GEOLOGY/GEOPHYSICS 101 Program 4
EARTH'S INTERIOR 
They're all models that we can use in various ways to understand the Earth's interior.
Over here, the mass vibrating on a spring. This is a very useful physical model for understanding lots of processes that go on inside the earth.
For one thing, we can use this as a way of modeling wave behavior. The vibrating mass behaves very much in the same way as a piece of rock behaves as a wave passes through it. You can also use an instrument based upon this mass on the spring to measure gravity. Let me stop this for a second. Notice that when this is hanging without moving, the spring is stretched by a certain amount. If I reach over and pull down on the mass, I cause the spring to stretch further. A machine called a gravimeter can be built based on this principle to measure Earth's gravity or differences in Earth's gravity between one place and another.
If I have a machine like this, and, first the gravity is a little stronger in one place than another, then the stronger Earth gravity has the effect of pulling harder on the mass, which stretches the spring. So, really, measuring the length of stretch of the spring when the mass is standing still is a way of measuring the strength of gravity. The actual machine, the gravimeter, that measures Earth's gravity is much more sensitive than we can do with a simple machine like this.
You can also use this to model the operation of a seismograph. A seismograph is basically a machine that measures earthquake waves. It operates on a very simple principle that if we take a mass on a spring like this and connect the base very firmly to the ground, when the ground shakes, the mass begins to move. You can see that if I shake this up and down. The mass begins to wobble up and down. If we attach a recording device, such as a pen, to this mass, we can basically make a continuous trace of a picture of the wave motion of that particular earthquake as it passes the seismograph station.
Here I have two rocks. These rocks are also models. Though they're not model rocks, they're real rocks, but you might say these are model citizens of two major rock groups that make up the Earth's crust.
Granite, the lighter colored rock is also lighter in weight and makes up the continental crust.
The dark colored basalt is heavier in weight and makes up the oceanic crust.
Over on this side are all things that relate to magnets. In the center, this rather common looking rock is actually very important. This is a piece of lodestone. It's a type of iron oxide or iron ore, which has a natural magnetism. You can see that one pick up the paper clip, and, in fact, the lodestone is also attracted to a magnet. It's the existence of lodestone that helped people understand magnetism in the first place and allowed us to build artificial magnets but also to help us understand the Earth's magnetic field.
The Earth's magnetic field, of course, is measured with a compass, and you can see that the compass is influenced by the magnets. In today's program,
I want to remind you of the text assignment.
I'll go over the objectives with you very briefly. Before I do that, I want to note that this lesson contains quite a bit of information, and the video covers a portion of it. I'll cover a portion of it in the program, but some of of it has to be left to reading.
The objectives are:
Before we look at the video, let me introduce some material to you a little bit. What we're doing in these lessons in addition to studying the Earth's interior is setting up to understand the model of plate tectonics.
Plate tectonics turns out to be the most important model in geology, and helps us to explain a alot of things, which were unexplained until this model was put together by geologists in the last 25 or 30 years. We get much of our information about the model of plate tectonics from studying the Earth's interior and from studying the sea floor.
The sea floor is the topic of our next lesson. As you might expect, studying the Earth's interior is really difficult. If we could somehow slice the Earth in half like we could do with the small marble, it would be very easy to see what it's made of, but we really can't. We do drill wells and mines and so forth into the upper layers of the earth, but this is insignificant. In fact, the deepest wells that have yet been drilled only go about one-fourth of one percent of the way to the center of the Earth. In other words, we're literally only scratching the surface. We just simply do not have a good way to directly observe the interior of the Earth.
We must rely upon combinations of various kinds of methods in order to make these studies. We so have some direct evidence of the material of the Earth's interior.
I'll make a distinction here between direct evidence and indirect evidence. Direct evidence basically is pieces of the Earth's interior, which we can hold in our hands and examine. Indirect evidence, on the other hand, relies using various models likes the ones here on the desk and trying to understand how we can use what we know about those models to understand this interior.
