GEOLOGY/GEOPHYSICS 101 Program 7
PLATE DYNAMICS 
In the last lesson, we saw how the Theory of plate tectonics was built, and if you're studying the text and study guide as you should be, it should be clear by now that the evidence for plate tectonics is overwhelming, but now's the time to put the pieces together and see how plate tectonics really works.
Plate tectonics is really a modern version of old ideas. It combines the theories of continental drift and sea floor spreading along with new evidence. Plate tectonics is a more comprehensive theory than either continental drift or sea floor spreading. It ties many facts together and explains earthquakes and volcanoes. But it also adds to our understanding of mountain building and the forces, which create them.
Plates are large sections of the lithosphere, which are bounded by earthquake belts. Interactions occur at the edges of the plates, and these edges occur coinciding with ridges, trenches, faults, and volcanism.
In general, the Earth's surface is made of seven major plates and numerous smaller ones, which are rafted over a mobile asthenosphere and interact with each other at the edges. The plates sometimes carry continents, and when they do, when the continents collide, they crumple at the edges to form mountain ranges, and in the process, new sea floor is created as spreading centers, and old sea floor disappears at subduction zones.
Before we begin the lesson, I want to remind you of the lesson assignment.
Today's assignment in the text is Chapter 4 beginning on page 79 through page 98, the last half of Chapter 4 beginning with the section on "Diverging Plate Boundaries." This is a long chapter, and there are a lot of concepts to learn, so it'll take more than one reading, but plate tectonics is a fascinating subject, and its understanding is crucial to understanding Earth revealed. This material also sets the stage for the rest of the course, and we'll be referring to plate tectonics a lot as we go into the course, so if you need to,
There are several objectives to today's lesson, and I'd like to list these for you:
Many geologic features occur at plate boundaries, and basically there are three ways in which plate can interact.
and these plate boundaries can be illustrated with simple models like these pieces of foam that I have in front of me.
For divergent boundaries, the plates are simply moving apart leaving a gap between, which is filled with magma, which becomes new sea floor. Examples of this can be found most notably at the mid-Atlantic ridge but also in the East Pacific Rise in the Pacific Ocean and the mid-Indian ridge in the Indian Ocean. There are also examples of divergent plate boundaries on the continents. If a divergent boundary happens to appear under a continent, then the continent is rifted apart and splits as new ocean floor is created between them, much as must have happened during the break up of Pangea 200 millions years ago when the current Atlantic Ocean was formed. A most notable example of a rift is in East Africa, and it was in the East African rift valleys that volcanic ash has preserved the fossil remains of early hominids. In other words, the East African rift valley is really the cradle of mankind.
Associated with these diverging plate boundaries are shallow focus earthquakes from fracturing of the surface rocks as magma is injected into the dikes. "Dikes" are simply vertical tabular shaped bodies of magma that solidify below ground.
Also associated with the divergent boundaries are basaltic volcanism and high heat flow. This comes as a liquid residue from partial melting of the mantle rocks and carries heat upward with it at the ascending portion of a convection current, so magma, then, can either fill an already expanding rift or work its way into the rift to wedge the plates apart.
There are also positive gravity anomalies associated with divergent boundaries because the rising convection current is pushing up on the crust of material from below.
Convergent boundaries, on the other hand, are where plates come together. When plates come together, if they collide, several different things can happen, and we'll take a look at some of the examples of this in the next lesson on mountain building.
The edges may simply crumple, but usually what happens is that one plate begins to subduct onto the other and is gradually thrust down into the mantle. The active earthquake region, as learned earlier, is along the Benioff zone, which represents the sliding of the descending plate as it moves passed the mantle that's already down there.
Convergent boundaries are found mostly in the Pacific Ocean, but there are some examples in some of the other oceans as well. These are associated with subduction zones where we find andesitic volcanoes and relatively low heat flow as the cold lithospheric plate plunges into the warmer mantle. These andesitic volcanoes form island arcs, such as the islands of Japan and the Philippines, the Aleutian Islands off of Alaska, the Malay Peninsula, and New Zealand.
It's thought that the andesitic magma comes from partial melting of this basaltic crust, which also incorporates some water and sediments along with it. The earthquakes associated with subduction zones are generally deeper, and you can see as the plate descends, the earthquake zone becomes gradually deeper along the descending Benioff zone.
We also find associated with convergent plate boundaries negative gravity anomalies. Negative gravity anomalies are caused when a portion of the Earth's crust is being held down, and here the holding down comes from the descending plates being forced down at the descending portion of convection currents.
