It is the interaction between the lithosphere and the asthenosphere that makes all of these interesting things happen. Volcanoes, earthquakes and, over a much longer time scale, moving continents. The tremendous heat at the center of the Earth, compared to the less deep layers, can set up convection currents in the asthenosphere. As hotter materials rise, cooler ones sink, and currents, though incredibly slow moving ones, begin to form. The currents in the asthenosphere pull along the base of the lithosphere, moving the continents above. As well, the solid cool crust is more dense than the hot layers underneath, and so the solid plates are sinking at the edges, pulling the plates along as they go. >> Which of these cities do you think experience regular, strong earthquakes? Check all the answers you think are correct. A) San Francisco. B) Santiago. C) Chicago. Or D) Moscow. Places like San Francisco and Santiago experience regular and sometimes very powerful earthquakes. These places are near where two plates meet. On the other hand, Chicago and Moscow are relatively distant from the edges of continental plates. While Chicago and Moscow can still experience earthquakes, they are nowhere near as devastating as those in more active earthquake areas. >> The reason the lithosphere moves is because it is a series of individual pieces or plates that can move independently of one another. These plates often corresponds roughly to continents and ocean bases. However, just because these plates can move independently, doesn't mean they don't interact. If you look at where the edges of these plates fall, you might also notice that these are the same areas where things like earthquakes and volcanoes are concentrated. Earthquakes and volcanoes are often the result of two different plates interacting, either colliding, sliding away or slipping past one another. One way to visualize how the plates are arranged, and move past one another, is to compare them to an orange peel. On this orange, I've split the peel into different pieces, each of which can move independently, just as the tectonic plates are arranged on the Earth. However, if I move just one piece, you can see how it interacts in different ways at different spots. In some places, it moves away from the others, leaving a gap. On earth, these gaps are filled in by the molten rock below, and are how the mid-oceanic rifts are created. In this area, the peel is running into its neighbors, and this can create a build up, which would lead to a mountain building event on Earth. As for these areas here, the peels move past one another. However, like the plates, they are not smooth, and rub as they move past. Sometimes as tectonic plates get stuck to one another, pressure builds up, and the pressure is released in the form of an earthquake. >> Let's take another look at our New York city structures example. If we know that the statue of Liberty and the Empire State Building have moved about three meters in 100 years, how far would they move in a million years? Remember, these distances are measured relative to the center of the Earth, not surrounding features on the surface of the earth. 300 meters, 30 kilometers or 3000 kilometers? B is the correct answer. Over the course of a million years, these famous structures would move about 30 kilometers. >> Even in the not so distant past, the position and shape of the continents was very different than it is today. Within just a few million years, the continental plates can change dramatically. If we were to observe the Earth moving backwards through geologic time, it might look something like this. If we go back as far as the earliest dinosaurs, about 240 million years ago, all of the continents of the Earth were connected into one giant super continent called Pangea. Journeys that today would require crossing thousands of miles of ocean could be traveled in a single step. One giant ocean called Panthalassa encircled the globe. Although Pangea was the most recent super continent, there have likely been others in the Earth's past, and may even be more to come in the far future. If we move the clock forward again, Pangea begins to break up into its constituent parts. First, the northern and southern sections begin to separate from one another, with a large sea forming between the two parts. These two small super continents are called Laurasia and Gondwana. Laurasia was made up of present-day Asia, Europe, and North America, while Gondwana was composed of South America, Australia, Africa, Antarctica, Madagascar, and India. Eventually, Gondwana and Laurasia broke apart into the smaller continents that we know today. You'll note that the last one I said, India, is today considered part of Asia. However, during the Mesozoic, it was separate from the rest of Asia, and at one point was the next door neighbor of Africa and Australia. As India broke away and moved north, it eventually plowed into Asia, and the collision between the two is what gave rise to the present day Himalayas. >> Today on the eastern coast of Canada and the US, stretching from Nova Scotia to New Jersey, is a large rock formation that was created by a huge volcanic eruption about 200 million years ago. These same kind of rocks can also be found in Africa. What region of Africa do you think would contain these same rocks? Check the box next to the area you think is correct. The answer was here, on the northwestern coast of Africa. These rocks date from a time when New Jersey and Morocco were next door neighbors, and you could've walked between North America and Africa quite easily. >> But, let's bring this back to the main topic of the course, dinosaurs. What did moving continents have to do with dinosaurs? Actually, a lot. The first dinosaurs appeared during the Triassic, while the continents were still joined into a single super-continent. Because there were no major sea barriers, dinosaurs had the ability to spread across the land, and indeed they did. However, this also meant that there was a general mixing of dinosaur faunas, with similar types of dinosaurs being found all across the world. This similarity continued into the Jurassic. For example, most people have heard of the dinosaurs Brachiosaurus, Allosaurus, and Diplodocus, all from North America. Very close relatives of all of these dinosaurs can be found in rocks of Tanzania, Africa. However, if you remember, an important event was going on at this time. The northern and southern parts of Pangea were beginning to split apart, creating a sea way between them. Eventually, as we get into the early Cretaceous, we can start to see the effects of this split on the types of dinosaurs we find in these different areas. Certain groups, such as Tyrannosaurs, Pachycephalosaurs, and Ceratopsians become more and more common in the northern hemisphere's super continent, called Laurasia. While other groups like Sauropods, Abelisaurs, and Carcharodontosaurs become more common in the southern hemisphere's super continent of Gondwana. Finally, in the Late Cretaceous, the continued break up of both Laurasia and Gondwana led to a similar, further break up of the different dinosaur groups. Because each of the major continents become relatively separate from one another, the groups of dinosaurs on each of these separate continents became more and more different from one another. It is likely that the fragmentation of the continents may have lead to an increase in the number of dinosaur species.
