[BR-Crater] Lava Beds area

[Brad Douglas]

Bah.  I just realized I made some errors in my previous post.  I'm sure
Scot will point them out for me. :-)

On Sat, 2007-03-10 at 23:46 -0600, Scot Wilcoxon wrote:
> http://www.rgj.com/news/stories/html/2004/04/07/67965.php
> 
> The Lava Beds has granite with a tendency to crumble.  That sounds familiar.

It's just a product of time and weathering.  Nothing special there.
Biotite and hornblende, two primary constituents of granite, are more
prone to weathering than silica.  It's why you sometimes see "rust
stained" granite...those products are being chemically weathered away
and a byproduct of the reaction is FeO.

BTW, anything that is no longer under the conditions it formed in, is
prone to weathering.

> However, a BLM geologist says the rock is considered to be an extension 
> of the Sierra Nevada batholith.  I wonder how they know that; maybe they 
> have some detailed info about that area.

I saw this in another document, but don't recall offhand.  IIRC, it was
based more on theory than fact.  There was a reason I discounted it.
Even if that is true, that's something that happened in the last 20MYa
when the Pacific and North American plates collided.  This was also when
the San Andreas fault system began to form.

> The Lava Beds area is described as 30 miles south of Black Rock Desert.  
> But the location of the columnar jointed bluff is quite a bit nearer to 
> the Lava Beds.  So "30 miles" is from the center of the desert.

I remember seeing a few small ones within or near the bounds of the
crater/desert playa.  Ian will have more details as to numbers and
locations.

> I wonder what kind of rock the columnar bluff is made of, what is under 
> it, and what the Lava Beds rock is like.  One map shows a mine symbol on 
> the west side of the Lava Beds, but I haven't found its name nor what 
> kinds of rock they found.

Basalt with lots of time to cool (probably in a lava tube).  See also
Devil's Postpile[1] in CA.

[1] http://wrgis.wr.usgs.gov/parks/depo/dpgeol1.html


-- 
73, de Brad KB8UYR/6 <rez touchofmadness com>