The third type of plate boundary is where plates slide past each other in what are called "Transform Boundaries." The sliding of the plates is a horizontal sort of motion. Now, this transform or transcurrent motion may consist of two plates sliding past each other or pieces of the same plate that are moving at different speeds. They may be moving in the opposite direction, or they may be moving in the same direction at different speeds. The transform boundaries are also active earthquake regions, but here the earthquakes are shallow. They extend only down to the depth of the lithosphere. The best example of this that we have on land is the San Andreas Fault that extends through Western California, and here the Western part of California is moving north while the Eastern part of California is moving south. We'll also see in the video that the presence of fracture zones which represent the Transform Boundaries on the sea floor allow the ridge to bend while still spreading at the same time. Associated with the transform boundaries is little or no volcanic activity, but we often find escarpments or cliffs across the ridge due to this offset.
Well, we'll consider more details after the video, but for now, lets watch the video. Music. Major funding for "Earth Revealed" was provided by the Anneberg CPB Project. Music.
On an active lava pool like this one in Hawaii, crustal fragments separate, slip past one another and collide, continuously rearranging and deforming the surface. The collisions that we've just seen may seem completely unrelated to this serene and beautiful landscape, but, in fact, this valley and these mountains are the direct result of very similar types of interactions between two of the Earth's adjacent plates.
Plates are the rigid slabs of rock that comprise the outer surface of the Earth. They're tremendous in size, sometimes encompassing entire continents, and they're moving and interacting with one another at their boundaries. Because the plates are rigid, their interiors are relatively inactive tectonically. The boundary between plates, however, is defined by a high degree of techtonic activity.
Plate boundaries coincide with narrow zones of earthquakes, and active volcanoes, rapidly rising mountain ranges, and deep sea trenches. This long linear valley is one such plate boundary. This is the San Andreas Fault in California. On this side, the North American plate is grinding fitfully against the Pacific plate over here.
The result of this interaction are sudden and sometimes devastating earthquakes. Over time, this interaction has resulted in the steady uplift of these mountain ranges. Plate boundaries are directly related to geologic hazards, to the formation of petroleum and mineral resources, and to the geologic development of the landscapes on which we live. So understanding how plates move and interact at their boundaries is not ony one of the most interesting, but also one of the most important goals of the current generation of Earth scientists.
Wrapping like a net around the globe, the boundaries of the plates in most places do not correspond to the edges of the continents in oceanic basins, so most plates contain both continental and oceanic crust. In all, about a dozen large plates and many smaller microplates have been identified by Earth Scientists. In few places can one see a plate boundary better exposed than in Iceland atop the mid- Atlantic ridge.
In a broad zone across Iceland, great tensional cracks called "rifts" break the landscape. Frequent small shallow earthquakes occur beneath this rift zone. Geysers and hot springs are evidence that the crust is hot. From time to time, great volumes of fluid basaltic lava and ash spew from fissures and volcanic craters. Here the crust is being pulled apart as basaltic magma fills the fissures that open, it solidifies, adding new crust to the edges of the plate. As the older rock is pulled away, new magma rises to cool, harden, and form additional crust in its place.
This type of boundary is called a "divergent boundary" for here the plates separate from one another. A dramatic example of this can be seen on the African Continent. In Eastern Africa a divergent boundary partially splits the landscape to the north. So much new crust has formed that Africa and Saudi Arabia, once joined together, have split apart with the Red Sea flooding the wide valley in between. Rifting has also opened up the Gulf of Aden to the East. In time these young seaways will grow larger, some day, perhaps, becoming as wide as the Atlantic today. Oceanic crust comprises about 70 percent of Earth's surface. Most of this crust originated by injection and eruption of magma at divergent boundaries expressed as mid-ocean ridges capped by rift zones.
This process of crystal growth it called "Sea Floor Spreading." Earth's volume has remained essentially the same for billions of years. As a result, plates can grow larger by sea floor spreading only if other plates are growing smaller. Plates are reduced in size or destroyed where they converge, creating some of the most dramatic topography anywhere on Earth. An important land form marking the collision between plates is a deep marine trench. Here one plate slips beneath the other in a process called subduction. Some plates, however, are too buoyant to subduct and simply crumple together. There are three basic types of converging plate boundaries, and this is based on the types of crust that are involved on both sides of the converging boundary.
In the three types of plate convergence described, oceanic rock always sinks beneath continental rock because oceanic crust is typically denser and heavier than continental crust. Once subduction starts, it may be sustained in part as the weight of a downgoing slab drags the rest of the plate with it. Subduction is also sustained by slow moving currents of hot mantle rock, which tug against the underside of the plate. As the plate sinks, earthquakes occur within it. These quakes range from shallow events at the trench itself to very deep cataclysms near the heated end of the descending slab as much as 700 kilometers beneath the surface. These earthquakes can be used to trace the descent of the subducting plate into the mantle.
