GEOLOGY/GEOPHYSICS 101GEOLOGY/GEOPHYSICS 101
Program 9 EARTH'S STRUCTURES 
The best direct evidence we have of tectonic forces is in the deformation of rocks.
Even before plate tectonics,the way in which rocks respond to stress was well understood. These results are observed in the folds and faults of mountain belts, such as we discussed in the last lesson.
Folds come in all sizes,their wave life structures that may be hundreds of miles long down to microscopic in size, and all are formed in essentially the same way by compressional forces.
Geologists study rock structures for several reasons. Number l, they help us to understand the geologic history of a region, and they also help us to understand the history of tectonic movements and the plate movements. The analysis of folds, faults, and joints are all in a branch of geology called "structural geology," and it's from these studies that we learned about mountain building forces and were able to interpret them in the light of plate tectonics.
The folds and faults and other geologic structures also help us to make geologic maps, which we use to infer underground structures where we can't see the rocks and to help us to understand the formation of geologic resources to locate and manage them.
Like other geologic structures, folds and faults can be classified, and we learn from the classification that similar structures are formed by similar processes. By classifying the geologic structures and understanding how they were formed, we can learn to interpret the story left behind in the rock. It's actually the geometry, that is the shape, and size, and the orientation of folds, and faults, and joints that holds the key to this interpretation. It's not only the geologic structures themselves, but it's also erosional features, gaps in the geologic record, that also give us information about the geologic history of a particular area.
When erosion was occurring over a particular region, for example, the material that was removed from that region was put some place else, so by studying the sediments in one place and the missing record in another place, we could make connections between the two regions.
Our module of the lesson assignment,
There are several objectives to this lesson, and I'll review these with you. Basically, this lesson will help you to
Forces and their effects are studied by many different types of scientists: Engineers and material scientists try to create forces of different types in the laboratory and see how material responds to those forces. Geologists apply the results of those laboratory studies to Earth materials, and the fact that geologists often go into the laboratory and take Earth materials and do the same things to them that engineers do to structural materials. We learn from this that each material responds a little bit differently, but there are some characteristics in common that we can use to classify the various types of forces and the responses of materials.
We've all heard the term "solid as a rock." Well, unfortunately, the term can't really be taken literally. The fracturing and folding is apparent in older rocks, and even young rocks like we find here in the Hawaiian Islands show some evidence of deformation. The structural geology, which deals with the stresses and strains in rocks is one of the oldest branches of geology and can be used by geologists in the field to make maps and to analyze the maps by various techniques to understand where these forces came from and the nature of the forces, and the strength of the forces.
We reconstruct geologic events, in other words, by looking at the nature of the deformation of the rocks. Geologists use the terms "stress" and "strain" to talk about the concept of force and the relationship of the cause of force and what happens as the result of the force.
"Stress" basically is the amount of force per unit area; whereas, "strain" is the amount of deformation of a material. So on one hand we have force being the cause of something, and, on the other hand, strain being the result of that cause.
Most materials behave in several different ways to the different types of forces. There are basically three types of forces that we have to concern ourselves with.
There are basically three types of deformation as a response to these stresses, or three types of strain, we might say, that we want to concern ourselves with. These are "elastic strain," "plastic strain," and "brittle strain" or "brittle fracture."
I'll give you a little bit of a background on these different types of strains before we see the video. Okay, the first type I mentioned is "elastic deformation." "Elastic deformation" is a recoverable deformation. "Elastic," well, everybody knows what "elastic" is. "Elastic" is a material like a rubber band that stretches, but when you let go of it, it snaps back into place. The resistance to elastic deformation is called "rigidity," so material that's rigid like a rock is very highly elastic, where a material like a rubber band that's not very rigid has low elasticity. We might also note here that the recovery from elastic stress may not happen right away. If I take a piece of styrofoam and squeeze it, after I release it, it may take a few minutes for it to spring back into place, and many rocks behave this way elastically. Elastic deformation is not particularly important in understanding folds and faults or geologic structures in rocks, but it is important because when earthquake waves pass through rocks, the rocks are deformed elastically, and you may remember an earlier animation from one of the early lessons that showed the motions of the rock as earthquake waves passed through it, and we'll see more of this in the next lesson when we study earthquakes.
