David Proctor, age 11, of Lansing, Michigan, for his question:
How thick is the earth's hardened crust?
You may ask how high are the mountains or how deep is the sea, how thick are the woods and the winter snows. The four questions may yield a zillion answers, depending upon where you made your measurements. Today's question also has a wide range of correct answers.
It is not hard to imagine a solid planetary globe almost 8,000 miles wide. This is the earth below its crusty surface skin. But young thinkers, trained scientists and almost everybody else are baffled by different problems. Beginners grasp the fact that the crust is the surface ground on which we live. It rests on layers of heavier and apparently hotter levels below. We feel more comfortable when we ignore them, together with the seething core in the center of the globe. Nevertheless, our early concept of the crusty skin prods us with tantalizing questions.
We wonder whether it is secure, or whether it may suddenly slip and dunk us into the unthinkable levels below. The answer is no.. Earth scientists are still baffled about many features of the crust, but they know for sure that it rests safely on the surface and cannot, just cannot sink. For one thing, it is made of the planet's lightest rock, which floats on heavier layers like rafts of cork on the sea.
The sea teases us with another set of questions. We may suspect that the ocean beds dip below the crustal skin. Here again the earth scientists have comforting promises. The deep sea basins lie almost three miles below the waves but none of them go through the crust. Now ate can relax and consider the thickness of the crust. This, however, happens to be one of the topics most baffling to the experts.
The deepest mines and wells have not pierced through it and nobody has been down to take on the spot measurements of its thickness. But we have indirect evidence from earthquake vibrations on the density of deep levels. Plumb lines that reveal variations in gravity also yield useful clues. These indirect clues prove without doubt that the thickness of the earth's crust varies from maybe five to 40 miles. Experts are still working on the immense job of measuring the total 197 million square miles of the earth's surface.
Our present evidence reveals thin patches of soil and scratchy dirt on the land and thin layers of oozy mud on the sea beds. The rocky crust of the continents has a lighter layer above a heavier layer. Its thickness varies from 10 miles under the flat plains to 40 miles under the massive mountains. Its overall average is about 20 miles. The crust under the oceans is estimated to be from three to five miles thick. It is a single layer of the heavier crustal rocks and very, very sturdy.
The uneven thickness of the crust poses a lot of questions and suggests a lot of answers. With other data, it suggests that earth forces try to even up the world¬wide weight of the crusty skin. The lighter continents appear to balance their weight against that of the thin heavier crust under the sea. Massive mountains are lightweights. They appear to balance the thinner, heavier crust of the plains and their roots dip down like anchors, sometimes 40 miles into the deep, denser layer of the crust.