 In this second discussion of plate tectonics and plate boundaries, we want to focus on what makes a plate, what a plate boundary is, how we can identify them, and begin to get a little bit of a sense of the type of geological signal that they may provide. In the last time, we looked at some of the evidence that was used in the history of plate tectonics to develop it, and one of the key pieces of evidence was earthquake data. When one looks at the global pattern of earthquakes, we see that it is not just randomly distributed, but rather falls in very discrete zones. We have locations such as along the various mid-Atlantic Indian Ocean ridges, etc., in which we have a linear trend of earthquakes away from any of the continents in the middle of the oceans, and we saw previously that associated with those linear trends of earthquakes in the oceans, we also had magnetic anomalies which were symmetric on either side of the oceans, and we could see that there was a symmetric age pattern on either side of the ocean. We also have another style of earthquake activity, and that tends to be along the margins of many of the continents in general, and sometimes with the oceans, but like for example here in South America, there the trend of earthquakes is relatively broad, but still it's focused in one area. And then in between these locations of large earthquakes, there are these large areas that have very few earthquakes, such as through most of Africa, Australia, major parts of North America, etc. And so that was one of the observations people had made that in general most earthquakes with a few notable exceptions such as this area in Central Asia and some parts of Western North America which have diffuse seismicity, in most places the earthquake activity is fairly concentrated in local places. So that was one piece of evidence about what is going on in terms of plates and plate tectonics and was used as people were developing the ideas. A second concept that was developed early in the 20th century it was first proposed around 1911 is the idea that when we think about the composition and the behavior of the earth we think of it in terms of shells or layers and compositionally in terms of types of rocks we have a very discreet boundary between what we call the crust and then the mantle below. And the mantle makes up a very large volume of the earth and the crust is a very thin layer on top of that. And that is based on composition. There's another way of describing the behavior and the layering of the earth and that is in terms of how it behaves mechanically. So we talk about a mechanical layering and in that case we have a layer which we call the lithosphere which is the outer 100 or so kilometers of the earth and it's composed of both crust and some parts of the mantle. The term lithosphere was coined to describe the rocky layer so it's the layer that is relatively strong and rigid and beneath that is a layer that's been termed the asthenosphere and that means a weak layer, asthenosphere means without strength. And so we have this rigid strong layer sitting on top of a weak layer and that becomes a very important part of the whole plate tectonic model in that we will have the lithosphere being rigid, hard to deform and it will be moving around on top of a layer of the upper mantle that is relatively easy to be deformed. So between those two concepts and some of the other things we saw one was able to develop the basic plate tectonic hypothesis. So let's just go over what that is and see what some of its manifestations are. So our plate tectonic hypothesis and remember a hypothesis is a statement of a concept. You say it as though it were facts but then it is something that can be tested by other data. So it's a hypothesis. You can also think of it as a framework and we will think of plate tectonics as a framework that we use to help understand other geological and tectonic processes. So the plate tectonic hypothesis can be basically divided up into a couple of key points. One, that the Earth's surface is covered by a finite number of rigid plates, hence the term plate tectonics and these rigid plates are made up of the lithosphere. So the lithosphere can be broken up into somewhere between about 8 and 15 major plates that are relatively rigid, relatively strong that would be sitting on top of the asthenosphere. Secondly, the plates move with respect to each other. And so that's a very important aspect. It's not simply that we're dividing the lithosphere up but these blocks that are sitting around then tend to move with respect to each other. And remember when I talked about Begner and some of his ideas he was focused on looking at things like ice flows in the Arctic, these blocks of ice that are relatively rigid, moving around in the Arctic Sea and then interacting with each other. But the third aspect of this which is very important is that since the plates are rigid so if we say, okay, since plates are rigid that implies that all the deformation must occur at the boundaries. And so that's a very important aspect of this and that gets back to our linking it to the pattern of earthquakes is that the plates themselves are relatively undeformed and it's only at the boundaries that most of the action is. And that is really very important from our geological analysis because if we want to understand plate tectonics it becomes very important to study what's happening at the boundaries, not necessarily what's happening in the middle of the plates. Now as a hypothesis or as a framework there are things that would follow from that that could be of importance and we call those corollaries. If you think back to some math classes you might have had a corollary is something that results from a theorem and so the corollaries from this, well the first one is that the earthquakes are going to represent the