Scientists believe that 3.8 million years ago there were a lot of volcanic eruptions, which formed clouds of ash.
The water in the clouds was acidic and fell to the earth in huge rain storms.
The acid in the rain dissolved the rock. Sodium released from the rocks mixed with chloride in the acid to form sodium chloride, which is a kind of salt. The result was salty water, and it filled in the lowlands and formed the oceans.
When the volcanoes stopped erupting, the rain became less acidic, and so the creation of salt slowed down.
When sea water is warmed and evaporates into the atmosphere, the salt that it contains is left behind. When this water precipitates over land in the form of rain or snow, it feeds freshwater bodies such as rivers and lakes.
When fresh water moves over rocks, it dissolves a little of the rock, thereby picking up salt. The rivers then carry this salt back out into the ocean.
In general, fresh water has only a few milligrams of salt per litre.
The ocean is thousands of times more salty than that: ocean water contains about 3500 milligrams of salt per litre.
A Pinch of Salt!
Most of the salt in oceans is sodium chloride. That is the same salt that we use in cooking.
In fact, if you sprinkle enough of your salt shaker into tap water, the result will be close to ocean water.
However, sodium chloride is not the only type of salt in the ocean. The sea contains other types of salt, including salts made from sulphur, calcium, magnesium and potassium.
Not All Oceans Are the Same
Oceans cover almost three-quarters of Earth's surface. The oceans are all connected. Some oceans are saltier than others.
The salinity (saltiness) of an ocean, or an area within an ocean, depends on many things, including:
- how much fresh water enters the ocean from rivers
- how much water leaves the ocean by evaporation.
For example, the Atlantic Ocean is not as salty as the Pacific.
The Atlantic is smaller than the Pacific. Consequently, the Atlantic is more affected by fresh water from rivers. It receives more fresh water for its size than the Pacific, so the salt in it is more diluted.