什么是 Fast-, slow- and ultraslow-spreading ridges？

A ridge is classed by its full spreading rate. An ultraslow-spreading ridge opens at less than about $20\,\mathrm{mm}\,\mathrm{yr}^{-1}$; a slow-spreading ridge at 20 to $55\,\mathrm{mm}\,\mathrm{yr}^{-1}$; an intermediate ridge at 55 to $75\,\mathrm{mm}\,\mathrm{yr}^{-1}$; a fast-spreading ridge at more than about $75\,\mathrm{mm}\,\mathrm{yr}^{-1}$.