Where does the sea get its salt from?

Dr Will Homoky from the University of Leeds talks about the process of chemical weathering and the transport of water by rivers into the ocean.

Rivers contain just a tiny amount of salt compared to ocean waters so there must be something else that accounts for the difference.

The salt comes from weathering and volcanic activity. The ocean formed very early on in Earth’s history, as soon as water comes into contact with rock then weathering processes start. These leach (dissolve) the soluble elements preferentially out of the rock (sodium, calcium, magnesium, potassium etc).

There isn’t very much chlorine or sulphur in rocks but there is lots in volcanic gases and it readily dissolves in water in the atmosphere to form chloride and sulphate that rains into the ocean. If these processes go on for billions of years, we get a salty ocean.

If evaporation outweighs precipitation

Where evaporation outweighs precipitation, surface seawater will become more salty. Local seawater salinity is also enhanced if there aren’t many rivers nearby, and/or if the basin is restricted, as the high salinity seawater can’t mix very well with seawater of normal salinity in this case.

The Red Sea is a good example and has an average salinity of 40. Eventually high salinity water from restricted basins does escape and enter global circulation. Higher and lower salinity waters are still recognisable a long way from their source due to slow ocean mixing.

The reason river water is fresh is also due to evaporation. When water evaporates from the ocean surface, the salts don’t evaporate with it. It’s this freshwater that eventually ends up in rivers.

That’s why we end up with a totally different balance of salts in rivers and the ocean.

Land and sea salt

Salt that is found on the land can come from evaporated ancient oceans. For instance, in the south of Spain today, there are large deposits of gypsum and salt that formed during a period of time known as the Messinian Salinity Crisis.

During this period, several kilometres of seawater evaporated from cut-off basins and left behind a series of minerals, including halite (rock salt) in thick layers. This salt would have been in similar concentrations to the salt in the ocean today.

However, there are other sources of salt on the land. The photographs of volcanoes on this site are incredible. It is run by a team of volcanologists who travel the world and photograph volcanic activity.

In Dallol, there are vast salt lakes that are formed in a completely different way. Here, the groundwater is heated from below, as the plates are pulling apart in the region, and the magma is close to the surface, hence the series of Great Lakes and volcanoes throughout the Rift Valley.

When this water is heated, it dissolves a wide range of minerals in the pore waters, and brings them up to the surface. When they reach the surface they cool, and the salt in the waters returns to its crystalline form.

Because this salty water is effectively “erupted” at high temperatures, it forms a variety of unusual, and often very short-lived geological features, such as the “hornitoes”.

The end result is a region very rich in salt, which has been mined for several centuries. Its colours come from a range of accessory minerals that are also dissolved by the hot waters.