This morning's trip was down to a place called South Mountain. South Mountain is a prominent feature of the Phoenician skyline. In fact, it can be seen from just about anywhere in the valley. It is most easily identified as the mountain that has all of the antennas on it. Anyway, I have known about south mountain for about as long as I have lived here in phoenix. However, this trip was remarkably interesting because it really opened my eyes to the great geological history that I live on top of. Here is an aerial of where we met:
This location, as it turn out is quite unique. It is the juncture of 1700 million year old gneiss and 22 million year old granite.(geologists use the term Ga, an acronym for giga-annum, which is latin for "billion years". I will use the abbreviation Ga for billion years old, and Ma for million years old from this point on.) Anyway, what happened is a LONG time ago, more than seventeen hundred million years ago rocks were formed through various means. Over the millions of years, and at a depth of about 10km under the surface, these rocks were heated and deformed. This intense pressure and heat transformed, or more appropriately, metamorphosed these rocks into the gneiss that is now found at the surface. Fast forward to 22 million years ago, a large plume of hot magma from deep within the earth is ejected and intrudes the gneiss. When the rock cools, it becomes granite.
Here is another view of the boundry:
Up to this point things have been relatively dull. All of this activity takes place 10km underground. On the surface, life is flourishing. It is early in the Cenozoic era. Then something rather catastrophic happens. There is an series of earthquakes. Huge earthquakes! To give you an idea of just how big these quakes were think about this. The typical earthquake in california, for example the 1989 Loma Prieta quake (you know the one that happened during the world series that year?) made the earth move only a couple of meters. They call this movement displacement. Anyway, the series of quakes, which can be called rifting, displaced this mountain by more than 40Km. Here is another aerial image of how far that is:
Incredible, if you ask me. The south mountain range used to be homogeneous with the McDowell Range. The big gray area in between the ranges is metro Phoenix.
Well, next week will take me to Payson, AZ in the search of mississippian fossils.
3 comments:
I like the way you've done this new blog. Very nice looking header. I always think of the grand canyon when I think Arizona geology. Thanks for dumbing it down - I spent a lot of time googleing terms in your last entry.:)
I just posted but somehow lost it. Here it goes again. Now that you have dumbed this down a little bit-I am really starting to understand a little of it...and it is pretty cool. Didn't we climb South Mountain when we were out there once? 4th of July? Pretty neat about the whole section moving 40k - how awesome is that? Keep the page going - I really like it.
Dad
Ok, that is a huge earthquake! Has anything like that ever occurred in mankind's recorded history (as opposed to prehistoric times)?
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