[MUSIC] Hi and welcome to week two of Origins. In last week's episodes, you canvassed nearly two-thirds of the history of the universe. The origin of the universe itself, in the Big Bang, the origin of nuclides, that is, the elements and all of their isotopes, the origin of stars, and of our star in particular, the Sun, and the origin of the Solar System around it. This week, we're going to take you through the next two great origin events that led to life as we know it today. That is the origin and the early evolution of the Earth, and the origin of life. Now, these two topics are presented together because it turns out that the evolution of life and the evolution of the planet on which they originated, at least as we know it, are intrinsically and inextricably linked. My name is Emily Pope. I'm an assistant professor here at the Natural History Museum of Denmark and I work in a research group called geobiology and mineralogy in which we explore the idea of how the evolution of life has shaped our planet and vice versa. Now everything that makes up life, the elements that our bodies are composed of, the air that we breathe, the water we drink. The nutrients that we consume and the environment that we live in is determined by what materials accumulated from the proto-planetary disc of dust around the early sun that you learned about last week. And how those materials distributed themselves and continue to distribute themselves, around and throughout the Earth. So to know how life got here, how it evolved and became you and I, we have to understand how Earth works, how it evolved and became what you and I live on. And that is its own exciting story because, just like life, the Earth is unique in our known universe. In science, the way to understand how things work is to bring them into a laboratory and measure how they respond to changes in their environment, or to take them apart and look at what they are composed of. Or to go out and collect many samples of something and measure it and compare them and identify some kind of pattern, if it's indicative of a process or a phenomenon. For example, you could study in a laboratory how a mouse responds to a specific medicine or you could go out and collect thousands of leaves and see how their size correlates to the average temperature of the region that they're from. But the Earth cannot be brought into a laboratory to be studied, it's 12,700 kilometers across. And we can't go out and measure many Earths to identify patterns because the Earth is the only planet we've found so far that looks anything like an Earth. Instead, for geologists to study how the Earth works, we have to go outside, carefully observe what is on the Earth's surface today. And then deduce backwards through both space and time to what is happening that we cannot see. For example, the next time you're walking along a beach take a look at the sand beneath your feet. Now if somebody were to come up to you and hand you this little bit of sand and ask you to guess what kind of environment the sand came from, it probably wouldn't take you long to guess a beach. You could look at the sand and say, I see seashells, so it must be close to the ocean. And all the grains are about the same size. They're all small grains, not gravel like I would see in the mountain rivers. But they're also not mud like I see in deltas. They're sand. And they're all kind of round. Each of the grains is pretty round, as though it's been rubbing against each other and smoothing each other out. Maybe because the waves of an ocean hitting the sand on the beach makes them abrade against each other and weather down into these little spheres. And once you've guessed that this sand comes from a beach and sorted out why you think so, it wouldn't take a large leap to suppose that sand from most beaches on Earth will have characteristics in common with the sand that you're looking at. And the sand would have gotten these characteristics from common processes. Similarly, ancient beaches must have formed in similar ways to modern beaches. And that is something you can prove to yourself when you look at a rock like this. Made up of many sand grains that are all all pretty rounded about the same size and there's even a few shells in there. The sand in this rock might have formed in a beach just like this sand. But a long time ago, after which it was buried deeply enough that it compressed and solidified into a solid rock, aptly named a sandstone. Now this little thought exercise actually has a name, we're describing the Law of Uniformitarianism. It's a concept that was first coined by James Hutton, a Scottish farmer who lived in the 18th century and is considered the father of modern geology. The law simply means that the present is the key to the past. The physical processes that shape our globe today are the same processes that shaped the other side of the globe, where we can't see. And that have shaped the globe for millenia prior. This rule means that to understand how Earth has worked in the past, we need to study the rocks that formed in those ancient environments, something that is collectively termed the rock record. Now of course, there are many different types of rocks in the rock record. And just as sandstones tell us that there were ancient beaches, the other types of rocks explain other geologic phenomenon on Earth. So, let's take a look at some of these rocks and what they can tell us, because this rock record is what we use to explore the history of Earth and the origins of all things terrestrial, including ourselves. A good place to learn about the variety of rocks and what they can tell us about the ancient Earth is here at the Natural History Museum, two levels below the ground floor of the geological exhibits. This is the petrographic collection, the museum's collection of different rock types from all over the world. And two of our museum's conservators of this collection, Zina Fihl and Niels Hald were kind enough to pull some samples for us to learn from. Now, the rocks that we have already looked at, the sandstone, come from a family of rocks known as sedimentary rocks. These are rocks that form at the Earth's surface, via one of two mechanisms. They're either the result of the amalgamation or the cementing together of individual mineral grains and rock fragments that were once sediments such as sand or mud or silt or gravel, or they're rocks that form by the precipitation of minerals out of a water solution. So the first group is called clastic sedimentary rocks, because they form from the clasts of sediments getting