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. Things build up for awhile and then suddenly slip and so it is a lurch type motion. But, it builds up for awhile and then suddenly that. Now all that motion isn't happening at once, because when earthquakes occur, it's not a continuous, smooth motion. 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. 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. Now because the San Andreas fault for much of its life is on land, you can stand on it, or near it. And that means that the Pacific plate is moving to the north, relative to North American along that fault. Notice the sense of slip on the San Andreas fault is that it is a right lateral fault. 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. The great earthquakes that happen in California that were responsible, for example, for the 1906 distraction of San Francisco. And that is the Continental Volcanic Arc due to the subduction of the Juan de Fuca plate beneath North America. 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. 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. 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. Here's an example of probably one of the most famous transform faults on the planet, San Andreas fault, California. Not all of them are associated with steps between ridges. Now, not all transform faults cut across ocean floor. So the little red segments represent the spreading ridges and the yellow segments represent the transform faults. And we see that it's not just a single curving but it's broken by east west trending breaks. If we look carefully, we can see the mid-Atlantic ridge. Because it turns out that in general divergent plate boundaries are not just a continuous band of normal 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. So basically, transform plate boundaries are big strike-slip faults. 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. We're simply having one plate slide past another. 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. It is a plate boundary where one plate slides past one another. That leaves the third type which we refer to as a transform plate boundary. So far, we've talked about two of the three types of plate boundaries, divergent plate boundaries and convergent plate boundaries. Finally, we will delve into the processes that produce the energy and mineral resources that modern society depends on, to help understand the context of the environment and sustainability challenges that we will face in the future. We consider volcanoes, types of eruptions, and typical rocks found there. We will emphasize that plate tectonics-the grand unifying theory of geology-explains how the map of our planet's surface has changed radically over geologic time, and why present-day geologic activity-including a variety of devastating natural disasters such as earthquakes-occur where they do. We begin with earthquakes-what they are, what causes them, what effects they have, and what we can do about them. Planet Earth presents an overview of several aspects of our home, from a geological perspective. Earthquakes, volcanoes, mountain building, ice ages, landslides, floods, life evolution, plate motions-all of these phenomena have interacted over the vast expanses of deep time to sculpt the dynamic planet that we live on today.
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