 series of lectures on plate tectonics and the last lecture which is also on this YouTube channel we devoted introducing Alfred Begner and his hypotheses on plate tectonics. Now we're going to talk about some of the evidence that was compiled by others to build up to the theory of plate tectonics to lay a lot of foundational groundwork. So, Alfred Begner talked about continental drift that was his hypothesis and eventually that was validated and improved by others including Marie Tharp and Bruce Heisen. They compiled the symmetry data from military and other oceanic surveys and made a set of beautiful maps right around when the science was just starting to come to a point where geologists could agree on this idea of plate tectonics. Here is the Tharp map, a beautiful representation of the ocean floor especially across the Atlantic and you see this amazing mountain range that goes right down the middle of the ocean and with a series of what look like the indentations on the side and then one indentation running right down the middle that almost looks like a stitching on a baseball but just think about the scale of this mountain range it's enormous right it travels all the way down through Iceland all the way down the middle of the ocean you know it's as long it's longer than North America and South America combined right and eventually with the help of Harry Hess we've come to realize that these mountain ranges represented plate creation zones where you're generating new oceanic floor along these mountain ranges and it's a conveyor belt of new ocean plate and here is Harry Hess himself working on drilling dregs like the cuss one and he is the one who hypothesized about this this idea of seafloor spreading with these mountain ranges do represent the plate generation areas right and this topic it's whole idea of continental drift continents moving around and seafloor spreading heavily debated all the way up into the 1960s a number of interesting papers came out in the late 60s including one by John Holger but many other contributors to this field keep in mind right and and you get these amazing patterns showing up in earthquake record right so one of the issues with seafloor spreading if you think about it is that if you're constantly generating new ocean plate wouldn't that mean that the crust of our planet should be getting bigger over time and there would essentially be an empty volume inside of our planet so there had to be some mechanism to take care of the old plate to eliminate that extra surface area that you're creating at these these divergent boundaries what which you'll learn that they'll be called right and if you look at iris's amazing webpage you can take a close look at patterns of earthquakes that we've been measuring and we have the ability to figure out exactly where these earthquakes are happening and if you notice something this purple area is near the surface of our planet and you get from blue to green to yellow as you go deeper and you can see that earthquakes are essentially outlining what we later understood to be what's known as subducting plates for pieces of oceanic crust diving down into our planet along deep ocean trenches right so that's that's kind of what we started to figure out and we get these things called bini off zones which delineate the subducting ocean plate so it kind of supports and provides an explanation to where all of that extra plate is going after we create new plate just a quick refresher for those of you unaware there's two major types of crust on our on our planet right on and that would be continental and oceanic continental you could say is generally comprised of granite which is a felsic igneous rock we'll talk about those more in later episodes but all you have to know is that it's thick like a blanket that keeps heat in it's always low density more squishy than oceanic lithosphere and like I said it's a felsic type rocket that describes its composition the chemistry that's made it in ocean lithosphere is generally thinner and denser not as good at retaining heat and it gets even denser as it cools off in ages and it's mafic in its composition this is a map showing how thick different parts of the plates are the different parts of the crust are and you can see that the continents are generally much thicker than this is units are in kilometers and that the oceanic plates the oceanic crust is generally thinner and then here's a USGS figure showing just the different layers of the planet just so you can help visualize what's going on with this plate tectonic theory and and those are different layers and I won't get into this in great detail here but just keep be aware that our planet is broken up into these different layers including a solid intercore liquid outer core mantle and then the lithosphere so that's kind of a quick overview on some of the foundational stuff and we'll get into more detail on additional piece of evidence in the next lecture and that concludes this this lecture here