create more rugged seafloor with deeper rift valleys
• Ultra-slow spreading ridges (Southwest Indian Ridge) have complex morphology and exposed mantle rocks

Mechanisms of Seafloor Spreading

Mantle Convection and Magma Generation

• Convection currents in Earth's mantle drive seafloor spreading
• Temperature differences between hot core and cooler surface cause upwelling of hot material and downwelling of cooler material
• Upwelling mantle material undergoes partial melting due to decompression, generating magma
• Buoyant magma rises through fractures in oceanic crust, forming magma chambers beneath ridge axis
• Extensional forces at divergent boundaries create space for magma intrusion and eruption, forming new oceanic crust

Plate Driving Forces

• Ridge push contributes to plate motion through gravitational sliding of newly formed crust away from elevated mid-ocean ridge
• Slab pull exerts downward force by subducting plates at convergent boundaries, creating tension at ridge
• Basal drag from mantle convection currents influences plate motion
• Trench suction pulls plates toward subduction zones, enhancing spreading at opposite plate boundaries

Evidence for Seafloor Spreading

Geophysical Evidence

• Magnetic striping patterns on ocean floor provide strong support for seafloor spreading
• Paleomagnetic studies reveal alternating bands of normal and reversed magnetic polarity in oceanic crust, symmetrical about mid-ocean ridges
• Heat flow measurements indicate higher heat flux near mid-ocean ridges, consistent with presence of newly formed, hot crust
• Seismic studies reveal thin crust at mid-ocean ridges, thickening with distance from ridge axis due to cooling and contraction

Geological and Observational Evidence

• Radiometric dating of oceanic crust shows systematic increase in age with distance from mid-ocean ridges
• Bathymetric data reveals presence of elevated mid-ocean ridges and associated rift valleys, consistent with seafloor spreading model
• Direct observations of seafloor spreading include underwater volcanic eruptions and hydrothermal vents at mid-ocean ridges
• Sediment thickness increases with distance from ridge axis, supporting concept of crustal aging and seafloor spreading

Seafloor Spreading vs Oceanic Crust Age

Age Distribution and Patterns

• Age of oceanic crust directly relates to distance from mid-ocean ridge where it formed
• Newly formed crust at ridge axis essentially zero-age, with crustal age increasing linearly with distance from ridge
• Seafloor spreading rate determines age gradient of oceanic crust
  • Faster spreading rates result in more gradual age increase
  • Slower spreading rates produce steeper age gradients
• Oldest oceanic crust found in western Pacific Ocean, with ages up to about 200 million years
• Absence of oceanic crust older than ~200 million years due to continuous recycling through subduction

Crustal Properties and Age Relationships

• Age-depth relationship of oceanic crust follows predictable curve
  • Older crust deeper due to thermal contraction and increased density
  • Subsidence rate decreases with age, following square root of age relationship
• Variations in crustal age across transform faults used to determine relative motion of plates and history of plate reorganizations
• Crustal thickness generally increases with age due to cooling and contraction
• Heat flow decreases with increasing crustal age as lithosphere cools and thickens