Earth, People & The Environment Book One: Beginnings Chapter 5: Our Layered Earth A. Violent Beginnings B. Birth of a Dynamic Planet C. What The Earth Is Made Of D. The Story In The Rocks "When I break a rock open with my pick, I'm a prophet. I see the past. I see the future. I know where the world is going, and where it's been. And I always, always want to know more." Christina Dodd,Virtue Falls Accretion & BombardmentGoldilocks conditions aside, the Earth was a veryinhospitable place during its first billion or so yearsof existence! The process ofaccretionfrom spacedust was slow, and for quite a long time the planet remained molten. Not helping matters was the constant pounding by rocky space debris, much more prevalent in a less "tidy" youthful solar system. Book One: BeginningsOur Layered EarthThis attack from space was so ... impactful, that it warrants its own name in the geologic history of our planet - the Late Heavy Bombardment. The energy and heat delivered by this pelting prevented the Earth from fully solidifying. This would prove a fortuitous twist in our story, as would the materials delivered by these visitors from space. How Did Our Moon Form? It was originally assumed that our moon formed from the same solar debris that created the Earth, but now scientists think otherwise - that the Moon is in fact the remnants of a galactic wanderer! Book One: BeginningsOur Layered Earth This hypothesis is called The Giant Impact Theory. It suggests that during the early days of the Earth our planet was struck by a body named Theiawhich was the size of Mars. The impact threw parts of both worlds into orbit around the Earth, where the debris coalesced into the Moon we observe today. This too would prove to be a huge help to future inhabitants of Earth, for the Moon has acted much like a good friend for the past 4 billion years, keeping its sister planetoid safe and stable, enlivening its oceans with rhythmic tides, and generally enhancing the "habitability" of our home world.
Planetary DifferentiationThe early portion of Earth's history is called the Hadean Eon (fromHades, forhell). It was a time when seas of lava belched from volcanic fissures, the cruststruggling to harden in the face of vaporizing impacts from space.Book One: BeginningsOur Layered EarthYet the Late Heavy Bombardment did bring metals likeironandnickelto Earth, where they were able to sinkthrough the planet's moltenupper layers and establish a metallic, semi-liquid core. Thisdifferentiationwould have at least two major implications for our planet. First, it led to the formation of distinct layersinside the Earth. Second, it created an engine at the core of this world that would prevent it from becoming like the dead rocky masses that litter our galaxy! Makings of a Solid Earth Despite the violence of the Earth's beginnings, solidcrustbegan to form as long as 4 billion years ago. The planet's oldest rock is found in ancient Book One: BeginningsOur Layered Earth These "shields" have been able to avoid the various fates that befell most other formerly solid pieces of the planet's crust. They remain today, as this slab ofCanadian shield in Hudson Bay, a type of time travel back to the opening chapters of the Earth's story. formations called cratons, often referred to ascontinental shield. These were the shores the first oceans lapped upon ... The Early Oceans & AtmosphereMuch of the Earth's future as a livingplanet would be dictated by the concentrations and fluctuations of three gases in the early atmosphere - oxygen, carbon dioxide, and water vapor. Book One: BeginningsOur Layered EarthCO2 accumulated in the atmosphere, allowing the planet to retain a layer of nurturing warmth. Scarce in the early atmosphere, oxygen began to accumulate significantly around 2.5 billion years ago thanks to photosynthetic organisms, "seeding" the planet for more complex life.Icy comets may have contributed to the early oceans, but more likely they formed when volcanoes expelled huge amounts of water vapor into the atmosphere by a process calledoutgassing. A Cross-Section of Our Planet This is our world, as it has looked for most of its history. We live on a thin skin above layers of molten rock, which in turn shield an internal heat engine! If you think nothing interesting happens beneath our feet, think again, because events at the surface have always been shaped by what happens below ... Book One: BeginningsOur Layered Earth
Rocks & MineralsOur daily experience tells us that the Earth's crust is indeed solid, but what exactly is our home planet made of? The Earth itself is made of rock, and rocks Book One: BeginningsOur Layered EarthMinerals arecrystalline, forming under conditions of extreme heat. These same conditions forge rock - igneous rock as we'll soon see - new earth! Just as proteins and amino acids are the building blocks of life, minerals are the buildingblocks of the solid, inanimate Earth. Much of thecontinental crustis granite(see right) ... this forms the heart of our mountains, and the foundation for our homes. in turn are composed of minerals. Minerals are combinations of certain elements, like oxygen, silicon, iron and calcium.The Rock Cycle The way rocks form illustrates how the planet works as a system. The basic cycle is a journey first from molten rock ormagmato igneous rock, then to layered sedimentary rock, (perhaps) to warped metamorphic rock, and back to magma. The processes involved include things like crystallization, weathering, deposition, lithification, heating, and melting! Book One: BeginningsOur Layered Earth Rock is born molten by volcanism (far left), weathered away, deposited, and re- hardened (left). "No Vestige of a Beginning, No Prospect of an End"James Hutton wrote those words in 1788 to encapsulate a brilliant and groundbreaking career in geology and to introduce a new perspective on our planet, known as uniformitarianism. The idea, illustrated in the graphic below, Book One: BeginningsOur Layered Earth50 years later Charles Lyell developed Hutton's ideas into a theory called gradualism, an idea that stuck with one young and avid fan of Lyell's named Charles Darwin ... foretold a new and controversial story of the Earth; one with a nearly infinite timeline, and one that clashed violently with the religious orthodoxy of the day. Reading the Story of Geologic Time Of the three types of rock we've identified, sedimentary rockis by far the most conducive to studying the planet's history. The tendency for sedimentary rock to form in layers calledstratamakes them a real life geology textbook! Not only are fossils most common in sedimentary rock (they wouldn't survive the formation of igneous or metamorphic rock), but the strata allow for their dating. Book One: BeginningsOur Layered Earth A story, as we know, requires a timeline. Once the timeline exists, we can utilize it in two ways - by placing events at specific points on the timeline, or byordering eventsalong it. Geology - the rocks - have always provided an opportunity to do the latter. Without sophisticated equipment, the rocks provide "relative dating" for events throughout Earth's history.The idea that younger rock sits atop older rock is calledThe Principle of Superposition.