Plate Tectonics: A Paradigm Under Threat?

earthseed · Dec 6, 2004

Finding isolated examples of compression on the Mid Atlantic Ridge (MAR) and representing them as common or typical seems to be a standard technique of Pratt's paper. I have never heard that the Earth's crust deforms by a foot due to tides. I would be interested in seeing some confirmation of this, because it would certainly have a significant effect.

silylene old · Dec 6, 2004

Here's an article from 2002 correlating minor earthquakes on ocean floors and the Juan de Fuca fault with tidal stresses: http://www.geotimes.org/aug02/NN_tide.html <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

earthseed · Dec 6, 2004

Thanks for the article. If these earthquakes aid the movement of plates, we have an example of how the moon helps plate tectonics.<blockquote>These findings, published in June issue of Geology, imply that the seafloor crust “breathes” with ocean tides, since the release of seismic energy and fluid flow take place on a periodic tidal basis. The team did not find a correlation between earthquakes and earth tides, caused by the deformation of Earth’s crust by the gravitational attraction of the moon and sun. This implies that ocean tides, which precede earth tides by two hours, play a more significant role in the day-to-day spurring of earthquakes than crustal deformation.</blockquote> Crustal deformation is mentioned here. Another article I found basically confirms what maddad stated:<blockquote>Earth's crust is elastic. It bends under the influence of the tidal potential. It also bends under the weight of oceanic tides. As a result, the seafloor, and the continents move up and down by about 10cm in response to the tides. The deformation of the solid Earth influence almost all precise geodetic measurements.</blockquote>

alkalin · Dec 7, 2004

Earthseed, Consider that up to 20 inches of water are measured by satellite relative to a standard ocean depth. There should be a considerable deformation on the ocean floor due to this extra weight, implying the actual tide depth is more like 24 inches. This a considerable extra weight on the ocean bottom. Also to consider is that due to the Earth’s rotation, the moon can be located over the continental area to the west, pulling upward on the land while the MAR tide is maximum. Crust differential becomes about like 8-12 inches?? Somethings gotta give. And thanks for the links.

earthseed · Dec 7, 2004

To put tidal forces on the ocean floor into perspective, it is important to understand that although the mantle below it is elastic, it is also very viscous. This is well illustrated by the geologically recent history of where I live, which is Ottawa, Canada. About 18,000 years ago the land where I sit was under 2 km of ice. By 12,000 years ago the ice had melted, and we were under an arm of the ocean. Since then the land has risen (due to isostatic rebound) by over 200 m, and is still rising at 3mm per year. This show how very viscous (resistant to change) the mantle is. It reacts to the weight above it, but very slowly. Compare that to ocean levels that rise and fall by one meter at mid-ocean (yes, that is all, read this for details) twice a day. I do not have figures for the effect this has on the level of the ocean floor, but I suspect it is minimal. The same paper claims the amplitude of tides in the earth's crust is about 20 cm, one fifth that of the ocean.

JonClarke · Dec 8, 2004

Keep in mind that the ocean is on average 5 km deep, so a change in depth of one metre is a change in loading on the crust of only 0.02%. It is not surprisingly that the correlation between most earthquakes and tides is so weak. Jon <div class="Discussion_UserSignature"> Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars Arthur Clarke </div>

Saiph · Dec 8, 2004

actually that viscosity is in the crustal plates, not the mantle, since the crustal plates are expanding now that the compressional pressures of the glaciers are gone. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

earthseed · Dec 8, 2004

It appears that the viscosity is in the asthenosphere below the crust, rather than the mantle. The crust itself is rigid, as explained in this introduction and this detailed discussion.

Saiph · Dec 8, 2004

That may be the case for overall uplifting of the plate. However I do know that local regions of the crustal plates can expand when the load is removed (called exfoliation IIRC). It's how you get the odd dome shaped bluffs that have "sheets" of rock falling off them. Same thing with roadside cliffs created when they cut away part of the mountain. The rock has had pressure removed, so it expands, often causing parts to fall off, creating the falling rock hazards. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

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