Spreading ridge

An accreting divergent plate boundary. In the symmetric normal-spreading model, its normal velocity equals the mean normal velocity of the adjoining plates; motion parallel to the ridge does not change that normal compatibility condition.

Seafloor spreading creates oceanic crust at a spreading ridge and carries it away with the adjoining plates. With symmetric steady spreading, an age interval maps to distance by the half-spreading rate, not the full relative opening speed. A time sequence of field polarities can therefore become paired marine magnetic stripes.

The speed of one side of newly formed crust relative to the ridge axis in a symmetric spreading model. It is half the full relative opening speed; the width formed during a polarity interval is this speed times the interval duration.