Wednesday, November 12, 2014

Re: [Geology2] Dark Magma a new concept- Connections



The answer is simple as to why that is there: remember Pacific Rim? Well just think "Atlantic Rim."

On Wed, Nov 12, 2014 at 7:44 AM, Lin Kerns linkerns@gmail.com [geology2] <geology2@yahoogroups.com> wrote:
 

Thanks Eman, for another great lesson in geology... now, to throw in a twist to this contoversial and hypothetical situation of dark magma: Iceland. Any ideas why Iceland, which sits atop a divergent plate zone, also sports a hotspot, which we are currently witnessing as the erupting field in Holuhraun? I've read so much about this situation, but the more I read, the less I can comprehend how both can exist in the same area. I know that no one really has a solid solution, but I am interested in your own personal theory.

Thanks in advance...

Lin

On Wed, Nov 12, 2014 at 1:04 AM, MEM mstreman53@yahoo.com [geology2] <geology2@yahoogroups.com> wrote:
 

There was once a science series called "Connections" where one seemingly unrelated historical let to another seemingly unrelated even or discovery but taken as a whole showed a connection.

But first a digression: in the article Mt Etna is cited as one of these dark magma/Super volcano examples but, Etna is--as is Italy itself in the volcanic forearc arising from the collision of the Africa and Eurasian plates which is also pushing up the Alps.  Lots of interesting thing squishing about in the mantle, including a huge slab of crust thought to have been subducted I think 250 mybp(?) which has yet to be fully assimilated by the mantle--but more on this later..  

 This dark magma theory as the article says, is derived from a lab experiment and not empirical observation.  It is further suspect because we know of at least two zones in the mantle where the pressure is so high it forces crystal lattices into a more compact form chemically the same but physically different,  Occum's razor suggests that the simple answer is that glass--which is the least well packed assemblage of molecules, can't exist in anywhere in mantle.  

Solids and liquids exist in an envelope of combinations of temperature and pressure.  At some pressures of depth, even molecule-splitting temperatures can't be overcome by their internal heat and remain solid. The higher the pressure the more compactly the molecules are stacked even to the point of squeezing atomic bonds into helical forms.   On the surface they would spring out rigidly.  Ergo:  Surface molecules are less dense than those of the interior and are therefore more buoyant. This is why crust floats and mantle convection currents do exist but they move slower than molasses in January.  Were this not true we could surf on plate bow wakes!

  Vice versa:  As plates are pushed downward in subduction they get compacted: e.g. olivine converts to its spinel form ringwoodite* at about 140 miles,  This appears to be the reason many of the deepest earthquakes come at that depth where no faults or voids could exist.  Those quakes are massive blocks of olivine changing over to the spinel form--ringwoodite.  Rising blocks probably generate quakes when massive blocks of ringwoodite pop back into olivine. 130-140 miles is also the downward leg of the triangle where water leaves the subducted slab to form a less dense mass  which propels magma at and ever increasing rate to form volcanoes approximately 140± miles landward of the plate edge.  And that is why we have volcanoes where they are in Northern  California. Western Oregon and Western Washington State

Eman

* The only samples of ringwoodite we have is from some meteorites which went through some really intense T-bone collisions in the asteroid belt. And then only in veins that were injected into outer zones of the asteroid during breakup.
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