Seafloor spreading

Earlier theories by Alfred Wegener and Alexander du Toit of continental drift postulated that continents in motion "plowed" through the fixed and immovable seafloor.

The idea that the seafloor itself moves and also carries the continents with it as it spreads from a central rift axis was proposed by Harold Hammond Hess from Princeton University and Robert Dietz of the U.S.

Seafloor spreading helps explain continental drift in the theory of plate tectonics.

When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere.

The motivating force for seafloor spreading ridges is tectonic plate slab pull at subduction zones, rather than magma pressure, although there is typically significant magma activity at spreading ridges.

[4] At a spreading center, basaltic magma rises up the fractures and cools on the ocean floor to form new seabed.

[9][10] This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred from data gathered with a magnetometer towed on the sea surface or from an aircraft.

[11][12] This is thought due to temperature gradients in the asthenosphere from mantle plumes near the spreading center.

[16] The process starts by heating at the base of the continental crust which causes it to become more plastic and less dense.

These areas are named triple junctions and can be found in several places across the world today.

Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge.

As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch.

At this point basaltic oceanic crust and upper mantle lithosphere begins to form between the separating continental fragments.

The East African rift was thought to be a failed arm that was opening more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment[17] reported that in the Afar region, September 2005, a 60 km fissure opened as wide as eight meters.

Seafloor spreading can stop during the process, but if it continues to the point that the continent is completely severed, then a new ocean basin is created.

[20] The Mid-Atlantic Ridge is a slow-spreading center, while the East Pacific Rise is an example of fast spreading.

[6] The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced.

When Alfred Wegener first presented a hypothesis of continental drift in 1912, he suggested that continents plowed through the ocean crust.

[22] Since then, it has been shown that the motion of the continents is linked to seafloor spreading by the theory of plate tectonics, which is driven by convection that includes the crust itself as well.

The magmatism at the ridge is considered to be passive upwelling, which is caused by the plates being pulled apart under the weight of their own slabs.

The age-depth relation can be modeled by the cooling of a lithosphere plate[24][25][26][27] or mantle half-space in areas without significant subduction.

The simple result is that the ridge height or ocean depth is proportional to the square root of its age.

Due to its continuous creation, the lithosphere at x > 0 is moving away from the ridge at a constant velocity v, which is assumed large compared to other typical scales in the problem.

is given by the error function: Due to the large velocity, the temperature dependence on the horizontal direction is negligible, and the height at time t (i.e. of sea floor of age t) can be calculated by integrating the thermal expansion over z: where

is the effective volumetric thermal expansion coefficient, and h0 is the mid-ocean ridge height (compared to some reference).

due to isostasic effect of the change in water column height above the lithosphere as it expands or retracts.

By substituting the parameters by their rough estimates: gives:[28] where the height is in meters and time is in millions of years.

Analysis of depth versus age and depth versus square root of age data allowed Parsons and Sclater[27] to estimate model parameters (for the North Pacific): Assuming isostatic equilibrium everywhere beneath the cooling plate yields a revised age depth relationship for older sea floor that is approximately correct for ages as young as 20 million years: Thus older seafloor deepens more slowly than younger and in fact can be assumed almost constant at ~6400 m depth.

Parsons and Sclater concluded that some style of mantle convection must apply heat to the base of the plate everywhere to prevent cooling down below 125 km and lithosphere contraction (seafloor deepening) at older ages.

[27] Their plate model also allowed an expression for conductive heat flow, q(t) from the ocean floor, which is approximately constant at