Gravitational Anomalies Beneath Mountains Point To Isostasy of Earth's Crust 95
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.
Can't You See? (Score:2)
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Gonna climb a mountain. The highest mountain. Jump off, nobody gonna know.
Is that you Aron Ralston [wikipedia.org]?
medium.com (Score:5, Informative)
Nothing to see here. Move along.
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Exactly what I was thinking.
Why are they posting every single article on Slashdot? Do they have a deal with the owners?!
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He's got a shaved head and a beard and paints himself like one of those avatar things that look like Bendydick Cummerbund. The 7 digit crowd love shit like that.
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Isowhat? (Score:5, Informative)
"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? (Score:4, Informative)
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.
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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."
Gerorge Airy died in 1892, so I don't think this is really 'news'.
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The work dates from M. Bouguer (he of the eponymous Anomaly [wikipedia.org]) and the geodetic expedition [wikipedia.org] to the Andes in about 1730).
It's from Medium.com ; not worth further consideration.
Re:We've known that mountains 'float' for a long t (Score:5, Insightful)
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.
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I think is was discovered that mountains 'float' when they did the survey of India back in the 1700 and 1800's. The summary makes it sound like it is news. It is not. I learned about it as a geophysics undergraduate.
Astonishingly, not everyone was ever a geophysics undergraduate.
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Astonishingly, not everyone was ever a geophysics undergraduate.
It's not news, regardless of anyone's ignorance.
I don't know the first thing about programming in C++; but I don't pick up C++ for Dummies and call it news.
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I thought it was a reasonably good article.
Hold on, you seem to be implying that you actually read TFA. Have you no shame?
What does that even mean (Score:4, Insightful)
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 (Score:5, Informative)
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.
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The other important thing about gravity is distance to the mass
when you're on a mountain you're further away from most of that mass.
if there was a mountain that was 40,000 Km tall you could jump off it and never reach the ground
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That would put a dent in SpaceX's clients list.
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if there was a mountain that was 40,000 Km tall you could jump off it and never reach the ground
Show your math please. 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.
Re:What does that even mean (Score:5, Informative)
Show your math please.
A mountain at 42,164bkm would have the peak in geosynchronous orbit. http://en.wikipedia.org/wiki/G... [wikipedia.org]
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 (Score:4, Informative)
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.
More detail on geostationary (Score:2)
This StackExchange [stackexchange.com] question has a nice answer showing why you have to be at the equator to have a geostationary orbit...
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This StackExchange [stackexchange.com] question has a nice answer showing why you have to be at the equator to have a geostationary orbit...
And this one addresses exactly the question posted in the summary: Lowest gravity on Earth's surface? [stackexchange.com]
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Just clear the path. Nobody likes Phobos anyway.
Cute, but on Red Mars Phobos fears YOU!
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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.
No, but you'd get a great view of the outer solar system... at a good clip of 27km/s or so if my math isn't too wrong.
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Don't count on never, this isn't kerbal space program, I don't think that would garauntee you a stable orbit. Your jump would likely be vertical, so you would see the top of the building move forward, as if you slowed down.
Pretending it is kerbal and there are no other bodies with gravity or uneven gravity etc.... your periapse would be slightly higher where you jumped, and slightly lower at the other side, where you would have a higher speed.
With the right parameters for roof size and starting height, you
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ahhh missed that one, yah that tends to mean you are going to have a bad time.
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You can also have hot blobs in the mantle providing buoyancy. The Rocky Mountains are mostly held up by hot blobs. They don't really have roots.
Post Wrong and 100+ years Out of Date (Score:3, Insightful)
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 Britis
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An alternative theory for the same conclusion, which I favour because experimental data is more accessible is:
Climbing mountains implies the increase of possibility of falling to great depths. Which means that, statistically speaking, when you go to the mountain you have indeed less mass beneath you than if you walked and occasionally fell elsewhere, where the depth is lower, or in the sea, where you float in mass denser than air.
Gotta love science.
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More mountain = less mantle = less dense.
So, in a way, it would behave light an iceberg floating on water. However, what I don't get is why there's less mass beneath, rather than equal amount. Indeed Archimedes stated:
Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
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As I said to the other person who brought up the same point:
The mountains are not free-floating on the mantle. They are attached to the entirety of the continental plate and thus not only are they supported by the plate the mantle displacement is not solely local.
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The only way it could work is if actually the plains were "supported" by the mountains rather than the other way round, but that somehow sounds unlikely...
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You appear to be assuming some sort of mountain which consists of only up moving plate.
What did I miss? (Score:1)
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.
Mass is the same regardless, weight changes, so how is the mass less on top of a mountain?
So what did I miss here?
An object on the Moon would weigh less than it would on Earth because of the lower gravity, but it would still have the same mass.
https://en.wikipedia.org/wiki/... [wikipedia.org]
Re:What did I miss? (Score:5, Informative)
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.
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Ugh... I knew that but for some reason it didn't click in my head.
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No.
The mountains are not simply floating on the mantle. The mountains are attached to, and forced into position by, the entirety of the crustal plate. The mantle displaced is not entirely local.
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The mantle is denser than the crust. Mountains have more crust under them, hence less mantle. Therefore, since there is less of the dense material, the mass under the mountain is less than the mass under the ocean. Nothing to do with weight.
