Density Currents
Density
If you a drop a rock into water it will sink. Why?
Let's consider the proposal that rocks sink because they are heavier than water. Suppose I had a small glass of water that weighed 100 grams. If you drop a rock weighing 200 grams into the water there is no doubt it would sink. But what if you chipped off a small piece of that rock which weighed just 10 grams. Would it sink? Of course.
Alternatively, can you come up with an example of something that floats even though it is made out of something "heavier" than water, say steel? A battleship would be a good example. It floats because it is lighter than the water it displaces. This is called Archimede's principle. Similarly, the reason that objects sink is because they are denser than the water.
So it is pretty clear that objects don't sink because they are heavier than water. The reason that objects sink is because they are denser than the water. Objects that are less dense than water float. This is called Archimede's principle.
Density is the ratio of mass to volume. Consider a simple example. A well-conditioned member of the U.S. Marines may have only 3% body fat, but may weigh exactly the same as an out-of-shape oceanographer with 10% body fat. Fat is less dense than muscle so that means that the trim marine will take up less volume than the pot-belleyed oceanographer. So we could accurately say that the marine is denser than the oceanographer, although you certainly would not want to say this to their face!
But that's not the end of our story. Body fat is also less dense than water, but muscle is slightly more dense. So the less dense oceanographer will have an easier time floating in a pool than the marine. We know of at least one oceanographer who put this theory to good use in easing through a Navy water survival course, while a few of the marines taking the course struggled!
So much for rocks, marines and overweight oceanographers. Can the density of water be changed? How?
What would happen if we made some water at the surface of the ocean denser than the waters below it? Can you think of anywhere in the world's oceans where this might happen?
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