Why is the sea salty?
The sea is salty because minerals from the earth’s crust are dissolved by water and washed or blasted into the sea.
Four cups of seawater contains about 7 teaspoons of salt—that’s only a little over an ounce. Only about 2.5 teaspoons of this is table salt (sodium chloride); in fact, there are at least 72 chemical elements dissolved in seawater, which almost certainly contain every naturally occurring element on Earth. There is enough salt in the sea to cover the land to a depth of nearly 500 feet.
Rivers and most lakes are nowhere near as salty as the sea. Seawater is 220 times saltier than fresh lake water. So why are the oceans so salty, while lakes and rivers are not?
In fact, water from lakes and rivers is a bit salty. When rain falls onto the ground, seeps through the soil and rock, and flows over the ground in streams and rivers, it picks up and dissolves minerals. Eventually, these are washed out to sea. Rivers and streams flowing from the United States alone dump about 248 million tons of dissolved solids into the sea every year.
Salt also gets into the ocean from hydrothermal vents—fissures in the earth’s surface. So much water blasts out of these vents that if the oceans were drained dry, it would only take 10 million years for the vents to refill them. Once the salt gets into the sea, it is concentrated by evaporation.
With all this salt flowing into the ocean and so much water evaporating, the real question is not why is the ocean salty, but why isn’t it getting saltier? If the oceans lost all their salt overnight, it would take about 250 million years for the world’s rivers to replenish them, yet the rivers have been emptying salt into the oceans for a lot longer than that.
The saltiness of the sea stays the same because the oceans lose salt at pretty much the same rate as they gain it. Salt is lost when it gets laid down as part of new sedimentary rock at the bottom of the sea, and it is filtered out when seawater seeps down into the crust of the earth through cracks in the sea floor.