deformation represent the deformation at the boundaries and so essentially the earthquakes are really our present day manifestation of the fact that the plates are moving and that as they move with respect to each other earthquakes occur at their boundaries. Secondly, there will be little permanent deformation outside of the boundaries, so little permanent deformations in the interior and so although there might be something we don't expect much in terms of the many earthquakes the size of them, etc. And then the third and a very important aspect of our corollary is that rigid plates will move in specific ways. That is once you become a rigid plate you're constrained and you want to stay on the Earth's surface you can only move in particular ways the same way when you're driving in your car you're in a rigid vehicle you can only go certain ways you can't bend around a corner you have to turn around a corner and this has led to something which is a very important mathematical constraint and it was really the identification that one could describe plate motions using this mathematics that helped it along and this is known as Euler's fixed point theorem. Now Euler was a swift mathematician of several hundred years ago but what he recognized was that on the surface of a sphere any motion of a rigid body can be described as a rotation about a pole or fixed point and so you can think about any motion of any plate any rigid body as being a rotation about a pole so that means you can turn around the other way and you can say I can describe the motion of North America by simply describing the pole and that became one of the very important tools because then we can apply the mathematics that comes out of Euler's fixed point theorem to all of our discussions of plate motions and we'll use that to actually figure out some of the things that are otherwise difficult to know so with this now we have the hypothesis that we have rigid plates that will move with respect to each other the earthquakes will be a modern example of the relative motion of them so we can use earthquakes and the patterns of earthquakes to define the actual sense of motion or where the boundaries are and also we can use the motion of the plates in terms of they have to behave in a particular way according to Euler's fixed point theorem so when we look on a map we should expect to see a big plate like North America will move in something that looks like an arcuate or rotation about some other point and so this will be a way to test some of our ideas so we can then look at the patterns of earthquakes and what the different types of plate boundaries are so with that sort of a conceptual framework people have been able to break the earth up into many major plates and so the major plates are shown here with the black things like North America South America Eurasia Australia India now Africa is sort of interesting because we do have the East African Rift Zone and so the eastern part of Africa this part in here we call the Somalia plate and then the main part of Africa we call the Nubian plate so or sometimes just called the African plate but so we see that Africa actually has a boundary with internal of it but otherwise we can define these plates this way now most plates have both ocean and continent like North America is both the continental part of North America and part of the North Atlantic the Nubian plate has Africa plus some of the Atlantic South America similarly but there are several plates in particular the Nazca plate the Kokos plate and very importantly the Pacific plate that are almost entirely if not entirely oceanic in their nature and so that will be important in the way they behave so we have these different types of plate boundaries we have different plates and they're going to interact with each other in different ways and we can then use our identification of such things as the ages in the ocean any evidence we have for the direction of motion to try to pin down how the plates move with respect to each other and so one can take that picture and superimpose it now on the actual earth and the topography and we can see that for example the mid ocean plate boundaries that are defined by earthquakes also are defined by topography there's shallower bathymetry or higher topography near the ridges and as you move away from the ridges the oceans get progressively deeper and we get closer to the continents so that's very important we have these boundaries say for example if we look at the boundary here the plate in South America we have this boundary here where the two are meeting in some way and we're going to want to understand how that happens so there's a lot of tectonic activity and we can see that a lot of the areas of say mountain ranges high elevations are associated with the active plate boundaries now with the onset of such things as GPS we can now determine how the continents are moving with respect to each other we can only do it where we have land so we can observe above sea level but as we can see we can as we break the plates break the globe up into plates we can see that the different plates are moving in different directions with respect to each other so we can see that for example in this reference frame here Australia is moving to the northwest whereas the Pacific northeast whereas the Pacific is moving to the northwest America is moving to the northeast Eurasia is moving principally to the east and North America is moving to the west southwest and so we can see that the plates are moving with respect to each other if we zoom in on the area of North America that's our principal area of interest for this course we can see that the details tell us a lot here is the GPS field of motion for North America we have lots of data so it's pretty clear this beautiful, arcuate nature of the plate motions of most of North