One type of direct evidence that's very important is a type of rock called "Xenoliths." Xenoliths are foreign or alien rocks. The word "xeno" means stranger. Xenoliths are basically small pieces of small fragments of rock usually enclosed in volcanic rocks that are thought to be pieces of the Earth's mantle, which are brought up along with the lava during the volcanic eruption.
Another type of direct evidence is a type of rock structure called "Ophiolites". The video will show you some of these ophiolites and discuss them in some detail, but I'll note there that the ophiolites are thought to be pieces of the mantle connected to pieces of the crust, which have been uplifted and embedded by tectonic processes in the crust of the Earth. They have a particular structure, which involves a transition from one rock type to another, and, as I mentioned, the video will help us understand this.
I'd rather concentrate on the indirect evidence. One of the things we learned in the last lesson is about the Scientific Method and the making of models. Since we can't access the interior of the Earth directly, we have to be kind of clever in figuring out what's down there, and these various combinations of methods involve all of the things that are on the desk top here along with some other information.
One way we can get a sense of what the interior of the Earth is made of is simply to look at the density. Now, density is a measure of the heaviness of material. It's a little bit more than that. It's a measurement of the amount of material in a particular place. We define this officially as mass divided by volume. It's fairly easy to go into the laboratory and measure the density of something. I could take this rock into the laboratory and within a few minutes have made a fairly accurate density determination, and it turns out that an average piece of granite weighs about 2.8 times as much as water.
A piece of basalt, on the other hand, is a little more dense than granite and weighs about three and a half times as much as water for the same size piece.
If we were to go along the surface of the Earth and collect rocks, just pick up pieces of rock that are lying around the surface, and measure their density, and find an average density, we'd find that the average density is about 2.8 times water, about the same as granite.
If we could only somehow find out the density of the entire Earth, we could compare the density of the surface rocks with the density of the whole Earth to find out, maybe the interior of the Earth is made of the same materials.
As it turns out, we can't measure the density of the Earth directly. We can't take the Earth into the laboratory and use the same techniques we used for the rocks. What we can do is to calculate the density of the Earth by knowing its mass and its volume. It sounds simple on the surface, but in order to calculate the density knowing mass and volume, we first have to know what the mass is, and then we have to know what the volume is. So how do we measure the mass of the Earth?
I guess we could take it home and put it on the bathroom scale, but that seems to be kind of a lot of trouble and not very feasible anyway. It turns out that in his study of gravitation, Isaac Newton back in the 1600s came up with the law of gravity, which allows us to calculate the mass of the Earth. It turns out the same law of gravitation, which allows us to understand the motions of the planets around the sun, can also be used to measure the masses of the planets, so basically we can calculate the mass of the Earth by studying its motion around the sun and also by studying the motion of the moon around the Earth.
When we do these calculations, we find that the mass of the Earth is a very large number, almost an incomprehensibly large number. In fact, it's about six times ten to the twenty-fourth kilograms. That's the number "6" with 24 zeros after it, kilograms, an incomprehensibly large number, but it's still one we can work with.
To find the volume of the Earth is much simpler. We might remember from mathematics that you can calculate the volume of a sphere if you simply know the radius us the sphere. The radius of the Earth is fairly well known. It's about 6,400 kilometers. So if we do the calculations, the formula is four thirds "pi" r cubed. We're not actually going to do the calculations here, but were we to do those calculations, we could then find the volume of the Earth.
So we take the mass of the Earth divided by the volume of the Earth, we have a number, which represents the density of the Earth. Okay, then, so what is the density of the Earth.
Well, it turns out that the results of these calculations show us that the density of the whole Earth is about five and a half times that of water. Compare that with 2.8 times water for the granite and about 3 times that of water for the basalt.
The point of all these numbers is not for you to memorize the numbers but to understand that whatever the Earth is made out of, it's made out of something more dense than those rocks that we find at the surface, so if we're going to have a model, which tells us what the interior of the Earth is made of, we know that that model must be something other than the granite or basalt that we find at the surface.
