You may recall my side job as an airline pilot. Well, climbing out of Denver International Airport, high over bolder, and headed to the west, maybe 15, 16,000 feet in the climb, we often get a great view of our iconic flat irons. These tilted layers in the foreground of the picture here, and seeing through a few clouds. Everything behind the flat iron rocks is really the uplifted Precambrian basement of very old granite and metamorphic rock. The uplift tilted the flat irons and associated sedimentary strata that's in the foreground. The highest region back there, the snow-capped peaks at around 13, 000 feet are what we call the front range, because this was the first range of the Rocky Mountains that settlers and trappers and adventurers encountered when going West in the 19th century and going West a lot slower than jet aircraft. Here's a close up of the peaks in the front range. Trust me, by the time we're flying over these, we are well above the peaks which topped out in the low 13,000s we're probably already climbing above 25,000 feet on our way up to cruise between 32 and 37,000 feet. These rocks are all in the neighborhood of 1.7 billion years. That's the age of continental crust in this region. Of course, the uplift was far more recent. The sculpting by glaciation is something that took place just over the last couple million years. On my right, a cartoon of the geology in the area just West of the University of Colorado Boulder campus. In this cutaway or side view, what geologists call a cross-section, I'm effectively out here in the Great Plains, looking back westward. I see some undulating ridges of tilted sedimentary rocks. The ridges being formed by the more resistant layers. Behind the sedimentary layers, we find the Precambrian granite metamorphic rocks that had been pushed upwards by various periods of Rocky Mountain uplift. What if we want to know what's beneath these old rocks and how the first crust of Colorado actually started to form. Getting information about the deeper earth and even the moderately deeper Earth, say, lower crust and uppermost mantle. Not that easy. How do we probe the depths? Well, seismic waves are actually really useful and can give information down to enormous depths. In this image here about my left, picture of seismic waves are fast-moving. We get blue, green, cooler colors. When the seismic waves are slow, the warmer colors. In fact, this image of seismic velocities beneath North America suggests what may well be the remains of an ancient subducted slab. But seismic waves just don't allow us to get our hands on actual pieces of deep crust or mantle. Things we'd like to put under a microscope or check their composition or determine isotopic ages. What about deep boreholes? Well, very expensive and not that effective with the very deepest being only about 10 kilometers down. Well, sometimes volcanic eruptions will bring up some deep seeded material, rocks that were ripped off the magma chamber conduit and brought to the surface. But even here, magmas form and come up from relatively shallow levels, maybe on the order of 5-50 kilometers down. It turns out though, that there are some rare kinds of eruptions that bring magma and associated pieces of the deeper upwards from great depth. These are called kimberlite eruptions, named after the location in South Africa where first investigated in detail. These magmas can come from as much as 200 kilometers down. Still outer part of the Earth, the little purple patch here, but vastly deeper than any other magmas. Kimberlites form in a region of the mantle so deep where elemental carbon at high pressure and high temperature converts into a densely packed arrangement. For geologists, kimberlites are a treasure trove, not just because of the diamonds, but because of other bits and bobs of the deep earth that also are brought up to the surface. These hitchhiker rocks are pieces of the lower crust and the upper mantle, and they're called xenoliths. Northern Colorado and Southern Wyoming contain a whole slew of these kimberlites well over 100 documented localities. The upper right image here shows some Colorado diamonds from near the state line. The lower right-hand images, well, yours truly quite a few years ago. Sitting on a kimberlite just outside of Boulder Colorado. Unfortunately, no diamonds, but still a wealth of mineral material in these rocks from the deep earth beneath the front range. Interestingly, most of these kimberlites have ages in the vicinity of 350-400 million years. This is cool because it's before the major phases of mountain building got going in the Rockies. However, we did some age dating on a couple different kimberlites, one being the kimberlite near Boulder, and another one located here in Northern Colorado called the Chicken Park Kimberlite, where we found this adorable mascot next to some of the material that we sampled. At the Boulder locality, we use the radioactive decay of rare element samarium into neodymium via Alpha decay. To determine a much older age than the other kimberlites, around 570 million and not to go into the details of these analyses, but we also obtained an age at around 600 million for that Chicken Park Kimberlite. It suggests that there were probably multiple phases back even before major mountain building, where kimberlites were punching up into this part of North America. With them, they brought pieces of that lower crust and mantle. Yes, we did release the mascot of the Chicken Part Kimberlite, off it goes. To sum up, in effect, these kimberlites have served as probes into the deeper, knowing about their ages and studying their entrained [inaudible] of lower crust and upper mantle material allows us to build a picture of the roots of the Rocky Mountains. As in the model here, at depth, there appears to be a candid boundary sloping downward to the South beneath Colorado. It's a boundary that separates the Archean, meaning older than 2.5 billion year continental crust from the younger Proterozoic, 1.7 billion year old continental crust. This structure produces a wedge or kill lithosphere beneath Northern Colorado. At least we think it's this super old mantle that seems to be the source for diamonds that show up in the kimberlites near the Colorado, Wyoming state line. At any rate, we think we're looking at a suture cell where subduction zones have brought together the younger crust from the South and slammed it into existing continent to the North. A classic example of continents forming by implicated subduction zones and the removal of intervening ocean crust, ultimately ramming fragments of buoyant continental crust together. Are you still there? That's some complex geology, a geo chronology and geochemistry. Module 2 has been a whirlwind. We've gone from the origin of the universe to that of stars and planets. We've looked at the birth of continents and in general. Then also the very guts of North America beneath the Rocky Mountains. But hang on tight. In the next module, Module 3, we'll take a look at the strange and sometimes wild conditions towards the end of the Precambrian, a world nearly encased in ice, barren continents lifting and eroding away. In the midst of all this exceptional developments in the evolution of life that would set the stage for the modern world and all the animals and plants that exist today. Finally, the third module then wrap up with the rise of the Ancestral Rocky Mountains and the deposition of rocks that would become the flat irons.