Plastic deformation, on the other hand, is a permanent strain. Plastic deformation is--"Well, the word "plastic" comes about in the first place because plastic materials like those used in plastic bags can be molded; that is, they can be injected into a mold, and they fill up the mold. The word "plastic" actually means a nonrecoverable stress that if you deform something plastically and release the forces, then it stays deformed. This is equivalent to flowing like a liquid. The plastic material differs from liquid in one very important way, and that is the plastic material has what we call "yield strength." That means that most materials require a certain amount of stress to be applied to them before they begin to behave plastically.
A third type of deformation is "brittle deformation." This results in fracturing and faulting or breaking. This is one of the things that can happen when the yield strength is exceeded by stresses. Now, it's important to note here that most materials behave elastically at least over a small range of stresses. Once a material has reached the elastic limit, it may behave either plastically, or it may behave brittlely. Different materials behave differently over a different range of conditions.
One of the important differences between the way materials respond to stresses is the amount of time over which the stress is applied; in fact, many materials like rocks and other Earth materials have what we call "duality of response." That means that they may show elastic or brittle behavior over short time periods, but they may show plastic deformation or plastic strain over long time periods, and many metamorphic and sedimentary rocks testify to this plastic deformation of rocks.
Movements in the mantle, such as the rising convection currents, are also a form of plastic deformation. The mantle is actually solid; it's not a liquid, but it behaves like a very viscous fluid over time. Isostatic adjustments are also examples of plastic deformation. When the weight of a glacier depresses the continent crust, the mantle material below must flow out of the way, and it does so plastically.
I have a model today that I can use to illustrate the relationship between these different kinds of stress. What I've got here is a piece of silly putty. You can buy this silly putty at your favorite toy store. I want to roll this silly putty into a ball, so I can illustrate these various types of response. Okay, we'll roll this into a ball. It's not a very good ball, but it's sort of ball shaped, and the silly putty, first of all, in order to get it into this ball shape, I had to deform it plastically; that is, it started out as a flat piece, and I deformed it plastically into a ball, but now if I take the ball and bounce it on the board in front of me, you'll see that it actually behaves elastically.
In fact, I can bounce it--Not a very good ball, as I said. I can bounce it many times, and every time I bounce it, it behaves elastically, just like a rubber ball. What's really going on here, of course, is that when the ball hits the board, it deforms elastically over a very short period of time. It's not in contact with the board for very long though it behaves elastically and rebounds back to its original shape, but I can also take this same ball and apply stress to it slowly. As I apply the stress slowly, you see that it deforms plastically, and, in fact, it maintains that it does not rebound. I can also draw it out like a rubber band. I can stretch it. You can see that it is a plastic deformation; in fact, if I just let it stand under its own weight, it begins to stretch like a piece of taffy, so when the stress is applied over a longer time period, this particular material behaves plastically.
Now, what happens if I apply the stress over a very short time period, even shorter time period than the bouncing of the ball. I'm gong to take this and stretch it out a little bit. I'm going to snap it very quickly and let's see what happens. I'm going to apply and stretch very quickly.
Well, let's try that again. Sometimes this works and sometimes it doesn't. I'll stretch this out. I'm going to snap it very quickly. There it goes, and you see that it breaks just like the rubber band breaks, and you notice the cuts on the end here are clean, just as if they had been cut by a pair of scissors. So what's the difference in this material between plastic, and elastic, and brittle structure?
In this case, like Earth materials it has to do with the rate at which the stresses are applied. I want to mold this back into a ball again, applying the stresses slowly enough that it deforms plastically, and I'll roll it back into a ball and set it here on the board for a minute and see what happens to it. We'll come back and look at this in a minute or two. There, it's back into the ball shape. I'll put it back here on the board, and we can see what happens to it. Okay, let's just leave it there for a couple minutes, and we'll come back to it in a minute or two.
Most rocks possess different kinds of strengths; that is, they behave differently depending on whether the stress is compression, tension, or shear. Generally, rocks are less strong in tension and compression. In other words, rocks withstand stress better if they're squeezed than if they're pulled apart, and most rocks are much weaker and shear than they are in either one of the other two. The response of a particular rock very often depends upon its temperature in the depth of burial; in fact, one of the reasons why I had trouble breaking the silly putty or causing it to behave brittlely was the fact that it's kind of warm in here, and it behaves more plastic over a wider range of stresses, so as rocks are buried deeper and deeper in the Earth's crust or into the mantle, they heat up, and as they heat up they tend to have a lower elastic limit, which means that they begin to flow plastically.