So where does that leave us? How can we find out the composition of the inner part of the Earth? It would be very convenient if we had an Xray machine. We could put the Earth on the table like you do in the doctor's office and turn on the Xray and put the Xray up on the wall, and look at the picture to see what it's made out of. Unfortunately, Xrays don't work very well for penetrating the Earth, but there are natural waves called seismic waves, which result from earthquakes, which can be used to study the interior of the Earth.
Basically, as we'll see in the video, the speed at which waves travel through the Earth depends upon the type of wave and also the type of rock that it's traveling through, so rocks that are more dense, for example, allow waves to travel faster through them than rocks that are less dense. Not only that, but earthquake waves are reflected from boundaries. If there's a change in material or a change in density, earthquake waves are reflected from the surface, and we can use that to find the depth through that boundary.
It's in this way that we find the Earth is layered. Even knowing that the Earth is layered, it still doesn't tell us what the composition is. We have to be even more clever to figure out the composition. One way is to go back to the direct evidence. We can look at the composition of xenoliths. We can look at the composition of the lower parts of the ophiolites to see how those things match up with the seismic properties that we observe as earthquakes happen, but there's yet another way, and, this,
I think, is one of the more clever ways that people are going to have to model things. As you remember from the last lesson, we assumed that the Earth was created at the same time as the rest of the Solar System by the same processes and from the same materials. If we could find pieces of the rest of the Solar System and find out what it's made out of, we might assume that the Earth is made out of the same things.
What I'm getting at here is that meteorites provide us with another piece of evidence. Meteorites are thought to be remnants of the formation of the Solar System, which, for one reason or another, did not collect into planets.
It turns out there are two main types of meteorites. The two main types, one of which is almost entirely metal. In fact, it's almost entirely iron and nickel. Another type of meteorite called "Stony" meteorite is very similar in composition to the material that we find in the xenoliths and at the bottom of the ophiolites.
In other words, we can take the material from meteorites, compare them chemically with the direct evidence that we have from the mantle, and at the same time, we can measure their seismic properties. When you put all this together, what we determine is that the inner part of the Earth core is probably made of iron and nickel.
In other words, it has seismic properties that are very similar to the iron nickel meteorites. On the other hand, the Earth's mantle is probably made out of the material we call peridotile which is the same material that's found in the ophiolites and in the xenoliths and in the stony meteorites.
Another way that we can study the Earth indirectly is by looking at the Earth's magnetism. This topic's covered well in the video, and I will come back and give you some demonstrations of magnetism after we see the video. We can also use the Earth's gravity; both variations in the Earth's gravity and gravity anomalies.
The word "anomaly" simply means a deviation from what's expected or what's normal, and, again, I'll come back after the video and we'll talk a little bit about variations in the Earth's gravity.
The last indirect method is by studying heat flow. The Earth is warm inside, and so there's always heat being transferred from the interior of the Earth to the exterior. By measuring the Earth's heat flow, we can get an indication of the types of rocks, how much heat is concentrated at various places, and so on. I won't be covering that in much detail in the program, so I encourage you to especially read this section in the text about the geothermal gradient and the heat flow, and this is found on page 39 to 42 in the textbook.
Okay, so with all this in mind, let's watch the video.
Major funding for Earth Revealed was provided by the Annenberg CPB Project.
Each of these oil wells is producing about 100 barrels of oil every day. Oil from wells like these has fueled the engines and economies of the industrialized world for over a century, but for geologists oil wells serve an equally important function. They act as vital windows to the Earth's interior. There are limits, however, to how far these windows allow us to see. These wells are about a kilometer and a half deep, and the deepest oil and gas wells penetrate the Earth's crust to about 8 kilometers. The world's deepest well is now being drilled in the Soviet Union.
Designed a scientific laboratory, it has a planned depth of 15 kilometers, which seems like a long way down until we consider that this is only one quarter of one percent of the distance to the center of the Earth. Also, drilling deep wells involves a technological complexity that rivals space exploration. The deep Soviet well, for example, penetrates rocks at pressures thousand times that of the Earth's surface and at temperatures of 300 degrees centigrade.
