[ RadSafe ] Radioactivity from volcanic eruptions?

[Dan W McCarn]

A good question, but I don't know the exact answer. "It depends."

What I can suggest is that as far as basaltic-type eruptions, basalt
normally contains only a small amount of uranium and is higher in
ferromagnesium content with relatively little free quartz and water, so I
imagine that the answer would be a relatively small amount of radiation in
gas (radon / thoron) associated with basaltic-type lavas.

Andesite, a quartz-rich volcanic rock, on the other hand, may contain
significant amounts of uranium, and I suppose, radon. It also contains a
larger quantity of free quartz & water and is generally far more explosive
than basalts.  Uranium is frequently derived from these acidic (quartz-rich)
volcanic tuffs or ash which is reconcentrated in sandstone deposits.  So the
genesis of quite a few sandstone deposits is associated spatially with
uraniferous tuffs and ash falls.

The Hawaiian volcanoes are basaltic, deriving their magma from the upper
mantle / lower crust.  The andesitic volcanoes, such as those in the Andes,
is generally remelted, subducted crust in which uranium has probably been
concentrated.

I would imagine that in zones of elevated crustal abundance of uranium, such
as the Western USA, the effusive vulcanism would reflect the elevated
concentration. Enrichment mechanisms, if present, could further increase the
concentration.  Generally, the late-stage differentiation of a magma will
retain water, quartz and uranium thus allowing for a processes of
reconcentration.  Volcanic rocks derived from such a magma would show
significant uranium enrichment.

http://tinyurl.com/BowenReactionSeries


The very large, 1 million year-old caldera near Los Alamos, the Valles
Caldera, has associated hot springs with about 10X background radiation,
probably from radium / radon and progeny.  Hot water bubbles-up through
fractures carrying minerals forming the travertine.  The water loses both
H2S and CO2 as it arrives at the surface and immediately precipitates CaCO3.
The travertine at the "Soda Dam", on the Jemez Spring side is a good place
to observe elevated levels of radiation. The incredibly thick and extensive,
welded tuff of the Bandelier Tuff was derived from successive, massive
explosions from the center of the caldera, emptying the magma chamber and
allowing the caldera to collapse.  The heat source for this feature is
probably the Rio Grande Rift, a zone of crustal rifting, thin crust and high
heat flow.

http://tinyurl.com/VallesCaldera

The Solar System abundance of elements ranks uranium as #92 of 92.  The
crustal abundance is about #34, so the processes of forming the earth's
crust have enriched uranium to about the same level as arsenic whose crustal
and solar system abundance is about the same (#35, as I recall).

So, in summary, the process of crustal enrichment and magma differentiation
would increase the amount of uranium in late-stage volcanic rocks.

Dan ii

--
Dan W McCarn, Geologist
2867 A Fuego Sagrado
Santa Fe, NM 87505
+1-505-310-3922 (Mobile - New Mexico)
HotGreenChile at gmail.com (Private email)
-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Bjorn Cedervall
Sent: Friday, April 16, 2010 08:28
To: radsafe at health.phys.iit.edu
Subject: [ RadSafe ] Radioactivity from volcanic eruptions?


Any comments regarding amounts and whether they can effect the background
levels in any sense?

 

Perhaps some Radsafers good point at good sources of information for those
of us who aren't too familiar with the topic?

 

Bjorn Cedervall   bcradsafers at hotmail.com

 
 		 	   		  
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