The global ocean is an immense, interconnected body of saltwater covering over 70% of Earth’s surface, holding approximately 321 million cubic miles of water. This vast liquid layer is fundamental to the planet’s climate, geology, and biology, yet its presence makes Earth unique among the rocky planets in our solar system. Understanding how the ocean formed requires examining the dual sources of the water itself and the geological forces that created the enormous depressions needed to contain it.
Where Earth’s Water Came From
The origin of Earth’s water remains a topic of scientific debate, but current theories focus on two main sources: water trapped within the planet’s building blocks and water delivered by extraterrestrial impacts. Early in Earth’s history, intense volcanic activity released large amounts of water vapor from the planet’s interior during a process called outgassing. As the planet cooled, this vapor condensed and fell as rain, contributing to the first surface water.
A competing theory suggests that a significant volume of water arrived via impacts from water-rich celestial bodies. This delivery occurred during the Late Heavy Bombardment, a period roughly 4.1 to 3.8 billion years ago when asteroids and comets frequently struck the inner solar system. Carbonaceous chondrites possess a deuterium-to-hydrogen ratio that closely matches Earth’s ocean water, making them a likely primary source.
Recent research suggests that much of Earth’s water may have been present from the beginning, incorporated into the material that formed the planet. Studies of enstatite chondrites, meteorites chemically similar to Earth’s original building blocks, show they contain enough hydrogen to account for the planet’s oceans. This evidence supports the idea that the ingredients for water were naturally present in the inner solar system, with later extraterrestrial impacts contributing only a small percentage of the total volume.
Creating the Ocean Basins
The presence of water is only half the story; geological mechanisms were needed to create the deep hollows that became the ocean basins. This formation process began with the differentiation of the Earth’s crust into two distinct types: a denser, thinner oceanic crust and a lighter, thicker continental crust.
Oceanic crust is primarily made of basalt and gabbro, which are rich in iron and magnesium, giving it a density of about 3.0 to 3.5 grams per cubic centimeter. Continental crust is composed of less dense, silica-rich rocks like granite, with a density closer to 2.7 grams per cubic centimeter. This density difference means the continental crust floats higher on the mantle, forming the landmasses, while the denser oceanic crust sinks lower, creating the vast depressions that collect water.
These depressions became permanent features as early plate tectonic movements began to operate during the Hadean and Archean Eons. These movements and subduction zones started to form the large-scale topography of the planet’s surface. This allowed the condensed water vapor to accumulate in the lowest-lying areas, establishing the first stable ocean basins.
The Salinity and Chemistry of Early Oceans
While the water initially condensed as freshwater rain, it rapidly began acquiring its characteristic salinity through interaction with the solid Earth. This process started immediately as water flowed over the newly forming continents. River runoff and surface weathering dissolved minerals and salts from continental rocks, carrying them into the accumulating bodies of water.
A second source of dissolved solids came from the seafloor itself through hydrothermal vents. These underwater geysers released mineral-rich fluids directly into the water column, contributing significantly to the early ocean’s chemical makeup. Reactions at these vents introduced ions like chloride, sodium, and sulfate into the water, driving the salinity upward.
The chemical composition of the oceans stabilized over billions of years, reaching a long-term balance. While ions are constantly introduced through weathering and vents, they are also removed through processes like the formation of evaporite minerals and the absorption of elements by marine organisms. The modern concentration of salt, approximately 35 parts per thousand by weight, is a result of this equilibrium, which has remained stable throughout much of geological time.
How Plate Tectonics Maintains Ocean Structure
Plate tectonics, the process that helped create the original basins, continues to maintain and reshape the global ocean structure. New oceanic crust forms continuously at mid-ocean ridges, which are underwater mountain ranges spanning tens of thousands of miles. At these divergent boundaries, magma rises from the mantle, cools rapidly, and solidifies to form new basaltic seafloor, pushing the existing plates apart in a process called seafloor spreading.
This constant expansion of the ocean floor is balanced by the process of subduction, which occurs at deep-ocean trenches. Here, the cold, dense oceanic crust is forced downward beneath a less dense plate and recycled back into the Earth’s mantle. Subduction maintains a constant planetary size and regulates the depth and volume of the ocean basins.
The interplay between seafloor creation at the ridges and crustal destruction at the trenches defines the continuous tectonic “conveyor belt.” This mechanism drives the movement of continents, causing ocean basins to constantly open and close over geological timescales, such as the formation and breakup of supercontinents. The ocean is therefore not a static feature but an integral component of a planet-sized geological cycle.