The composition of rocks also affects their response to stress. In the video, it will show you how marble is deformed, and marble is used very often by geologists in the laboratory because it's especially plastic; in other words, it has, for rocks, a very low elastic yield, and it makes a good model for the varieties of response, which rocks undergo over both long and short time intervals.
Let's take a look at this piece of silly putty again. It's only been sitting there for a minute or so, but if I pry it off the board, even when it's sitting on the board, you notice it's kind of flattened on the top. If I pry it off the board, it's sticking a little bit, notice that it's already flat on the bottom. What caused this to deform in this case?
Well, the answer, of course, is it's the weight of the putty. The weight of the putty has the same effect as if I was squishing it down on the top like this, and, in fact, if I was to let it sit for a couple of hours or overnight, when we came back and looked at it, it would be a puddle very much like water. So when you watch the video, try to imagine Earth materials behaving like this piece of silly putty, and the difference, of course, is that rocks although they behave like silly putty do it over a much longer time period. This again goes along with the difficulty we have in visualizing geologic time. Earth materials respond this way over a much longer time scale, so with all this in mind, let's watch the video.
Major Funding for "Earth Review" was provided by the Annenberg CPB Project.
Dramatic landscapes like mountain ranges, mid-ocean ridges, and the Grand Canyon are created by extraordinary forces in the Earth's crust. These forces generally arise from the movement of tectonic plates, and they not only shape the landscape, they also permanently deform the rocks of the crust.
The rocks either break or under certain circumstances actually flow like a very thick liquid. When rocks are deformed in this way, geologic structures, such as faults and folds are produced. Rock deformation is often accompanied by vertical motions of the Earth's crust, causing it either to rise or subside. Understanding rock deformation and geologic structures is fundamental to the Science of Geology. These structures are evidence of important events in Earth history.
Because they are often responsible for concentrating deposits of important resources, including petroleum, metals, and ground water, they can be of immense economic value.
Geologic structures are patterns in the arrangement of rock inside the Earth. Among the most common patterns is parallel layering seen in sedimentary strata and some volcanic deposits.
One of the key insights leading to the birth of Geology as a modern science concerned the nature of this layering. In the early Seventeenth Century, Nicholas Steno, a Danish military engineer living in Italy, published an important observation. He noted that in most places at the bottom of water bodies sediment settles to form continuous flat lying layers. This explains why young sedimentary strata tend to be horizontal with the youngest layer on top and the oldest at the bottom. Steno's observation became known as the "Principle of Original Horizontality."
Geologists find this principle useful as a basis for measuring how much deformation has occurred in ancient strata. If layers are folded, geologists assume that they layers were once nearly horizontal, and that the folding came later.
To accurately measure and record deformation, geologists use a small instrument, which combines a compass with a hand level. This instrument called a "pocket alidade" measures two aspects of the orientation of any tilted layer, the "strike" and the "dip."
One of the things we always want to do with structures is to measure their orientation in the field, and the way we usually do that is to measure two angles.
Those angles are called the "strike" and the "dip" of a surface. I'm going to take the upper block and just remove it and get rid of it. Then, we can look at the surface of the fault itself right here, which is a planer feature, and our two angles again are the "strike."
The strike is measured from a horizontal line lying within that plane to true north, so it's an angle between that line and whatever direction north is and affix the orientation of the plane in this direction.
The other angle that we need to measure is what's called the "dip," and that's just the angle between a line perpendicular to the strike line and the horizontal plane.
Okay, so it's an angle from the horizontal down to the plane itself, and it fixes the orientation of the plane in this direction.
