The first oceans on Earth formed about 3.8 to 4 billion years ago through a process primarily involving volcanic outgassing and subsequent condensation of water vapor. Early in Earth's history, the planet was molten and extremely hot, preventing water from existing in liquid form. As the Earth cooled below the boiling point of water (212°F or 100°C), water vapor released from volcanic eruptions condensed into rain, which filled the basins on the planet's surface to form the primitive oceans
. This volcanic outgassing theory suggests that water vapor and other gases trapped in magma were released as the magma rose and pressure decreased, contributing to the early atmosphere and eventually surface water. Additionally, some water may have been delivered by icy comets and meteorites impacting the young Earth, adding to the water supply
. Geological evidence, such as ancient rocks from the Isua Greenstone Belt and the Nuvvuagittuq Greenstone Belt, indicates that liquid water existed on Earth at least 3.8 billion years ago, supporting the timeline of early ocean formation
. In summary, the first oceans formed when Earth's surface cooled enough for water vapor—mainly from volcanic outgassing and supplemented by extraterrestrial sources—to condense and accumulate in surface basins, creating the global oceans that have persisted since then