By diving into the mantle, the subducting plate creates friction and heat, and the basaltic oceanic crust partially melts under the intense pressure. The molten rock rises, ultimately reacting with overlying rock to form andesitic magma, which erupts in curving or arc shaped chains of volcanoes. The volcanic chain parallels the ocean trench and at sea forms a string of volcanic islands known as an "Island Arc System." Where subduction occurs along continental margins, the andesitic volcanic chain is built atop dry land, sometimes towering kilometers above the landscape, rarely very far from the sea.
Eruptions are frequently hazardous, unlike those at divergent boundaries, because andesitic magmas are more viscous and gas rich then basaltic magmas. Earthquakes, too, are more powerful at convergent than at divergent boundaries because of greater levels of stress resulting from plate collision. It is this earthquake activity together with crumpling and folding of the crust that creates the mountain ranges like the Andes along continental margins overlying subduction zones, but Earth's largest mountain ranges and plateaus form when two continental masses are brought together by subduction of intervening sea floor. Little volcanic activity accompanies this type of mountain building because subduction stops occurring once the continents collide.
Along some plate boundaries neither divergence nor convergence occurs; instead, two plates slip past one another, their edges marked by a special type of rupture known as a "Transform Fault." There are really two major environments that we see transform faults occurring.
Most geologists are in general agreement regarding plate tectonic theory. However, there is considerable debate about the forces which drive plate movement, and a great deal of current research focuses on finding a mechanism. This is a particularly challenging problem because the mechanism operates deep within the Earth's interior, which we cannot see or sample directly. The Earth's crust and uppermost mantle act together as a rigid unit.
This layer which comprises the plates is called the "lithosphere," and it's about a hundred kilometers thick.
These lithospheric plates float crowded together like lily pads in a pond in a deeper layer of the mantle called the "asthenosphere." Unlike the lithosphere, the asthenosphere is soft and partially molten, so the relatively cool rigid lithospheric plates can move through the asthenosphere if enough force is applied.
Several different theories have been advanced to try to explain the driving force behind this process. All agree, however, that the mechanism for plate movement is somehow related to the unequal distribution of heat within the mantle.
When mantle rocks are heated unevenly from below, most geologists believe that they can circulate in a cyclic fashion called "convection." Currently, some form of convection in the mantle is the most widely accepted mechanism for plate movement. Convection occurs because rock inside the Earth flows like a gradually moving fluid. Just as the application of heat to a pot of water causes the warm liquid to rise and the cooler liquid on top to descend. So, too, does heat inside the Earth cause portion of the mantle to convect. The fundamental force driving convection whether on the stove top or inside the Earth is gravity. Cool matter is more dense and, therefore, heavier than warm matter. Under gravity it will sink displacing the warmer material beneath. It may seem surprising that rock should act like a viscous liquid and flow even on the most slow moving time scale, and yet this is precisely what happens in the Earth's mantle.
Okay, we can use silly putty as an analogy for rocks, particularly if we're interested to see the effects of the rate of distorting or straining the rock. I can give you several examples. I've got three different piles here of the exact same material. In one case I can take the silly putty, and if I apply a very fast rate of deformation to it, we can just make it fracture.
Okay, this might be a good analogy for what would happen near the surface of the Earth where our rocks are cold, and if we deform them quickly, they would fracture like that. We could take the same material, and I could again apply stress to it, but if I do it at a much slower rate, essentially I have a slower strain rate, then we can see that my material will flow instead of fracture, and this might be a good analogy to what would happen at depth where the rocks are hotter and we're deforming at very slow long term rates, and we could get folds and things like that.
We can even go to a more extreme case, and we'll again take the same material, and I can just place it on my block right here, and we'll essentially let gravity be the only stress operating on that silly putty. In this case, we'll see with time that the rock will still flow although at a much slower rate than we saw in the previous example. In the same way mantle rocks can deform plastically because of their high temperature when stressed over very long time periods.
It is the ability of rock to flow that in part allows the convection process to take place inside the Earth, but that still does not explain why convection occurs in the first place. The reason that the Earth's mantle is convecting is that first there is still primary heat left from the formation of the Earth, and then there is heat being generated in the Earth by the radioactive decay of a number of elements, primarily uranium, thorium and potassium. This heat within the interior of the Earth wants to escape and flow out, and the most effective way for it to flow out is by convecting just as a pot of boiling water on a stove would convect.