Okay, so the strike from true north and dip from horizontal. Geologic maps require more than strike and dip symbols to indicate deformation. They also include faults and the contacts between different layers and bodies of rock. When colored in, such maps provide powerful insights into the overall geological structure of an area. To locate the geologic structures in an area like this, the geologist first looks for patterns in the distribution of rocks at the Earth's surface because soil and vegetation usually conceal most of the rock we need to see. This type of analysis can't be done from airplanes or satellite. Instead, geologists spend a great deal of time studying the rocks on the ground. Information about each individual rock exposure or outcrop is recorded and then plodded on a base map. Constructed outcrop by outcrop, this information eventually becomes a geologic map. The different colors show how rocks of different types and ages are distributed throughout the area. These rocks are described here in the map explanation. Faults are shown as dark lines, and special symbols indicate where rocks are tilted and folded.
Here at the Grand Canyon, the Colorado River has cut down through the rocks showing us what's underground, but exposures like these are very rare. In most places, surface information from the geologic map is used to infer the underground distribution of rocks and structures.
This is done with the geologic cross section, where the geologist has conceptually sliced the Earth open to reveal the structure of its interior. Many different cross sections can be drawn to fit the same pattern of surface exposure. The geologists recognize that the simplest one usually turns out to be the most accurate, and, in most cases, evidence from drilling or seismic sounding validates this assumption.
The process of choosing the simplest explanation from a group of possibilities is not unique to geology. This approach is used throughout the sciences for solving different problems. But drawing cross sections involves more than just applying this technique. A knowledge of common types of geological structures is also essential.
Geologists recognize three main classes of structure caused by deformation in Earth's crust: unconformities, faults and fractures, and folds.
We usually think of rocks as being very hard and brittle. They break when a force, such as the blow of a hammer, exceeds the strength of the rock itself, but if rocks are very hot or under great pressure, or if they're exposed to stress very gradually over a long period of time, a rather surprising type of deformation takes place.
The rocks can actually bend or flow forming a geologic structure called a "fold." Folded strata can assume many different shapes ranging in size from a few centimeters to several kilometers across. Among the many complex patterns of folding, however, geologists recognize several basic forms. These include: "synclines" with down folded layering and "anticlines" having upfolded layering.
The line of greatest curvature along any layer in a fold is called the "fold hinge." Linked together, the many different hinge lines of a fold make up the fold's hinge plane. In an outcrop, the position of the hinge plane can be seen at a glance. The orientation of the hinges and hinge plane of a fold and the amount of folding itself serve as a basis for further classifying the fold.
We can classify folds based on their orientations, whether certain parts of the fold tend to be upright, or we can turn the fold over, or turn it on its side, those would all be different types of folds that give us different information.
We can also look at the geometries of the folds; for instance, we can look at how tight the layering is folding. Here is something where the layering is rather open. We get these curved hinges in here and very planer lems.
We could contrast this type of fold with something that looked like this where the folds are much tighter. You could probably guess from comparing these two rocks that this one has been deformed a lot more or shortened a lot more in this direction than the other fold that I showed you.
The same tectonic forces that fold rocks can also caused rocks to break. Rocks tend to fracture instead of fold when the force is applied rapidly. This is especially true in the shallow portions of the Earth's crust where rocks are relatively cold and under low pressure.
Where tectonic stress is applied constantly over a long period of time, the fractures are concentrated along a discrete zone called a "fault." Some faults remain active for millions of years resulting in hundreds or even thousands of kilometers of displacement, and fault movement generally occurs in a series of steps or jumps generating a series of earthquakes.
Like folds, faults are classified according to their physical orientation. This includes the dip of the fault plane and the direction of offset created by movement along the fault. For example, ruptures along which vertical motion has occurred, are called dips slip faults.
Ruptures along which horizontal motion has occurred are called "strike slip faults."
Many faults show some combination of both dip slip and strike slip motion. These are called "oblique slip" faults.
Geologists have found it useful to subdivide fault types even further. Strike slip faults, for example, are subdivided according to whether their direction of offset is to the left or the right.
In the case of steeply inclined dip slip faulting, there are two main types. We can have what we call "reverse faults" where the upper block above the fault's surface is moving up relative to the lower block like this.
The other possibility, obviously, is that we can take this upper block and move it down, and that is what we would call a normal fault, okay.
We can also if we take that third orientation where I rotate my fault surface to a horizontal direction, or often they're not perfectly horizontal, but the dip is very low, we can have again what we call "thrust faults" if my upper block is moving up with respect to the lower block, and if we move it in the other direction to the upper block is moving down the dip of my low angle surface, we can have low angle detachment faults.
