Transform boundaries. Transform boundaries are the third type of plate boundary. In this type of plate boundary, the plates are not coming together or moving apart, they are sliding past each other as shown in this block diagram here.
So there's not much in the way of vertical motion in a transform boundary. The motion is mostly horizontal. Transform boundaries form strike-slip faults like the San Andreas fault. The San Andreas Fault is a transform boundary between the Pacific Plate and the North American Plate. Now transform boundaries don't tend to be nice straight boundaries like shown in this diagram here.
They tend to have bends in them, like is demonstrated in this diagram here. So what we're looking at here, this would be the transform boundary, and we're looking in map view here, like we were looking down on the surface from above. So we have two different scenarios here where the plates are moving opposite to each other, but in both cases, this transform boundary has a bend in it, right? So let's take a look at the top one first.
In this scenario, the top plate here is moving off to the southeast, and the bottom plate is trying to move off to the northwest. But there's a big bend in this region, okay? So what's going to happen right here...
is that rather than just sliding past, like on these straight areas of the fault, there's going to be compression, almost like a convergent boundary, where these two pieces of crust are going to converge upon each other. That is going to cause uplift, and that can form mountain ranges. So this is a different process than the continent-continent convergence, but it is due to compression of the crust. due to the plates not being able to smoothly slide past each other and forming uplift in this region. Now, in this scenario, the bend is in the same orientation, but here the top plate is moving to the northwest and the bottom plate is moving to the southeast.
So in this case, in this region, what's going to happen is that you're going to have tensional stress or pulling apart. because of this bend, this area of the crust is going to get stretched out. And what ends up happening when that occurs is that the land drops down, forming a basin or a low area.
So the San Andreas Fault has both of these features. The Salton Sea is an inland sea out here in southeastern California, and that results from a bend. in the very southern part of the San Andreas Fault. So the San Andreas Fault runs through here and ends at the opening of the Sea of Cortez.
This is the Pacific Plate over here, right? This is the North American Plate over here. So the Pacific Plate is trying to move to the northwest relative to the North American Plate. But because of this bend in here, we get that tensional stress.
and a basin forms, a low area. This is referred to as a pull-apart basin, like what we can see here. Okay, so because these blocks are pulling away from each other, there's going to be a dip, a low area. And that is where the Salton Sea now is.
Farther to the north, the San Andreas Fault bends in the opposite direction. That's called the Big Bend. And we get compressional stress here. That causes the San Gabriel Mountains to be uplifted, right? So the San Gabriel Mountains are directly due to compression along a bend in the San Andreas Fault.
So we can see in the map here, here's the San Andreas Fault. So down here's where the Salton Sea is and then up here as it starts bending in this direction, remember that the Pacific Plate here is trying to move to the northwest. relative to the North American plate, but there's that bend right there.
And so you get some compression. That's the compression zone that's shown here. So here, what we're looking at is this is north, right? Here's the Los Angeles Basin.
Here's the San Andreas Fault. So this piece of the Pacific plate is trying to move to the northwest, but it's stuck because of this bend here. And so you get this whole region of compression, which... uplifts the San Gabriel Mountains.
Okay, so we can see that if we look in satellite view, here are the San Gabriel Mountains here, and they are being actively uplifted because of compression along this bend in the San Andreas Fault. And then if we can take a look down here, where there is a bend going this way, and that causes tensional stress. And in fact, what happens here is that the San Andreas Fault does what is called, it steps over, the fault steps over onto a new strand and then continues down. to the Sea of Quartets. Okay, so let's look at the situation that we have here on the west coast of North America.
We have all three types of plate boundaries. So we've just been discussing the San Andreas Fault. That's a transform boundary that comes all the way through California, off of the coast to this point here, which is called the Mendocino Triple Junction. So a triple junction is a place where three plates come together.
In this case, it's the Pacific Plate, the North American plate, and this small plate which is called the Juan de Fuca plate. So that's kind of the end of the San Andreas Fault right there. Right here starts a convergent boundary.
So remember that those teeth indicate a convergent boundary. So the oceanic plate, the Juan de Fuca plate, is actively subducting beneath the continent of North America. So this is an ocean-continent convergent boundary, and we have active subduction occurring. and an active continental volcanic arc, the Cascade Range, which starts in the south with Lassen, Mount Shasta in California, Crater Lake, and then all the way on up through Oregon and Washington into British Columbia.
Those volcanoes, including Mount St. Helens and Mount Rainier, are all due to this active convergent boundary. Now to the south, we see some different behavior. We have a divergent boundary where we have active spreading centers in the Sea of Cortez.
So new oceanic crust is being formed here, and the land of Baja California is actively being pulled away from mainland Mexico here. So this divergent boundary is really kind of the northernmost strand of the East Pacific Rise, which is the big mid-ocean ridge in the Pacific Ocean. So there are a couple big differences between convergent and divergent boundaries and transform boundaries. First, Remember that convergent boundaries are directly responsible for volcanism, as in this case here with the Cascades. So anywhere that we have subduction happening, we will have that hydration melting and we can form volcanoes.
Divergent boundaries also have volcanism. As those spreading centers open, the mantle partially melts due to decompression melting, magma is formed, and it erupts onto the surface. at mid-ocean ridges and continental rifts.
Transform boundaries, however, are not directly related to volcanism. So there's no partial melting process that is directly caused by the motion of a transform plate boundary. The other thing is I told you that anywhere that you have a divergent boundary where you're creating or constructing new crust, you have to have somewhere a convergent boundary.
where that crust is then being destroyed or recycled back into the mantle. So divergent boundaries are constructive. Convergent boundaries are destructive. Transform boundaries, however, are conservative. They don't create new crust and they don't destroy it.
You will have a lot of grinding of the rocks right along the fault, but there's no formation of new crust and there's no crust that's being pulled back down. to be recycled in the mantle. So we live in a very tectonically active area of the earth, active convergence and volcanism to the north, a long 1200 kilometer long transform boundary, the San Andreas Fault, which connects to a divergent boundary in the south where new oceanic crust is being formed.