WATCH: Unit 4 Overview

Far out in the uncharted backwaters of the  unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting  this at a distance of roughly 92 million miles is an utterly insignificant little blue green  planet whose ape-descended life forms are so amazingly primitive that they still think  digital watches are a pretty neat idea. Douglas Adams, The Hitchhiker's Guide to the Galaxy Yikes. If aliens are ever going to visit us we need better tourism marketing. I mean, come on, take a look at this it lights up how neat is that! [instrumental music] Hi, I'm Rachel Hansen. And this is Unit 4, our  solar system and earth.   Our earth is a very, very small part of the universe, but it probably  seems familiar to you. You've been here your whole life, after all. But don't get too comfortable.    What if I told you that the ground you stand on is constantly moving? What if I insisted that our homes, our nations our continents, everything   rests uneasily on a surging sea of molten magma.  You'd probably say, "Duh, Rachel. Plate tectonics. Everyone knows that." Congratulations you paid attention in your earth science class. But you also had several thousand years of collective  learning on your side. The meteorologists Alfred Wegener wasn't so lucky. He claimed that 300 million years ago all the continents were kind of  smushed together and they've been drifting apart ever since. He used interdisciplinary evidence to support his theory, which you'll learn about in  this unit. But the world wasn't ready to listen. His colleagues mocked his theory. Most geologists  continued to believe that the continents were   fixed in place and that the ocean floor was smooth. But 20 years later, a geologist named Harry Hammond  Hess happened to be captaining an American warship  in World War II. His ship had new sonar technology that could locate enemy submarines. But Harry was a geologist at heart and he kept the sonar on continuously, creating a detailed map of the ocean floor.   What he found was far from smooth. Valleys, trenches, and volcanoes surged beneath the  waters. By the end of the 1960s we understood that the earth's crust was divided into dozens of huge  plates.   Drifting in on the molten mantle of the earth, causing earthquakes and eruptions, excavating  ocean trenches and raising mountain peaks. [instrumental music] Plate tectonics wasn't an accepted scientific  concept until the 1960s. Think about that. By the 1960s humans had nuclear weapons  capable of ending life on this planet, The Beatles were the most popular band in  the world. I mean, humans had been in space, we were about to set foot on the moon.  And still, we didn't understand a fundamental part of our home. We didn't know how mountains were made.   Stop me if this sounds familiar. Wegener built on the theories and collective learning of past generations.  Other scientists had proposed something similar during the 19th century.   And it took maps made by early  explorers, sailing across oceans before people began to notice that parts of the Americas  look like they fit together with Afro-Eurasia like puzzle pieces. And earlier thinkers, including  Leonardo DaVinci, often wondered why they found the fossils of sea creatures on the tops of high mountains.   Plate tectonics tells us how mountains rise and continents move.   It also reveals more about the earliest days of the solar system and the birth of our planet 4.56 billion years  ago. The molten core of our earth and the sea of super hot magma our thin crust of continents  and oceans moves around on are reminders of our   planet's earliest days, and how ridiculously  lucky we are that life developed here at all. [instrumental music] In Unit 3, the universe got a whole lot brighter when stars lit up. It also got a lot more complex as some of those  stars died and released new chemical elements. We also crossed over two thresholds in the last unit. As the first stars formed and then as stars began to die, generating the intense heat and pressure  needed to create new elements and more complexity. We examined how scientists have dealt with  the complexity of all these elements by   organizing them into this handy table that every  science teacher has hanging in their classroom.  And we discovered how those elements are the building  blocks for everything in our lives, including us. [instrumental music] In Unit 4 we'll cross over a new threshold. You'll learn how new chemical elements came  together under the force of gravity to create new   stars and planets, like our solar system and earth. That's all thanks to a process called accretion. Gravity pulls together space gases and clumps of  matter into a spinning disk.   As this disk spins, its center gets hotter and hotter until gases fuse  together and light up a new star. But, since we now have heavier elements in matter in the disk  this matter crashes together and the force is   so strong that these chunks get bigger and bigger  until they form planets that orbit around the star. The universe was over 9 billion years old by the  time enough space stuff had accumulated in our   little slice of the milky way to form earth. And it took a while for earth to become a place that could support life.   For the first few million years  pummeled the planet as fiery collisions combined with extreme radiation and heat to make  the early earth a lava death trap. Scientists call this lovely time the Hadean Eon, after the  Greek God of the underworld Hades. Luckily for us, and everything else on the planet, Hades didn't stick around too long.   But it did leave behind a lot of molten lava lying beneath the relatively  thin layer of rock your school sits on right now. That's the stuff that spews from the top of  volcanoes and moves the earth's crust around. We spend our entire lives, all of human history, surfing on lava. Now there's a nice tourism slogan. We'll end this unit by discussing how  scholars like Wegener and others from   different disciplines piece together the  origins of our planet and solar system. The plate tectonics developed by Wegener and  Hess is yet another example of the collective   learning that helps us understand the history  of our planet and how it impacts our lives. How about "Earth: It's been surfs up for hundreds  of millions of years." Or, "Earth: so hot right now." Y'all, I think this is a real career opportunity  for me. Extraterrestrial Tourist Liaison I've got work to do. Posters to make. Bob, how  many more of these videos do we have to make? Six?!