Hello, my name is David Schultz, this is Our Earth, It's Climate, History and Processes. In this lecture, I want to continue talking about super continents past, and these cyclical formations of super continents that we saw in the last lecture. Now this cycle is called the Wilson Cycle. What is it? Well, we start out with a super continent. Let's say, it's Pangea some 200, 300 million years ago. Now the thick continental crust that exists under this super continent inhibits the heat flow from the earth's interior, and this leads to the super continent to start doming and this doming then leads to cracking, deformation, and eventual rifting. This rifting splits the continent leading to the creation of new ocean crust in between. The oceans will then flood in between this, separating these pieces of a former continent into two continents. Now as these continents start spreading apart from one another, eventually some other place on the planet has to have, convergence. These plates are coming together. And in this case, there's an ocean plate being subducted underneath a continental plate. And this would create a volcanic island chain, and eventually, new land, on the surface. And so, as this island chain gets welded to the continent, we start forming more area of continent. And these are called terranes T-E-R-R-A-N-E and this process of terrane accretion is how the continents grow in size over time. Now eventually the subduction closes up and you have two continents colliding, forming an orogeny, thickening the crust and building the mountains and forming a new super continent. So that's the Wilson Cycle. We go from a super continent to rifting to subduction, eventually that subduction closing up the ocean basin forming a new continent. So is there a modern analogy to this supercontinent rifting that broke up Pangea? Well, the answer is yes. It's happening in East Africa along what is called the African Rift Valley. This is a place where, continent is breaking apart and volcanism and mountain building, and the creation of new ocean crust is occurring. Here we see a geologic map showing the basalts flooding into this weakened crust, and the formation of ocean crust, here in north of this rift valley. We also see volcanoes occurring, such as Mount Kilimanjaro. Looking back on this slide, we see this period of supercontinents occurring on the planet, about every half billion to billion years. This is the time scale of the Wilson cycle. That's how long it takes for a supercontinent to form, dome up, rift pieces of continents be spread around the globe, reform back again into another supercontinent. So to summarize today's lecture. We saw that Pangaea is just one of several super continents that have existed on Earth, in geologic time, we see that these Cratons that serve as these continents grow in to continents by accreting these new terranes along their margins. The rifting that starts the Wilson cycle is initiated by heat flow being inhibited from the interior and this is due to the large mass of thick continental crust.
