Dear Chuck,
Thank you for your reply to my general request for comments.
Perhaps we have a question of semantics. Perhaps not.
When I use the word "pulse," I am trying to describe the application of sudden impact pressure to a relatively closed system. I would argue that this system is similar to a hydraulic system, in that the rock would be liquid enough to transfer energy much more easily than most solids would.
I have previously commented about how the industrial cold heading process can extrude solid (room temperature) steel through the force of impact. The steel is trapped in a closed system and forced to extrude through a hole in the die at the antipode of the impact.
When a very large object impacts the Earth, I believe that a similar situation occurs. Gravity keeps the system from flying apart. The integrity of the crust traps the liquid mantle, except at the antipode. The liquid rock extrudes through the pulverized rock at the antipode and, if the impact is great enough, a continent is uplifted via crack propagation from the antipodal hotspot to and around the perimeter of the uplift.
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. If the word "pulse" is not properly descriptive, what word would you suggest?
There is one other point that may be of significance. You write of seismometers recording waves even when there is sudden displacement in one direction. When I am describing the effects of a very large impact, I am writing about deformation, not displacement. Displacement may not actually depress the Earth's crust in any serious way. Deformation, by definition, does. It is the deformation of the crust that produces the force that would transfer energy to the antipode.
Regards,
Ben Fishler
From: ChuckB <gumboyaya@cox.net>
To: geology2@yahoogroups.com
Sent: Saturday, May 4, 2013 2:40 PM
Subject: Re: [Geology2] Re: The Earth Moved
Ben,
You wrote:
You wrote:
...I believe that a really large angled impact would transfer this energy in a pulse (not just a wave) through the hydraulic system that is the liquid mantle of the Earth....
1. The mantle is not liquid, not even close.
2. A pulse is technically a wave, or more properly a superposition of many waves. Each frequency will have a different speed inside the Earth, and each will be refracted by a different amount. The medium is dispersive. Thus, the energy of a sharp impulse is broadly spread out, both in space and in timing. What you actually detect over the opposite hemisphere is an interference pattern that takes much longer to record than the initial pulse.
3. Since propagation speeds increase with depth, both in the mantle and core (liquid and solid parts), each layer acts as a sharply divergent lens, further spreading the energy of a localized impulse and creating large shadow regions where no wave reaches at all. In the case of the transverse component (S waves), the opposite hemisphere is nearly completely shadowed by the liquid core. All of the S energy is directed elsewhere or dispersed in the liquid core. For the longitudinal component (P waves), only a very diluted and unfocussed amplitude will reach the opposite node. What had started out in a one degree "beam" of vibration will be spread over a region the size of a continent.
4. Since long distance transmission will transform a pulse into a series of vibration modes, there is no net force for uplift. Instead, the fraction of energy that persists will just create alternating up and down motion. This is why distant seismometers tend to record oscillations rather than pegging to one side, even when the source was a sudden displacement in one direction.
Chuck
__._,_.___
No comments:
Post a Comment