[MUSIC] In module one, we saw how humans first mapped the earth and then not much later they began to chart the depths of space. And finally in the 19th and 20th centuries came the ability to measure the immensity of time. At the bottom of this slide in broad brush and showing just the most major divisions is the fruition of that ability to measure deep time. It's the geologic time chart. The origin of planet earth is on the far left at 4.5 billion years ago. And the divisions are sewn in GA which just means giga annum or billions of years. So 2.5 GA the division between the Arche in and the pro tour is OIC is simply 2.5 billion years ago. In subsequent lectures, you'll see that I use either BY for billions of years or the somewhat more formal GA. Which means the same thing. The chart breaks up geologic time into to really major divisions. The Precambrian and phanerozoic on in Greek. Phanero means visible, and Zoic refers to life initially meaning visible life. So phanerozoic rocks and time were strata with shell or bone fossils. In contrast, Precambrian refers to before the Cambrian period and for the early geologist, Cambrian rocks were the oldest fossil of various strata. Of course today, the origin of these terms is a bit dated and misrepresentative. We know that there was plenty of life back in the Precambrian, albeit dominated by tiny single celled organisms. If we go way back in the subject of this lecture, we hit the Hadean part of the Precambrian era. The Hadean was originally intended to represent the earliest phase of earth history where the surface was barren, frequent impacting took place and remelting events occurred. Such that no rocks of that age remained. But that too has kind of changed and we now have mineral materials that date to nearly 4.4 billion years. So today the Hadean simply refers to that first 500 million years or so of earth history. And it's in the Hadean that we find some significant first, the origin of our moon, oceans, atmosphere and life a lot to consider. So we'll just hit the highlights. On worlds with no tectonic plate motion or with little in the way of atmospheric weathering. Surface features are neither removed nor modified and the hallmark of these bodies are craters. You might be surprised that the upper left image here is not the moon, it's the planet Mercury. But speaking of the moon, the Apollo missions to the moon now, well over half a century ago were certainly a boon to national pride and all kinds of technological advancements. But one of the key contributions to science that came from the return of moon rocks was the discovery that these lunar rocks were nearly identical to earth mantle. The lunar rocks turned out to be basically just dehydrated or water free earth mantle? This was a huge discovery. It begs the question, how did earth mantle get to the moon? And the answer is that it appears that very early in earth history and during a period of extensive bombardment and impact, probably all within 100 million years of earth formation, a massive impactor whacked into our planet, the debris forming a cloud and then a disk and then consolidating into a world of its own our moon. Below is a link here to an animation provided by the American Museum of Natural History. Let's check it out. Shortly after moon formation, the volatile constituents of our planet start to show up. In other words, materials that are either liquids or gasses and ambient surface conditions are very earliest atmosphere would have consistent of lingering hydrogen and helium from solar system formation, but we think this would have been quickly removed by the strong wind of charged particles flying off our youthful son. The earth's interior was undoubtedly a major source of all kinds of volatile materials and these oozed out during a phase called internal de gassing that still continues today. But less actively gasses like water, carbon dioxide and ammonia were rather unceremoniously burped and belched to yield the first atmosphere and oceans. Recent work on the minerals of earth mantle, some done at the University of Colorado by our colleague Joe Smith suggests that even today, earth mantle likely contains more water than all the surface oceans combined. And it's unclear how much, but some of earth's water may well have come from objects like this comments, commentary impact and based on isotopic composition, these comments were probably ice balls that roamed the inner solar system, not the distant icy ghosts of the so called or cloud. Where we get most comments today. You may recall that the very hot early earth during planet formation underwent segregation. This is shown in the diagram on the left, molten iron rich droplets sunk into the interior to form what would eventually be the core layers of mantle and crust also formed. And layering is the key to a hallmark feature of planet earth plate tectonics, as shown in the diagram on the right and probably active within a few 100 million years of planet formation. Plate tectonics takes place within a couple 100 km of earth's surface. It's really just the outer few percent of our planet's radius. The outermost layer, which can be upwards of 100 km thick, is called the little sphere it slides atop the underlying kind of gushy, taffy like Christina sphere. The earliest lift, this fear was probably quite a bit thinner and moving around faster than plates today, somewhat like the modern earth lift. This fear was made at melt zones where mantle was rising, it was lost and destroyed, sinking back into the mantle at subduction zones, like at the trench circled in this diagram interestingly. Even before a modern style of plate tectonics, some rocks managed to form and not get recycled back into the deep earth. In other words, the first vestiges of a stable crust were forming way back, but how? Making stable continental crust. The details are complicated and even a bit controversial whether it's linked to fancy sounding stuff like here in the center image