Re: [Geology2] Re: The Earth Moved
Ben,
You wrote:
...When I write about a "pulse," I am speaking of the transferred force of the impact as illustrated in both of the two scenarios described above or as occurs in a hydraulic cylinder. I don't see how waves are involved to a significant extent. It is just the sudden impact pressure.
I used the word "pulse" to describe this. Is there a problem with the word itself, or is there another problem that I am not understanding here....
A "pulse" is a waveform. An impulsive deformation will propagate. Whether the pulse shape is a stair step, a square wave, or a Gaussian impulse. If you perform a Fourier analysis on any pulse, you will find it is composed of a linear combination of harmonic waves (sines and cosines). The amplitude associated with each frequency is dependent on the shape of the pulse, but any shape of pulse shares this property.
What you do not seem to understand is that the interior of the Earth is dispersive to propagating deformations. It refracts and reflects these wave components very differently. Consequently, what starts as a pulse on one side of the Earth will be very diffuse and of different character when it reaches the other side. The deformations which constitute such a pulse will generally be either in longitudinal compression (P waves) or transverse shear (S waves). As previously described, these will also propagate very differently from each other and will result in much of the energy of the original pulse being dissipated and converted to heat away from the antipode. This contrasts starkly with an extrusion system for which the force of compression is confined and focused.
In the language of wave analysis, cold extrusion is a "near field" phenomenon. The far end of an extrusion system is close enough to the source to preserve the essential character of the pulse as though the deformation were directly applied to the moving material. Wave phenomena in the "far field" will be very different with each frequency component taking a different path and speed. A pulse could be preserved over a distance comparable to the geographical scale of the source, but P and S waves from earthquakes separate in just a few miles. Propagation into the mantle will separate the different frequency components as well.
The only way the opposite side of the Earth would display near field impulse phenomena, is if the bolide collision were on a planetary scale, enough to overwhelm the dispersive properties of the rock. This would necessarily throw enough ejecta into orbit to create a satellite and liquify a portion of the planet. The evidence, in the form of a single moon, suggests that this at most only happened once, and it was at a time when the mantle was already mostly liquid. Isostasy would therefore have immediately returned any upward thrusting continent to its original equilibrium level.
On the scale of the planet, the mantle is an example of a non-Newtonian fluid. It will "flow" at a rate of 1-3 cm/year, which again would restore any raised continent to its original level over a relatively short geological period, if that raising were possible in the first place. However, on the time scale of a propagating wave, mantle rock is quite rigid, and displays the refraction and dispersive effects I have described on any propagating wave or pulse.
I appreciate that you are attempting to gain insight into orogeny by metaphorical thinking. You suggest that the Earth might act like a giant mechanical extruder. However, the danger of metaphorical thinking is that all metaphors eventually break down. In this case, the extension of a factory scale process to properties of a planet just does not work.
Chuck
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