There are a great many questions concerning the convection process. Perhaps the most fundamental of these is the role of convection in plate motion. According to one theory, plates are pushed apart as the hot rock beneath them convects upward. Another view suggests that upward convection creates the mid-ocean ridges. Plates slide away from the ridges by a combination of gravity and drag from the convection currents themselves, but some geologists believe that there is a third possibility, that the plates are literally being pulled down into the Earth. There are those who believe that because the lithospheric plates where they descend in the trenches are of greater density than the material they displace that, in fact, like the covers falling off the edge of the bed, they are being pulled down, so plates push at the ridge because of hot rising material sliding off the ridge and trench pull are all aspects of the process.
What we know for certain is that the lithospheric motions are as described in all those models, but none of them is a paramount argument or model for explaining the process of actual plate motivation, why plates move. I personally believe that because the plates that have trenches, that are moving down trenches, are going fairly fast, 10, 12 centimeters a year; whereas, those that are simply moving away from ridges that are not tied or hooked to descending limbs are only going a few centimeters a year, that, in fact, there is an important trench pool, but I believe it's also possible there is a small at least plume drive or rising asthenosphere column which causes the plates to move away from the ridge crests as well.
The depth at which mantle convection occurs is another controversial issue within the geologic community. A single convection cell extending throughout part or all of the mantle was first proposed as part of the sea floor spreading hypothesis. Most current models, however, use a two tiered approach known as the "boundary layer" theory of convection.
According to this theory, one set of cells in the upper mantle is driven by another set in the lower mantle. Although the issue of mantle convection is still unresolved, evidence from seismic studies of the Earth's interior points toward a two tiered process. I think the mantle's split into at least two layers that we call the "upper" mantle and the "lower" mantle. The evidence for this is the plates that subduct around the ring of fire, around the Pacific Ocean, and generate mostly earthquakes. We can trace these earthquakes down to about 700 kilometers depth; then, the earthquakes stop. We also use seismic images to find out where these slabs are, and they appear, the slabs themselves appear to stop at 700 kilometers.
Below that we have the lower mantle which is also convecting and to some extent influencing convection in the shallow mantle, but it probably has a higher viscosity; it probably has a higher density, and it's likely that the slabs cannot sink into the lower mantle but have to stop at the 700 kilometer boundary, which is a very sharp seismic discontinuity, so, in my view, we have two layers. The upper mantle provides mid-ocean ridge basalts. Then, as the lithosphere cools and gets denser, it sinks back down to the bottom of the lower mantle; then eventually heats back up again and comes up to the surface. The lower mantle is convecting very, very slowly, but because it's convecting very slowly, the parts of the lower mantle that are particularly hot stay hot for a long time and tend to heat up the upper mantle, so I think the lower mantle is definitely influencing convection in the upper mantle on the locations of where there's hot upwelling material, but I don't think any material's coming directly from the lower mantle into the upper mantle, nor is upper mantle material sinking into the lower mantle.
Whether there is boundary layer convection in the mantle or not, there is another kind of convection that occurs in the outer portion of the Earth's core. Then, as we cross the Kormel Boundary we're into a very low viscosity fluid molten iron core. This is convecting very, very rapidly and forming the Earth's magnetic field, so basically we have two parts of the Earth that are convecting. The mantle is a very sluggish high viscosity fluid, more or less like tar that's trying to convect on a hot day; whereas, the core itself has more the viscosity of water, and it's convecting very, very rapidly, so there's two scales of convection and two kinds of convection in the Earth. Current measurements of Earth's surface from space have provided additional evidence that convection is occurring within the planet. Interestingly enough, convection changes the shape of the Earth. If we look at the Earth from a satellite and look at it with very detailed radar techniques, we find out that the Earth is not a smooth sphere at all. Even if we get rid of the mountains and the continents, we find out that what's left over is a very bumpy object. Now, the upwelling convection currents tend to make bumps and swells in the surface of the Earth, and downwelling cold currents tend to make depressions in the sea floor, for example, so by looking at the shape of Earth, we're able to map the convection pattern.
In the late 1960s, it was suggested that there are places beneath the middle of the plates where a special kind of convection takes place. Comparatively narrow columns of hot mantle rock rise from below and spread radially outward as they reach the lithosphere. These are known as "mantle plumes." This kind of circulation can be seen in other more familiar phenomena that occur in our atmosphere. When thunderhead clouds form, for example, a similar type of plume convection is taking place but in an accelerated manner.
