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.

Comments

medium.com

[narcc]

Nothing to see here. Move along.

Isowhat?

[sinij]

I had to read TFA to figure out what isostatic is.

"Bizarrely enough, if we wanted to reach the Earth’s mantle, our best bet would be to dive down to the ocean floor and dig there; we’d “only” have to go through maybe 3 km of crust, as opposed to upwards of 25 km atop the Himalayas. This concept is known as isostatic compensation, and was actually uncovered by the famed British astronomer George Airy."

Re:Isowhat?

[lgw]

No different than an iceberg: when you stand on an iceberg your height above the seawater is much higher than the visible height of the iceberg above the sea.

What does that even mean

[drinkypoo]

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.

Re:What does that even mean

[phantomfive]

I think you're missing the point:

The earth's mantle is significantly more dense than the crust. Mountains are made of matter that is less dense than the mantle, so when they go deeply into the earth, there is less mantle "beneath" your feet.

More mountain = less mantle = less dense.

Re:What does that even mean

[AK Marc]

Show your math please.

A mountain at 42,164bkm would have the peak in geosynchronous orbit. http://en.wikipedia.org/wiki/G...

The moon is 384,000 km up and even it has to maintain an orbital velocity considerably faster than a jumping person to avoid falling to Earth.

But if someone built a tower 384,000 km high, it would travel faster than the moon. And if you jumped off that tower, you'd also never reach the ground.

Re:What does that even mean

[lgw]

A mountain at 42,164bkm would have the peak in geosynchronous orbit

But not geostationary (unless the mountain were at the equator) so while you might not fall down, you'd be in a bit of an awkward orbit yourself, relative to that mountain. Quick, someone try it in Kerbal Space Program!

But if someone built a tower 384,000 km high, it would travel faster than the moon. And if you jumped off that tower, you'd also never reach the ground.

One of the problems with building a space elevator on Mars is that it would be higher then the (innermost) moon, which would come say "Hi!" every few hours, moving quite fast.

Re:What did I miss?

[BoogieChile]

Mass has a relationship with density. The crust is less dense that the mantle, so more crust=less mass. The mountains float on the mantle in a similar way to icebergs on water, ie they displace mantle beneath them, resulting in a 1-you-wide segment down to the core of the earth that contains more crust and less mantle, therefore containing less mass.

Floating mountains

[BevanFindlay]

It's interesting the implications of this: we think of mountains as these giant, immovable things, culturally and linguistically used as a reference point of something solid and immutable. And yet, the reality is that they are comparably the soft fluffy marshmallows floating on top of a dense, thick liquid. I don't think it detracts from their majestic nature any, but I won't look at mountains the same, knowing they are in fact the "lighter" parts of the Earth - and the reminder that they float!

Science is fun, especially when it comes up with things that to the casual, uninformed observer are so counter-intuitive. This paints a beautiful picture.

Also, it goes to show that mountain-climbing is a great way to lose weight!

Re:We've known that mountains 'float' for a long t

[Eravnrekaree]

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.