Although progress is being made, there are tremendous technical obstacles that must be overcome before wells like this can be extended and offer geologists a direct view of the Earth's interior.
Fortunately, there's a great deal that we can learn about the Earth's interior without having to see it or sample it directly. This branch of Geology, called Geophysics, uses indirect techniques to deduce the composition and behavior of the interior of the Earth.
These methods include studying seismic vibrations from earthquakes, analyzing variations in the Earth's temperature, magnetic field and gravity, and using laboratory and computer models to simulate conditions deep within the earth, and although these don't tell us everything we want to know about the Earth, the indirect approach of geophysical research has painted for us a remarkably detailed picture of the interior of our planet.
At 5:04 p.m. on October 17, 1989, just 20 minutes before game three of the World Series, the ground beneath the San Francisco Bay area was convulsed by a severe earthquake. The death toll was 63. The damage, an estimated $6 billion, but although they can be catastrophic, big quakes provide valuable information as their shockwaves radiate through the Earth.
Seismic waves are like sound waves in air. They travel through the body of the Earth from a source of the waves to all points within the body of the Earth. It's very similar to my talking and the waves then moving from my mouth to all points in the room. Like sound waves in the air, seismic waves move in three dimensions forming spherical wave fronts as they pass through our planet. Where the seismic waves encounter rock layers of differing densities, the waves reflect off the boundaries between the layers in much the same way light reflects off a mirror. These waves are recorded on seismographs during earthquakes allowing geophysicists to see how long they take to travel down to the boundary, reflects off it, and return to the surface.
From the amount of time needed for the round trip, scientists can calculate the depth of the boundary, and thus learn more about the deep structure of the Earth. So the principal information we get then are distances of objects from the surface of the Earth, and this, in effect, defines the geometry of an object because as we illuminate or shine our seismic waves on different parts of the body, those different parts might be at different distances from the surface of the Earth, and, therefore, were outlining the shape of the body.
But not all seismic energy will bounce off the boundary between rock layers. Much of it will pass through the boundary. Since the rock layers differ in density, the velocity and direction of the waves will change as they cross the boundary causing them, once again, to behave like light waves and to bend or refract as they move from one rock layer to the next.
By studying the travel times of both reflected and refracted seismic waves following great earthquakes, geophysicists can gradually piece together the general structure throughout Earth's interior.
There's hundreds of thousands of earthquakes every year, and there's thousands of seismic stations scattered around the Earth, so we constantly listen for these earthquakes and constantly process them through our seismic stations and through our computers, and slowly we're building up an image of what actually is down inside the Earth, building up three-dimensional images.
And I say "building up" because it's very much like building up an image on a television screen. As the television scanner sweeps across the image, you slowly see what it is you're looking at. Earthquakes aren't the only source of seismic waves. People sometimes create their own earthshaking devices. Seismic waves can be generated artificially at the Earth's surface by explosions or by devices which pound on the Earth's surface, and the waves, then, transfer through the Earth.
This is very similar to Xray, for example, and when we Xray a human body, the Xray instead of being destructive to the body are used as a source of energy which can penetrate to the interior of the body and then scatter and reflect off bones or internal organs and then can be documented on a film or some other kind of sensor, and we can then see the interior of the human body, and similarly we see the interior of the Earth from the information that comes back from seismic waves.
Based on seismological research, geophysicists have deduced that the Earth's interior is divided into three layers.
The Earth's crust itself occurs in two varieties:
An important distinction between these two types of crust is that they are mostly made up of different kinds of rock. Granitic rock is typical of continents, while basalt predominates in ocean basins.
Also, the continents are somewhat thicker than in the oceans. The continents are between about 25 and 40 kilometers thick. The oceanic crust is something between 5 and 10 kilometers thick.
The continental crust is made up of materials, which have been swept together during dynamic processes on the Earth. They include igneous rocks from volcanic and magmatic events, sedimentary rocks that have been pushed in from the oceans, and metamorphic rocks, which have been modified by heat and pressures. They've been taken to depths on the continents into the Earth's interior and then regurgitated again.
