When my parents took me to the sea for the first time, I immediately realized two unquestionable facts: that it was a very large pool and that the water was salty. Something fascinating, because the water of the rivers, from which the sea is nourished, is not.
Of course , river water is not just water . As anyone can tell by reading the label on a bottle of water, in addition to water there are different types of mineral salts. The problem is that river water has more dissolved calcium, potassium and magnesium than chlorine and sodium , the components of common salt, so there should be more of the former in the sea. Because is not like that? It all has to do with time. Calcium remains in the sea for more or less a million years, because marine organisms use it to form skeletons and shells. Something similar happens to magnesium and potassium, which combine with the clays and sink to the bottom. But the times of chlorine and sodium are longer. Thus, sodium remains dissolved in ocean water for about 60 million years and chlorine even longer.
Standing by the sea is one of life’s little pleasures, but we may never have wondered where all that water comes from. If you have the opportunity this summer, look at the sea for a moment. You will be facing one of the rarest scenarios that our Universe offers: large amounts of liquid water .
Seas and oceans emerged from the interior of our planet shortly after it was formed, 4 billion years ago. The water, trapped inside the Earth, came out through the numerous volcanoes and geysers present on the surface. This degassing was so rapid that in just one hundred million years the water needed to form the oceans was released.
However, not all the water came from inside the Earth. Between a third and a half of the water came aboard comets – big balls of dirty snow – that fell on our planet when it was still young. Taking all this into account, isn’t it fascinating to discover not only that sea water hasn’t changed at all in several billion years, but that some of it has an extraterrestrial origin?
What color is the sea?
The sea is blue… sometimes. Anyone who has been sitting by the sea will know that it also has greenish hues and can even discover purple or glaucous. The color of the sea is indefinable . However, something curious happens: the higher the height with which we observe it, the more intense the blue, as happens when looking at it from an airplane. This does not mean that the blue color appears the higher we are, but because we observe it from an increasingly vertical angle . The same thing happens with the surface of the waves: the areas with waves are a much more accentuated blue.
Perhaps you have already realized the solution: the sea is blue because the sky is blue . The surface of the sea reflects the light scattered by the atmosphere that gives it its characteristic color.
To all this we must add the fact that water absorbs red more than blue and that, in the same way, it disperses more blue than red: the conjunction of both phenomena –along with the mud, algae and oils in suspension- contribute to creating that indefinite color that the sea possesses.
Waves, Venturi effect and refraction
Let’s leave the blue of the sea aside and focus on what is characteristic of seas and oceans: the waves. It is obvious to any of us that it is the wind that is responsible for the waves . However, if we want to give a more concrete explanation, we will have to pay more attention to the details. Imagine one of those dead calm days. The surface of the sea is practically smooth and there is no wind blowing, but at that moment it begins to rise. That means that the air above the water is set in motion; but it is not uniform but it presents some turbulence where the air goes at a higher speed. At this moment, an effect identical to the one that blows up the roof of a house during a hurricane begins to play, the so-called Venturi effect . This happens not because the speed of the air starts it, but because if the air travels at high speed, the atmospheric pressure on the roof decreases: a pressure difference appears between the outside and the inside of the house that, if it is high enough, the anchors of the roof roof cannot support it. At sea, when the wind blows, the pressure on the surface decreases and rises : a wave has formed.
Let’s continue standing on the seashore, watching the waves break. If we are perceptive observers we will realize something very curious: all the waves advance to the coast in a direction perpendicular to the beach , regardless of the direction in which the wind blows. Why?
Do you remember that experiment we did at school, where when we put a pencil in a glass of water we saw it break? Well, oddly enough, this phenomenon, refraction , explains our question. As the waves approach the coast, to shallower areas, their speed decreases. According to the laws of refraction, when a wave passes from one medium to another where the speed is lower, the direction of the wave tends to approach the perpendicular of the line separating the two media. In other words, the wave, moving in shallow water, will slow down and then approach the beach perpendicularly . This is particularly interesting on islands: no matter where the beach is and which way the wind blows, the waves will always arrive perpendicular to the coastline. If the beach is curved, the breaker will reproduce the profile of the beach, since this tendency to perpendicularity must be fulfilled at each point.
Trefil, J. (1984) A scientist at the Seashore, Dover Publications