8 The Oceans
Explore how ocean-floor features, seawater properties, currents, waves, tides, and exchanges with the atmosphere shape the global ocean.
Mapping the ocean floor
The global ocean is one connected body of salt water, but it occupies distinct ocean basins. An is a broad depression in Earth's crust, and its floor is a varied landscape rather than a flat plain.
From a continent toward deep water, the seafloor commonly crosses a , the shallow submerged edge of the continent, and then a steeper . Sediment can build a continental rise at the slope's base. Farther offshore, extensive abyssal plains may be interrupted by seamounts, underwater hills and mountains.
Mid-ocean ridges are long underwater mountain chains where new oceanic crust forms. Ocean trenches are deep, narrow depressions commonly associated with subduction, the process in which one tectonic plate sinks beneath another. Seafloor spreading at ridges creates new crust, while subduction recycles older crust. Together, these processes continually reshape ocean basins.
Why seawater varies
Seawater is water with dissolved substances, chiefly salts. measures the concentration of dissolved salts. Average ocean is about parts per thousand, or roughly by mass, though it varies from place to place. Evaporation tends to raise ; rainfall, river inflow, and melting ice tend to lower it. When sea ice forms, much of its salt remains in the surrounding water.
Water depends mainly on temperature and . Cold, salty water is generally denser than warm, less-salty water, so it tends to sink beneath it. These differences help organize ocean layers and contribute to deep circulation. The ocean also stores heat effectively, warming and cooling more slowly than the air above it.
How ocean water moves
A current is a sustained movement of ocean water. Surface currents are driven mainly by winds. Earth's rotation deflects moving water through the , while continents steer currents into broad rotating systems called gyres. Major gyres generally rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Warm and cold currents transport heat between the tropics and higher latitudes.
differences also drive circulation. involves water moving in response to temperature and differences: cold, salty water can sink and flow through the deep ocean, while other water rises through mixing and . brings colder, often nutrient-rich water toward the surface. can also generate currents, especially in bays, estuaries, and narrow coastal passages.
Together, wind-driven surface currents and -driven deep circulation move water and heat over large distances.
Waves carry energy
Most surface waves form when wind transfers energy to water. Wave size depends partly on wind speed, how long the wind blows, and —the distance over open water that the wind blows. A wave carries energy across the ocean; in a typical surface wave, water particles move in roughly circular paths rather than travelling forward with the wave over long distances. Near shore, interaction with the seafloor slows the lower part of a wave, making it steeper until it may break.
Not every wave is wind-driven. An earthquake or underwater landslide can displace water and generate a , which is distinct from an ordinary wind wave. are also very long-period waves, but their main cause is the gravitational pull of the Moon and Sun.
The rhythm of
are the regular rise and fall of sea level, caused mainly by the Moon's gravity and also by the Sun's. As Earth rotates, coastlines pass through changing tidal water levels. The horizontal flow linked to a rising or falling tide is a tidal current: it is called flood during a rising tide and ebb during a falling tide.
When the Sun, Moon, and Earth are approximately aligned, their tidal effects reinforce one another and produce a larger tidal range called a . When the Sun and Moon are at right angles relative to Earth, their effects partly offset, producing a smaller range called a . Local coastline and seafloor shapes affect the actual height and timing, so tidal patterns differ among coasts.
Ocean and atmosphere connections
The ocean and atmosphere exchange heat, water, momentum, and gases. Sunlight warms the ocean surface, and evaporation transfers water and heat into the air. Water vapor can condense into clouds and later return as precipitation. Winds push on the ocean surface, helping generate currents and waves; in turn, ocean temperatures and currents influence air temperature, winds, and rainfall.
Because the ocean stores substantial heat, it redistributes energy from warmer regions toward cooler ones and moderates climate, especially near coasts. Carbon dioxide also moves between air and seawater. Ocean circulation can carry dissolved carbon into deeper water or return it toward the surface. As a result, changes in winds, currents, or sea-surface temperature can affect weather and climate over wide areas.
Takeaway: Ocean-floor structure, seawater properties, and ocean motion are connected, and exchanges between the ocean and atmosphere help shape conditions across the planet.