What does Wavelength have to do with Crustal Structure?

[RJLiberator]

Forgive my ignorance, I am learning about topics like the geoid, geoid anomalies, gravity anomalies as it relates to lithospheric composition.

In my studies, I repeatedly find talk of show wavelengths and long wavelengths having different effects on crustal composition.

One example I can give that is at the focal point:

Lithosphere can support short wavelength, but cannot support high wave length.
What does this mean?Short wavelengths do not depress the lithosphere, long wavelengths result in flexure and a depression of the Moho.

I guess, my question is why are we talking of wavelengths here?

 

[jim mcnamara]

Are you reading about P and S waves? What exactly are you reading? ...it helps us to help you.

 

[davenn]

Are you reading about P and S waves? What exactly are you reading? ...it helps us to help you.

I suspect this is along the lines of what he/she is talking about ?

Dynamics of crustal compensation and its influences on crustal isostasy
http://onlinelibrary.wiley.com/doi/10.1029/97JB00956/full#references

Geodynamics Turcotte Schubert - geosci.uchicago.edu
http://geosci.uchicago.edu/~kite/doc/Geodynamics_Turcotte_Schubert_part_of_ch_5.pdf

there's a number of other articles, some behind paywalls

this is not a subject that I am well versed in

@jim mcnamara ... are you able to expand on this subject ? as in what the wavelength is that they are talking about ? ... it's been 25yrs since I last studied about isostasy and lithospheric rebound etcDave

 

[RJLiberator]

Yes, that is a paragraph directly from the text that I am reading (chapter 5 of Geodynamics). I will check out the article you presented.

 

[johnbbahm]

I am not sure if I know what your are asking, but some crystals are wavelength sensitive,
Acousto-Optic Modulators work on the idea that at Braggs angle, the acoustic waves in a crystal
can change the path of a beam of light.
https://wp.optics.arizona.edu/milst...sites/48/2016/06/acousto-optics-modulator.pdf

 

[berkeman]

I am not sure if I know what your are asking, but some crystals are wavelength sensitive,
Acousto-Optic Modulators work on the idea that at Braggs angle, the acoustic waves in a crystal
can change the path of a beam of light.
https://wp.optics.arizona.edu/milst...sites/48/2016/06/acousto-optics-modulator.pdf

No, I misread his title as "crystal" as well. He is talking about the Earth's crust...

 

[johnbbahm]

No, I misread his title as "crystal" as well. He is talking about the Earth's crust...

That's funny, I completely misread that!

 

[johnbbahm]

I will give this one more try after reading your question again.
In dense structures many of the optical wave equations apply, except C (the speed of light) becomes the speed of sound in that medium.
different types of rock and soil reflect and refract differently like different optical indexes.
I am not sure what would support a shorter wavelength over a longer one though, because generally
the Earth has a fairly severe high frequency cutoff at about 110 hz

 

[davenn]

I will give this one more try after reading your question again.
In dense structures many of the optical wave equations apply, except C (the speed of light) becomes the speed of sound in that medium.
different types of rock and soil reflect and refract differently like different optical indexes.
I am not sure what would support a shorter wavelength over a longer one though, because generally
the Earth has a fairly severe high frequency cutoff at about 110 hz

Please have a read of my links to get an understanding of what the OP is talking about
The subject is about lithospheric loading not reflection/refraction of say seismic ( sound) waves in the crust etcDave

 

[jim mcnamara]

@davenn - I learned about this a long time ago as well. One look at the resources tells me to shut up and read.

 

[RJLiberator]

Hi guys, sorry to get back to this thread so late.

I've learned that hte wavelength is describing the load on the lithosphere. I believe it is the wavelength of the load, but I should have a more concrete analysis when I read chapter 3 in Geodynamics by Turbcotte (sp?).

 

[RJLiberator]

Okay, so I made a mistake in my above post. It is not the wavelength of the load, but rather the wavelength of the resulting deflection of the plate under a force.

From chapter 3, second 11 of Geodynamics mentioned above:

"When an elastic plate is subjected to a horizontale force P, the plate can buckle if the applied force is sufficiently large. Fold trains in mountain belts are believed to result from the warping of strata under horizontal compression. We therefore consider the simplest example of plate buckling under horizontal compression to determine the minimum force required for buckling to occur and the form, that is, the wavelength, of the resulting deflection. "

 

[davenn]

Okay, so I made a mistake in my above post. It is not the wavelength of the load, but rather the wavelength of the resulting deflection of the plate under a force.

From chapter 3, second 11 of Geodynamics mentioned above:

"When an elastic plate is subjected to a horizontale force P, the plate can buckle if the applied force is sufficiently large. Fold trains in mountain belts are believed to result from the warping of strata under horizontal compression. We therefore consider the simplest example of plate buckling under horizontal compression to determine the minimum force required for buckling to occur and the form, that is, the wavelength, of the resulting deflection. "

but that is a totally different process to what you and I have originally discussed
I'm sure you will find lithospheric loading is a vertical load ... ie. from whatever is sitting on top of it. Not horizontal pressure/stress.
This process is one of the things studied when looking at continental ice sheets and how the crust and lithosphere rebound when the ice starts melting
( after a ice age)

I vaguely remember being taught that the North American Plate, particularly nthrn USA and Canada, the rebound is still occurring
so long after the ice age.

https://en.wikipedia.org/wiki/Post-glacial_rebound

http://www.antarcticglaciers.org/glaciers-and-climate/sea-level-rise-2/recovering-from-an-ice-age/

http://www.tulane.edu/~sanelson/eens1110/glaciers.htmtho these still don't answer your original Q on "What is wavelength in this context"
it is more info on the general subject

@Astronuc are you able to help out here please ?

Dave

 

[billiards]

The wavelength is simply the spatial extent of the gravity anomaly. A gravity anomaly that spreads over a large area has a large wavelength. If the lithosphere were infinitely strong then a load would not cause a local depression (and hence a localised "short wavelength" gravity anomaly) but the lithosphere would bear the weight of the load over its entire extent (effectively creating a super "long wavelength" anomaly). Because the lithosphere, in reality, has a finite strength (characterised by its elastic thickness) it buckles locally under a load and cannot sustain a long-wavelength anomaly.

 

[jim mcnamara]

Thanks @billiards - is there a source with more detail on this?

 

[billiards]

No worries @jim mcnamara -- here's a starting point: https://en.wikipedia.org/wiki/Lithospheric_flexure

 

[jim mcnamara]

Question removed.