America that's because remember they're rotating a belt this pole of rotation somewhere to the south in this case that is related to the Euler's fixed point theorem the other very important thing we see is this boundary between two distinct styles of motion in this reference frame most of North America is moving in this west southwest motion until we get to California and then we see a fundamental boundary between stuff that's moving to the southwest and material that's moving to the northwest and this is then the plate boundary between the Pacific plate and the North American plate and so we have a very distinct and a very sharp boundary and so we can see that based on plate motions moving with respect to each other we'll then be able to look at say earthquakes or other evidence of what the nature of this boundary between the Pacific and North America looks like so with that let's look at the different types of plate boundaries talk about them a little bit here is a map of active plate boundaries around the world one produced by the U.S. Geological Survey and we can see the various plate boundaries are defined and we zoom in and there's behavior associated with the boundaries between in this case the Nazca and South America plates but we can look at those in a little more detail and we'll do that both this week and next week so if we zoom in on this map a little bit and look at the behavior that we we see in each of these plate boundaries let's look in particular at several different plate boundaries we have plate boundaries for example down here in the middle of the ocean those will be interesting to look at we have plate boundaries say along Alaska South America the Western Pacific another and then we also have the boundary for example that we've seen already in the GPS data in western most North America so let's first look at this boundary in the South Central Pacific we zoom in on that area this is what the seafloor bathymetry looks like the colors are depth the red colors are shallow the blues and purples are deeper and we see a very distinctive a very linear region of very shallow seafloor that's offset from each other with a boundary between them and then the seafloor gets progressively deeper as we move away from this linear region of shallow bathymetry other features for example this particular ridge here that are associated with other aspects of the plate tectonics but we see this very simple thing and so we can identify the location of the East Pacific rise by the bathymetry by the elevation of the seafloor and it provides not only we would see this with magnetic anomalies also but we get to see this pattern moving away we also see that the East Pacific rise is offset in places and then we will have a feature connecting those and remember this is the feature that Tuzo Wilson find as a transform fault that in this case is connecting to mid ocean ridge segments now what is happening at the mid ocean ridge well that's still a point of active research but our general picture is that as the plates move apart mantle material comes up to fill the space and as it does that it melts and so the situation we have can be seen in this cartoon here in that this would be the position of the ridge that shallow region of bathymetry underneath that is a region of partially molten rock it's called here a crystal mush so there will be both some melt and crystals in it as the plates move apart these plates are moving each other a gap opens up this material can flow up fill that gap some gets erupted and forms this top layer others get injected here as a series of near vertical dikes and then the main magma chamber itself just slowly cools along its edges producing an additional crustal rocks and so we end up with a very systematic pattern of rocks three or four different types and then there's a boundary here which is the boundary between the crust and the mantle but remember even this upper mantle is still all part of the lithosphere so all of these two sides are moving away from each other even though it involves both the crust and the mantle because of the heat associated with the magma here and the system being overall quite warm this area will be much shallower than the area to the side and hence we see this nice well defined region of high elevations so that's one type of boundary the mid ocean ridge type of boundary we can then go and look at some other boundaries in particular there are the ones that we refer to as subduction zones and subduction zones are where we have two plates meeting in a convergent direction and then the question is what happens when you do that do they just collide with each other and crumple up well in general that may happen sometimes but in general one plate goes beneath the other and when we talk about subduction we're usually talking about one going beneath the other the question really comes down is how do we know that that is happening so the first thing we can do again is look at the earthquakes and so to do that we'll look specifically at this region right up here off of Alaska and look at a profile from the ocean to the continent across Alaska so if we go to well before we do that let's just look at some of the complications down in this area this is the area between the Pacific and the Australian plate and if we just look at that quickly we can see that the area has a lot of earthquakes the earthquakes are all the various color dots in this as it clarifies itself and what we can see though also earthquakes are color coded by depth and so the red or the shallowest earthquakes green and yellows are intermediate and the blue are the deepest and what we see is we go from shallow say here in the Tonga trench we go from shallow earthquakes to very deep earthquakes over a distance of 500 to 1000 kilometers so if we look at that you know where all these earthquakes