Where does the salt from the oceans come from?
Posted on August 28, 2016
Have you ever considered where does all the salt of the oceans come from? Think about it. We assume that the oceans are salty, but most of the people never thought about the cause of it. Some hypothesis considered that oceans are salty because of all the tears cried by sharks that only wanted to cuddle. As much as I love sharks, I have to admit that that hypothesis can be easily discarded.
The most abundant salt in the ocean water is sodium chloride (NaCl). If you consider just for a moment the amount of life found in the oceans (although not all parts of the oceans are populated evenly), it is fun that NaCl is so abundant, as it is composed basically by two poisons: sodium (Na), and chloride (Cl). There are other salts present there, of course, but they cannot steal the spotlight from NaCl. Some of these would be sodium sulfate (Na2SO4), calcium chloride (CaCl2), and calcium carbonate (CaCO3). The common concentration of sodium chloride in the oceans is 23.9 grams per kilogram of water, although it is very common to express it as ‰ (per mill).
So, where is this salt coming from? First of all, we have to consider that the sodium chloride in the oceans is not present in small crystals, like the ones that we use for cooking. When in contact with water, they dissolve, so the water contain Na+ and Cl– (sodium and chloride in their ionic forms). Although sodium and chloride are not the most abundant elements on Earth, they are very soluble. This means that other compounds that more common in Earth’s crust, like iron, silicon or aluminium, with very low solubility, will not be present in ionic form in the oceans, leaving the dominant role for sodium and chloride. Sodium is present in a wide variety of minerals, and when in contact with water, release Na+ in the water. On the other hand, chloride is a different story, as there are not many minerals containing Cl–, which in common conditions chloride is found as gas. Can you imagine any event that occurs in nature where there is a lot of gas, and that may be related somehow to the creation of rocks and minerals? Yes, volcanoes!
Maybe you are thinking that there are not so many volcanoes to fill up the whole oceans with chloride. But think it from the geological perspective. Earth is 4.8 billion years old, so that accounts for a lot of time for filling up the oceans with chloride. And even if a volcanic eruption emits many more things than chloride, those compounds are not as soluble as chloride, so it is just easier for Cl– to stay in solution in the ocean waters.