The Invisible Mountains of Earth (Synopsis)

“Journalists often ask me when I go to the field, ‘What do you expect to find?’ And my answer always is, ‘The unexpected,’ because we’re just looking at the tip of the iceberg; we’ve just scratched the surface.” -Donald Johanson

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?

Image credit: © 1999–2014 Michael Pidwirny, via http://www.physicalgeography.net/fundamentals/10k.html. Image credit: © 1999–2014 Michael Pidwirny, via http://www.physicalgeography.net/fundamentals/10k.html.

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.

Image credit: Christoph Reigber, Roland Schmidt, Frank Flechtner, Rolf König, Ulrich Meyer, Karl-Hans Neumayer, Peter Schwintzer, Sheng Yuan Zhu (2005): An Earth gravity field model complete to degree and order 150 from GRACE: EIGEN-GRACE02S, Journal of Geodynamics 39(1),1–10. Image credit: Christoph Reigber, Roland Schmidt, Frank Flechtner, Rolf König, Ulrich Meyer, Karl-Hans Neumayer, Peter Schwintzer, Sheng Yuan Zhu (2005): An Earth gravity field model complete to degree and order 150 from GRACE: EIGEN-GRACE02S, Journal of Geodynamics 39(1),1–10.

Come and find out why!

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@Ethan: I don't know if you can edit these things after you have posted them. You've got one very silly error towards the middle:

"Add something like 1% to the distance, and the force between these objects increases by approximately 2%."

That should be "decreases", of course.

By Michael Kelsey (not verified) on 20 May 2015 #permalink

@ Ethan

' where the Earth’s elevation is lowest (closest to the Earth’s center) experience the greatest gravitational acceleration, and the places where the Earth’s elevation is ( lowest ) (farthest from the planet’s center) experience the smallest acceleration. ' { :

Maybe it is just me but I don't see the inverse correlation between mountains and gravitational acceleration reflected in the maps. The Indian Ocean just south-east of the Indian Subcontinent is deep, yet that area is far more blue than even the tops of the nearby Himalayas. Conversely, the Atlantic Ocean between Greenland and Europe is also deep,but that deep spot is bright red.

I totally get the concepts of less dense roots of cratons displacing more dense material in the lithosphere and sometimes even down into the asthenosphere. I can go along with the counter-intuitiveness of mountains not being more mass under your feet, but I don't see your counter of an inverse relationship either. From where I stand, the two seem to be discoupled.

I did read the linked material and was dissapointed to find the WaPo piece was just another bit about Global Warming.

Ethan, thank you for this article.

I'm 51 years old and I still remember the time in fifth grade when I was asked on a test whether gravity would be stronger at the top of a mountain or in the valley below. Naturally I chose the top of the mountain because of all the extra mass I assumed was beneath my feet. I never argued the point, but I always believed Mr. Chesla, my teacher, had erred. Now I know he was correct, but I can give myself point for good intuitiveness.

By Brian Cox (not verified) on 21 May 2015 #permalink