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To the author below (tmount). Read -- Terra infirma: Understanding salt tectonics - Hudec and Jackson, 2007...Its ability to flow is related to the thickness and density of the surrounding rocks. Salt cannot "intrude" into a thick overburden without a trigger (either regional extension or regional shortening). (Try to nail a piece of rubber into a board...)
If a diapir is at the surface it will continue to rise until the source layer is exhusted ("downbuilding"). When the source becomes exhausted, or the rate of depsosition exceeds the rate of diapir rise, the diapir could stop growing and get buried...(read Vendeville and Jackson, 1992 - The Rise and Fall of Salt Diapirs). The only way to reactive the diapir, again is either by extension, either detached or basement-involved, or regional shorteing...The latter works much better.
There is a density inversion with sedimentary rocks at depth... Rock salt, halite, has a density of about 2.2 kg/m^3, and does not change with depth...Look seismic from any Paleozoic salt basin (i.e. North Sea, eastern Canada) the salt is still there at the bottom...
...I just realized that these posts are quite old...Oh Well...at least there is some clarification...
Nbgeo1 (talk) 01:29, 21 May 2009 (UTC) ____
I don't believe that 'bouyancy' is essential for intrusion. The driving force for any intrusion is the weight of the adjacent overburden/cover which 'sinks' into the source layer, driven by gravity. The one factor that allows this release of gravitational energy is the MOBILITY of the intruding material. For example, sea water will rise up a hole/crack in an ice floe, forming an 'intrusion', yet the ice is less dense than the water otherwise the ice would sink. Even mercury will rise up a hole in a styrafoam block floating on it.
Sure, salt is less dense than typical host rocks/overburden, but its most important property, for intrusion, is its ability to flow(especially if wet and hot)in response to very low stresses. It is a time-dependant fluid, or 'rheid', rather like pitch/bitumen (and glass, marble etc). A tombstone (see old marble examples) or window (see glass in old cathedrals) of rock salt will droop and flow like treacle, given enough time
A mobile intruding material that is LESS dense than the overburden simply has the potential to EXtrude (the cover sinks completely). If more dense, the intrusion will penetrate the cover to some point, then cease.
This is especially the case in extensive terrains where intrusion can be 'permitted' by tensional zones in the cover; even 'pull aparts' in compressive zones.
Forget about 'low density'; mobility is the key to intrusion.
Perhaps we should forget about 'diapirs' and just call them 'intrusions (salt, granite, shale, mud, breccia etc)'?
Cheers tmount 00:41, 27 February 2007 (UTC)
Retrieved from "http://en.wikipedia.org/wiki/Talk:Diapir"203.3.197.249 (talk) 21:12, 9 April 2008 (UTC)
- (Also copied from Diapir - talk should continue there so the comment can be addressed in its original context.) This is over a year since the comment was posted, but I'd like to clarify that the density difference IS driving this. What is described above uses examples of holes in the upper material. Because of buoyancy and isostasy, no less-dense material will hover above a more-dense material - a volume proportional to the ratio in densities will be submerged. But for rocks, we are looking at layers - there are no holes cut in them, and if this density instability didn't exist, the salt would be very immoble because it would only be able to move upwards at a distance of the density ratio - so much less than the thickness of the salt layer. The importance is that, with a layer of salt covered by a denser layer, the density contrast allows Rayleigh-Taylor instabilities to form and cause domal structures of salt to intrude the overlying rocks
- The mobility is also important for setting the short time-scales required for intrusion, but the fact that salt domes form is set by the density imbalance.
- Second, and less related, glass flowing over ~1000 year time-scales is a myth. It's viscosity requires orders of magnitude more time. The "thinning" is related to the Medieval process of thinning glass for windows on a rotating table - the outer edges of the glass became thicker, and were put at the bottom of the windows, probably for stability.