Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 106.Seafloor spreading at mid-ocean ridges was recognized more than 30 years ago as the key which unlocked the plate tectonic revolution. Ridges are not only the locus of the most voluminous magmatic activity on Earth, but they are also the largest and most active extensional tectonic regime on the planet. The abyssal hills of the seafloor, formed at mid-ocean ridges, are the most widespread morphologic features on the planet. Beneath these ridges, oceanic lithosphere forms nearly 60% of the volume of the Earth's rigid outer shell. As impressive as this may be, it is worth recalling that similar volumes have been created repeatedly in the cycle of spreading and subduction of oceanic tracts that characterizes plate tectonics.