Evidence of mantle plumes has been found in the form of hot spots, regions of concentrated volcanic activity, which are roughly circular in shape. When you just look at the topography of the general sea floor, every now and then you find a sea mount. There'll just be little round cones. They range from real small to huge. Hawaii is one. And we don't know about all of them, but a lot of them seem to be related to hot spots. Hot spot volcanism happens when there's a place way down in the mantle that for some reason produced extra lava, so much extra that it bubbles right up right through the plate and builds a sea mount on top, and often it builds an island like Hawaii and the whole chain of islands that's strung along behind Hawaii. The reason they're interesting is that the hot spots seem to be still or nearly still down in the mantle, and so when the plate moves over it, it keeps making new volcanoes in a line, so that right now the Big Island of Hawaii is being built. The islands up the chain get older and older and there's, in fact, that stretches way across the sea floor all the way up to The Aleutians, which are older and older, showing us the motion of the Pacific plate over that Hawaiian hot spot.
As the islands of Hawaii age and weather away they ultimately become flat-topped submerged sea mounts or Guyots. Thousands of such sea mounts dot the ocean floor. One thing that's interesting about oceanic islands is that they don't last very long. The erosive power of the waves is so strong that any island that isn't being continually built will be eroded away in a few million years, planed right down to wave base. As plate movement carries the islands away from hot spots and mid-ocean ridges,the underlying sea floor cools and subsides.
In lower latitudes coral reefs build up completely capping the remnants of the sinking islands. Such caps are called "coral atolls." Ultimately, the atolls themselves may sink from view. The Big Island of Hawaii is being built right now, and, in fact, there's new land being added out to the ocean, but even all the other Hawaiian Islands in the chain, even though they're still islands are planed away, so when you drive up on a ship with a sonar you come up the side, and then there's a big flat surface, and then the island is the last little erosional remnant that hasn't been chewed away by the waves yet.
If the theory of Hawaii's formation is correct, Hawaii will drift off the hot spot in a few million years as they Pacific plate carries it away in a northwesterly direction. The mantle plume will stay where it is and eventually create a new island over itself. Indeed, a young submarine volcano dubbed "Loihi" has been discovered forming southwest of Hawaii. It rises some 8,000 feet from the sea floor but has another 3,000 feet to go before it breaks the surface and becomes a real island. Loihi should build up to the surface some time between 19,000 and 100,000 years from now. The enduring mystery is why a mantle plume, such as the one underlying Hawaii should remain in the same place for over 75 million years.
Not all hot spots occur under the sea floor. There is good evidence that the area underneath Yellowstone, which has long been famous for its geothermal hot springs and geysers is occupied by a hot spot. This hot spot lies beneath the interior of a continent, and while it isn't responsible for island formation, it certainly contributes to the geothermal activity there. Plate tectonics is a model of the way the Earth works. The significance of this theory lies in the fact that it connects many seemingly unrelated geologic phenomena, such as earthquakes, volcanic activity, mountain building, sea floor spreading. Yet certain questions remain. For one thing, we don't fully understand the mechanisms that drive plate movement. Also, it's undeniable that there are certain places where the geologic relationships don't fit easily into the plate tectonic model.
The western margin of North America between the Pacific and the Rockies is one such example. However, unanswered questions like these don't mean failure. Instead, they propel Science forward forcing scientists to reevaluate the assumptions of existing theories.
Nearly 100 years ago Alfred Wegener challenged the scientific orthodoxy with his theory of continental drift. That challenge resulted in nothing less than a revolution in earth sciences.
Music. Major funding for "Earth Revealed" was provided by the Annenberg CPB Project.
I like those animations. They really show how the process of plate tectonics operates. Well, let's take a closer look now at some of the details to see how plate tectonics explains some of the Earth's features and processes. One of the things I want to talk about first is "rifting." "Rifting" simply means the spreading apart, usually referring to a continent. It's a spreading zone where lithosphere splits and separates and moves apart. In other words, at divergent boundaries. The lithosphere is much thinner at spreading centers. This has to do with the fact the essentially lithosphere is being created at the spreading centers by magma.
You remember from an earlier lesson, we discussed "ophiolites" and how ophiolites can give us clues to the Earth's interior. The ophiolitic structure is characteristic of rifting at divergent boundaries. The characteristic structure of ophiolites has gabbro on the bottom followed by sheeted dikes on the top. The sheeted dykes are the pipes where the volcanic eruptions actually occurred, and, finally, on top are a pillow of basalts, which represent the lava being erupted onto the ocean floor under water. Beneath the layer of gabbro, the lithosphere thickens as magma cools and sticks to the bottom of the lithosphere, and the gabbro forms, of course, when the magma actually cools. We often find old remnants of ophiolites stuck to the continents. But one good example is the island of Cypress. Another one is the coast of Alaska.
