[MUSIC] Welcome to Module 3. By the early 1800s, scientists knew at least approximately the immense distances to the stars and planets. Using accurate observations in Newton's laws and mechanics, it was possible to calculate their sizes and their orbits and motions with exceptional accuracy. But as you may have read in the previous slide quotation, it appeared that with things so far away and that we could never touch, there was basically no way to know anything about their composition, densities or temperatures. Of course, Auguste Comte and most of his contemporaries were wrong. By the mid 19th century, the tools of stellar spectroscopy use light to reveal the elemental composition of stars, planets and galaxies. In the same vein, geologists have found tools and techniques to probe down thousands of miles beneath the surface and to effectively travel millions and billions of years into the past and see the world as it once was. In this third module, we bring the tools of geological science to bear on deciphering the early history of the North American continent, rocks and fossils and magnetism and radio metric dating. All come together in order to pull back the mists of time that even scientists used to think would forever obscure the deep past, and what we found is quite a story. Before we delve into this story, let's take a moment to think about where we've been. Module 1 considered the activity of science, what it is and how it's a unique approach to understanding the world, quite distinct from things like philosophy, religion, or metaphysics. And we learned about how science managed to crack the code if you will on deep time, revealing an earth whose age is far outside of human experience and measured in billions of years. Module 2 went back to the beginnings of things like the universe itself, stars and planets and the very early earth. We looked at how our planet formed, got its first crust, oceans and atmosphere, the stage on which life itself emerged. We reviewed some of what we know about the formation of the North American Creighton and even looked at its crustal age patterns. But although it's nearly 90% of geologic time, we need to work our way out of this lengthy period of geologic history called the Precambrian. Module 3 is going to focus on this really unique and eventful time period. The last few 100 million years of the Precambrian and officially called the Neoproterozoic and the first era of the Phanerozoic, the Paleozoic, essentially my little red circle down here. The official boundary, the vertical red line is identified in the rocks by a major change in life, in the fossils themselves. The beginning of the Phanerozoic Eon is marked by the emergence of fossils with hard parts, things like shells and skeletal structures. To learn about this time period with its wild changes in climate, massive continental erosion and the emergence of new forms of life. Well, go looking at the rocks to the myriad layers of stone that tell the story of the past. So much like reading an old book, we have to learn to read a challenging and old forgotten language. And in many cases, the pages themselves are smudged beyond recognition or simply torn out i.e missing. So first things first, we'll spend the rest of this lecture with a quick survey of rocks, things that geologists and geology students are supposed to be able to read. If this looks familiar, great. Most people encounter some rudimentary earth science somewhere in middle school or high school and if not, no worries, the basics are not too complicated. Igneous and sedimentary rocks are pretty straightforward types. There are rocks that were once molten now solidified, we call igneous rocks. Complexities involved where they form, where they cool, how they cool. But if once molten, we're talking about igneous rock. Sedimentary rock, my daughter many years ago, having a seat on some, are just solidified sediment. The ground up rock pieces big and small and in between that have been moved around by air, water or ice or in some cases precipitated out of lakes or oceans and then glued together by compaction and sedimentation. As we'll see metamorphic rocks are a bit trickier, probably because they form over long time periods and it's deep in the earth where we can't really see things. These metamorphic rocks by virtue of elevated temperature and typically also elevated pressure don't actually melt. But their chemical constituents quite literally come apart and recombined to make new minerals that were not there to begin with. Okay, we'll take a closer peek at each type. If you randomly drill a hole into the earth, you won't find liquid rock even way down below the crust in the mantle, melting in the earth is luckily relatively rare. Since magma is generally less dense than surrounding rocks it rises. And if there were loads and loads of melting in the mantle or deep crust, then we'd have volcanoes popping up everywhere. When rocks do melt making magma, then that magma may end up cooling and crystallizing either on the surface, like my rhyolite above or beneath the surface underground