glued together. The second group is called chemical sedimentary rocks because they're formed by chemical precipitates from a solution. Now there's a huge variety of kinds of chemical sedimentary rocks that form in the environments that they form in, but the most common form from precipitation in sea water or ground waters. Like carbonate rocks, this one here, such as calcite or dolomite. And those form from the precipitation of calcium or magnesium carbonate. Or rocks that are formed from microcrystalline grains of silicon dioxide, and include chert, flint, opal, chalcedony, and jasper, such as this one here. Another rock that forms at the Earth's surface is not a sedimentary rock. It's something you might expect to see if you visit Hawaii or Iceland, and those are volcanic rocks. Volcanic rocks form when lava erupts or flows out of the Earth onto its surface. Lava is the liquid that forms if you heat rocks to high enough temperatures that it melts and then is released onto the Earth's surface. Of course, the rocks don't melt above the Earth's surface. They melt deep below the surface where temperatures are much higher. And before the lava erupts above the Earth's surface, it's actually called magma. Now volcanic rocks are the result of lava erupting and cooling so that it crystallizes. That is, minerals freeze out or crystallize out of the melt due to the change in temperature. Now of course, it's possible for magma to cool and crystallize before it erupts as lava from a volcano to form a volcanic rock. Both rocks that erupt from volcanic eruptions and rocks that form from the crystallization of magma beneath Earth's surface are called igneous rocks. Volcanic rocks are extrusive igneous rocks and rocks that formed under Earth's surface are intrusive igneous rocks. An example of an extrusive igneous rock that you might be familiar with is basalt. This is the volcanic rock that erupts from most Hawaiian volcanoes, as well as in most other volcanic islands. And it's also the rock that makes up most of the ocean crust. This is an example right here. It's characterized by being rich in the elements iron and magnesium, which is why it's so dark. It's usually a black rock. And sometimes it has these green minerals called olivine. Now, another rock that's igneous that you might be familiar with, is granite. Granite is a name that is used rather loosely when we're referring to kitchen countertops, but in geology, it has a pretty strict definition. It's an intrusive igneous rock that is very poor in iron and magnesium, but rich in the element silicon. And there are actually a huge variety of igneous rock types between granite and basalt. But the spectrum of this variety is controlled by two things. Either intrusive and extrusive and its chemical composition. The more iron and magnesium in an igneous rock, the more mafic it is. The more silica rich a rock is and iron and magnesium poor, the more felsic it is. And just like ocean crust is made up mostly of the mafic rock basalt, continental crust is made up of the much more felsic rocks like granite. Now I know that was a lot of terminology to have to learn in a short period of time. But trust me, these are useful rocks to be familiar with as you continue in the class. There is one final classification group of rocks, metamorphic rocks. These are rocks that form when a pre-existing rock, called a protolith undergoes textural and mineralogical changes due to changes in its physical or chemical environment. Such as increases in temperature or pressure, or the addition of fluids such as water or CO2, or it could also be some combination of all of these things. Now, as you can imagine, metamorphic rocks comprise a huge variety of rock types, because there's an almost endless number of combinations of starting rock. And metamorphic conditions of temperature, pressure, and fluids, that can exist on the planet. Zina has pulled a few examples, just so you can see the variety that I'm talking about. All of these rocks formed by the metamorphosis of the same kind of rock, a mudstone, which is simply a sedimentary rock made of mud or clay particles. They look so different, because they formed at different pressures and temperatures. So, this rock, formed at low temperature and pressure, and it metamorphosed to these other rocks during increased temperature and pressure. Now, alternatively, all of these rocks formed at about the same pressure and temperature. But they look different because the original rock that was being metamorphosed, such as a granite, or a basalt, or a limestone, or a mudstone, all have different chemical compositions. All of the rock types: metamorphic, igneous, and sedimentary, can be related to one another by the different geologic processes through which they are formed or destroyed. For example, igneous rocks can be exposed on the Earth's surface through volcanism, or through uplift of mountains. And once exposed to the atmosphere and hydrosphere they can be weathered, broken down into sediment which is transported by rivers or wind and deposited as clastic sediments that are eventually buried deeply enough to compress and cement into sedimentary rocks. Those rocks or the chemical sedimentary rocks that precipitate out of the ocean water form from evaporation of the salt lake. Once they're buried deeply enough, they'll heat up and they'll start to metamorphose into a new type of metamorphic rock. If the rock heats up enough, it will actually melt into a magma which then becomes the source for a new intrusive or extrusive igneous rock. Collectively, this is called the rock cycle, and there are many alternative pathways connecting the different rock types. Metamorphic rocks and sedimentary rocks can also be uplifted and weathered. Igneous rocks and even other metamorphic rocks can be further metamorphosed, and so forth. The bottom line is that each of the three rock types that we have looked at, igneous, sedimentary, and metamorphic, tells us something specific about the environment in which it formed. So that, when we look at any one of these given rocks, we can say something useful about what Earth was like when this rock was made. And because we have tools to determine the age of these rocks, either by using radiogenic isotopes similar to the ones Jim and Henning told you about last week, or by estimating the age of a rock based on its position relative to a well dated rock. We can use the rock record to build a chronology of the history of the Earth. And this is what we're going to talk about next, starting from the beginning. The origin of Earth. [MUSIC]
