How deep is the earths crust?
The earth’s crust is thinnest under the oceans, thicker under the continents, and thickest under large mountain ranges. Under the ocean, the crust may be only a couple of miles thick, but under the continents it is more likely to be on the order of 15-20 miles thick. Under a mountain range such as the Rockies it may be 40 miles or so thick, but it is probably not quite that thick under the Appalachians. Think of mountains like an iceberg; you see part of an iceberg floating above water, but most of it is below water, and the higher the part that you see, the deeper the bottom of the iceberg is.