We also find associated with this rifting significant mineral deposits. We'll look at mineral deposits and Earth resources in the last lesson. The black smokers and hypothermal vents are ways of depositing material where super heated seawater comes in contact with colder water, so minerals, mostly iron-bearing minerals, are redeposited. The copper mines of Cypress, which were used to provide copper for the Roman Empire and also for early Bronze Age use of tools comes from the ophiolites, which were associated with the island of Cypress. At the rifting zone, then, the plate bows and stretches. It bent upward from both the heat below and the upper pressure of the magma and begins to crack under tension. When it cracks, magma can fill those cracks and also help to wedge apart the spreading center. Now, this rifting also can occur on land and is responsible for splitting land masses. Geologic structures called "grabens" are basically down dropped fault blocks. What that means is that as the continental crust above the divergent boundaries split, the crust simply collapses under tension and eventually will become filled with magma or lava. When it becomes filled with magma or lava, we then call it a "rift."
The African rift valley that I mentioned earlier is a "graben" that's filled now with oceanic volcanoes; in other words, basaltic volcanoes, and there are regions within the African rift valley where sea water is slowly working its way in, and people have predicted that within a few hundred years, the African rift valleys may actually be sea floor and become filled with water. The Red Sea is also part of this rift system, and, in fact, is a brand new ocean basin, which is only a few hundred miles wide at this point. The continent of Africa is splitting away from the Arabian Peninsula as the Red Sea opens.
There's a beautiful illustration of this at the beginning of the chapter on page 64 if you look at the satellite picture on page 64. Processes associated with these divergent boundaries also shape the continental shelf on the trailing or passive margins of the continents, so to explain what that means, we need to take a look at what we mean by continental margins or the different types of continental margins. So, let's review the effects of plate motions on the continental edges.
Basically, a continental block as it's rafted across with the process of sea floor spreading embedded in the lithosphere has only two types of margins. One margin is on the front edge of the continent; the other is on the trailing edge of the continent. On the trailing edge we find passive margins. The word "passive" means that the coast line or the edge of the continent is not greatly modified by tectonic processes. The passive margin is the part of the continent on the side closest to the spreading center. There are usually wide continental shelves--very few trenches and very few island arcs. Wedges of accumulated sediment from the continent up to a few kilometers deep are very orderly and pretty much undistorted. Coral reefs and other types of deposits may also add to the building of the continental shelves. We also here find thick turbilites on the continental rise, and many of these coastlines are eroded or drowned shores, as most of the coastlines along the Eastern part of the United States are and Atlantic coastlines in general. Active margins, on the other hand, tend to be on the opposite side of the continental from the spreading center. In the case of North America, this would be the Pacific Coast. Here the edges of the continent are modified at the subduction zone or a fault boundary. The rocks are deformed or subducted. We generally find narrow and rapidly changing continental margins. Here the sediments are deposited in the trenches instead of on abyssal planes. The trenches are right adjacent to the continental blocks, and so the sediments that are traveling down the submarine canyons are simply trapped in a trench before they can reach the abyssal plane. When plates collide this way at the active margins, the sediments are either scraped, folded or sliced, we find the earthquake patterns characteristic of the Benioff zone, and we find the continents being crumbled and deformed to form folded mountain belts at the edges. These folded mountain belts will be the topic of the next lesson.
At depth we find that partial melting occurs. Sometimes the process of melting creates granitic magmas, which rise and emplace themselves as batholiths. Sometimes the magma rises to form andesitic lava at volcanic arcs. It's interesting to note that if you look at the composition of volcanoes in the Japanese islands, as you go inward from the trench; in other words, toward the Asian Continent, the volcanoes actually change composition. This composition changes in such way that confirms the fact that the melting of the crustal material takes place at deeper and deeper depths as you go toward the continental side of the Japanese islands. For this reason, the trenches or the subduction zones, or converging boundaries, or active margins, are often called the "andesite line," and here at the andesite line it marks a clear demarcation between continental type volcanoes or andesitic volcanoes, and oceanic-type volcanoes or basaltic volcanoes. And again, we'll spend quite a bit more time on the volcanoes and associated features in a later lesson. The best example of these active type margins are on the Pacific Rim; in fact, the entire region around the Pacific Ocean is sometimes referred to as the "Pacific Ring of Fire," relating to the fact that these andesite volcanoes are very common here.
The driving mechanism for plate tectonics is not really well understood, but there are only a few different possibilities. One possibility is that the spreading plates are actually forced apart by the upwelling magma; in other words, the upwelling magma forces its way into cracks, and the pressure simply slides the plates apart as new oceanic crust is created.