sinking peripatetic SCLM subcontinental with a spirit mantle or the melting of depressed wealth of ocean crust. This is all from Kranendonk science 2011 paper, good stuff. But who knows? The big picture idea is easier to share and it's kind of on the right, just like on earth today, the rocks that are stable and don't get stuffed back into the mantle are what we call continental crust. Continental crust is made out of lighter and more buoyant, meaning less dense rock than ocean crust, which in turn is lighter and less dense than mantle rock. So it's all a little bit like making whiskey, making whiskey? What are you talking about? I'll leave it parked up for now making whiskey. Another complicated process, but fundamentally, whiskey is a distilled spirit. You take an alcoholic beverage to start with and you heat it up and the vapor coming off comes into the bottle, making it even more alcoholic in the earth. A kind of natural distillation takes place whereby mantle rock undergoes partial melting. In other words, the lighter minerals melt first, and these rises magma to make ocean crust then in places like subduction zones, the ocean crust melt and this magma gets even lighter and more buoyant materials forming typically granite type rocks that ultimately make up the ocean cross. So continental crust, by virtue of its lower density, doesn't easily sink back into the mantle and it can stick around for billions of years. When we want to find old rocks, we go looking in continental interiors. The oldest rocks are on continents like North America and Australia. In most cases the oldest rocks have been smushed and smashed and cooked up a bit. And so these tend to be what we call metamorphic rocks. In the upper right, one of the oldest true rocks, in other words an amalgam of various mineral grains. The Acosta nice from the Northwest territories of Canada and dated with radioactive isotopes at nearly 4.1 billion years old. But there are older earth materials, individual mineral grains inside of rocks that date back to 4.34 point four billion years. Just 100 or 200 years after earth formation. For example, this grain in the lower left of the image of zirconium silicate, a mineral called zircon from the Jack Hills region of Western Australia. I made a sort of cartoon here to show the story of the zircon grains. The zircons are special because they don't easily melt or transform into new rock and they retain the radioactive isotopes from their time of origin. We can envision super old super ancient crust eroding to make a sediment that was later squished in metamorphosed this metamorphic rock would have a bulk rock age of somewhat younger. Perhaps like the Acosta nice, but the included so called Detrital zircons. The little red triangles here are shown and can retain their original isotopic composition and their original age of formation. Okay, we're almost there. A couple of the last slides are going to cover an important origin story life itself. It's something that geologists and biologists are becoming more and more convinced arrived exceptionally early in earth history. What sort of evidence do we have for early life? Well, first off, we have fossils based on the work of Bill Shop and colleagues, it appears that structures found in the 3.5 billion year old Apex Church initially found in the early 19 nineties are almost certainly strings of fossilized cells perhaps akin to the blue green algae or cyanobacteria, that still exist today. But there is also a chemical fingerprint for life that predates these fossils by quite a bit. It primarily has to do with the element carbon and without going into complexities. It turns out that living things prefer a kind of carbon called C12. In contrast to the slightly heavier carbon isotope called C13 which simply contains an extra neutron. Various investigators, including Steve voyages from the University of Colorado have found C12 rich sediments from Greenland in the acicula and Isela meta sedimentary sequences. These carbon isotope anomalies perhaps generated by cellular processes suggest that life might well have arisen by 3.8 to 3.9 billion years ago and started changing Earth's chemistry. That's a few 100 million years after planet formation. So life is quite ancient. But where did it come from? How did life first arise? We don't really know. To be fair though, quite a bit of headway has been made. Back in the 1950s Young Stan Miller, this is old Stan Miller here, Young Stan Miller, under the direction of his PhD advisor, Harold Urey did a pretty simple yet amazing experiment. They took the primordial non living components likely to be present in the earlier things like methane, ammonia, hydrogen added some electrical sparks and voila. The concoction turned kind of brownish and ended up containing the whole suite of amino acids, fundamental building blocks of proteins in life. Since that time, scientists have used other starting materials and other sources of energy but still end up with those amino acids. Additional work has been shown and has gone forward that some of these fundamental carbon molecules will spontaneously prelim arise, in other words, making bigger and more complicated molecules. But unlike the suggestion of the cartoon on the right here, nobody has come even close to making life in a testing. So the first several 100 million years of earth history was quite eventful. The moon formed, as did the first continents and oceans and atmosphere, and somehow through all the commotion, including major phases of extraterrestrial impacts, which may well have erased its first vestiges. The microbial ancestors of all life that exists today showed up. We're on our way to learning about what scientists have revealed regarding the beginnings of the North American continent. But with the next lecture, we're going to set the stage with a little bit of a primer on plate tectonics. Much like evolution for biology, plate tectonics is effectively the reigning principle of geology.
