Phoenix, a brief history.

Todays little jaunt was pretty fun. I took James along and so we both got to learn all sorts of cool geology stuff. I am taking notes from Emily's last comment and will be "dumbing down" my blogs so that they are easier to understand.

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.

Hayden Butte Revisited

Back to the butte. Tuesday took us back to Hayden butte, but this time to stratorgaph the lower sedimentation layers (notably from the base up to the ash layer from the superstition caldera eruptions. This lab took a bit longer than usual, however it was great to get up close and personal with various rock outcropping. Generally speaking, the bedding we looked at consisted of about 70 layers of alternating sandstone and shale. Only 3 of of the layers contained trace fossils. The oldest layer had trace fossils of worm burrows, which was again repeated in a layer about 10 meters above. The other trace was the mudcracks. Upon closer inspection it appears that the mud cracks formed on several sandstone layers in superpositioned horizons, in my estimations there were 4 distinct horizons that them. I'm gonna try and get a scan or two of the hand drawn stratographic sections that we did (about 6 pages in all). I'm looking forward to Oct 27, when we will be on a fieldtrip to south mountain to checkout some sweet synclinic action. ohh yeah.

In a lighter note, I really enjoy the names geologists come up with for things. For example gneiss (pronounced nice) is a common metamorphic rock. Schist is another common metamorphic rock. Anyway there are many others with strange and unusual names. But i couldn't leave these two alone. So i came up with an idea for a t-shirt:

Hayden Butte Lab

This is the first outdoor lab for the historical geology class. Located on ASU Campus, Hayden Butte (A Mountain) is a well known site for and ASU student. The butte holds the Sun Devil Stadium (Home of ASU Football and Super Bowl XXX) on a saddle on the south east side. The lab began by looking at the oldest rocks on the butte, located on the north side in a drainage canal parallel to Rio Salado Pky. The exposed sandstone layers here are estimated to be only 17 Ma. The oldest sandstone beds are perforated with trace worm burrow fossils.

Just up the rock strata we found:

This is the trace fossil of mud cracks. Cracks were formed into a layer of mud, that has since turned into shale. Sand filled the cracks and formed a layer of sandstone. Later, the underlying shale eroded and left the negative impression of the cracked mud.

A little farther up the strata we found:

This section of Sandstone / Shale Interbedding shows a fault. Although the age of the fault is unknown, is can be assumed that it was caused by the tilting event.

Now this was unusual:


This is a very perculeier formation. What this is is a 2m thick volcanic ash formation. However as you can see, the bedding has been convoluted into ball and pillow shapes. This can be caused by either seismic activity on the wet rock, or by being compressed from above (also while the rock was still wet. This formation was so cool that I had to grab a smaple for my collection.

Finally the youngest rocks on the butte were a nice andesite lava flow. A short ways down from the top I found some eroded chunks of the andesite:


and here is what I thought was really cool; this is where a lava flowed over sandstone, resulting in contact metamorphisim:


And here is a rip-up clast of sandstone that had been metamorphosed into quartzite:

Introduction

In order to log my various geological excursions, I will attempt to maintain what I call my geo-blog. Comment if you want, ask questions if you like.