海水为什么是咸的？

Salt in the ocean comes from two main sources: runoff from the land and openings in the seafloor. Rocks on land are the biggest source of the salts dissolved in seawater. Rainwater is slightly acidic, so when it falls on land, it slowly wears away rocks. This releases ions, which are carried into streams and rivers and eventually flow into the ocean. Many of these ions are used by living things in the ocean and removed from the water. Others are not, so their concentration builds up over time. Another source of salt is hydrothermal vents in the seafloor. Seawater seeps into cracks in the ocean floor and is heated by magma deep below. The heat sets off a series of chemical reactions. The water loses oxygen, magnesium and sulfates, and picks up metals such as iron, zinc and copper from the surrounding rocks. The hot water then flows back out through vents in the seafloor, carrying the metals with it. Underwater volcanic eruptions also release minerals directly into the ocean. Salt domes add to the ocean's saltiness too. These are huge deposits of salt that formed over millions of years, and they are found underground and under the sea in many parts of the world. The two most common ions in seawater are chloride and sodium. Together, they make up about 85 percent of all the dissolved ions in the ocean. Magnesium and sulfate make up another 10 percent. Other ions are found only in very small amounts. The amount of salt in seawater, called salinity, changes with temperature, evaporation and rainfall. It is generally low near the equator and the poles, and high in the middle latitudes. On average, salinity is about 35 parts per thousand. In other words, about 3.5 percent of the weight of seawater comes from dissolved salts.