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Gravitational Anomalies Beneath Mountains Point To Isostasy of Earth's Crust
StartsWithABang writes: Imagine you wanted to know what your acceleration was anywhere on Earth; imagine that simply saying "9.81 m/s^2" wasn't good enough. What would you need to account for? Sure, there are the obvious things: the Earth's rotation and its various altitudes and different points. Surely, the farther away you are from Earth's center, the less your acceleration's going to be. But what might come as a surprise is that if you went up to the peak of the highest mountains, not only would the acceleration due to gravity be its lowest, but there'd also be less mass beneath your feet than at any other location.
Less mass beneath my feet? That depends very much on how you measure "beneath", right? I'd argue that if your load is being distributed into something, it's beneath you. If I'm standing on a mountain which is sufficiently sharply pointed, then almost the entire mountain might be engaged in supporting my weight — cue fat jokes. But anything it's standing on is going to be the same thing, so wouldn't that make it more mass "beneath" my feet?
Anyway, I RTFA (my geek card is in the mail, it should be back at the processing facility shortly) and the article is all gushily excited that "thereâ(TM)s far more crust underneath the mountains than there is in the oceans!" Wait, was this a surprise to anyone? Mountains happen when earth gets shoved up into the air. They're not pimples.
So in short, the article comes to completely the opposite conclusion of the truth: they say that "if you wanted the least amount of mass beneath your feet, youâ(TM)d climb up to the peak of the highest mountain" when in fact, there is more mass beneath your feet if you stand on a mountain than if you stand on the seabed or in a valley, because of all the mass that by definition can't be beneath your feet if you're standing at a lower altitude.
"You're right," Fisheye says. "I should have set it on 'whip' or 'chop.'"
All of the earth crust floats. Mountains are higher because the rock beneath them is lighter, hotter, or thicker than elsewhere. Continental crust does not subduct because it is the result of island arc subduction related magmatism which produces metal poor rocks due to fractional melting, producing a rock that is lighter than mantle. Oceanic crust is mafic and contains more metals, is more dense, and has similar composition to the mantle, so it tends to subduct easily. Good we are covering basic geology 101 for the benefit of all here.
I think you're missing the point
Actually he has a very good point. The article is wrong: there is just as much mass "beneath your feet" since technically the entire planet is beneath your feet. The point is that the mass is, on average, located further from your feet near a mountain because of the thick crust which floats on, and displaces, the far denser mantle. The gravitational field depends not just on the mass but on the distance as well.
What I don't understand is how this counts as 'news'. The effect was discovered by the British Trigonometric Survey of India where they noticed a discrepancy in their measurements caused by the fact that the 'vertical' was not the same near the Himalayas. This was well over 100 years ago...hardly news.