Okay, in each case we are changing the shape of the rock in a different way depending on again the orientation of the fault surface and the direction of displacement.
The many categories of folds and faults were developed long before 3 geologists had a clear knowledge of how and why these structures formed. Even today, the origin of many geologic structures is not fully understood, but geologists do know that stress, which is as concentration of force, plays a role. Compressional and tensional stresses are we're really talking about the stresses around a point or the stresses operating on a block of rock. I might use my little foam model here in the case of compressional stresses, that's where we have stresses directed towards our low block of rock in every direction. Okay, so it's squeezing the rock in all three dimensions. Tensional stresses are the exact opposite. Instead of squeezing the rock together, what we're trying to do is just pull the rock apart so the stresses are directed outward away from the rock.
Geologic structures like folds and folds are examples of strain, a change in the shape of the rock caused by stress. Stress is the application of force on an area. If you lean against a wall, you're putting some stress on it. A strain is when the wall moves, so strain is the change in shape or volume. Plastic strain is where stress has been applied to an object, and it's deformed, and then it stays in that same shape. It doesn't resume its original shape. Elastic strain is where the object is deformed, and then when the stress is removed it returns to its original shape. Now, if you exceed its elastic limit, then the object will break and shatter. That's when we see folded rocks. They have been subjected to a plastic strain. Fracturing occurs when the strain exceeds the elastic limits of a material, and the rock breaks or fractures.
Different types of geologic structures result from different types of stress. For example, shear stress in which one mass of crust slips laterally past another causes very large strike slip faults to form, and most folds are formed by compressive stress. Where the stress occurs is also significant.
Now, when we talk about stresses and different types of structures, when we're looking at structures forming at depth below the surface of the Earth, just because of the overlying weight of the body of rock, we're almost always dealing with compressive stresses and so the types of structures we get there really are just a reflection of the different magnitudes of stresses, compressive stresses in different directions. However, when we get up near the surface of the Earth where we're dealing with a free surface with no overlying weight of rock, we can sometimes get tensional stresses, and these stresses will try to pull apart our rock in some direction.
Under those circumstances, we can get things like joints, or fractures, or little veins of different types of minerals that flowed into these fractures that do give us some clues about the directions and magnitude of tensional stresses, so in just by looking at different types of structures, or folds, or joints, or things like that, we can get some clues about what the stress field is like during the time that these structures formed.
"Unconformities," the third great class of geologic structure are not as useful as folds and faults in analyzing past crustal stress. Nonetheless, they, too have proven to be an important key to the past. Just as astronomers are preoccupied with the immensity of space, geologists are uniquely fascinated with time and Earth history. The first geologist to actually recognize the scale of geologic time was a Scottish intellectual named James Hutton. Over a century ago, Hutton recognized that a sequence of layered rocks is a physical record of some portion of Earth history. He also predicted that in places where mountain building has occurred some part of that record would be destroyed by erosion. Armed with this hypothesis, Hutton found places where old rocks had been covered by much younger sedimentary rocks. A contact between these two rock formations marked a period of Earth history with no rock record. This geologic structure is called an "unconformity."
Unconformities are formed when rock is first removed by erosion followed by burial of the erosion surface by younger sedimentary rocks. The horizontal contact separating the lower dark rocks from the overlying chocolate brown rocks is the great unconformity of the Grand Canyon. The lower rocks are over 1.5 billion years old. After they were deposited as sediments, they were deformed and tectonically uplifted during a mountain building episode. Erosion then carved away at these rocks until sea level finally rose and flooded the area about 500 million years ago. The younger rock layers covering the unconformity's surface are sandstones that were deposited at the bottom of that sea. The great unconformity represents about a billion years of geologic time. It reveals an important chapter in the geologic history of the Grand Canyon.
The classification of unconformities is less complex than that of folds and faults. Erosion rather than stress causes them to form. There's three kinds of unconformities, three major kinds of unconformities.
Geologic structures are also useful, not only for what they reveal about Earth's past, but because of their economic role as well. In this regard, folds are especially important. Understanding folds and the way they form is not only intriguing from a scientific point of view, it can also have enormous economic benefits as well. The compressive stress that folds rocks can also contribute to the formation of petroleum and to the structures that trap it.
