[MUSIC] So far, we've talked about two of the three types of plate boundaries, divergent plate boundaries and convergent plate boundaries. That leaves the third type which we refer to as a transform plate boundary. It is a plate boundary where one plate slides past one another. Now, think about it for a second. If this is the motion that is taking place, we're not creating new plate material as we do a diversion boundary, and we're not destroying old plate material or subducting old plate material, as we have at the conversion plate boundary. We're simply having one plate slide past another. And if you remember our description of faults, a fault on which there's horizontal motion, such as shown, is called a strike-slip fault. So basically, transform plate boundaries are big strike-slip faults. Now, where do they occur? Well, I was a little bit careful when I talked about all earthquakes at divergent boundaries being associated with normal faulting. Because it turns out that in general divergent plate boundaries are not just a continuous band of normal faults. If we look carefully, we can see the mid-Atlantic ridge. And we see that it's not just a single curving but it's broken by east west trending breaks. Those are the transform faults. We can label a few of them here. So the little red segments represent the spreading ridges and the yellow segments represent the transform faults. Now, not all transform faults cut across ocean floor. Not all of them are associated with steps between ridges. Here's an example of probably one of the most famous transform faults on the planet, San Andreas fault, California. It extends from a spreading ridge in the Gulf of California just south of the US-Mexican border, northwestward across much of California to where it connects with another spreading ridge off the coast of northern California and southern Oregon. There's a little plate between the west coast of the United States and the Pacific plate, and that little plate is called the Juan de Fuca plate. There's a chain of volcanoes along the west coast of Oregon and Washington and a little bit of into Northern California called the Cascade Volcanic Arc. And that is the Continental Volcanic Arc due to the subduction of the Juan de Fuca plate beneath North America. The great earthquakes that happen in California that were responsible, for example, for the 1906 distraction of San Francisco. And even bigger earthquake that happened in the area that is now Los Angeles back or near the area that is now Los Angeles back in 1857 are due to the stripe slip displacements along the San Andreas fault and related faults. Notice the sense of slip on the San Andreas fault is that it is a right lateral fault. And that means that the Pacific plate is moving to the north, relative to North American along that fault. Now because the San Andreas fault for much of its life is on land, you can stand on it, or near it. And you can have for example, it's not really just a hairline, but you can pretty much out one leg on North America, and one leg on the Pacific. That portion of California that's to the west of the San Andreas Fault is moving north at about six centimeters a year relative to the plate on the eastern side. Now all that motion isn't happening at once, because when earthquakes occur, it's not a continuous, smooth motion. But, it builds up for awhile and then suddenly that. Things build up for awhile and then suddenly slip and so it is a lurch type motion. We call it stick slip, and so when I say that the plates to the west of the San Andreas fault is moving six centimeters per year, that's on average, averaged over many years. Another major strike-slip fault that cuts across land is the Alpine fault in New Zealand that cuts across the south island. Motion on the Alpine fault is not completely strike-slip. There is some uplift along it, and that has led to the development of the Southern Alps in the south island of New Zealand. You've seen that the theory of plate tectonics is a revolutionary idea that explains many, many earth phenomena. We've seen already that it explains why most earthquakes occur, it explains why volcanoes occur. And we can even use it to understand perhaps subtle differences such as why different kinds of earthquakes happen at different locations. [MUSIC]