Weathering at the Boulder's - Arizona

  I don't know if you are like me or not.  But when I go to a new area I like to go not really knowing much about it.  Then I like to explore and figure things out on my own.  After I've come to some conclusions then I like to do some research on the area and see if what I came up with is in agreement with other people interpretations.
   Well, that's exactly what I've done, with my trip to Arizona this past March 27-31, 2011.  I went there knowing very little about the area where we were going to. I imagined myself being with John Westley Powell and what he would have to say seeing this stuff for the first time. Only I was going to an area that was more southern then where he was famous for studying,- so I just would have to do this on my own.  The area is called ' The Boulders  in Carefree Arizona so I mused I'd see some boulders.
  But what type of boulders I wondered?  Since it was in the southern dessert area I did not expect the boulder to be like the type of boulders I was used to seeing when I was growing up in Ohio.  Those boulders were the result of glacier processes and left as erratics.  You could see all different types of rocks and sizes with glacier processes. The problem is the glaciers didn't get that far south so unless we were going to be in mountainous region I realized it would have to be some other mechanism causing those Boulders.  Could they be deposited in river beds? or are they the result of wind erosion?  I just had to go there and figure it out based on what I could see in the area.

(I must put in a caveat here.  I have not worked in the geologic field for about 18 years.   The last time I actively studied geology was back in 1981, and all of my stuff is based on things I remember studying back then. I am very rusty with things.  I'm sure a lot has changed since then but  I am still using material from back then since that is what I am familiar with.  Any corrections to my interpretations would be greatly appreciated.) 

So here's some of the things I mused upon while I was at
the Boulders, Arizona (33 48'00.96" N & 111 55'25.76"W elevation 2301'.)
We were not even to the place before I couldn't resist taking a photo of the rocks I was seeing.

Yes these were definitely boulders, but they did not look like there were brought to the location but formed at the  location.

    As my husband and I were being taken to our place the bell boy was giving us a talk about the resort and all the amenities that they had.  Once he finish his prepared talk, he said that the most frequently asked question about the boulder was why they are there - how did they form.  He said it was all due to weathering and that the rocks were very very old.  They were 12 billion years old.  I started to snicker to myself at that and couldn't resist asking him was he sure they were 12 billion years old.  He said yes mam. A geologist had told him so.  I said that was very interesting because the earth is only 4.5 billions years old, could he have meant 12 million years.  The guide insisted that they were 12 billion years.  I told him I also had been a geologist at one time.  That shut him up and I didn't hear anything more on how others had figure things out.  That left me to my own devices.

  I couldn't wait to get out and see what type of rocks they were.  If they were Sedimentary, Igneous or Metamorphic. It wasn't long before I found out they were........

IGNEOUS!!

   I could see distinct crystals in the rocks. The crystals were not held loosely together like a sedimentary rock but seemed to have formed at the same time. I could see lots of feldspars, quartz and a low percentage of mafic minerals. I was intrigued by how the crystals were weathering out of the rocks and how angular they were as they came out.
  Then I saw this one with a lot more dark minerals in it, but yet still igneous in nature.

What was interesting about this one was some of the minerals seemed to be lined up with each other.  Could it have been a metamorphic rock?  I wondered if it was darker due to weathering or minerals that were present.  It seems to be the minerals that were in it.  Oh how I wished I had a microscope where I could look at it under it and get a better idea as to what its true composition was. And to decide if it was igneous or metamorphic.   I couldn't spend long examining it because I had to be somewhere in a couple of minutes so I settled for a picture.  Hoping I would have more time to examine it later, or see others like it.
      Then I came upon this one.  What got my attention with this one was how the crystals were so much smaller than the other rocks I had seen along the way.  Most of the boulders I was seeing were very similar to the second picture in this post.  Seeing the finer crystals that indicated to me that this rock cooled at a faster rate than the other boulders I had seen.   Probably due to the fact that it was closer to the surface when the crystals were formed.

I had to get going but while I was at the function I was suppose to be at I took this setting sun picture of the boulder outcrop.

 While I was trying to fall asleep I wondered about whether these igneous rocks were due to a dike, a sill or a volcano.  I ruled out them being in a dike, because usually when I saw a dike it was more linear  than what I was seeing here.  Plus they just didn't fit in with what I recalled about seeing dikes.
  I ruled out a sill because I had never seen a sill outcropped before.  I've seen them on seismic logs and in drilling but never as an outcrop.  I figured it would be unlikely for this to be a sill outcropping, since they are so rare.
  That just left it being a remnant of an old volcano.  This made sense to me because I knew we were in the Basin and Range geologic providence.  When I had studied the Basin and Range I knew frequently volcanoes would come up through fault lines.  I remember studying about the area is one of tension (pulling apart) verses compression (coming together).  Normal faulting is seen as a result of the tension.  These fault lines would allow the molten material to migrate upwards thus forming volcanoes.
  Since they are on the continent they should have a high siliceous content.  The molten material would have been very slow moving and thus would allow time for slow cooling.  The slower it cooled the larger the crystals that could be formed. Also the type of crystals that would be formed more in the dacitic to rhyolitic range. All of this was in agreement with what I had already observed.  The rocks had a lot of feldspar in it.  Because the crystals that were there I thought they were orthoclase but could have been plagiclase (but without proper equipment I couldn't tell for sure). 
  Once I decide it was an volcanic remnant I then was curious as to what type of volcano.  That would take more looking.

