Sunday, October 24, 2010

Sitting and Thinking (it's all about the geology)

Since my last post (“Field Tripping,” Oct 20, 2010) I have been doing a bit of thinking (ooh, that’s scary) about what I had written.  What was that post all about?  I strive (honestly, I do strive) to make my writing mean something and not just be a collection of groovy images - well, unless groovy images are the point, such as with the bears of Katmai.  

But what meaning could anyone without a geology background possibly derive from “Field Tripping?”  Could those anyones (and you know who you are!) even decipher what I was trying to say?   For that matter, what was I trying to say?  I finally realized that there was no context; it was all a mumbo-jumbo-gumbo and that was about it.  For sure, I had included lots of nice photos of geologic structures commonly found in southwestern Utah and the surrounding environs, and by inserting some cool lines and boxes into the photos I thought that would explain everything to everyone.   But finally I figured out that wasn’t enough.  The pictures were awesome but they probably didn’t mean anything to anyone except me and the folks who were on the field trip.
Beaver Dam Mtns. as seen from the Virgin Mtns.
I live here in southwest Utah.  I have spent the better part of the past fifteen years looking at its rocks; the first ten were spent wondering how in the heck geologists actually know all this stuff they know, and the next five were spent sitting in a classroom, studying my buttons off trying to understand at least some of that “stuff.”  I spent upwards of three years doing undergraduate field research in the Beaver Dam Mountains, mapping the ≈1.74 billion-year-old metamorphic rock outcrops in my study area and trying to figure out what it all meant.  I followed Mark C. everywhere and asked questions until both of us were blue in the face.  I started by mapping outcrops of amphibolites (metamorphosed basalts) and continued mapping  migmatites (partially melted metamorphic rocks from deep within the Earth’s crust), schist (a metamorphic rock in which its minerals exhibit foliation like the layers of an onion) and gneiss (a metamorphic rock exhibiting alternating light and dark bands of foliated and less-foliated minerals).  I hammered out hand samples of rocks from the outcrops; took gps readings; drew pictures in a field notebook; took strike and dip measurements (the orientation of an inclined rock surface) with a Brunton compass on every possible outcrop that displayed anything even remotely resembling a flat bedding surface; filled multi-gigabyte cards worth of digital photographs;  took my samples from the field back to the lab where I made hundreds of thin section slides which I examined under a petrographic microscope; and with Mark’s help identified the multitude of minerals displayed within the .03mm-thick rock sliver.  My favorite classes were mineralogy and igneous-metamorphic petrology because it was in these classes where I started to put “it” all together.  My last summer at field school tied “it” all into a bow as I mapped many of the sedimentary, igneous, and metamorphic rocks that comprise this little corner of the Colorado Plateau/Great Basin transition zone.

Now I realize that many of you reading that post were probably wondering “What in the world is she talking about???”  You may even be asking that very question right now.

So I’d like to explain by starting at the beginning.  Well, not at the Big Bang beginning – you have Discovery Channel and PBS Nova for that.  First I want to define the 3 different types of rocks.  You probably already know them (I know you do!) but we’ll just do a quick review anyway.  Next we’ll get oriented to this Colorado Plateau/Great Basin transition zone that I keep raving about, and then we’ll just take it from there.  My goal is that the photos taken on last week’s field trip will become less of a mumbo-jumbo-gumbo to you.

I sincerely hope you enjoy this particular geologic journey and that you find it all as interesting as I do.  Who knows where we will end up!  If you have any questions or comments please do not hesitate to tell me because I love feedback.  The photos are all mine but  the diagrams were gotten off the internet.  Unless specified, the text is all from my head and notes I’ve taken over the years.

Will there be a quiz on this?
Sedimentary rocks form from fragments or particles (“clasts” in geologic lingo) eroded from somewhere else.  Limestone, sandstone, and shale are all examples of sedimentary rocks.

 Igneous rocks form from magma which is molten or melted rock from deep within Earth’s mantle (Hey – wake up!  Remember crust-mantle-core layering from those Nova programs?).  Lava is magma that has erupted onto the Earth’s surface.  Basalt (like in Hawaii) erupts above ground, cools quickly, and has no crystal formation (it is fine-grained).  Granite (like in the Sierra Nevada Mtns) doesn’t erupt above ground but cools slowly underground and so has time to form crystals (it is coarse-grained).  The pumice of Novarupta was “frothy magma” that contained massive amounts of gas when it exploded.

