Final answer:
Abyssal plains are flat, sediment-covered areas of the deep ocean, more extensive in the central Pacific due to tectonic activity. The oceanic crust beneath is composed of mafic igneous rocks, with sediment layers thicker near continents.
Step-by-step explanation:
The abyssal plains are expansive flat areas of the deep ocean floor. They are typically found at depths of 4000 m or greater and are covered with fine sediments. These sediments are composed mostly of muds and the shells of tiny sea creatures, becoming thicker near the continents due to the deposition from rivers and wind currents.
The oceanic crust, on which these plains rest, is mostly composed of mafic igneous rocks such as basalt lava flows or gabbro, an intrusive igneous rock. In the central Pacific basin, these plains are more extensive due to the tectonic activity of numerous hot spots, which has resulted in various ages and sizes of seamounts beneath the thick sediment layers. Comparatively, the North Atlantic has less extensive plains due to different geological formation processes, such as the presence of the Mid-Atlantic ridge where plates are diverging.