The geologic history of Western United States with Southern California in particular. First it's necessary to understand a day's orientation of tectonic plates. We have the North American plate, here made of continental crust and the oceanic Pacific plate. In addition, there are two little oceanic plates subducting underneath North America. In the North there's the Wandafuka plate and in the South, tiny little Riviera plate subducting under Mexico. We'll begin with a mesozoic. From now on I'm going to take advantage of the animations and narration created by Tanya Atwater of UCSB. So we'll begin with the late critatious about 85 million years ago. This is a time when a series of oceanic plates are subducting underneath North America. During the late mesozoic, the western edge of North America was a subduction zone. Most of the basement rocks in Southern California were created in this environment. As you look at the mountains of California, like the Sierra's and the coastal ranges, you'll notice that they have a distinct North-South orientation. This is a direct result of the subduction that went on during the mesozoic and the early senozoic. Now we're going to move ahead to the senozoic, 40 million years ago. The ferrolone is the last of a series of plates that's subducted underneath North America. In the following sequence, we will keep the North American plate stable and watch as the Pacific rise moves toward North America. Remember, we're keeping it stable, but in fact, all plates are moving relative to all other plates. Notice that Nevada is really skinny and that Baja Mexico doesn't even exist yet. Here, the East Pacific rise has hit the North American plate and split the ferrolone plate into two smaller plates. The contact between those plates is the San Andreas fault. The upper plate is now called the Juan de Fuca plate and the lower plate is the Riviera plate part of the larger Cocos plate. 18 million years ago, the San Andreas fault has grown as the Mendocino triple junction and the Riviera triple junction have grown apart from each other. Finally, today we have a San Andreas fault that extends all the way from Mexico to Mendocino and a segment of the continental crust has been torn off of North America and carried along with the Pacific plate. Notice also that Nevada and California are wider and that Western United States has been stretched. Notice how the East Pacific rise spreading center separating the ferrolone plate from the Pacific plate migrates toward the subduction zone. The contact becomes the San Andreas fault and the Pacific plate breaks off pieces of North America and carries them away, creating the San Andreas Gulf of California plate boundary inside the continent. Now what happens to the interior of North America during the late Cinezo could expand forming the basin and range province, which is hot week and easily stretched and broken. Notice that skinny Nevada filled out as the rim of the continent got pulled away by the movement of the Pacific plate. Now we will concentrate on Southern California. Notice the locations of San Francisco, Santa Barbara and San Diego, 20 million years ago. At present, Santa Barbara is rotated and Baja California has pulled away from Mexico. Let's see how this was accomplished. During the Maya scene coastal fragments of California were transferred to the Pacific plate. The block containing Santa Barbara was rotated in the process. 5 million years ago, Baja California was transferred to the Pacific plate as one coherent block. How do we know that the block containing Santa Barbara rotated? The most compelling evidence is paleo-magnetic. As lava cools, the magnetite crystals line up with the Earth's magnetic field, so they act as compasses pointing north. 20 million years ago, igneous rock containing crystals pointing north are shown by these dark blue lines. Watching the animation, what happens to these rocks as well as new igneous rocks that formed in the process? You can see the block starting to rotate, even new rocks form with their orientation, until finally that block is totally rotated and the old rocks are pointing east and instead of north. That rotated block became the transverse mountain ranges of southern California, which contain more than a dozen east-west trending mountain ranges starting in the west with the Channel Islands off the coast of Santa Barbara, and includes the Santa Monica Mountains near Los Angeles and all the way to the San Bernardino Mountains in the east. So the next time you see Semitinational Park in the Sierra's, you can think about subducting plates during the Jurassic. Or the Hollywood sign in Los Angeles, you'll know that it sits on top of a slice of North America that was captured and rotated by its new owner, the Pacific Plate. And finally, there's a San Andres fault, one of the very few transform fault boundaries located within a continent. Since we live so close to a transform fault boundary, we're subjected to earthquakes. But that's another story.