 Metamorphic rocks (my favorite!) are those that have been altered by heat and/or pressure.  Schists and gneisses are examples of metamorphic rocks.  We try to figure out what they were in the distant past by examining their mineral composition today.  There are thousands of fascinating yet confusing pressure-temperature diagrams to help in this effort but this too is another story for another time.

Additionally, there is a geologic principle called “original horizontality” which describes how rocks are deposited.  Gravity is the ultimate driving force here, and so those eroded particles or clasts (of sandstone, limestone, whatever) must be deposited horizontally.  If any kind of rock is tilted, folded or faulted, as they are in mountains, something happened to bring them to that configuration.  How that something happened to bring about the tilting, folding or faulting of the rock layers can best be explained by plate tectonics. 

My bad – I forgot to mention I need to mention plate tectonics
 If someone asked if you know what plate tectonics is, you’d probably say yes but might not be able to actually define it.  But geology is all about definitions so I’ll present the basic one from my Glossary of Geology: 
Plate tectonics theory holds that the lithosphere (the crust and upper mantle) is divided into a number of brittle plates which move about on the surface of the Earth relative to each other and interact at their boundaries causing seismic and tectonic activity along these boundaries.
Whoa there!!!  What???
What this means is that the brittle plates of the Earth’s crust move about relative to each other and cause, among other events, earthquakes and volcanoes to occur and mountains to form.  This is the something happened to our flat-lying rocks. 

Now put on your Physics hats for a moment.  Subduction occurs when a denser plate collides with and dives or “subducts” beneath a less dense plate.  In this manner, some of the rocks from the less dense plate are pushed up to form mountains while the others that were subducted deeply enough then melt and eventually rise as volcanoes.  We see this along the Oregon-Washington coast as well as in Alaska.

I used this diagram in my evening program this summer at Katmai.  You can see that the plate on the left (oceanic lithosphere or Pacific plate) is subducting beneath the plate on the right (continental lithosphere or North American plate), a mountain range forms (#14) as the N.A. plate is pushed up, and rocks subducted deeply enough to melt (#12) rise as magma  (# 13) to become a volcanic arc.  Viola!  And so we have the Cascades with Mt. St. Helens and Mt. Ranier;  we also have the volcanoes of the Aleutian Islands and Katmai National Park. FYI:  #5 and #6 show where two plate diverge or move apart such as occurs along the mid-Atlantic Ridge.

Here's more than you probably want to know at this point:  The mechanism for subduction is convection!  Like a pot of water set to boil on a stove burner, heated magma rises to the surface from deep within the Earth (#4 and #5), cools at the surface, then descends again to depth - this movement of heated/cooled magma is thought to contribute to plate motion.  

A common type of fault is called a normal fault in which the hanging wall drops down relative to the footwall (block diagram on left).  These occur as tension stretches the brittle crust to the point of failure.  This is a major type of fault that separates the Colorado Plateau from the Great Basin – in southern Utah it is called the Hurricane Fault and it runs from near I-70 south across Grand Canyon where it disappears near Kingman AZ.

block diagrams courtesy of USG
Thrust faults occur when the crust is compressed (as opposed to
extended or stretched) and older rocks are “thrust over” younger rocks.  Think of a rug on a hardwood floor – if you push on one end of the rug, what would happen?  The rug would soon fold over on itself over and over again.  This is pretty much what can happen when continents collide (wasn’t that the name of some B movie?).  Thrust faults are a type of reverse fault and often occur in mountain building events.  On last week’s field trip we saw this type of fault where the 240 million-year-old Permian limestones had been thrust over younger Early Jurassic (≈ 190 million-years-old) Aztec (Navajo) Sandstone.  Just remember that the older rocks are always on the bottom unless something happened.  This is another of those geologic principles that is pounded into students from day one.  A ramp fault is a small thrust fault – there is a picture of one from the field trip with a cool and groovy box pointing it out.

