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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.
Gonna climb a mountain. The highest mountain. Jump off, nobody gonna know.
Nothing to see here. Move along.
Required reading for internet skeptics
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."
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.'"
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/...
"If any question why we died, Tell them because our fathers lied."
"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.
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: Git off my lawn
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!
For me, it's always been 9.8 m/s^2 underneath various laptops and phones I've owned.
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.
This StackExchange question has a nice answer showing why you have to be at the equator to have a geostationary orbit...
"There is more worth loving than we have strength to love." - Brian Jay Stanley
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.
I do not fail; I succeed at finding out what does not work.
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.
That is so trippy, I totally just got done watching the Bill Nye Science guy episode about crust on Netflix.
Really? You'd prefer to literally fly off on a tangent?
Confucius say, "Find worm in apple - bad. Find half a worm - worse."
http://iopscience.iop.org/0295...
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 *might* be some sort of unknown systemic errors in (a) how gravity is measured (b) how the length of day is measured which *happen* to coincide and give the *impression* that there is a statistical correlation between gravitational variation and the length of the earth's day. he does however state that in light of how the measurements are taken it would seem to be very unlikely that there are such systemic errors.
so, anyway, the point is: gravity appears not to be as simple as we assumed, hence why some long-distance space probes (Pioneer for example) have anomalous unexplained behaviour.
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.
To have a right to do a thing is not at all the same as to be right in doing it
I thought it was a reasonably good article.
Hold on, you seem to be implying that you actually read TFA. Have you no shame?
To have a right to do a thing is not at all the same as to be right in doing it
>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.
of that fat assistant of yours that comes with his/her own gravitational field.
My acceleration is zero. If I took my chair to the top of a mountain and sat in it, my acceleration would still be zero at that time. My speed might change, but my acceleration is relative. I'm not speeding up or down (actually, if I recall correctly, the rotation of the Earth is slowing by a very small amount, so technically my acceleration would be negative...I could be wrong about that.) As for the amount of matter below me, ummm...from a relative perspective, the entire Earth is below me, so the amount of matter is the same. Now, if you consider all of the matter that is visible in the Universe from the opposite side of the world at any given time, that probably changes with the rotation of the Earth throughout each day, but the measurement would be so gargantuan that it really doesn't matter (pun intended.)
The correct answer is 42.
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.
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 drag.