Abyssal Hills and Abyssal Plains

Abstract

Abyssal hills and abyssal plains makeup the majority of the seafloor, and thus cover vast amount of the Earth’s surface. Abyssal hills form in the young oceanic lithosphere near mid-ocean ridges. These elongate, ridge-parallel hills and intervening valleys provide the characteristic fabric of the recently accreted and sparsely sedimented seafloor. Near-bottom investigations document that abyssal hills owe most of their morphology to extensional faulting. Their tectonically-driven growth continues as far as ~35 km from the spreading axis, thus defining a broader plate boundary zone within which the parting plates acquire their steady-state motion. Abyssal hill morphology is sensitive to key aspects of seafloor accretion, and thus preserves accurate records of changing spreading rate, lithospheric thermal structure, and plate boundary geometry. In general, the slower the spreading rate, the larger their dimensions are. This relationship is modulated by regional variations in the thermal structure of the lithosphere, such as may be produced by proximity to hot spots, cold spots, or transform faults and non-transform ridge offsets. As divergent plate motion rafts the aging, subsiding oceanic lithosphere away from the mid-ocean ridge, abyssal hills are generally slowly buried beneath layers of sediments. However, extreme variability in sedimentation rates means that the burial of abyssal hills by sediments is not predictably related to the age of the lithosphere. In fact, the rugged fabric of the abyssal hills is transformed into the remarkably flat surface of the abyssal plains only where oceanic basins are within reach of the fast-moving turbidity currents that originate along the continental margins.