A. Through erosion. Rainwater dissolves minerals in the soil and rock that it passes through, carrying them until it reaches a stream or river. Eventually this water will flow into the sea taking with it the dissolved minerals, or salts.
Basically seas are like huge lakes: they have no outlet, so what flows in cannot flow out. Lakes such as the Dead Sea and the Great Salt Lake are lakes without an outlet, and they're very salty, 10 times saltier than the oceans. A similar process that created these salty lakes is partly responsible for the saltiness of the world's oceans. Water flowing into the sea can only escape by evaporating, which leaves the salt behind.
Because of the huge volume of the oceans, hundreds of millions of years of river input were required for the salt content to build to its present level.
Q. Are rivers the only source of salts
A. No, in fact they only provide a small amount of the sea's salt.
Hydrothermal processes have a big impact on the oceans salinity. Hydrothermal vents are found on the seabed where seawater has seeped through into the hot rocks of the earth's crust, becoming extremely hot and melting some of the minerals in the crust. This water then flows back into the sea bringing the dissolved salts with it.
The reactions between seawater and the ocean crust, are not one-way: some of the dissolved salts react with the rock and are removed from the water.
Q. Will the seas become saltier
A. No, the amount of salt in the sea has remained relatively constant for millions of years. Although water evaporates from the sea, leaving salt behind, dissolved salts are removed from seawater to form new minerals at the sea bed, which are in turn replaced by new salt from river wash and hydrothermal processes. The net result is that the amount of salt in the sea remains fairly constant.
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