like my granite below. We call these exclusive and intrusive igneous rocks. And the interesting piece here is that even though these are radically different looking rocks, the rhyolite picture and granite have the same composition. The same magma has just undergone different cooling rates. Sedimentary rocks again, just glued together particles of preexisting rock that have been broken up and moved i.e eroded or in some cases like limestone, a matter of calcite, precipitating directly out of lake or seawater. The difference between sediment just ground up unconsolidated particles basically muck on the bottom of say river or lake or sea. And actual sedimentary rock is this business of squeezing often via the weight of overlying sediment and the pressure serving to squish out the water and precipitation of superfine mineral material between particles. What geologists call cement. This cement is the glue that holds sedimentary rocks together. And in the hypothetical close up view here, the cement is the intergranular material between the particles. Well, the cool thing about sedimentary rocks is that they form in all kinds of settings, right down from the mountaintops and glaciers to rivers, lakes, deltas, beaches and ocean bottoms. Each of these settings will produce a characteristic type of sedimentary rock. Because of this, sedimentary rocks are the key to understanding what past surface environments might have looked like and therefore integral to understanding the geologic history of a place. Okay, lastly metamorphism and metamorphic rocks. As I intimated earlier, these just aren't quite as intuitive or familiar from our everyday experience living here as we do on Earth's surface. In a nutshell, metamorphic rocks are rocks that were once other rocks and have undergone change due to heat and pressure. They generally form at depth in the roots of mountain zones or continental interiors. Here's one that we've seen before, the world's oldest rock, the Acasta Gneiss from Northwest Territories Canada, dated at over 4 billion years old. During metamorphism, an initial rock which might contain, say hypothetical minerals A, B, and C, gets heated and squeezed and out of these original constituents elements recombined. And we get a new suite of minerals and in this case looks like mineral A stuck around, but we also now have D, E and F. In the real world A, B, and C, might be something like quartz and mica and clay. And after metamorphism, we have quartz and some new minerals like [INAUDIBLE] and garnet, and feldspar. The key idea however is that, the mineral changes take place without melting. The elemental constituents that make up these minerals, silicon, oxygen, aluminum, potassium, they just move around in the solid state. Hence solid state recrystallization. And if that sounds challenging or science speak, just consider cooking. You start with some flour and yeast and water and salt and you heat it up and out comes something different maybe even better like a pizza crust. Metamorphism occurs in many plate tectonic regimes and kind of like sedimentary rocks at earth's surface. Each of these places makes representative kinds of metamorphic rock. Location a, a mountain belt, location b mid ocean ridge, location c subduction zone etc. Here are some examples of minerals that we might find in a metamorphic rock. Each of these quite different minerals has the exact same composition. They're called polymorphs and they're all composed of aluminum, silicon and oxygen in the same proportions. But kyanite here forms at relatively low pressure and temperature and andalusite forms at relatively low temperature but high pressure and sillimanite forms at high temperature and high pressure. A metamorphic rock might contain more than just one of these or other minerals that are indicative of particular pressures and temperatures in one rock. In other words we might see vestiges of early forming minerals and then later mineral overgrowth. This kind of information allows us to build a pressure temperature history of a rock. With metamorphic rocks, we can use early and later forming minerals to envision how a rock underwent burial, reaching maximum pressure and then maximum temperature at unique times and then being uplifted and cool and brought to the surface. On the right, this is what we call a pressure temperature time plot. All right, last slide, still with me? On the left, the old rock cycle from elementary geology textbooks. I think it's poorly named because it's not really a cycle per se. There's no directionality. Any rock can turn into any other kind of rock. I prefer thinking about these different rock types, igneous, sedimentary and metamorphic in a plate tectonic sense, like on the right hand diagram, where we see them in their quote, natural habitat forming in all kinds of different environments. And then we use them to determine what things were like, where they formed and when they formed. So if we are geologically literate, we can take our old and tattered book of earth layers and we can read the story of earth history and that's where we're going.
