Re: extinction => Flood Basalts; Iceland

[xm9@sdcc12.UCSD.EDU (richard g. adair)]

> On Apr 27, 15:25, richard g. adair wrote:
> } Subject: Re: extinction
> > > 
> > > Similar flood basalts (layers of laval sheets) are found in Idaho
> > > (Craters of the Moon Park), Siberia, Antartica, and a few other
> > > places.
> > 
> > Much of Eastern Washington and some of N. Oregon comprise notable
> > flood basalts as well, a much greater volume of rock than any of the
> > others you mentioned, expect for the Deccan Traps. The Columbia River 
> > cuts through these basalts along most of its American course.  
> 
> I assumed that the "Idaho" reference was the Columbia flood basalts.
> Are you saying they are different events?

Perhaps; the jury is still out on this. The Snake River (Idaho) Basalts 
seem to be clearly related to a hot spot, whereas the Columbia River
Basalts are more closely related to Cascade vulcanism and the events
responsible to the Modoc Plateau in N. Oregon.  Also, these more
western basalts are tholeiitic (ocean crust like), whereas the
SRBasalts are not.

> > Iceland appears to be a spreading center 'filtered'
> > through continental crust.  
> 
> What does this mean?  How can continental crust maintain itself at a
> spreading center? It ought to be rafted away within a few million years. It
> sounds like you are describing East Africa (the Rift Valley), not Iceland.
> Would you care to elaborate?

Iceland appears to be a continental fragment that got in the way of
the Mid-Atlantic Ridge.  This is supported by:

=> seismic reflection survey interpretations

=> numerous granitic xenoliths ejected from volcanic vents 

=> quiet magnetic field, in contrast to prominent magnetic striping
at other sites on Mid-Atlantic Ridge. That is, low levels of oceanic
crust material which, when cooled through the Curie temperature,
captures the orientation of the current geomagnetic field.

That the continental fragement has not yet rafted away is probably
due to the complex tectonic history of the area, which includes
several ridge jumps as Greenland and Norway rifted apart, with
Iceland between them. That is, first Iceland was on one side of the
ridge, and then the other, ping-ponging it west, then east.  
The most prominent piece of continent underlies an oceanic basin north 
of Iceland and west of the Jan Mayen Ridge.  Certainly, though, 
the Mid-Atlantic Ridge passes through Iceland today.

A couple of references:

Johnson, Southall, Young and Vogt (1972). Origin and structure of
the Iceland Plateau and Kolbeinsey Ridge.  J. Geophys. Res., vol 77,
no. 29, pp. 5705-5719.

Gilluly (1971). Plate tectonics and magmatic evolution.


regards,

rick adair