 section 2 of plate tectonics for geology 1010 introduction to geology. This section was on convergent plate boundaries. These are very interesting plate boundaries. They created a number of interesting features in some cases volcanoes, deep ocean trenches, and some of the tallest mountain ranges in the world. Of the three types of plate motion convergent boundaries are boundaries where plates are moving toward each other. These are compressional forces. So convergent meaning two things moving towards each other for things come together. These are known as destructive boundaries because in many cases especially when seduction is occurring the plates are essentially destroyed. There are three types of convergent boundaries and it all has to do with the types of plates that are interacting with each other. When you have an oceanic crust ramming into a continental crust you get ocean continent. When you have ocean crust running into ocean crust you get ocean ocean. And finally when continent crust runs into continental crust you get continent continent. All types of convergence where the plates are moving towards each other. Ocean ocean and ocean continent both create trenches volcanoes and very large earthquakes. Continent continent still creates earthquakes but does not create volcanoes. It also creates very large mountain ranges. Here is the first scenario continental crust versus oceanic crust. If you predicted that the oceanic crust depicted here by the guy with the football would be subducted below the continental crust then you're right. Some of the features that you see when you have continent and ocean converging is a deep ocean trench from where the oceanic plate is being subducted. An accretionary wedge this is material that has been smeared onto the side of the continent as the oceanic crust is subducted it smears or accretes material onto the side of the continent. This can be anything from ocean sediments to micro continents islands things like that. As the ocean crust is subducted it brings with it water and once the ocean crust is been subducted down into the planet that water starts to be driven off by the heat of the planet and in turn the water essentially contaminates the mantle allowing for the melting of that mantle wedge above the subducted plate and what that will do that melted material rises up slowly through the continental crust until you get a line of volcanoes known as a volcanic arc. Oftentimes it's arc shaped on the globe. These areas have some of the largest earthquakes ever recorded and like I said they also create these explosive arcs of volcanoes. Where is this happening? South America is a great example. Note the deep ocean trench all along the western coast of South America and then right after that trench you have the Andes and these the Andes are a number of active volcanoes explosive volcanoes all along the margin of the continent related to the subduction zone here oceanic plate and continental plate converging and the ocean plate gets subducted you get these arc shaped zones of volcanoes and some of the biggest earthquakes ever recorded on our planet is in this area in the subduction zone. An American example North American example of this same kind of scenario is where the small Juan de Fuca plate is being subducted under the Pacific Northwest creating all of the majestic mountains known in Washington state and Oregon including Mount Hood and Mount St. Helens and Mount Rainier and a number of other major mountains in the area. As this plate seducts it creates these volcanoes and the earthquakes experienced in the area. Notice that this tiny plate is relatively small and extremely new relatively speaking as compared to some of the other plates that are seducted. So it is seducting out of what we call a relatively shallow angle since the plate is so young and so hot it's riding high. Here's a depiction of what we think is going on. The Juan de Fuca is just a remnant of what used to be a very large plate known as the Farallon. The Farallon was also seducting shallowly and pushing the whole western edge of the North American plate in helping create the ancestral Rocky Mountains and a number of the volcanoes that are left over here in Utah. As the Farallon plate was completely subducted the the divergent zone that was creating it got subducted as well and that slowly turned into what is now a transverse zone where San Francisco is creating the San Andreas Fault and we'll talk about those types of plate boundaries in the near future. What happened to that Farallon plate as it was seducted? We have seismic tomography images or basically a seismic CAT scan of the inside of our planet. There's the western edge of North America and there that blue blob we think represents remnants of the Farallon plate. We're still trying to understand exactly what happens to these plates after they're subducted. Terrains goes back to that picture of those accretionary wedges, those accretionary zones where things are smeared onto the side of the continent. A terrain is a large part, large piece that's smeared onto the side of the continent. Accretion is the process of that smearing, it's the smearing process. As you accrete things you often create mountains, this process is known as orogenesis. Larger buoyant features like some microcontinents don't subduct easily and they're more likely to get smeared onto the side of the continent as depicted here. Here's your accretionary wedge, you're subducting oceanic plate, your continental plate with volcanic arc and here's a little microcontinent or island. As this approaches the subduction zone it gets accreted onto the side of this continent here, the green layer here in the accretionary prism or accretionary wedge right here. Okay, another scenario is when you have oceanic plate and oceanic plate and we'll pick that up in the next video just to keep these videos short and easy to upload.