[MUSIC] So we've definitely done some geology. We've learned a bit about plate tectonics and the formation of earth's oldest rocks. We've discussed how earth rocks and minerals have isotopic dates that go back well over 4 billion years. But to find materials that actually match the age of planet formation at 4.5 billion, we need to go look at lunar rocks or meteorites. Okay, back to planet Earth, on my right, the present world and the colors on this map are places we'd have to go in order to find really old rocks. Certainly not the ocean floor once believed to harbor old rocks. Plate tectonics tells us that ocean crust is continually forming at mid ocean ridges and removed by subduction zones. It comes and goes. The oldest ocean rocks are not much more than a couple 100 million years old. A far cry from the oldest continental rocks whose ages are measured in billions of years. Okay, on this map, the green represents continental regions with the very oldest rocks, 2.5 billion years old or greater of what we call Archean Eon. Orange indicates tracks of continent between 1 and 2.5 billion and the yellow regions are between 500 million and 1 billion years old. Keep in mind these yellow regions, the young stuff are rocks that are still generally much older than the earliest skeletal and or multicellular life on planet earth. The large scale exposures of these rocks are called continental shields. You can see major precambrian shields in Northern Europe on this map, Northern Asia, Australia really on all the major continents, we're going to pop over and take a look at the Canadian shield in North America. But first a little perspective on rock ages and geologic time. This is a pretty standard geologic timeline. Even our intro students have to memorize these top bars, the eons and eras. And of course we want them to know that Earth origin, its accumulation out of the solar nebula occurred about 4.5 billion years ago. Precambrian time. The precambrian Eon extends from earth origin to about 540 million years ago, where the first abundant multicellular life and fossils appear that have like skeletal parts right there at the vertical red line. On this again, pretty standard chart. It looks like the Phanerozoic Eon is a bulk of earth history, but this is not the case. It's an artifact of how the chart is presented with a somewhat understandable focus on the time period that contains things kind of like us. Well, okay, not that much like us, but things that are at least macroscopic and skeletal. In fact, the precambrian amounts to about 88% of geologic time. In other words, nearly 9/10 of earth history, all are precious fossils of trilobites and fish, amphibians, dinosaurs and mammals and human beings are crammed into a little over 1/10 of earth history. The phanerozoic, okay with that in mind, we roll back to this ancient and dominant part of earth history, back to the precambrian and the foundations of continents. Shown in purple the Canadian shield is a huge tract of land, admittedly covered here and there by lakes and trees and soil. But in general it's a place of exposed precambrian age rock, like these on the left here located at Georgian bay and here's the arrow, shows Georgian bay's here on the northeast edge of Lake Huron. These rocks are dated as archean at between 2.5 and 3 billion years old. Well, since the advent of radio metric dating literally thousands of age determinations for rocks across North America have revealed a jigsaw of continental pieces with distinct precambrian ages. We envision that this pattern here is a result of continental fragments being effectively swept together and sutured onto the proto North American continent as intervening ocean crust was removed via subduction. It's worth noting that these old rocks are sometimes covered by a veneer of overlying and younger sedimentary layers. It's as if the old rocks were covered by a carpet. In the case of our continent, the carpet formed as various washes of ocean came onto and off the continent. Each incursion of ocean water depositing sediment. To be clear the main portion of continents that are floored by old rocks are referred to as a Craton, where the Craton is widely exposed. Well, those we call the shield regions and here's a cartoon showing that even outside of the shield regions on the Craton itself, we may get a glimpse of these deep cratonic rocks, the guts of the continent. All it takes is a deep canyon, like where my little boat here is on the left, slicing through the carpet of sediments. Many examples, but perhaps the best is the famous Grand Canyon that will take a peek at in a moment. Alternatively, the deep and old basement rocks can get pushed upwards via mountain uplift with the overlying carpet of sediments being removed by erosion. Speaking of uplift, here is perhaps my most favorite mountain range and a great place to go climbing, although it does take a lot of uphill hiking to actually get to the climbing. It's within the rocky mountains and it's the Teton range in Western Wyoming. The mountains are the result of relatively recent uplift, say, within the last 10 million years. But the rocks that have been uplifted are very, very old are archean age in excess of 2.5 billion years. On the right, my wife Melissa a number of years ago and we used to spend large portions of our summers climbing amongst these peaks. She's leaning on abandoned metamorphic rock called nice, in her right hand a dark metamorphic rock called amphibolite and in the left hand, another piece of nice all 2 and a half billion years old. Well, and here we are at the quintessential Canyon of Canyons, the Grand Canyon. The oldest rocks down here in the inner gorge represent rocks about 1.7 billion years. And they represent the age of basement rock across a huge chunk of North America, most of Nevada, Utah, Arizona and Colorado. And here above the inner gorge, we see the overlying carpet of sedimentary cover layers. Everything in these layers are the early paleozoic, so watch sandstone at the bottom to the kaibab limestone of late paleozoic age at the top. Well, the Grand Canyon and in fact all the rocky mountains in Colorado are within a huge tract of old rock shown here in the brown. Between about 1.6 to 1.7 billion year-age range where the overlying sedimentary cover has been removed we see these rocks. Now to the North where we were looking at the Teton Mountains pop over here, we have an even older tract basement rock in excess of 2.5 billion years old. Now you might notice on this map that in a general sense and is particularly As one moves from the North to the South out of the Canadian shield, there's a gradual decrease in Craton ages. And the reigning hypothesis shown below is that these age provinces represent pieces of continent or island arc material that have been swept together due to the subduction of intervening ocean crust. It's basically the model for how we think continents grow. Light continental crust does not sub ducting easily, so it gets secreted onto the existing continent.