Sometimes, the relative motion of plates to each other is sideways in opposite directions – this is called a strike-slip fault  A world-famous example is the San Andreas Fault in California where the Pacific plate is sliding northwestward while the North American plate is moving westward.  

In this image the relative motion along the fault is shown.

 The action of the two plates along the SAF is thought to be partly responsible for the thinning crust and fault-blocked mountains of Nevada and the Great Basin (right side of image) – as the Pacific plate moves northwest it shears and draaaaaaags the western edge of North America with it. The crust is so stretched and thinned in Nevada that it is thought to be only a few miles thick there. 

The relative movements of the SAF have been occurring for about the past 20 million years.  Prior to that, the tectonics was different along the west coast - the oceanic plate was moving eastward, carrying along land masses that would eventually collide with North America.  A period of mountain-building lasted from about 140 million years ago until about 35 million years ago.  It is during this time that many of the structures  seen on field trips in the Beaver Dams and Virgin Mtns. came to be.  

That’s enough of plate tectonics for now!  It’s time to get oriented.

Where in the world are we, anyway? 
I had two lovely red stars to indicate the location of St. George in southwest Utah on these maps but sadly I couldn’t get them inserted.  What a drag.  But with your keen observation skills you can still see that our area of interest is smack-dab on the boundary of the Colorado Plateau and the Great Basin – the transition zone between the two.  Cedar City is close enough and also in the transition zone so let’s go with it.

Colorado Plateau (courtesy of USGS website)
The Colorado Plateau consists primarily of flat-lying sedimentary rocks that have, over the past several hundreds of millions of years, undergone very little tectonic activity – it is considered a fairly stable part of the continent.    It does have a slight dome shape due possibly to melting of crustal rocks deep beneath it but that is another story for another time.   However, many areas along the western and southern edges or boundaries of the Plateau have undergone much compression and uplift or extension and faulting over geologic time as landmasses of different size and composition have collided into and attached themselves to or “accreted” onto the more stable core.

What does your mind’s eye see when you think of the Colorado Plateau?  “Layer-cake stratigraphy” is a common description of its sedimentary rocks that have maintained their horizontal configuration pretty much intact since they were laid down millions of years ago.  These rocks are a window into the past;  they tell of advancing or retreating shallow sea floors, coastal plains, flood plains, mud flats, lagoons, inland freshwater lake beds or massive fields of desert sand dunes. 
Point Sublime, Grand Canyon National Park

Grand Canyon AZ is the textbook example of flat-lying sedimentary rocks of the Colorado Plateau.  This is a view from Pt. Sublime on the North Rim (probably one of the better photos I have ever taken!).  All of the well-known national parks in Utah are on the Colorado Plateau – Zion Canyon, Bryce Canyon, Arches, and Capitol Reef as well as Grand Staircase-Escalante National Monument and Cedar Breaks National Monument.  Zion is actually on the eroding western edge of the Plateau and so is considered to be in the transition zone.  Cedar Breaks is also on the eroding western edge of the Plateau –a barely distinguishable drainage divide along the park road sends run-off draining eastward ultimately into the Colorado River while run-off draining westward goes into the Great Basin. 

           Virgin River cutting through upper Navajo Sandstone and lower Kayenta Formation in Zion National Park

50-million-year-old lake bed sediments of Cedar Breaks National Monument

 The Great Basin, contained within the larger Basin and Range, is characterized by having no surface drainage to the sea.  Any rain or snow that falls within the confines of the Basin finds itself either evaporating before it hits the ground, evaporating on the ground, or disappearing into subterranean rivers or aquifers.   It is an area of stark desolation and dramatic scenery – Death Valley and the Great Salt Lake are both found here as is Great Basin National Park.   The hundreds of fault-block mountains of the Basin, their sedimentary beds tilted due to tension or stretching of the crust, are generally oriented north-south, causing one early explorer to describe them as “marching caterpillars.”