For example, many folded regions are composed of alternating permeable and impermeable rock layers. Some of the the permeable strata contain water and oil, which actually flow through the rocks themselves. In fact, this flow is sometimes driven by the same compressive stress that folds the rock layers. Because petroleum is lighter than water, it tends to float to the highest point in the fold, and it will be trapped there if the overlying layer is impermeable. The intensely folded rocks of the western Appalachian Mountains produced the first commercial oil well in the United States in 1859. Since then, folded strata have been recognized as superb petroleum structures all over the world and have yielded tremendous quantities of oil and gas.
Typically, petroleum is found in certain sedimentary rocks forming from the decomposition of organic matter. Petroleum, of course, is generated from the dead or decayed remains of living organisms. Most of that comes from microorganisms. Okay, and as these organisms die and they're buried with sediments, they become part of the sedimentary section. Those organisms may the organic matter may also be from leafy materials, wood materials, that sort of thing. Anything that has carbon in it, carbon based materials; hence, the name "hydrocarbon," "liquid carbon." Lighter than both rock and water, petroleum drifts upward through the porous spaces in fractures in rock. Some finds its way all the way to the surface of the Earth where it dissipates into the oceans or collects in pools.
The remainder, however, becomes trapped by geologic structures within the Earth. One of the most effective structures for creating a petroleum reservoir is known as an "anticlinal" trap.
Here an impermeable layer of rock forms a cap over a layer of porous permeable sedimentary rock. If petroleum is present, it drifts upward and is caught within the fold of the anticline. Natural gas, which is the lightest form of petroleum, collects at the top. Next comes oil. Water, which is heavier than petroleum, forms a layer underneath. Other geologic structures can also create oil traps. A fault, for example, or an unconformity.
Locating geologic structures, which can trap migrating petroleum, is just one of the considerations taken into account by geologists looking for oil. We have to assess the basin for its source rock potential. Once we assess that there are source rocks appropriate for that, then we have to say has the timing been appropriate. Has that source rock set down there long enough and become buried deep enough such that the temperature has allowed the hydrocarbons to generate, so we have to look at source, migration, and the migration has to be timely. We have to look at appropriate traps, and we have to look for reservoir quality.
Author John McPhee once tried to reduce the study of geology to a single sentence. He wrote "The summit of Mount Everest is marine limestone." This statement summarizes centuries of human fascination about geologic structures, including mountain ranges, folded and contorted rocks, and great faults. These structures are both the product of tectonic plate movement and a record of Earth's dynamic history. An understanding of geologic structures is not only essential to interpreting Earth's past, it's often the solution to practical problems as well. For example, faults are the record of ancient earthquakes, and the study of these structures is fundamental to earthquake hazard analysis and quake prediction.
The tectonic activity that creates mountain ranges is also responsible for the formation of oil and gas fields, so an understanding of geologic structures is essential to the search for these fuels.
In addition, structural geology is vital to landslide analysis and in planning disposal sites for the waste products of human civilization from spent nuclear fuel to household garbage. The study of geologic structures is one important way that the Science of Geology links academic knowledge to the practical concerns of people and civilizations.
The structure of the Earth beneath our feet is vital, both to interpreting Earth's past and to planning our own future.
Music Major funding for Earth Review was provided by the Annenberg CPB Project.
That's an interesting video, but there are some terms that are used in structural geology, which are kind of difficult to understand. Structural geology is a three dimensional science, and books by their nature are two dimensional. Well, television is two dimensional, too, but I can at least use three dimensional models and, hopefully, it'll help you to understand some of these things that the video is talking about.
It'll help you in this material to be sure to study Figures 6.8 through 6.20 on pages 127 to 131 to see these features. Also, study the photograph at the beginning of the chapter on page 122 because it shows you a very nice example of folded rocks. Other photographs in the chapter show folds of various sizes in both real and model form.
One of the things that geologists use to describe tilted rock layers is the concept of "dip" and "strike." Now, be aware that when a geologist encounters a rock in the field, he doesn't see the entire rock. What he sees is an outcrop, and that outcrop may be tilted like the board here on front of me. On the board I've drawn the symbols for "strike" and "dip." The strike, this way, and the dip runs this way.
