On solid ground? (2)

Continued from “On solid ground? (1)”:

The Earth’s crust is much like the skin of an apple — which is to say that it is very, very thin in comparison to the overall radius of our planet.  The oceanic crust, the crust under our major oceans and seas, is only about 8 kilometers (3-5 miles) thick and the continental crust, under and including the terrestrial land masses that extend above sea level (e.g., the seven continents), is about 32 kilometers (20 miles) thick.

The average radius of the Earth is 6,378 kilometers (3,958.8 miles), which means that Earth’s crust only represents between 0.005 and 0.00125 of that radius.  It is, in other words, very like the skin of an apple.

Earth’s crust is broken into many fragments called tectonic “plates.”  These plates “float,” as it were, on the soft and viscous or “plastic” mantle — much hotter and much more dense — that is located beneath them.  (It’s rather like the film that forms on top of a cup of hot chocolate as it cools.)  Usually, the plates move smoothly.  But sometimes they stick, they become caught, and pressure builds up.  When things suddenly give way, we on the surface experience an earthquake.

Obviously, we live atop the terrestrial crust.  But not even all of the crust is friendly to life.  On top of the crust, of course, things are (by definition) at air temperature.  In the deepest parts of the crust, however, the temperature reaches approximately 870 degrees Celsius, or about 1600 degrees Fahrenheit.  To put this in perspective, 350 degrees F. is a fairly common setting for baking a loaf of bread.  At 1600 degrees F., rocks begin to melt.  Thus, beneath the crust is the mantle, which is liquid and which can reach temperatures as high as 4000 degrees C. (or nearly 7,250 degrees F).

At the bottom of the world’s deepest mine, 2.4 miles down in South Africa’s TauTona, the ambient air temperature is 55 degrees C. (131 degrees F.) and the temperature of rock surfaces is 60 degrees C. (140 degrees F.).  Without artificial air conditioning, the ambient air temperature alone would soon kill the miners. (On average five miners die every year in the TauTona.)

So the lowest habitable depth on our planet is generously reckoned as about two miles down into the crust.  And, as I’ve tried to explain in my “The Thin Blue Line (1)” and my “The Thin Blue Line (2),”   we humans cannot usually function very well without supplemental oxygen beyond roughly three miles above sea level

Which means that we can only live in a thin region, roughly five miles thick, within the combined area of Earth’s nearly 3960 mile radius and its surrounding 500 miles of atmosphere.

That’s a stunningly narrow range.  The ratio 5/4460 equals 0.00112108.  That is the vertical portion of Earth’s combined mass and ambient atmosphere — slighly more than a thousandth of one percent — in which humans can survive.

I find that humbling.