The seafloor is the submerged base of the world’s oceans, covering roughly 70% of the planet’s surface. It is a dynamic geological feature with a varied and complex composition, not a uniform, flat plain. The foundation is solid rock, known as the oceanic crust, which is constantly being created and recycled. This solid base is typically blanketed by layers of loose material, called marine sediment, which varies in source and composition across ocean regions.
The Underlying Geological Structure
The solid material forming the ocean floor is the oceanic crust, which differs fundamentally from the continental crust. Oceanic crust is relatively thin, averaging 6 to 7 kilometers in thickness, and is significantly denser than continental crust. This higher density is due to its chemical composition, which is rich in iron and magnesium.
The rock is classified as mafic igneous rock, dominated by basalt and gabbro. Basalt forms the uppermost layer, often extruding onto the seafloor as “pillow lavas” due to the rapid cooling of molten rock in cold seawater. Beneath the basalt lies gabbro, which has the same chemical composition but a coarser crystalline structure because it cooled much slower beneath the surface.
Oceanic crust is continuously formed at mid-ocean ridges, where tectonic plates move apart in a process known as seafloor spreading. As the plates separate, magma from the underlying mantle rises to fill the gap, solidifying into new crust. Because of this constant creation and subsequent subduction at ocean trenches, the oceanic crust is geologically young; the oldest sections are generally less than 280 million years old.
Types of Deep-Sea Sediment
The solid crust is covered by layers of marine sediment, which is loose material deposited from various sources. This material is categorized into four main types based on the origin of its particles. The most common type is terrigenous sediment, derived from the erosion of landmasses.
Terrigenous material, composed mainly of quartz, feldspar, and clay minerals, is transported by rivers, wind, and glaciers. These particles settle, forming thick layers of mud and sand, especially near continental landmasses. Far from land input in the deep ocean, the finest particles accumulate slowly as pelagic red clay, which is rich in iron oxides.
Biogenous sediment consists of the hard parts (shells and skeletons) of dead marine organisms. This material is composed of either calcium carbonate or silica, forming “ooze” when it constitutes at least 30% of the sediment. Calcareous ooze is made from the calcium carbonate shells of organisms like foraminifera, while siliceous ooze contains the silica skeletons of diatoms and radiolarians.
Hydrogenous sediment forms through chemical reactions that precipitate dissolved minerals directly out of seawater. A key example is manganese nodules, potato-sized lumps composed of manganese and iron hydroxides, which grow extremely slowly on the abyssal plain. Other materials include metal sulfides formed near hydrothermal vents and phosphorite deposits.
The final, least abundant type is cosmogenous sediment, which originates from outer space. This material consists of microscopic dust and spherules from pulverized meteors and comets. Although spread thinly across the seafloor, its contribution to the overall mass of ocean sediment is minimal.
Distribution Across Ocean Provinces
The underlying rock structure and sediment types are organized across distinct geographical zones called ocean provinces. Continental margins (shelf, slope, and rise) are characterized by proximity to land and account for the largest volume of sediment accumulation. These areas feature extremely thick deposits, often exceeding 10 kilometers, dominated by terrigenous sediments delivered by rivers and turbidity currents.
In contrast, the deep ocean basins (abyssal plains, mid-ocean ridges, and trenches) feature much thinner sediment layers. Near active mid-ocean ridges, the sediment layer is thinnest or absent because the underlying basaltic crust is newly formed. Moving away from the ridges, the sediment progressively thickens as the crust ages, following the principles of seafloor spreading.
The abyssal plains are covered mostly by fine-grained pelagic sediments, specifically biogenous oozes and red clay. The distribution of biogenous material is influenced by the Calcium Compensation Depth (CCD), a depth below which calcium carbonate dissolves rapidly. Calcareous oozes are common on shallower plateaus, but below the CCD in the deepest basins, only silica-based siliceous oozes and red clay persist.