Over 60% of the Earth's surface is made of oceanic crust that formed by the spreading of the tectonic plates at mid-ocean ridges (MORs). Slow- and ultraslow-spreading mid-ocean ridges (spreading rates of <55 mm/yr) make up about 55% of the global ridge system; that is, roughly 30% of the Earth's current surface area has been created at slow-spreading ridges. In the last 10 years, exploration of slow- and ultraslow-spreading ridges has shown they are different from the better-understood fast-spreading ridges. Recent research suggests that ultraslow-spreading ridges are a separate type of mid-ocean ridge.