Where did Earth's oceans come from? That question has intrigued researchers for centuries. Unraveling the origin and evolution of the oceans is a greater challenge than it is for landforms. In the latter case, solid materials (rocks) that formed billions of years ago are still in place and can be studied to determine their age and evolution. Water that makes up the oceans presents a different problem. That water existed at one point in Earth's history almost entirely in the form of water vapor in the atmosphere, and at other times, it existed at least partially in the form of snow and ice. Determining the precise amount of water present at any one time in Earth's history, and its location and boundaries, is, therefore, a challenging task.
One thing we know for sure is that water did not exist when the planet was first formed about 4.54 billion years ago. Surface temperatures were so great that any liquid water present on Earth would have boiled away into space. Researchers now believe that it would have taken 200 million years for the planet to cool sufficiently so that bodies of water may have been able to form. But where did the water come from to form those oceans?
One popular theory is that comets and asteroids colliding with Earth may have deposited water that accumulated and created the earliest oceans. Research studies from time to time have provided some evidence for this theory. For example, a study reported by a team led by Jet Propulsion Laboratory scientist Dariusz Lis in 2019 found that a comet that had passed close to Earth contained water with chemical characteristics almost identical to that found in the oceans today. Whether this mecha nism could provide all the water that produced the primitive oceans is still not clear.
Another theory of ocean formation is that water released from the cooling Earth escaped into the air and then fell back to Earth in the form of rain. As this process continued, sufficient water accumulated to produce the earliest oceans. Some research has produced evidence for this theory as well. A third option, then, is that both mechanisms, deposition from comets and asteroids along with the release of water from Earth, contributed to the creation of the first oceans.
In any case, the first water that appeared on Earth was vaporized. It rose into the atmosphere above Earth's surface, along with carbon dioxide from the planet's interior, forming a greenhouse-like system that caused water vapor to change to liquid droplets. That water then fell back to Earth as rain, and that rain eventually accumulated on Earth's surfaces, forming its seas and oceans.
Much of what we know about the evolution of oceans comes from the study of changes in land masses throughout Earth's history. According to current theories, a large fraction of Earth's original land mass was originally contained in two supercontinents, large land masses from which smaller continents later broke off. The earliest supercontinents are thought to have been Vaalbara and Ur, both small accretions of cratons, large, stable blocks of Earth's crust from which continents and supercontinents were formed (“Is Vaalbara Earth's First Supercontinent?” 2019). Over the eons, the size and shape of supercontinents continued to change. The supercontinents about which we know the most, because they are the most recent, are Gondwana, Euramerica, Pangea (also Pangaea), and Laurasia. The gradual breakup of the last of these two supercontinents, Pangea and Laurasia, resulted in the formation of Earth's map as we know it today.
The movement of large land masses across Earth's surface is explained by the theory of plate tectonics. According to that theory, Earth's uppermost layer consists of a small number of very large slabs of solid rock. These slabs rest on top of the asthenosphere, the liquid upper layer of the mantle. In a way, then, supercontinents and other large surface features are able to move because they slide over the asthenosphere, much as a cork floats on the surface of water.
In their random movements across the asthenosphere, plates often come into contact with each other. They may collide more or less head on, causing one plate to slide beneath a second plate, a process known as convergence. Or they may move away from each other, a process called divergence. Or they may slide horizontally across each other, a change called transformation. Any one of these processes can cause two land masses, such as two parts of a supercontinent, to change their relative position to each other, to alter a map of the area.
Depressed regions surrounding supercontinents provided basins filled with water, the planet's earliest superoceans. The earliest superoceans were Nealbara and Gyrosia, bodies of water that surrounded the small supercontinents of Vaalbara and Ur. It is thought to have existed from about 4.404 to about 1.071 bya (billion years ago). Later superoceans were Lerova, surrounding the supercontinent Kenorland (2.523 bya–1.805 bya); Atlanta-Pacifica (Columbia; 1.41 bya–1.065 bya); and Mirovia (Rodinia; 1,380 bya–600 mya).
The superocean that is best known and most thoroughly studied (because it is also the most recent super ocean) is Panthalassa, the body of water that surrounded the supercontinent of Pangea. It is sometimes called the Mother of All Oceans, because it eventually broke up into the bodies of water we call “oceans” today. A small arm of the ocean extended into the Pangea landmass, forming a body of water known as the Tethys Sea. Over time, as Pangea itself began to break apart, the surrounding waters were also divided into groups of different shapes: Panthalassa evolved to form the modern Pacific Ocean, while the Tethys Sea spread out to form the Atlantic Ocean and Indian Ocean