Another possibility, equally likely, is that the cold lithosphere sinking into the mantle at the subduction zone pulls the plates along, very much the same thing that would happen if you put a towel on the edge of a desk, and the weight of the towel drags the whole thing off the desk. It's also possible that the entire lithospheric plate is dragged along the surface by horizontal convection currents.
Another possibility is that the plates are simply sliding downhill on the asthenosphere. Remember that the crust is elevated near the spreading center. The rock is much hotter there. Hot rock expands so the crust tends to be at a higher elevation.
As noted in the video, the actual mechanism is probably some combination of all of these things, but the rates operate at different rates in different regions, and it's very difficult at this point to predict or to even measure exactly which one of these different processes is responsible, but it is probably some combination. As far as the motion of the plates go, we can understand the changes--Not very well, but we can describe them pretty well. In fact, we can trace the motions of the continents, and we can also use linear island chains like the Hawaiian Islands to help us track the motion of the plates over millions of years. In general, the spreading rates are about one to ten centimeters per year, which doesn't sound like much, but it's enough to have opened the Atlantic Ocean in 200 million years, a distance of 2,000 miles or so. The rates actually vary in magnitude and time, and the models that we see like the ones in the video show a nice smooth progression, but in reality the motions are not smooth, and nor are they continuous at a given location.
Keep in mind that this process is at least partially produced by magma wedging its way into cracks. We also note that steeper ridge systems like the Mid-Atlantic Ridge actually have slower rates; in other words, the slower the rate, the more time material has to accumulate near the ridge and so the thicker the crust becomes.
In the mid-Atlantic the spreading rates are maybe one and a quarter centimeters per year, typically in the East Pacific, maybe five centimeters per year, and the fastest rates on the Pacific Coast of South America, maybe nearly 20 centimeters per year. One of the best lines of evidence both in support of plate tectonics and also to help us understand plate movements over time are hot spots and aseismic ridges.
Now, hot spots are scattered around Earth. They occur fairly deep in the mantle at depths of maybe 100 to 200 kilometers probably associated with the asthenosphere. The hot spots are centers of isolated volcanic activity, which are not associated with plate boundaries.
Now keep in mind most of the geologic processes, earthquakes and volcanoes, are associated with the boundaries of the plates. The hot spots are mid-plate features. The existence of hot spots is speculative, but the evidence as we'll see later is quite overwhelming for this as well. It seems that the hot spots are fixed in place in the mantle by descending plumes of dense material. What I just said will make more sense, I think, after we've studied igneous rocks, so we get a sense of how igneous rocks change composition. But for now we can just say that when rocks melt, they form different materials; in other words, when a rock melts, some of the the material that forms is light in density, and some of it is heavy in density.
The lighter material rises to the surface to become basaltic volcanoes. The heavier material sinks deep into the mantle to be reincorporated with the deep mantle, so you can think of a hot spot as a region of melting with an ascending portion, which becomes volcanoes, and a descending portion, then, which anchors the hot spot in place.
There are approximately 40 active hot spots at this time on the Earth's surface, and about half of them occur in ocean basins, but the other half occur under continents.
One of the important things that hot spots do is to resupply the asthenosphere; in other words, they bring magma up to asthenosphere, which aids the lubrication of the lithospheric plates. We might also note that the rising material from the hot spots helps to bring heat up from the mantle; in other words, it helps this process of heat transfer from the center of the Earth.
It's also interesting to note that hot spots from one place to another are slightly different composition. The hot spot that's currently active under the Island of Hawaii here in the Hawaiian Islands is slightly different in composition from the hot spot that's active under the island of Iceland. It also seems that hot spots have limited life times.
In some cases, they may fade away and reform again in the same place, but a typical life time seems to be about a hundred million years. Now, a hundred millions years, remember, is a short amount of time in geologic time, but it's a fairly long amount of time in human terms, and, as we'll see, the entire Hawaiian Island chain has been built in only a few million years.
The best evidence for hot spots comes from aseismic ridges. You may remember from our study of the ocean floor that aseismic ridges are linear chains of volcanic islands, sea mounts, atolls, and Guyots, which have little or no earthquake activity associated with them. They're called aseismic because of this lack of earthquake, and also keep in mind that being in the middle of the plate there are no plate boundaries to interact, and that's generally what causes the earthquakes. The interesting thing about the hot spots is that they form linear ridges as the lithosphere passes over them. Today we accept this idea of hot spots and linear volcanic islands, but when it was first proposed, not very many people took it seriously.
