[MUSIC] This lithograph is a rendering of The Royal Cabinet of Curiosities, a collection curated by the Danish royal physician, philosopher, and naturalist Ole Worm in the 17th century. The lithograph itself is an historical artifact recording an event of human history. You learned in the last episode that, similarly, this rock is a historical artifact of Earth's history. Now, one of the primary jobs of geologists is to catalogue the history of the Earth. And the condensed results of this job is this. The international chronostratigraphic chart is a record that documents the timing of major geologic or evolutionary events and organizes them into discrete units of time. Now, you will likely see this chart several times over the course of the class. So it's worth spending a few moments to understand it now. The numerical age of a boundary of units of time is shown here. The parenthetical Ma you see means Mega annum, or millions of years ago. So the beginning of the Cambrian, for example, was 541 million years ago. And the beginning of the Holocene, the current epic of time was 0.0117 million years ago or 11,700 years ago. The next thing to know is that Earth's history has been divided into large units of time, which have then been subdivided into smaller units and then even smaller units. Each of the divides represents a major geologic or evolutionary event. Just as historical time periods, such as the Dark Ages, or the Renaissance, or the Neolithic Period, are divided based on historical events. Now, perhaps the most famous of these geologic time divisions is the transition between the Cretaceous period and the Paleogene period, which occurred about 66 million years ago. This transition marks the major asteroid impact event that wiped out the dinosaurs and led to the mass extinction of about three quarters of all plant and animal species on Earth. Now this chart that we are looking at was published in 2012. And the chart itself is constantly being revised as geologists find better and better examples of geologic specimens recording the events that mark the transitions between these periods. Notice that the beginning of geologic time is marked at the formation of our solar system 4,600 million years ago or 4.6 billion years ago. The Earth likely forms due to the large moon forming impact you learned about in last week's lesson here. 4.5 billion years ago. Just think about that. By the universe's standards, Earth is pretty young. It has only existed for the last third of the history of the universe. But by our standards, by human standards, it is really, really old. This lithograph of Ole Worm's exhibit was made in 1655. It is nearly 360 years old. By comparison the Earth is 4,500 million years old. It is so old that it is nearly impossible for humans to comprehend. To just try to wrap our heads around how old that really is, let's pretend that one centimeter. This right here from there to there. One centimeter represents 10,000 years of time. That means all written history fits into this one centimeter. The Romans, the Greeks, the Incan and Mayan empires fit in this centimeter. Even the end of the last ice age, when glaciers covered half of Europe and North America, and mammoths still roamed the world, is within this one centimeter. Now, if you can imagine that, then the history of the Earth is 450,000 centimeters, or 4.5 kilometers long. That's the same distance the ferry takes to get from the town of Helsingor here in Denmark across Oresund to the town of Helsingborg in Sweden. So, keep in mind when we talk about four and a half billion years ago we are talking about a really, really long time. Now getting back to the stratigraphic chart itself there's another interesting thing that you might have noticed. It's not to scale. And let me make this a little clearer. All I've done is rearrange the chronostratigraphic chart so that it falls on a line. You can see from the ages of major chronologic period transitions, that this is not to scale. It's not even close to scale. In fact, it's even easier to see when you compare it to what it should be. What should be about the middle of the timeline is way over here, and about three quarters of the timeline represents only the last 12% of Earth history. Why is this? Why is the chronological chart that we have of Earth history not to scale? It's because the chronostratigraphic chart is as much a summary of the knowledge of Earth as it is of geologic time. Our Earth is in fact a very dynamic place. Rocks are constantly eroded away by the weathering processes of glaciers, precipitation, wind, or waves. And they're constantly being recycled back into the interior through plate tectonic processes. The result of this is that we have a lot of young rocks sitting at the surface of the Earth and easily accessible for geologists to investigate. But not a lot of old rocks. Taking a look at the distribution of exposed rocks by their age, you can see that only a small portion of the continents are covered by rocks that are from the Precambrian eon, that is, older than 541 million years. And most of these rocks are from the Proterozoic Eon, the next two billion years. There are only a handful of localities on Earth, older than 2.5 billion years old. And the vast majority of these rocks, come from the Archean Eon between 2.5 and 4 billion years ago. The Hadean Eon which goes from 4 billion years ago, back to the origin of Earth was originally defined as the age before the rocks. Now there are in fact two localities with undisputed geologic materials older than 4 billion years old. The Acasta Gneiss located in northwestern Canada and individual sand grains composed of the mineral zircon from a clastic sedimentary rock called the Jack Hills conglomerate that is located in western Australia. The Acasta Gneiss is a metamorphosed granite that is 4.03 billion years old. And the zircons in the Jack Hills conglomerates are derived from the weathering products of ancient granites that were as old as 4.4 billion years. That's it. That's the only geologic material we have to work with to learn how Earth originated and evolved in its first half-billion years. But of course when you have almost nothing you find a way to do a lot with what you have. In addition to extensive chemical data that we've obtained from these two rock localities, Earth scientists use our understanding of the solar system formation to make estimates of the bulk Earth composition. And we use our observations of the end result of Earth evolution, that is what Earth we have now. And our understanding of the basic chemical and physical processes that control our universe to develop a reasonable working hypothesis for how the Earth formed and what the very young planet Earth probably looks like. In the final episodes of last week Henning and Jim talked a bit about the hypothesis of the moon forming impact. Where an asteroid approximately the size of Mars smashed into a proto-Earth. The energy of the impact was so large, that both bodies partially vaporized. With particles being splayed out in a circular disc that quickly amalgamated back into a hot molten