The oceans are a very thin layer on the planet

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

29 August 2021

After reading last week’s article about the volume of water in all of our planet’s oceans, Jeremy Owuor did some calculations. He writes: “If the volume of water is 1.386 billion cubic kilometres, and we know that the two-thirds of the Earth is water, doesn’t that mean that the volume of the Earth is 2.079 billion cubic kilometres? But when I calculate using the known Earth diameter of 12,800km I get 1.646 trillion cubic kilometres. What is going on? Why such a big difference?”

Well Jeremy is right: the diameter of the Earth is about 12,800km and then you calculate the volume using the standard geometrical formular from high school, you ger 1.646 trillion cubic km. It is also true that the volume of all the water in the oceans is 1.386 billion cubic km and when you divide this figure by two then multiply the result by three, you get 2.079 billion cubic kilometres.

The mistake that Jeremy is making is in stating that “two-thirds of the Earth is water”. That is not true. The correct position is that two-thirds of the Earth’s SURFACE is covered by water – not the entire planet.

Water forms a very small portion of the Earth’s volume. The deepest part of the oceans – the Mariana Trench – goes down to about 11km below the surface of the ocean. That depth on the seabed, is still some 6,360km from the centre of the planet.

To get the proper perspective, think about the normal classroom globe of the world. It is about 30cm in diameter. This 30cm (or, 300mm) represents the 12,800km of the real planet. On that scale, one millimetre is equivalent to about 43km in reality (12,800km divided by 300mm).

So, the question is: how deep would the Mariana Trench be on the same scale as the classroom globe? Well, 43km are represented by 1mm; therefore, 11km would be about a 0.25mm – a quarter of a millimetre!

Clearly, the water in the oceans form a very thin layer on the surface of the planet. There are some sophisticated globes that have bumps and troughs to represent the mountains and valleys on the Earth. From what we have seen here, such globes are grossly exaggerated – the planet would not look like that to a giant alien visitor from outer space. It would appear like an extremely smooth and perfect sphere.