Abyssal plains are flat or very gently sloping areas of the deep ocean basin floor. They are among the Earth's flattest and smoothest regions and the least explored. Abyssal plains cover approximately 40% of the ocean floor and reach depths between 2,200 and 5,500 m (7,200 and 18,000 ft). They generally lay between the foot of a continental rise and a mid-oceanic ridge. The abyssal plain is formed when the lower crust (sima), melted and pushed up by the up-welling mantle, reaches the surface at mid-ocean ridges and forms new oceanic crust. This new oceanic crust is mostly basalt and has a rugged topography. The roughness of this topography is a function of the rate at which the mid-ocean ridge is spreading (the spreading rate). Magnitudes of spreading rates vary quite significantly, and are generally broken down into 3 rates (fast, medium and slow). Typical values for fast-spreading ridges are >100 mm/yr, whilst medium-spreading rates are ~60 mm/yr, and slo

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