Transcript of Earth as a Planet

as we look around the solar system there is no place like home earth is an oasis and the void there is no life at least not yet known on any other planet and in fact the conditions for life are really unique to earth we don't really see any evidence of life anywhere else in the solar system at least not yet so let's talk about our own home planet there's a few characteristics that we should consider first of all we have a mass of about 6 times 10 to the 27 grams and a volume of about 10 to the 27th cubic centimeters so if we were to divide the mass by the volume we would get an average density of about 5.5 to grams per cubic centimeter however when we look at the surface density that is the rocks and minerals right here on the surface of the earth we get something much less what's going on well it turns out that Earth is differentiated that means that one it was forming the earth was molten and the densest materials such as iron and nickel sank to form the core near the center of the core the pressures are strong enough to squeeze iron and nickel into a solid outside of that the pressures are just a little bit lower and therefore the iron and nickel sloshes around as a liquid surrounding the core is a mantle of semi solid rock it has almost like a dry putty kind of consistency to it and then layered on top of that is the crust where you and I live and then we're able to survive on the crust by virtue of the fact that we have the atmosphere surrounding the crust of the earth so how do we know this how do we know that there's a solid inner core and a liquid outer core and a semi solid mantle well we can use geologic activity to allow us to probe the interior of the earth and this is something that geologists and seismologists do all the time the two main forms a geologic activity that produce seismic waves are going to be earthquakes and volcanoes so let's imagine that there's some sort of seismic event maybe it's a earthquake or a volcanic eruption and seismic waves pour and radiates throughout the earth there are two types of seismic wave the first are the primary waves and these have the ability to bend or refract through any type of solid or liquid and we set up monitoring stations all around the globe to detect these types of waves but the second type of wave are called secondary waves and these are only able to penetrate through solid substances only in other words they can only travel through the crust and through the mantle so by examining the types of waves and how strong they are from different seismic stations at different parts of the globe we can build a picture of the interior of the earth and this is how we understand the earth has a outer liquid core but the earth is rotating and that liquid core plays a huge role in setting up the Earth's magnetic field it produces what's called a dynamo effect in other words as the core is rotating and heat is being exchanged from the solid interior through the liquid core to the mantle that sets up convection currents and also generates electric field so these are ribbons of rotating electric current and this in turn produces a magnetic field so this is what produces the Earth's magnetic field and the Earth's magnetic field extends all the way out into space giving us what's called the magnetosphere and it's this magnetosphere that really protects us from the hazards of the solar wind remember the solar wind is carrying with it all kinds of charged particles lots of plasma stuff that would be pretty dangerous to life here on earth so the location where the solar wind encounters the magnetosphere is called the magneto pause it kind of acts as a bit of a bow shock or an initial break against the solar wind and some of that solar plasma gets bound up in the magnetic fields and there's a really strong concentration of those charged particles in a series of magnetic field lines that are relatively close to the earth and these are called the Van Allen belt therefore whenever we send astronauts out to the moon back in the late 60s and early 70s passing through the Van Allen belt was a bit of a I see situation because that's where there was a fair amount of extra radiation so once I got through that they were they were relatively safe and when we talked about the surface of Earth we got to remember that the earth surfaces cracked into these tectonic plates and the tectonic plates move around on top of the mantle heated mantle material Rises and releases heat as it approaches the crust and then sinks back down again picks up more heat and the cycle repeats this is not unlike boiling water and this sets up a convection current several convection currents are set up such that they begin to move around in other words we have moving magma or moving mantle and the tectonic plates are situated atop this mantle so therefore they're being carried around on the mantle of Earth and this is a map of Earth showing the locations of all the tectonic plates and the oceans themselves have been removed or at least the waters been removed and you're seeing a topographical map of even the ocean floors so even without the red lines you can start to make out that there are some natural boundaries here and there are different types of boundaries depending upon what's going on with these tectonic plates so the first boundary we should talk about our rift zones this is when you have two plates that are basically being convected away from one another as is the case with the North American and the Eurasian plates so as these two plates spread apart mantle material rises up to fill it in giving us the mid-atlantic ridge this is like a giant underwater mountain chain running through the Atlantic Ocean the second type is when you have a plate that is tucking under another plate and dragging that plate down with it so the Pacific plate is dragging the Mariana plate down forming what's called a subduction zone and this is responsible for the Marianas Trench which represents the deepest part of Earth's ocean anywhere on the planet and sometimes that material is squeezed out through a volcano and these volcanoes as a cooling mechanism for the planet next up let's take a look at what's going on in California a fault zone here we have two plates that are sliding past one another giving us the familiar San Andreas Fault as a matter of fact if you look in this image you can actually see the fault line so you could just see where it's sort of flatland and then all of a sudden it just encounters this mountainous terrain and as residents of California are familiar with these two plates are still sliding past each other today and they get a lot of earthquakes out there so that's a fault zone let's take a look at one more and we head to the Swiss Alps and we find a convergent zone so here's the African plate and it's tucking under the Eurasian Plate but rather than drag the Eurasian Plate down as it as the plates do in the marianas the African plate is pushing up the Eurasian Plate building a huge mountain chain otherwise known as the Swiss Alps so this whole mountain chain is a result of two plates colliding so if these plates are still sliding and banging and grinding and separating away from each other today they must have been doing this for quite some time and computer simulations such as this one allow us to retrace our geologic history the number in the upper left-hand corner represents the number of millions of years ago so around 500 million years ago the entire ocean was in the North plates however began to smash around you'll notice that some of the present-day continents like North America were still underwater at this time and a new supercontinent reformed this is Pangaea literally meaning the entire Earth well sometime in the middle Jurassic era the North American plate separates from the African and South American plates by the Cretaceous era the African and South America plates separate and if you look in if you look toward the middle right-hand side you're going to see India the Indian plate coming up and smashing into the Eurasian Plate building up the Himalaya Mountains that we have today and here we are