Geophysical measurements have provided a wealth of indirect information about properties of the crust and have led to exciting hypotheses regarding crustal structure, materials, and deep environments. Direct observation of the crust below its surface has been limited. Industrial drilling has resulted in great numbers of shallow wells, but few have penetrated more than 5 km, and they are localized in sedimentary basins productive of oil or gas. No well in North America has penetrated as deeply as 10 km. Since continental crust is generally thicker than 30 km, it is clear that a vast volume of rock awaits direct observation.
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- A U. S. National Continental Scientific Drilling Program
Francis G. Stehli
- Springer Berlin Heidelberg