Abyssal hills – Abyssal hills are defined by the IHO (2008) as “An isolated (or tract of) small elevation(s) on the deep seafloor”. Hills on the seabed at abyssal depths having peaks that rise >300 to <1,000 m above the seafloor were mapped by Harris et al. (2014) as abyssal hills; they are abundant in the region between the mid-ocean ridge and the comparatively flat abyssal plains. These hills have their origins as prominent peaks of the mid-ocean ridge, but subsidence coupled with thick blankets of sediment obscures their original relief. They are the summits of larger features comprised of basaltic ocean crust that pierce through the layers of sediment that rain down from the water column to blanket the seafloor, forming thick deposits that drape over the topography.
Sediment deposits are generally thinnest near the mid-ocean ridge, because, for one reason, the ocean crust here is youngest and sediments have had less time to accumulate. With increasing distance from the centre of seafloor spreading, the ocean crust is older and the overlying sediment blanket is thicker. Sediment thickness increases (and abyssal hills appear to grow smaller in height) with increasing distance from mid-ocean ridges, in direct proportion to the amount of time that has elapsed since that particular piece of seafloor was created and moved laterally away from the spreading centre, cooled and subsided. The oldest sediment occurs at the far western edge of the Pacific and is around 200 million years old (middle Jurassic).