The theory was first proposed in 1963 by a geologist named J. Tuzo Wilson; 1963, by the way, was about the same time that Vine and Matthews were discovering magnetic stripes and about the same time that Hess was coming up with the Theory of Sea Floor Spreading. Because of the nature of the understanding of sea floor processes at that time, some people were jokingly, but affectionately, referring to Wilson's Theory as "Wilson's Donut Machine." You get the picture?
There's a hot spot, and an island forms above it, moves away from the ridge, another island pops up, moves away from the ridge, so you get a whole series of islands being formed like a conveyor belt. One island forms, moves away from the hot spot, another one pops up and forms, and moves away from the hot spot. The Hawaiian Islands are a very good example of this sort of linear island chain.
There are lots of lines of evidence in the Hawaiian Island, and we'll come back when we study volcanoes to talk a little bit more about the details of the Hawaiian Island volcanoes, but one of the things we notice first of all in examining the Hawaiian Islands is that the ages of the islands tend to increase in a very regular way starting with the Island of Hawaii on the southeast portion of the islands as we progress through the island chain, the islands become progressively older. Even a surface observation can tell us that the islands are older.
The Island of Kauai, for example, is much more eroded, has many beaches and a fairly extensive reef development. As we move down the island chain to Oahu, to Molokai, to Maui, and to the Big Island of Hawaii, we find that the islands become less eroded, they have fewer and fewer beaches, and have virtually no reef development.
If you visited the Big Island you may notice that there are virtually no reefs surrounding the island, and the beaches, if they exist at all, are generally black sand beaches rather than white sand beaches. The black sand comes from the fragmenting of volcanic material, which is then piled on to the beaches. Along with this line of evidence, we note the fact that off shore of the Island of Hawaii is a sea mount called "Loihi," which is currently extremely active. The Island of Hawaii contains Hawaii's only active volcanoes on the volcanic mountains called "Mauna Loa" and "Kilauea," so it seems that at this point the hot spot is situated somewhere around that area.
The three volcanoes, Mauna Loa, Kilauea, and Loihi form a triangle that's 50 or so miles on a side.
If we extend our study a little bit further past the actual Hawaiian Islands, we find that the so-called leeward islands of the Hawaiian chain look much more like the later stages of atoll development; in fact, many of these islands are not really islands at all but are simply reefs, or shoals, or pinnacles, where only single fragments of rocks protrude up from the coral reef. Tracing the Hawaiian Island chain all the way to Midway Island, we find that extending then northwest from the island is a chain of sea mounts called the Emperors Sea Mounts. If we follow these sea mounts, we find that they extend all the way to the Aleutian Islands, another 3,000 miles; in other words, the Hawaiian Island chain is not just the eight major islands.
We have eight major islands with a string, if you like, of Wilson's donuts with a bend at the Emperors Sea Mounts. The bend, as it turns out, has been age dated at about 40 million years. It's thought to represent a time when the plate direction changed. The Emperors Sea Mounts head much more northerly whereas the Hawaiian Island chain tends northwest.
There are many other examples, some of these in the Atlantic and some in the Indian Ocean. If we compare, by the way, the aseismic ridges or the linear island chains with island arcs, we find some significant difference. For one thing, island arc volcanoes are all pretty much the same age.
The volcanoes of Japan, for example, are all active. Well, not all active, but there are 40 or so active volcanoes that make up the islands, and they're all roughly the same age; whereas, the volcanoes of the linear island chains do show this age relationship, as I mentioned about Hawaii. Also, the island arc volcanoes are andesitic in composition; whereas, the aseismic ridges or the linear island chains are basaltic in composition, and the volcanic island arcs, such as Japan, generally lie perpendicular to the direction of plate motion; whereas, the island chains lie in the same direction as plate motion.
I should also note that hot spot activity intensifies if it happens to lie under a spreading center. The best example of this is the Island of Iceland, where it seems that there's a hot spot, but, also, a spreading center, which we know is the Mid- Atlantic ridge. This process of plate tectonics also can explain the sinking of islands during atoll formation.
You may remember we discussed Darwin's Theory of atoll formation, and I pointed out at the time that Darwin really didn't have any explanation for why the islands sink. He also wasn't aware of the age relationships of the Hawaiian Islands.
It seems that if islands form near the spreading center, their in shallower water because of the bulging of the spreading center due to the heat, and as they move away from the island, they basically move downhill, so you can think of this as a downhill conveyor belt where the islands are simply rafted along with the lithosphere into successively deeper and deeper water.
In our next program, we'll move our focus to the continents, and there we'll start by studying the process of mountain building, to see how this process relates to plate tectonics, and at the same time we'll set the stage for understanding the geology of the continents, so I'll remind you of the
I'll see you next time.