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USGS CMG InfoBank: Mountains Have Thick Crustal Roots

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Comment: 19:36 - 21:07 (01:31)
Source: Annenberg/CPB Resources - Earth Revealed - 7. Mountain Building
Keywords: "Eastern North America", "Appalachian Mountains", collision, erosion, mountain, orogeny, uplift, gravity, "Himalaya Mountains", crust, mantle, "convergent plate", compression, isostasy, "G. B. Airy"

Our transcription: In Eastern North America, the Appalachian Mountains continue to exist more than 200 million years after the plate collisions that formed them.
Given rates of erosion, these mountains should have worn flat tens of millions of years ago; yet they still stand, indicating that some uplift must be continuing.
The cause of this puzzling late stage uplift was discovered in 1859 by British surveyor, G. B. Airy.
While working in India, Airy discovered that plumb bobs, iron weights used to level sighting instruments, were less attracted by the gravity from the nearby Himalayan Mountains than they should be if the Himalaya were directly underlain by the same dense rock presumed to form most of the Earth's interior.
This suggested there was less mass present beneath the Himalaya than previously thought.
To explain this discrepancy Airy concluded that a low density root must lie beneath the range.
Geophysical studies have since confirmed that the crust beneath the Himalaya extends to a depth of 75 kilometers, twice as thick as ordinary continental crust.
It's now known that most mountain ranges are underlain by crustal roots floating atop the hot plastically deforming mantle.
The roots grow as a result of compression during plate convergence.

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