Is the Earth’s Crust Proportionately Thicker than an Eggshell?
 
 
In May, while I was doing some dino-bone prospecting in Utah for the Denver Museum, I wondered one morning as I peeled a hard-boiled egg whether the eggshell was thicker or thinner, compared to the size of the egg, than the Earth’s crust is as compared to the size of the Earth. Here’s the answer.
 
Yesterday I worked out the numbers. Typical poultry eggshell thickness is about 0.35 mm (as determined from some poultry journal references). The mean diameters (the arithmetic average of the long and short axes) of a set of a dozen chicken eggs from the grocery store came out at 50 mm (2 inches). The ratio of these values is (0.35/50) = (7/1000).
 
The Earth’s crustal varies between 8 km in the sea floors to 45 km in really mountainous continental areas. A decent estimate of average crustal thickness would be a little less than half of the average of these two numbers, as oceanic crust covers something like 70% of the Earth’s surface. So call the Earth’s crustal thickness 25 km, on average (and that’s being generous). The Earth’s diameter is 12800 km, to three significant figures. The ratio of the Earth’s crustal thickness to its diameter is therefore (25/12800) = (2/1000).
 
These numbers are fairly rough, but in round terms I can report that the eggshell is roughly three to four times thicker (more precisely a ratio of 7/2 = 3.5), proportionately, than the Earth’s crust.
 
To illustrate my result, I drew some continents on the surface of a hard-boiled egg, cracked the egg and partially peeled it, and then photographed it to show the eggshell thickness compared to the size of the egg. I used Adobe Photoshop to color the various eggshell surfaces for greater clarity. The image below is the result.
 
See the eggshell thickness in the picture? The Earth’s average crustal thickness is roughly 1/3 to 1/4 of that amount. Even our planet’s relatively thick continental crust is less than half the relative thickness of an eggshell. Wow! Our crust, which we might think of as fairly thick and rather solid, is nothing but an extremely thin layer of hardened silica scum riding on top of the upper mantle. 4.6 billion years after its formation, our planet is still mostly molten, kept hot inside by the ongoing decay of long-lived radioactive forms of the elements potassium, thorium and uranium.
 
For a standard 12-inch (30 cm) diameter globe, as found in classrooms around the world, the scaled crustal thickness would be just 3/5 of a millimeter. (By comparison, Mt. Everest at its proper scale would be a bump that is just 1/5 of a millimeter high and Challenger Deep in the Mariana Trench, the deepest place in the Earth’s oceans, would be just 1/4 millimeter deep.)
 
This illustration ought to make it easier to comprehend how our continents, riding on the water-lubricated asthenosphere at the top of the mantle, are able to slide around like bumper cars at an amusement park. It should also make it easier to understand the existence of our planet’s spreading zones where the mantle wells up and freezes to make new crust (as along the Atlantic centerline) and subduction zones (as around the Pacific rim) where our thin crustal plates balance the spreading activity by diving back into the mantle to be re-melted like soup scum turning downward in a cooking pot.
 
Cool, huh? Well, it kept me out of trouble for a couple of hours, anyway.
A chicken egg marked with a crude drawing of North America, cracked open and partially peeled so that the eggshell can be seen edge-on. Colors and annotations have been added with Adobe Photoshop to clarify the illustration. If our planet were the size of the egg, most of the crust would be only 1/3 to 1/4 as thick as the eggshell. Even thick continental crust would be less than half the thickness of the eggshell. Earth’s crust is in some sense the mechanical equivalent of scum on top of a hot pot of soup.
 
As thin as it is, water is believed to be the key to Earth’s crustal movement. The crust is believed to slide on the water-lubricated asthenosphere between it and the top of the upper mantle. Venus, a planet that is an almost exact mechanical replica of the Earth, lacks plate activity because it lacks water to lubricate crustal movement. (Venus’ water cooked off eons ago due to a runaway greenhouse effect.)
 
I worked out the relative thickness of an eggshell and the mean diameter of an egg, and compared that result to the relative thickness of the Earth’s crust and the diameter of the Earth. I found that the eggshell is three to four times thicker compared to the size of the egg as the Earth’s crust is when compared to the Earth’s diameter. Our planet’s crust, which seems thick and robust to us as we stand on it, is just a thin layer of hardened scum riding on the top of the upper mantle.
Tuesday, July 17, 2007
Which is proportionately thicker: Earth’s crust or eggshell?