 The next day I just had to get out and examine the boulders some more.
Here's some of the things I saw.

 People who were looking at this outcrop commented that it looked like a cap on a head.  What I saw was exfoliation fracturing, probably due to the the loss of the over head material.

 When I really examined the rocks they were mostly coarsely crystalline igneous, granite looking rocks. I couldn't find another rock like that third rock that I first saw.  I figure that must have been some of the cap rock that has been sloughed off since it was not next to others like it.

   Instead of figuring out what type of volcano it was I got more fascinated by the weathering that I was observing.  The most obvious weathering was how rounded all the corners were. And how few really sharp fresh outcrop faces I was seeing.  Even where there were joints I could see weathering going on between them.  I really wondered if it was due to water or wind.  With it being the desert, wind made the most sense to me. 

I was thinking wind until I saw this.  Then my mind went to water.

I couldn't believe how lucky I was to observe this.  Being in the desert you don't expect water to be a factor.  But when I was there in the morning, I saw some condensation coming off the big boulder above and forming in this small pool below.  Look at how much those slow dripping had eaten away.  I could just imagine what would have happen if there had been a big rain storm.  

Then I saw this little boulder sitting under this big one.  I could just imagine this one rolling around with both wind and water breaking the rocks away and it would impact on the other rocks around it.
    I started to think that if the rocks were 12 million years old (instead of the 12 by)  then they would have been around during some periods of glaciation.  The rocks them selves would not be in contact with the glaciers, but when there is glaciation the weather patterns change and there can be more rain water in areas that are now dry.  Their is evidence of this happening in AZ by the fact of the cliff dwellers near Montezuma castle.  There was obviously an extensive civilized society that used to live there but has moved on.  The main theory that they moved was because the land could no longer support them.  I could imagine there being more water in this area at one time and that has lead to the rounding, and weathering of the Boulders. 

  But what really convinced me was when we went to the pool side.  There they had put in a man made water feature on one of the Boulders.   I could see the water cutting into the rock.  The resort has only been in existence since about 1988.  I figured the water feature was installed at that time. I don't think they would have intentionally cut into the rock to create this feature but just would have let it flow over the top of the boulder. (look above the 2nd & 3rd umbrellas @ the bottom of the page, in the center of the next photo.)  There is definitely a worn groove from that flowing water.  If that much rock could have been worn away in just 23 years, you can just imagine how much was taken away over million of years. 

   Sometimes I was amazed that the rocks were the way they were and hadn't fallen off.  I just wondered when they would come down. 

After I got back from that outing, I went to the gift shop.  They did have a book there with some general information about the area.  It was called the 'The Spirit of the Boulders' by Pam Hait with photo's by Jerry Jackson, 1988.  I tried to retain as much as I could and wrote it down when I got back to the room.  I could have some of this stuff wrong, seeing as how I did not buy the book thinking I already had my pictures and the book was overpriced for what you got.
  The Boulders is near the Paradise Valley where the water is at the surface.  But at Cave Creek where the Boulders are located the groundwater is about 400 feet deep. 
The rocks are granite, with a large amount of feldspar.  She didn't write what type of feldspars.  
There is a 3 dimensional network of planar fracturing or joints that have weathered into rounded joints.  Carbon dioxide in the air makes an acid rain that does the eroding of the corners resulting in the formation of the boulders.
  The rocks are downthrusted.  I did not get the name of the formation or the time in which this occurred.  I did not see any mention of it being a volcano or what type.
  Also while I was at the shop I ran into one of the tour guides.  He told me that there was a geologist on staff that had given him a tour when he was here scouting out this location for the trip.  The only thing that he remembered was that the rocks were 20 million years old and that they were formed from weathering.
  20 million years seems a lot more reasonable to me.  I asked him about the geologist and he said he wasn't here at this time.  You have to make reservations with him and currently he was out of pocket.  I was very disappointed to hear that, because I definitely would have set up an outing with him if I could have.
  

             Now I'm back home trying to figure out more info.
There is nothing in Wikipedia about it.  Now to search the internet.  I can get info on the resort and all that it has to offer but not anything on the geology.

  I was able to google Earth it and when I look at it it is definitely round in shape which further leaves me to believe it was a weathered volcano.  Now I muse what type of volcano.
I don't think it's a remnant of a cinder cone because I was seeing some of those to the north.  Since it is so small I don't think it is a stratovolcano and also I saw no evidence of layering.  This leads me to believe its a endogenous lava dome.

  Now on to do more research to find out what others think about these boulders.  Oh how I wish I had a mass spectrometer to figure out the age of the material.  I would also love to get some thin slides of the rocks so I could see if there were any other hidden clues in the rocks themselves.
  This was definitely a fun place to visit.  I hope I can get back there some day now that it has caught my attention.