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An object on the Moon would weigh less than it would on Earth because of the lower gravity, but it would still have the same mass.
Because the mass beneath your feet would be lower.
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Because the mass beneath your feet would be lower.
Really? Why?
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LOL holy shit...
um... (Score:2)
"You might’ve learned—in some long-ago physics class—that objects at the surface of the Earth all accelerate downwards, towards the Earth’s center, at 9.81 m/s^2 (or 32 feet/s^2), without fail."
Only if you had a really bad physics teacher.
If your physics teacher was good and explained how 9.81 came about you would quickly realize acceleration varies depending where you are on Earth. A good example is a object weighs differently around the world even though it's mass doesn't change.
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Exactly. Your weight changes because it's not always 9.81m^2. No teacher worth their salt would teach their students that its 9.81m^2 all the time like the article states. The article might as well have said "You might’ve learned—in some long-ago English class—that I comes before E except after C, without fail. But it turns out...."
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Exactly. The acceleration changes which changes your weight. No physics teacher worth their salt would teach their students it's 9.81m^2 everywhere on the Earth.
9.81 m/s^2 at sea level (Score:2)
9.81 m/s^2 at sea level is how I was taught. ... which is the old school why is saying: Throwing straw-men at a problem with out understanding what the problem is ... aka your typical slashdot'er knee-jerk reaction to every problem in the last .. oh .. 5 years or so .. as this sight has slowly degraded into moron-o-city.
Anything above sea level is less and below is more.
AFAIK the article is backward as reason #4 is the most obvious and reasonable while 1, 2 and 3 are trying to 'over-think' the problem
Ob: Gi
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9.81 m/s^2 at sea level is how I was taught.
Anything above sea level is less and below is more.
This is incorrect.
If you are standing at sea level in a cave deep inside a mountain, acceleration will be less than 9.81 m/s^2. That's the point of the article. The mountain above you is pushing down into the mantle, displacing denser mantle material, so between you and the core is less mantle than if you were on a boat in the sea.
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And, the mass of the mountain above you is pulling up on you as well. Making you even more light-headed. (ok, light-everything, but that's not as funny.)
If you burrow deeper into the earth, the mass that lies "behind" you if you are facing downward lessens the imbalance of forces drawing you towards the center of mass of the earth. If you made it all the way to the center, you'd be "weightless"
Floating mountains (Score:4, Interesting)
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!
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Yea. It's pretty amazing to think that the rock making up was once deep inside the ground and it slowly raised to the level it is at. Or when you look at a natural diamond and think about how deep it must have been inside the ground for the pressure required to create it.
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Really? You'd prefer to literally fly off on a tangent?
I don't know about the rest of you... (Score:2)
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>By their statement they obviously mean directly under...so what area are they using? The area of the soles of your feet? The widest area looking down from a top view? I assume it doesn't matter and they are assuming any area projected towards and through the Earth.
The easiest way would be to define that as the center point of your mass, reducing you to a point, which at the size of a human is not an unreasonable assumption. Humans are not really large enough to have a barycenter.
Boys? (Score:3)
Measure. It.
I spent a very, very long week with developers and network architects arguing about the subtle disrepencies of their layouts and software and how their software works. And eventually, I took actual measurements and showed that for far less money, using the simplest tools provided the faster solution at a tiny fraction of the complexity and cost when you _actually measured things_.
This has been a consistent lesson throughout my career. People theorize and postulate endlessly with complex analysys and essentially fraudulent testcases, and don't examine it in the real world.
Just. Measure. It.
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Fluff piece for clicks. Perhaps non-intuitive, but not really useful from a practical sense as the applications where such precision is necessary are not really dependent on cocktail party conversation starters.
FWIW, The gravitational field of the earth has been fairly well characterized. It can be done very accurately from low earth orbit using a passive mass with a retroreflector following a lead craft and taking measurements of the distance between them using a high frequency pulsed laser. I worked on p
That actually makes sense (Score:2)
In order for the mountain to be pushed up, it has to be lighter than the mantle and therefore less dense. Just like something floating in water.
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>made out of metal styrofoam.
FTFY.
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The average density of an aircraft carrier is still less than the water it displaces. Even the ancient Greeks were smarter than you.
Trippy (Score:1)
correlation between gravity and length of day (Score:2)
http://iopscience.iop.org/0295... [iop.org]
just to throw an appropriate spanner in the works, it's worthwhile mentioning the above article which notes a significant statistical correlation between variations in the measurement of the effect known as "gravity", and the (appx) 6.5 year cyclic variation of the earth's length of day.
now, before you go all "ooer" or "waah! gravity varies! we're all gonna dieeee spinning off into space", it's worthwhile pointing out that the author mentions, in the conclusion, that there
also take into account the mass (Score:1)
I am sitting in my chair (Score:1)
This isn't news (Score:1)
The crust we see is less dense than the rock underneath it. A mountain is a bunch of less dense rock sitting on a thinner layer of denser rock. The mountain pushes some of the denser rock away. We've known this since we started using gravitometers on the ground. LAGEOS-1 in the 1970s confirmed it (http://ilrs.gsfc.nasa.gov/missions/satellite_missions/current_missions/lag1_general.html).
Try looking at the real news that GRACE is able to track sea ice, or that it's looking for general relativistic gravity