[MUSIC] Okay. The image on my right here is not a picture taken through the window of a time machine. It's an artist's conception of the very early Canadian earth, but we think it's a pretty accurate one. Nevertheless, I'll stay put over here. It looks a little rough on planet earth at 4.4 billion years ago. The early earth was so hot that not only was the interior largely molten. The surface would have been a cauldron of lava rock with the moon hanging large in the sky due to its vastly closer orbit and both lightning and meteors flashing above. In the previous lecture, we talked about our oldest rock and mineral materials and we touched on the idea that the first stable crust came from quite literally the distillation of mantle. In other words, multiple stages of partial melting to make the crust. Admittedly if the earth were scaled to the size of an apple the crust which ranges from 5 to 80 km thick, would be about as thin as the skin on the apple, not very thick. And it's where nearly all life and all surface processes take place in order to really understand the generation of earth's crust. Both ocean crust and our beloved continent we need a little primer on plate tectonics. Don't worry, we're going to just hit the highlights. Plate tectonics is an exemplary theory with loads of substantiating evidence. It's also a revolutionary theory. And like other revolutionary theories like those shown below here, it provides human beings with a whole new way to see the world. Our understanding of chemistry absolutely requires and understanding of these fundamental units called atoms. Physics is grounded on Newtonian motion and gravity, but it's tweaked by quantum and relativity theory. And biology needs organic evolution in order to make sense of life's variety and its changes through time. All of these theories remind me of the reversible chalice shown in the upper right. It's a lesson in perspective. You can look at something for a long time and only see it in one way. Then all of a sudden a whole new perspective arises the chalices a drinking cup, but it is also to face is pointing towards each other. When scientists came to really understand plate tectonics, it was a similar kind of Aha moment. A whole new way to see the world. Planet earth is unique amongst the terrestrial or rocky inner worlds. It's the one planet that appears to have a truly mobile outer shell, which is what plate tectonics is all about. These plots here are both called hypsometric curves. They show the amount of surface area in terms of elevation. On the left just occurred for earth, very little surface above 6 kilometers or 6000 m. You have to go to the Himalayas and very little surface that's below standard ocean level depth of about -4 to -6 km beneath sea level. The only deeper stuff would be those relatively rare super deep ocean trenches. So most of our surface, as you can see on the left, is it two distinct elevations, about a third or so is around a kilometer above sea level, typical continent elevation and 2/3 around 5km below sea level. The oceanic abyssal plains. Now on the right, this version of the hypsometric curve shows the bimodal distribution of elevations on earth, contrasted with elevations relative to a standard data on planet Venus. Which, like the other rocky planets, mars and mercury has just one standard elevation and hold onto your hats. Here's the punchline earth's curve is by model it's got two peaks because it has two kinds of crust, ocean crust and continental crust, courtesy of plate tectonics. Here's the gist of plate tectonics on my right here, ocean crust forms at mid ocean ridges. These ridges are huge undersea mountain ranges that encircle the globe like seams on a baseball where plastic. But not quite molten mantle rises and then melts to form ocean crust dominated by heavy, dense basalt. In turn, ocean crust sub ducts either beneath an already existing continent or beneath more ocean crust. And at subduction zones, a wedge of mantle undergoes melting and produces lighter, more granite type rock, which is the stuff of continents. So subduction creates continental rock and this is lower density and therefore more buoyant in comparison to ocean crust. That's why there are two elevations on earth. The ocean basins made of heavier ocean crust literally sink down lower compared to the continents that are made of lower density rock, which sit higher. Okay, the first serious discussion of moving continents came from a German explorer adventure meteorologist, basically all around renaissance guy Alfred Wegener. For years, map makers had recognized some inverse similarity between, say South American and African coastlines across the Atlantic Ocean but Wagner took it much further. He noted that not only did continental margins matchup, but so did rocks and fossils now separated by huge tracts of ocean and even scratch marks on the rocks from glacial movement. All seem to line up and emanate from a central source. When he reconstructed today's continents as an accumulation that he termed, and you're probably familiar with Panja. Unfortunately, Alfred died on an expedition to Greenland perhaps his motivation to go to Greenland was at least partly linked to the relatively sparse data for the northern continents as having been linked. The southern accumulation, often referred to as Gondwana land, had much better substantiated data. At any rate, he died in 1930 while trying to bring necessary supplies to a winter station deep into the Greenland ice cap. Well, poor Alfred Wagner is also rolled out as a great example of a hypothesis. In other words, a good idea what we call continental drift, but not a great theory. Why not so great? Because Wagner had no idea how these continents moved. He figured that perhaps they were linked to something like, icebreakers that smashed through blocks of ice. He suggested that maybe continents just smashed their way through ocean crust. His fellow scientists correctly didn't buy the continent as icebreaker model. They knew enough about the strength of ocean crust. Just wouldn't work. Today and long after Wagner was gone and largely because of better understanding of the ocean floor, we know the continents move around because the intervening ocean crust is coming and going. Continents move apart when new ocean crust forms between them and continents come together when ocean crust is lost between them at subduction zones. Here's a little of the evidence that brought about an understanding of the ephemeral nature of the ocean crust around the mid ocean ridges, a pattern here, these colored lines in the upper left diagram. A pattern of rock ages exist close to the ridge young ocean crust, further from the ridge, old ocean crust, with the oldest being a mere couple 100 million years old. Shown in the blue colors on the left hand diagram. A couple 100 million years is small change compared to continental crust, which could extend into the billions in other words, thousands of millions of years old. On the right, we see the pattern of earthquakes at the deep sea trenches. The earthquakes increased in depth with distance from the trench. And it all made sense when it was realized that the quakes were being created through the slipping and sliding of the down going ocean slab at subduction zone. Well, has earth always had plate tectonics? Between earth formation at 4.5 billion and maybe 3 billion years. It's unlikely that plates were moving around like they do today. Old Hutton on my left here would have to accept that his uniform earth existing in the past always as today might not be a very good way to look at the very earliest history of planet earth. Before 3 billion years ago, continents might have formed from blobs of rising mantle or perhaps welts of ocean crust sinking down like in the diagram I showed in the previous lecture. And on my right here, after 3 billion years ago, things changed and a cooler earth probably allowed for much more mobile crust and modern style plate tectonics to kick in. But in either case, and as we've already discussed, making continental crust is all about distilling partially melting mantle. And then ocean crust and distilling off these lighter, more buoyant rocks that characterize the continents. In summary here, stage one involves cooking up the mantle beneath ocean ridges to make slightly lighter kind of rock. The ocean crust a little bit more silica rich and iron magnesium pour. Then stage two cook up the ocean crust in subduction zones and make even more silica rich stuff, the light and relatively buoyant rock of the continental crust. Okay, we've got some background now and enough for this lecture. Next we tackle the formation of North America, what will eventually be the roots of the rocky mountains?