The "strike" is the direction of a level line on the surface of the rock or on the surface of the flat surface, the plane.
The "dip" is the direction that a ball would roll down the surface; in fact, I have a little ball here, and I can put this. There you can see that if I let go of it, it rolls right down the dip. Two points for that one, I guess.
The reason geologists use strike and dip is because planes' flat surfaces of dipping rock strata can be oriented in different directions. For example, I can rotate the board at different orientations both this way, and I can tilt it to different orientations this way, so geologists measure the strike in compass direction, usually registered as north and south, or east and west, or northwest and southeast, or various other orientations which refer to compass directions.
The dip is also measured by compass direction, but it's always perpendicular to the strike, so if the particular layer of rock strikes north and south, then the dip may be either east or west. The geologist measures the strike by using a compass in the field.
I have a geological compass here, and if I lay the compass along the strike of the rock, then in this particular case, the strike is east and west. I don't know if you can see the compass, but the compass is pointing almost east and west. If I rotate the board 90 degrees, now the strike is north and south, and I think you might be able to see on the compass there the north and south orientation, so for this particular orientation, the geologist will record in his field book that this particular layer of rock is striking north and south and is dipping, in this case, to the east.
The dip can also be measured by using the compass. I'm going to tilt this around the other way like this, and you'll notice that the dip can also vary from level to an angle, so in addition to reporting the direction of dip, the geologist also reports the angle of dip. The angle of dip is measured from the horizontal, so this would be level or a dip of zero degrees. This would be perpendicular or a dip of 90 degrees and anything in between, so if I were a geologist and I came across a layer of rock that was oriented the way this board is oriented now, the strike is east and west. In this case the dip is to the south, so I might record this as a rock layer striking north and south and dipping 20 degrees to the south.
Okay, so you might want to look at the diagrams in the textbook, which show you various examples of dip and strike. We will be using these throughout the course as a reference to describe the way rock layers are oriented.
Okay, the next thing we want to talk about here are folds. Folds obviously are common in folded mountain ranges; hence, the name, but the detailed structure of folds is extremely complicated. Often, these folds are folds within folds, within folds within folds, and much work is required in the field to determine the overall structure, and even more work is required to go back into the laboratory and use the orientation of the folds to figure out what forces might have caused these folds.
Folds generally are caused by compressive stresses, and, as we've seen since rocks behave plastically only over long periods of time, it requires a tremendous amount of time for the folds to form. It also requires burial to great depth in order for the rocks to become warm enough to behave plastically. There are several different types of folds that we can note.
Monoclines are simply single folds, which are sort of like a ramp between two level surfaces. The dipping part of the fold is called a "limb".
Anticlines and synclines are folds with multiple limbs. I have some examples here of models that I can use to illustrate the various types of folds. I have some layers of clay here that I bought from a local toy store, and on this model we can see various illustrations of the terms that are used to describe the orientations of folds, and as I mentioned before the video, it's the orientations of the folds that actually give us the information about the forces that created the folds.
Okay, you'll notice here that the folds are like a wave. The crest of the wave is called an "anticline"; the trough of the the wave is called a "syncline".
Associated with every fold is an axis. I've drawn the axis on these folds. The black lines here represent the axis, and they basically are lines, imaginary lines. The real folds don't have the lines drawn on them, but they're imaginary lines that run along the crest or along the trough of the wave, so here's the axis of an anticline. Here's the axis of a syncline.
Okay, we can also talk about the axial plane. The axial plane I can represent here with a piece of cardboard. The axial plane is an imaginary plane that would cut the fold in half. Well, the reason we talk about these terms, the axis and the axial plane, is because the folds can be oriented in many different ways. The terms that geologists use to describe these orientations are all related to the different ways in which the folds can be oriented in space. For example, the folds may be upright and more or less level; in other words, the axial planes may be more or less vertical, but the folds may be tilted to the side, so that the axial plane-- Let me put the card back up here for the axial plane--The axial plane now is tilted. If the fold is tilted far enough, it may become overturned. An overturned fold, the beds on both sides of the axis are dipping in the same direction. In extreme cases, the fold may become overturned.
