Why are the oceans salty?
This weeks I thought it would be interesting to discuss what makes the oceans salty. First-off, let’s look at how the oceans were formed. As the young earth was cooling, about 3.5 billion years ago, many gasses were being emitted from the earth’s core, including water vapor.
As the earth further cooled, the water vapor reached a temperature where condensation occurred resulting in rain that continued to fall until, as we know today, water covers about 70% of the earth’s surface.
So what makes the oceans salty? Well, the ocean basins are the lowest portions of the earth where the vast majority of fresh water flowing down steams and rivers will eventually terminate. As streams and rivers flow over the surface they slowly erode the rock carrying away dissolved salts and minerals to the seas. Salt also comes from tectonic boundaries on the bottom of the oceans and also hydrothermal vents along divergent plate boundaries that release large amounts of salts and minerals.
There are also basins of water that are land locked by mountainous boundaries that are known as terminal lakes. In the western U.S. there are a few notable terminal lakes that have high salt contents. There is Mono Lake in Central California, the Great Salt Lake in Utah and Owens Lake, that is actually just a salt bed as all the water has evaporated from this lake and the rivers that once fed this lake have been diverted to supply Southern California with fresh water.
Back to the oceans. The average salinity (the amount of dissolved salt in water) of the oceans is about 35 parts per thousand (PPT) or 3.5%. That means that one gallon of seawater contains about eight tablespoons of salt and a cubic mile of seawater contains about 120million tons of salt!
Salinity is not constant at all parts of the ocean.
The areas in red show the the parts of the ocean where salinity is above average. You’ll notice that the areas of higher than average salinity are in the areas where the Trade winds blow. These are some of the most consistent winds in the world and are in parts of the world where the air and sea temperature are relatively warm. This combination of warm temperatures and high wind speeds result in high evaporation rates resulting in higher salinity.
So the next time you take that trip to Hawaii and your eyes are stinging a little more than normal after duck-diving through a set it’s only because the water is a little saltier than at home in California.