2 Ocean Basins and Seafloor Geology
Explore how plate motion forms and recycles ocean basins, how seafloor features and sediments are distributed, and what they reveal about geologic processes.
How ocean basins form and change
Ocean basins develop as tectonic plates move and oceanic crust is created, displaced, and recycled. At a , plates pull apart. Hot mantle material rises, melts, and forms new oceanic crust at a . Continued separation makes the basin wider; the Atlantic Ocean is a modern example of a basin that has grown as continents moved apart.
At a convergent boundary, denser oceanic lithosphere can sink beneath another plate through . This creates a deep- and may produce earthquakes and volcanic arcs. Because oceanic crust is made at ridges and consumed at zones, ocean basins can expand, shrink, or eventually close.
Oceanic crust is generally younger than continental crust because it is continually recycled. It is youngest near ridge crests and becomes older farther away. Most present-day oceanic crust is less than about 200 million years old.
Features of ocean basins and margins
A cross-section from a continent toward the deep ocean commonly passes through several features. The continental shelf is the shallow, gently sloping, submerged edge of a continent. Beyond it, the continental slope descends more steeply toward the deep ocean floor. At the base of many continental slopes, a gentler, sediment-covered continental rise may occur; it can be absent or less distinct where a trench lies nearby.
Farther out, an abyssal plain is a broad, deep region of seafloor that can be relatively flat where sediment has buried much of the underlying relief. Mid-ocean ridges form extensive underwater mountain systems at spreading centers, while ocean trenches are long, narrow, very deep depressions commonly associated with . Seamounts are underwater volcanic mountains, and submarine canyons cut into continental shelves and slopes.
Continental margins differ according to their tectonic setting. A passive margin, such as much of the Atlantic coast of North America, lies away from an active plate boundary and can accumulate thick sediment. An active margin, common around much of the Pacific, borders a plate boundary and may have trenches, earthquakes, and volcanism.
Evidence of plate movement
The seafloor preserves evidence of plate movement. At mid-ocean ridges, magma cools to form new crust. Seafloor rocks preserve alternating magnetic patterns that form matching bands on either side of many ridge segments, supporting the interpretation that crust spreads outward from the ridge. Crustal ages provide further evidence: rocks are youngest at the ridge and older with increasing distance.
Where plates slide past one another, transform faults offset ridge segments and accommodate horizontal motion. At zones, the descending plate bends downward to form a trench. Together, ridges, transform faults, and trenches show how oceanic lithosphere is created, moved, and recycled.
Sources and distribution of seafloor sediments
Sediment covers much of the seafloor, but its thickness and composition vary. It comes from several sources:
Lithogenous (terrigenous) sediment comes from weathered rock, often carried from land by rivers, wind, or glaciers. It is especially common near continents and islands.
comes from the remains of marine organisms, including shells and microscopic skeletons. Deep-sea sediment rich in these remains is called ooze.
Hydrogenous sediment forms when dissolved substances in seawater precipitate into solid material.
Cosmogenous sediment comes from space, including tiny particles that reach Earth from meteorites or interplanetary dust.
These materials are distributed unevenly. Land-derived material commonly accumulates near continental margins, while biological remains may be important in deep-ocean deposits. Sediment layers can preserve evidence of past environments and changes through geologic time.
Investigating marine geology
Marine geology examines the seafloor, its rocks and sediments, and the processes that shape them. At spreading centers, seawater can circulate through cracks in hot crust, become heated, and emerge as mineral-rich fluid at hydrothermal vents. These vents link tectonic activity with the chemistry and geology of the ocean floor.
Mapping seafloor relief, sampling sediment, and studying rock ages help scientists reconstruct basin history, identify active geologic processes, and understand how material moves between the ocean floor and Earth's interior. Ocean-basin features and sediment deposits together preserve a record of ongoing geologic activity.