Well, when a fold becomes overturned, obviously the terms "anticline" and "syncline" don't really mean very much, and usually folds that are recumbent this way are continued off along the length of the fold. Okay, folds can also be plunging or level. I've oriented this one more or less level, but the axis may also plunge. In other words, the axis itself may be oriented at an angle like this or like this. One of the reasons we're interested in plunging folds is because most folds in folded mountain ranges actually do plunge.
When folds in the real world are eroded, erosion often accompanies the folding and more or less keeps up with it. When a fold that's plunging is eroded, it leaves a characteristic "u" shaped, or "v" shaped, or "horseshoe" shaped feature. I'm going to take this fold and cut it with the knife. We can see what the pattern looks like. Let me cut this more or less horizontally through the fold, and we can get a sense of this horseshoe pattern. See the horseshoe pattern that's revealed, and you'll notice that in this particular pattern the oldest rocks, the blue rocks in this case, are in the center, and they get successively younger toward the outside.
I note here "old" and "young" because one of the primary things that we'll learn about understanding geologic layers is that the oldest layer is on the bottom, so here we see the oldest layer exposed in the center, and the younger layers exposed successively along the point of the "v". If we were to cut the syncline, we'd get a similar pattern, but the relationship between the old and the young rocks would be reversed.
Okay, I have another model here, which I can use to illustrate movement along a fault. This is just a block of wood that I can use to illustrate the fault and the relative movement along the fault. Geologists use the same terms to characterize the orientation of the fault plane that they use the characterize different rock layers.
The dip of the fault is down the slope of the fault plane; the strike of the fault is a horizontal line along the fault plane.
Geologists also use two different terms to describe the two blocks. The footwalls on the bottom; hanging walls on the top. These terms come from mine terminology because mine shafts were often build along the fault trace, and if you're standing in a mine shaft, the footwall's at your feet, and the hanging wall is where you hang your lamp.
The faults can have basically two different types of movement. One of these is called "dip slip" where the movement occurs vertically along the dip of the fault; the other is a "strike slip" where the movement occurs horizontally along the strike of the fault.
Dip slip faults come in two types, which refer to the movements of the two blocks relative to each other. In one case, there's a normal fault or a gravity fault where the hanging wall moves down relative to the footwall like this.
The other type is a reverse or thrust fault where the hanging wall moves up relative to the footwall like this. The gravity faults are called "gravity faults" because it's the direction of motion that the hanging wall would naturally slide under its own weight due to gravity. These are generally caused in geological circumstances by tensional forces, which tend to pull the blocks apart.
Reverse faults are generally caused by compressive forces, which tend to squeeze the blocks together and cause the upward movement. Another thing to note here is that when we talk about these movements, we're talking about relative movements because we can't always tell which block has actually done the moving. It may be that the footwall has been stationery and the hanging wall moves down, or it may be that the hanging wall is stationery and the foot wall moves down.
Either type of movement gives the same sort of displacement.
Okay, on the other hand, strike slip faults--I'll turn the model around like this-- Strike slip faults are a movement in the horizontal direction. Strike slip faults are usually caused by shear forces along transformed faults, and, once again, there are basically two types of movement. One type of movement is left lateral offset, where the trace of the fault on one side moves to the left relative to the other.
The other type of movement is a right lateral strike slip where the movement is to the right. The terms "right" and "left" are kind of confusing, but basically you can characterize this if you're standing on one side of the fault, and you look across at the stripe on the other side of the fault, you see the displacement here is to the right in this case. The other displacement is to the left. The San Andreas Fault, by the way, is a right lateral strike slip fault where the western part of California is moving northward, and the eastern part of California is moving southward.
Many faults, by the way, actually combine both of these types of movements; that is, they might have a dip slip component and also a strike slip component, which amounts to the same thing as a diagonal movement, and these are called, then, "dip slip faults." Well, I hope these models help. It's easy enough to make these at home for yourself if you need to.
I want to remind you that our next lesson is Lesson 9 on Earthquakes, and you should read Chapter 7, pages 147 to 172 and also review Chapter 2, pages 26 to 32, the section on earthquakes and the Earth's interior. This is kind of a long chapter, but it contains a lot of key terms and concepts that we've already studied before. We're kind of putting these things together, so don't forget to follow the study plan and the study guide and study hard.
If you haven't been studying hard, well, now's a good time to start, but if you have been, keep it up, and I guess I'll see you next time.