 Welcome to the Open Geology. For this lecture, we will be discussing divergent boundaries, specifically those that occur in the ocean. Those are mid-ocean ridges. Mid-ocean ridges are places where you're making new ocean crust. This is kind of what one looks like. It's a linear ridge. And this ridge is essentially a slow oozing volcano, constantly making new plate. As new plate is being made, it pushes apart, it diverges, and more new plate is made. And so the newest plate would follow the high point of this ridge. These features, mid-ocean ridges, are characterized by medium-sized earthquakes that follow the axis of the ridge. The slow oozing volcanism that's making the new ocean plate. These are constructive boundaries, meaning they are creating plate, as opposed to the destructive boundaries, which are convergent boundaries, where you have things like subduction going on that essentially recycle the plate back into the mantle. These are also known as spreading centers. This is a nice gif demonstrating new plate being made. This is the axis here of the mid-ocean ridge. And on either side, at about the same rate, you are generating new ocean plate. The ridge is higher than the surrounding area because it's hotter and less dense. Ocean plate is extremely dependent on density. The young, hot ocean plate will ride much higher on top of the mantle than the stuff to the east and west that is cooler, older and colder. As the ocean plate cools off, it becomes more dense and more heavy. And eventually, in some extreme cases, it will actually start sinking as a result of being old and cold. These are some of the longest mountains on earth. And oftentimes, the small segments that are creating new plate are offset by transform faults, which we talk about in a different lecture, because they're creating plate at different rates. These often start out as continental rifts, which we'll also discuss in a different lecture, because the continent is ripped apart and begins to develop a number of down-dropping normal faults. And that eventually develops into one of these mid-ocean ridges. This idea of creating new plate on each side of a ridge was developed in a hypothesis by Harry Hess, where he talked about sea floor spreading occurring along these ridges. Harry did a large amount of mapping of the age and thickness of sediments and rocks around these ridges. Here's a map depicting the ridge going through the middle of the Atlantic Ocean. And this world map, comically depicted like a baseball, is showing where all of these mid-ocean ridges are on our planet. Notice how extensive they are. These are some of the longest mountain chains on our planet. Not only that, they're technically volcanoes. So, most of the world's volcanism is occurring all along these ridges. It's an enormous amount of new plate material being made, and these mountain chains are gigantic. Much bigger than North America and South America. Longer than North America and South America put together. We can also view these in Google Earth to get a better idea of their scale. For this representation of Google Earth, I decided to overlay Marie Tharp's beautiful bathymetric map of the ocean floors so you can get a better idea of how these ridges look. The red line depicts the plate boundary, and notice these mountains build their way up to these ridge lines. Look at all the lines crossing the ridge lines. These represent those transform faults allowing for offset between the two ridges. We can turn Marie's map off to get a different look at this bathymetry, or ocean floor height. This is the mid-Atlantic ridge. Notice it goes all the way down the Atlantic, and it has large jogs in some places where it's offset significantly, represented by the green here. There's volcanism occurring all along these ridges. Great! I hope you learned something about divergent boundaries and mid-ocean ridges. Hopefully you have a better understanding of how these are caused. Look at the sea floor age. You can see that the youngest plates, the youngest ocean material, is right near the axis of the ridge. As you move away from it, it gets older and colder. In this lecture, we discussed some of the common features you see around mid-ocean ridges, the age distribution around a mid-ocean ridge, and how heat controls how buoyant the ridge is. Thank you.