Earth's Crust Thin As Apple's Skin

Really. The Earth's crust in most textbook diagrams is exaggerated, otherwise it would be too small to see; it's as thin as the skin on an apple.

The distance from the surface of Earth to the center is about 6,400 kilometers. The crust of our home planet is as thin as 6 kilometers in oceanic areas and is rarely over 50 kilometers—in a few mountainous areas. On average it's about 25 kilometers thick.

Inside, Earth is a squishy ball of molten rock; tectonic plates and continents float around on top, moving at about the speed your fingernails grow. Where there's a thin spot we have volcanoes. When the plates stick and then suddenly move we have an earthquake. Where they bump into each other the crust wrinkles and we have mountains.

A typical McIntosh apple is about 2.75 inches (70mm) in diameter, and the skin is about .3mm thick. The skin to diameter ratio for an apple, then is .3:70 or .4%. For Earth the crust thickness to diameter ratio is 25:6400, or about .4%.

Earth's crust is about as thick as an apple's skin!



Anonymous said…
Your math is incorrect. You said the earth's skin:diameter ratio is 25:6419. 6419 is the earth's radius, no?
Tom said…
You're right! I should have used 25:12800. In fact Earth's crust is only about half as thick as an apples skin!
Anonymous said…
A bit late, but... the original math was correct. Only you had to call it the skin:radius ratio. It's the variation on the radius divided by the radius.
Antoine said…
The original math used diameter for the apple and radius for the earth. So Tom's comment that the crust is about about half as thick as an apple's skin is correct.
Anonymous said…
Cool example. Just one thing - the Earth is not squishy molten rock on the inside. It is very very hard and solid rock, HOWEVER on geologic timescales (millllllions of years) it DOES flow. At cms per year. Volcanoes erupt with fluid rock for example because the pressure is suddenly lower so that the solid rock can become molten.

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