Sunday, November 13, 2011

New Mexico Geology: Rio Grande Rift

I still vividly remember learning the words to the Beach Boys’ Surfin USA when I was five years old. I was due to graduate from my preschool and we were going to sing that song for our families at the graduation ceremony. For those of you who don’t know, the songs begin with the quintessential Californian Boys lamenting the lot in life for the rest of their countrymen. Specifically they bemoan that:

If everybody had an ocean
Across the U. S. A.
Then everybody'd be surfin'
Like Californi-a
You'd seem 'em wearing their baggies
Huarachi sandals too
A bushy bushy blonde hairdo
Surfin' U. S. A.

            While learning this song I remember trying to imagine a United States cut in half by a body of water large enough to generate waves like the ocean. I imagined cowboys surfing from Texas up to Kansas herding cattle who were chilling on rafts, or businessmen windsurfing through city streets to  their office buildings (I had a really overactive imagination as a kid). I didn't think an ocean in the middle of the continent would be such a great idea, but I thought it would be interesting none the less. On a side note, when we went over the Vietnam War in high school, guerrilla warfare was described as like your whole country being an ocean, and all your people floating in it which always made me think of this song. Yeah, I know I'm weird.
          However, what I didn’t know was that the interior of America did share some features with the ocean (the Atlantic though not the Pacific) and even more surprising, that this feature cut my home state in half. What could dry, land-locked New Mexico have with the Atlantic Ocean? It would seem that no two regions could be less alike. But somehow they are, but for that to be obvious, first we need to understand a very fundamental aspect of our Earth's surface.

Change in Location of  continental Plates from the Permian to the present, Photo Credit Geology.com
           
While the ground beneath our feet may feel solid and static, we know that it is not. Instead the surface is made up of tectonic plates which floats on the flowing plastic of the Upper Mantle. The plates can be divided into two groups, oceanic and continental plates; the continental plates are mostly composed of granite with a thin veneer of sediments coating it and the oceanic crust is composed mainly of basalt. Since granite is composed of lighter (in color and density) than the iron rich basalt, it stands to reason that the continental plates would float above the basalt. Now look at the changes our world has undergone, specifically look at how the continents were torn apart, forming the east and west hemispheres. This formed a divergent plate boundary, and you can almost make out the change in this picture from the initial valley (in the Triassic) that the rift formed to the ocean we see now.
Plates can interact in three primary ways. If two plates strike together head on, this is called a converging plate boundary, and in this instance three different outcomes are possible. If a light continental plate and heavy oceanic plate converge, then the oceanic plate will subduct, or sink, beneath the continent. In this instance a lot of oceanic water end up mixing with the magma, this creates some seriously violent volcanoes, like Mount St. Helens. If two light continental plates crash together, they push each other up like a rug bunching up when you slip on it; this creates massive mountain ranges a great example being the Himalayas. Two heavy oceanic plates can meet, here both plates get pushed down, partially melt and form a  volcanic arc. Plates can also slide past each other, like in California at San Andres Fault, and if those movement is interrupted then the plates will lurch forward in a jerking motion causing an Earthquake.

Photo Credit: cotf.edu
However, none of those are what makes the Atlantic Ocean similar to New Mexico. Instead we have to discuss the third type of plate interaction, divergent plate boundaries which are places where plates pull away from each other . Like we saw in the break-up of Pangaea, this small crack in the plates can grow into a huge rift valley and eventually into an ocean! Luckily we have the opportunity to see a Rift Valley in the process of growing farther a part; we see in the aptly named East African Rift Valley the convergence of a few rifts cutting up the eastern coast of Africa. While the mechanism which cause this phenomena are not well known, the popular model among Earth Scientists involves elevated heat flow from the mantles causing bulges under the plates, forcing those plates apart.

Photo Credit: Geology.com

In New Mexico we have our own example of this process, a massive structure that cuts the state in two and formed mountains  and volcanoes at its sides making it one of the two features that dominate the geology of the state (the other being the Jemez Lineament). It began about 35 million years ago and then entered a phase of major volcanism for around 15 million years, and the first phase of crustal extension occurred about 5 million years after the start of the volcanism and lasted for ten million years. The rift then took it easy for a few million years, until the northern and southern portions of the rift underwent a period of expansion in the last 10-12 million years.

Photo Credit: Originally produced by the USGS

The rift formed numerous interesting features, but I like the Sandia's the best, so that is a great place to start. Why do I like the Sandia Mountains the best? It might be because they translate to the Watermelon Mountains, it might be because my fiance is from Eastern flank of those mountains, or it might be because it was the first stop on my structural geology field trip. Whatever the reason, they are really cool. Here the crustal extension resulted in horst and graben mountains; the grabens were mostly filled in with sediments eroded off of the Sandias.

Photo Credit: academic.emporia.edu

The Sandia's culminate at the crest, which is a beautiful sight and I could not more highly recommend that anyone visiting Albuquerque drive up to the top and look out over the city.

From my Structural Geology field trip

In addition to being beautiful the Sandia's also hold some amazing geologic features. First and for most is the amazing unconformity which represents a 1.2 billion year gap in the geologic feature. I have to give you my sincerest apologies for not taking pictures of it on my trip, but I'll try to get some the next time I'm in the area. However, just imagine the ancient granite crumbling into a pile of grus at its base tilted slightly with sandstone and limestone laying on top of it. The bottom of the sedimentary layers have been exposed in some places by crumbling granite, and you can walk under the rock and pull out fossils buried above your head! The Sandias also contain hydrothermal vents perforating the area which have resulted in mineral deposits (including some gold).
On the western edge of the Rift are the Three Sisters, who are now kind of old and cold but they used to be HOT...get it because they are dormant volcanoes, I'm hilarious...In all seriousness these three volcanoes are not that large, but they are awesome. These were also caused by the tectonic forces which shaped the valley, and sit on top of a 5 mile long fissure. They are dormant now, but they once spewed out basaltic lava over the area - side note you can actually see the path and level of the rain water over these volcanoes, the calcium minerals in the lava have been weathered in some parts into calcite, so that you have rocks which are black on top and white on bottom.
It would be easy to write a book just about the Sandia's, but the rift gave us more than just this, including the Organ Mountains near Las Cruces which is made up of granite, rhyolite, some sedimentary rocks and even some welded volcanics. The rift dominated much of New Mexican geology and most the North-South trending features in New Mexico were caused by the Rift (East-West trending features are a result of the Jemez Lineament. Including the very river that named the rift.
You see the Rio Grande is a fairly unique river, in that it formed within the valley, instead of forming the valley it now resides in. When the rift was opening up it created several lakes which then began to start linking up, eventually they started to flow south, and provide part of our border with Mexico. These massive forces are at their most striking at the Rio Grande Gorge which drives home more than any other site the awesome power that was tearing apart the crust millions of years ago.

Photo Credit: NM BLM
I should really go into more depth about these features; however, when I first said I would talk about this feature I forgot about how freaking huge it was and just how much the rift affects the geology of the entire state, so when I take some trips out to the areas I'll write them up, but until then why don't we just try to imagine what if the rift had expanded farther until there was an ocean across the U.S.A, do you think everybody would be surfin' like Californi-a?


No comments:

Post a Comment

Post a Comment