The Great Basin bounds the Colorado Plateau on the west and encompasses western Utah, most of Nevada, parts of Idaho, Oregon and California, along with a tiny corner of northwestern Arizona.   If you have a highway atlas handy, take a look at Interstate 15 as it cuts north and south through Utah.  This road pretty much marks the boundary between the Colorado Plateau and Great Basin as it divides the state nearly in half.  Interstate 15 also generally follows a nearly billion-year-old fault line in the crust, so it is not really surprising that the boundary between the Great Basin and Colorado Plateau is located exactly here.  
Colorado Plateau and Great Basin transition zone near St. George, UT - snow-capped peaks are the Beaver Dams with red Kayenta Formation sandstones and siltstones in foreground

Virgin River Gorge  transition zone between Colorado Plateau and Great Basin, SW Utah

I’m striving to not make this too difficult to understand so hang with me
Over the course of those three years of metamorphic rock-hounding and thin-section grinding, Mark (mostly) and I (leastly) worked out some ideas of what had gone on over the past nearly 2 billion years.  We came to a number of conclusions which continue to be refined.  Field work continues sporadically as we are able to get out to the study area and map.  I am no longer a student but maintain a nearly obsessive interest in the area.  Can you tell?
Shearing textures seen in thin section under the petrographic microscope     
The metamorphic and tectonic history of the Paleoproterozoic (1600 to 2500 million years ago) assembly of North American continental crust is preserved in ancient crystalline basement rocks of the southwestern US - for instance, at the bottom of Grand Canyon along with these outcrops found in the Beaver Dam Mountains and Virgin Mountains.  We found that our field mapping and thin section analysis delineated a massive shear zone extending for tens of miles through AZ-NV-UT. This shear zone exhibits different mineral assemblages and exhibit diverse textures.  These textures (seen on the thin section slides under the petrographic microscope) tell much about the shearing and decompression (uplift) that different tectonic blocks have undergone from deep within the crust and possibly the mantle to the surface over the past ≈1.7+ billion years.  

Mineral studies suggest that these rocks came from depths near 27-29 miles (45-48 km).  This is why that “shear zone” in the Virgin Mtns. photographs is so beguiling.  To be able to get “up close and personal” with a shear zone in which the shearing occurred deep within the crust (because we are talking not about brittle, upper-crust rocks but about plastic-flowing, lower crust rocks) just blows me away.  Most earthquakes are shallow, brittle affairs – no disparagement meant here! – while these shear zones involved nearly inconceivable pressures of 188,549 pounds/square inch and temperatures approaching 1472°F (800°C).  And that ultramafic pod at which the students were milling about is thought to have been brought up during tectonic collision from mantle depths of possibly 34-35 miles (56-57 km). 

Remember earlier in this post I talked about different size land masses colliding with or “accreting” onto what already existed as ancient North America?  The evidence Mark and I and others found here in the southwestern Utah/southeastern Nevada/northwestern Arizona transition zone suggests to us that an ancient landmass referred to as the Mojave Province (think Mojave Desert) most likely accreted onto another ancient landmass referred to as the Yavapai Province.   There is much debate (of course) about what accreted on to what, and when.  But it is fairly certain that something happened those billions of years ago and that it occurred in this area.  

Wait there’s more!  
A mountain building event called the Sevier Orogeny (orogeny means “mountain building”) occurred ≈140 million to ≈ 35 million years ago; during this time more landmasses were “rafted in” from the west as the oceanic plate collided with and was subducted beneath what already existed as North America. 
Sevier Orogeny folding along Taylor Creek/Double Arch Alcove trail in Zion NP
This lengthy event resulted in folding, faulting, thrusting up and generally wreaking serious tectonic turmoil on a good many of these rocks.  That is why, for instance, those ≈ 240 million-year-old Permian limestones we saw on the field trip are thrust over Early Jurassic (≈ 190 million-years-old) Aztec (Navajo) Sandstone.  At the completion of the Sevier Orogeny, tectonics took a breather for a few million years.   Ultimately the plate configuration changed, the San Andreas Fault came into existence, and the western North American crust stretched and normal-faulted its way to the Great Basin/Basin and Range Province marching caterpillars that we see today.  

Take another look
I am leaving a whole lot of the story out, so if anyone is interested in learning more, just let me know.  My hope here is that those images from my field trip last week will now be less of a mumbo-jumbo-gumbo and will make more sense to you.  Take another look at them, if you will.  And know that I am always more than happy to go out and take more, wherever I go!

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