Final answer:
The salinity of oceans is attributed to the weathering of rocks on land and hydrothermal vents in the ocean floor, which provide ions that dissolve in seawater and contribute to its salt content.
Step-by-step explanation:
The sources of the oceans' salt are believed to include the weathering of rocks on land and from hydrothermal vents in the ocean floor. When it rains, water interacts with minerals in rocks, causing them to break down and release ions. These ions are carried to the oceans by rivers and streams.
Additionally, hydrothermal vents, which are openings in the sea floor, emit mineral-rich fluids. These minerals contain various ions, including those that contribute to the salinity of seawater. Over time, these processes have contributed to the relatively constant and high salinity of ocean water.