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!

TH

Comments

Anonymous said…
Your math is incorrect. You said the earth's skin:diameter ratio is 25:6419. 6419 is the earth's radius, no?

alec.ferguson@gmail.com
1:37 AM, October 13, 2009
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!
6:02 AM, October 13, 2009
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.
4:26 PM, January 09, 2011
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.
12:06 PM, April 25, 2013
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.
2:15 PM, May 12, 2014
Post a Comment

Popular posts from this blog

Your DNA would reach the moon

Image
Foolishness! Your DNA would actually stretch to the sun and back about 4 times! DNA is a fine, spirally coiled thread in the nucleus of every living cell that serves as a guidebook so the cells "know" what they're supposed to do. The strands are so fine you need a high power electron microscopes to see them. The human genome , the genetic code in each human cell, contains 23 DNA molecules each containing from 500 thousand to 2.5 million nucleotide pairs. DNA molecules of this size are 1.7 to 8.5 cm long when uncoiled, or about 5 cm on average. You have about 10 trillion cells in your body , so if you stretched the DNA in all the cells out, end to end, they'd stretch over 744 million miles. The moon is only about 250,000 miles away, so all your DNA would stretch to the moon and back alomst 1500 times. The sun is 93,000,000 miles away, so your DNA would reach there and back about 4 times! Interestingly, no more than 1.5 percent of the human genome contains DN
Read more

What's Outside The Universe?

Image
Bailey, When we were in Albuquerque you asked me what was beyond the edge of the Universe, and I didn't have a very good answer. To be honest, I felt like a baseball player that had practiced and practiced (I've read a lot of books and thought a lot about the same question myself), but suddenly I was standing in the outfield and a high fly ball was headed my way (your question)...and I dropped the ball. Your question is a really good one and a very hard one to answer because the ideas aren't simple and they aren't easy to understand--even for me. But I've been thinking about what you asked, and I'm really, really glad you did becuase it made me think hard about the question again too. Next time we see each other, maybe we can talk about this more, but here's my answer: think about the earth. It's a big ball, and when you look toward the horizon it looks like you can see where the earth stops. But you know from driving and flying that there's no edge.
Read more

Earth's curve tilts bridges

Image
Really. The top of towers of the Verrazano - Narrows Bridge at the mouth of the Hudson river in New York City are tilted almost two inches further apart than the base thanks to the curve of the Earth's surface. Actually, there are a lot of amazing things about that not-so-famous bridge. When it opened in 1964, at 4260 feet it was the longest suspension bridge in the world, exceeding the Golden gate completed in 1937, by 60 feet. Today it's the longest single span in North America and the seventh longest in the world. (The longest is the 6,529 foot Akashi-Kaikyo Bridge in Japan, completed in 1998.) The Verrazano - Narrows Bridge contains enough concrete to construct a single lane highway from New York to Washington, and if the wires that make up the suspension cables were laid end-to-end they would reach 143,000 miles, more than halfway to the moon. On hot days, the double-decker road bed expands almost six feet and sags 12 feet lower, something that ships passing underne
Read more