occurring and what do they tell us about the way the Pacific plate and the Australia plate are interacting with each other well let's go up to Alaska then where we have some very good data to look at and when we look at Alaska we see that there are lots of earthquakes those are all the red dots on this figure here and we can look at those in detail Alaska's had some very large earthquakes this represents the area of rupture from the 1964 magnitude 9 plus earthquake so if we look at that area in a little more detail and so we'll look at a profile that's basically running across here from the Pacific Ocean on to mainland Alaska going from this would be the right hand side to the left hand side of our profile if we look at that we see a picture that looks like this so what do we have well we have the trench which is right here the trench is located here and we see that the earthquakes tend to be fairly shallow but as we go progressively further in we have this deepening of the earthquakes and so this is the characteristic shape of a subduction zone and the interpretation is that this would be North America this over here would be the Pacific and the Pacific is coming in and subducting beneath it so the Pacific plate might be made up of something like that it's coming in and going down underneath and the earthquakes get progressively deeper and that's the color scheme here so we see that profile the other thing that's very interesting to note is that our plate tectonic in theory just talks about rigid plates moving and doesn't talk about other features such as volcanoes but associated with subduction zones we very typically have a series of volcanoes that sit above the subduction zone and we're going to look at those in more detail and on our field trip we'll go and look at Mount St. Helens Mount Hood and some others because they have a very important role both in terms of defining the plate tectonics of the area and a very important role in the evolution of the planet so we've looked at two types of plate boundaries we've looked at the mid ocean ridge type we've looked at the subduction type that occur in different places along the plate boundaries the third major type of plate boundary that we need to look at are the so-called transform faults and there are two types of transform faults they're the ones that are associated with the mid ocean ridges and then there are others that are big faults that as Tuzo Wilson said simply connect other segments of the plate boundary our most favorite of those is the San Andreas and so we'll look at both what happens at the mid ocean ridges and along the San Andreas in the field trip in the class we'll be going to this area so we'll actually be looking at the San Andreas in quite a bit of detail so if we look at this earthquake activity around the San Andreas and here's a zoomed in area we can see the San Andreas system in the vicinity of San Francisco San Francisco is right here and we can see this band of earthquakes coming down like this but we also see that there are other bands of earthquakes in the region that are associated with the plate boundary so in this case we think that the Pacific plate is moving to the northwest relative to North America and when we had the GPS data the GPS showed North America moving this way and the Pacific moving that way when you add those together you get the two slipping past each other so if we look at this we can see that the faults are very distinct of course if I'm going to say where is the plate boundary well all of these faults play a role in the plate boundary so really when I'm out here in the Pacific and when I'm over here I'm clearly in North America but somewhere between here and here I'm in a transition zone between the two so the entire area around San Francisco Silicon Valley etc is a transition between the Pacific plate and the North America plate now this is zoomed in just to the Bay Area if we zoom out just a little bit and look at the area south of the Bay Area we see that this the fault and the plate boundary through the San Francisco Bay Area is not something that necessarily is occurring everywhere as we go further south the San Andreas becomes a very narrow feature and the plate boundary is much simpler there are some complications like this cluster of earthquakes here that we'll talk about in the future these are not associated with the plate boundary but we see that as we go further south the plate boundary gets progressively simpler we're just studying the plate boundary even further north of this figure and in there we'll see that it becomes a very diffuse boundary so that's what the plate transform might look like on land if we look at a transform in the ocean it's a much simpler feature in this case we have the mid ocean ridge coming in here this is in the North Atlantic the two sections of the ridge are offset from each other by the transform fault this is the motion is something like this this ridge is spreading this way this ridge is spreading this way and so along the transform the motion is like this we would say this is left lateral because as I cross the fault the plate on the other side is moving to the left I can zoom in on this picture a little bit shown here and here's the same feature the mid ocean ridge this very narrow very discreet transform fault connecting them so we've seen the three major types of plate boundaries we have some hints on how we can detect them and so in this week's activity we will actually take some data from the Pacific Northwest and attempt to do that so we look at a map like this of earthquakes and we can say okay we know how to draw plate boundaries on it and in some cases we can also interpret what might be going on and in this week's activity we will zoom in on the area here right around western most North America to understand what's going on