Earth. And a smaller molten Moon. Understanding just how such an impact happened and the resulting effect it had on the formation of the Earth requires complex computer simulations. In which physical parameters, such as the size of the proto-Earth, the angular momentum of proto-Earth, and the size, speed and angle of the impactor are all varied so that the resulting Earth, Moon pair matches the physical characteristics that we can measure on these two bodies. Now, these characteristics include the relative masses of the Earth and Moon. The distance between them. The rotational speed of both objects, and importantly, their composition. We can measure the total mass of both the Earth and the Moon, based on their gravitational pull on one another. And, from this, we know that while the Earth has a dense metallic core, the Moon does not. We also know that the bulk isotope composition of several elements on the Earth and the Moon, like oxygen, silicon, tungsten, and titanium are all identical. Now how does this happen? Well, just like large asteroids, the proto-Earth and the large impactor that collided with it to form the moon, were large enough to heat up to the point of melting. Both from the transformation of kinetic energy, from collisions of asteroids into thermal energy, and from the decay of radioactive elements. Now, while initially a homogenous mixture of metallic and chondritic meteors that formed them, once molten, these components of these protoplanets began to differentiate. With more dense metallic elements sinking toward the center of the planets, and forming a metallic core. This was likely the physical state of these two bodies, proto-Earth and the collider, when they collided. And they must have done so with enough energy, that the metallic core of the impactor actually merged, with the core of the proto-Earth. And the remaining molten material of both the proto-Earth and the impactor were vaporized into a ring of debris around the new core. Now the debris mixed and completely homogenized. And then most of it recondensed into the Earth with the remaining outer disc material coalescing into the moon. Here you can watch one of these simulations, where a relatively small impactor, about half the size of Mars hits a fast rotating proto-Earth. The different color balls represent the cores and the residual mantles of both bodies. Note how, after the impact occurs the asteroid's core melds with the Earth's core. But the mantle material is scattered outward and completely mixed, becoming homogeneous and so hot that it's likely in vapor form. For up to 1000 years after the moon-forming impact, Earth was enveloped in this hot, silicate atmosphere that slowly cooled and consolidated. Now as the Earth cools and the silicate vapors condense, they do so in the order of their boiling point. For example, rock forming elements like silica, iron, magnesium, calcium, and aluminum, they condense into a liquid first. A homogenous or well mixed ocean of magma probably capped by a thin layer of hard rocky crust. Volatile molecules, that is molecules that are stable in liquid or gas forms even down to relatively low temperatures, such as CO2 or water, were probably mostly lost to space during the giant impact. And what remained would have been retained in a dense atmosphere until the Earth cooled even further. The magma oceans solidified, and the Earth temperatures dropped below the boiling point of water. So, that liquid oceans could actually rain out of the atmosphere. This differentiation happened very quickly, and then it stabilized. In fact, modern Earth is still characterized by this type of chemical differentiation. The core, made up mostly of iron, is solid in the middle, where pressure is highest. And, liquid, in the outer portion, where it convects, forming the magnetic field around Earth. The mantle, which makes up most of Earth's mass, is composed of igneous intrusive rocks that are very rich in iron and magnesium, called peridotite. The crust, the outermost, thinnest layer, is the least dense rock on the planet. And probably, the first crust to form from the very hot and magmatically active mantle was a basalt, which is slightly less rich in iron and magnesium and slightly richer in silica than the peridotite rock that makes up the mantle. We don't actually have any vestige left of this early basaltic crust. The Hadean rocks that we have fragments of, did not come from basalt but from granite. Remember the Jack Hills conglomerate from Western Australia? It is made up of small sand grains and gravel that were derived from older rocks. And all deposited together as clastic sediments before cementing together around 3.4 billion years ago to form this rock. And some of those sand grains are made out of a mineral called zircon. Zircon is a fantastic mineral for two reasons. First it is an incredibly strong mineral. It can go through a lot of geologic processes and survive with pretty much the same composition it had when it first grew. And second, although it is made up of almost entirely of the elements zirconium, silicon, and oxygen, it has the kind of mineral structure where other elements can substitute for the zirconium. One of the elements that likes to substitute for zirconium is the element uranium. And as you learned in last week's episodes, uranium has lots of radioactive isotopes that decay to form the daughter products, thorium and lead. That means we can date this mineral. The zircon grains found in the Jack Hills rocks of Australia are between 4.1 and 4.4 billion years old. And not only that, their trace element and oxygen isotope chemical composition tells us that they formed in a granitic melt, not a basaltic melt. And the only way to form a granite with the zircons that have the chemical composition we measure in Jack Hills is to partially melt either a hydrothermally altered basaltic rock or sediments that formed by the weathering of preexisting rocks into clay. And to have either of these materials on the Earth, means you had to have liquid water. Thus, the chemistry of these ancient zircons necessitates that the Earth had cooled off enough in its first 150 million years of existence for water to condense from the atmosphere into liquid oceans. Now, once a temperate environment has been reached with stable oceans present the emergence of early life was possible in virtually any near-surface environment on Earth. However, survival of these potentially early life forms was likely jeopardized by the high rate of bombardment by large asteroids throughout the Hadean. And perhaps especially in the last 200 million years of this era, during the late heavy bombardment. Based on statistical properties of impact craters on the moon the probability that there was a single impact large enough to re-vaporize all of Earth's oceans. Melt much of the exposed crust and destroy any nascent life forms during the Hadean is about 10%. So, life on Earth was certainly tenable, as long ago as 4.4 billion years, but it would have been a precarious existence. And still on a planet that looked almost nothing like the Earth we have today. [MUSIC]