The chemistry of continents as a result is very complex, but by and large, much of the crust is composed of silicates, primarily of aluminum, potassium, calcium, with some iron and magnesium.
The oceanic crust is a bit more primitive than the continental crust, more uniform. We believe it's largely iron and magnesium silicates. It has some structure to it, but much less structure than the continents. Although as a general rule, our knowledge of the Earth's interior comes from indirect evidence, there are some exceptions.
Occasionally, erupting lava will carry with it fragments of rock torn from the walls of volcanic conduits. These are known as "xenoliths" or foreign rocks. Some xenoliths come from the mantle. In other places, huge slices of mantle rock have been brought to the surface by techtonic activity attached to pieces of sea floor crust. These unusual rock slices are called "ophiolites".
An ophiolite is a sequence of rocks that geologists interpret as being a cross section through the oceanic crust, the part of the Earth's crust that underlies the oceans. It's a sequence of rocks that might be three to five kilometers thick, the bottom most layers consist of rocks like these behind us, which are rocks of the Earth's mantle composed of minerals rich in iron and magnesium.
Moving up through the three to four kilometer thick section, you move into rocks called "gabbros" which are richer in silica than these, and then, finally, two rocks, which are thought to be the rocks that form the actual floor of the ocean, rocks that were either erupted volcanically or deposited as sediments on the ocean floor.
Ophiolites are of special interest to geologists because they are the easiest way to see into the Earth's mantle. The crust of the oceans is much thinner than the crust on the continents. On a continent you have to be seeing 60 to 70 kilometers deep in order to get to mantle; whereas, under the ocean floor, the mantle lies three to four to five kilometers below the top of the oceanic crust, so ophiolites when they are exposed, as this one is, are a chance for geologists to peek down three, four kilometers deep in the Earth and actually be looking at the mantle.
So apart from some xenoliths, walking across a complete ophiolite section is the only other way geologists have of directly viewing rock from Earth's mantle. I have come about a quarter mile from our last stop, which was in the lower part of the ophiolite. There the rocks looked like this: dark, because they are rich in the minerals caleed pyroxenes, which are typical of the Earth's mantle.
The rocks I am standing on now are lighter colored, primarily because they are rich in the mineral feldspar, the white mineral you can see in this rock.
In coming the quarter mile, I have walked from the Earth's mantle into the Earth's crust. While ophiolite outcroppings are a rich geological find, they are rare and only provide a view of the uppermost mantle, so seismological research remains the primary tool geophysicists use to study the deep interior of the Earth.
Geologists divide the seismic waves that travel through the Earth's interior into two basic types, "primary" or "P" waves" and "secondary" or "S" waves."
A "P" wave" is a compressional wave that makes the rock vibrate parallel to the direction of its movement. Since it is a very fast wave traveling through rock at between four and seven kilometers per second, the "P" wave is the first wave to arrive at a recording station following an earthquake.
An "S" wave, on the other hand, has a shearing motion that makes the rock vibrate perpendicular to its path. this movement slows the "S" wave, so that it travels at two to five kilometers per second or about half the speed of the "P" wave. This is why "S" waves arrive as secondary waves at the Earth's surface.
There is another important difference between "P" waves and "S" waves. Although both can pass through solid rock, only "P" waves can also pass through gases and liquids.
As geophysicists probe deeper into the Earth's interior, the properties of the two types of seismic waves provide clues about what the core of this planet must be like.
When a large earthquake occurs, seismograph stations around the world record the arrival of its "P" and "S" waves, but for stations slightly more than half the distance around the earth from the focus of the quake, p-waves are not recorded. This region is called the "P" wave shadow zone. At almost the opposite point on the Earth's surface, the "P waves reappear.
The shadow zone exists because the waves are refracted as they pass through the boundary between the mantle and the core and are diverted from their original paths. The "S" waves leave a much larger shadow zone because none of them passes through the core. Given that "S"waves cannot traverse liquids, it seems probable that at least part of the Earth's core is liquid.
In fact, the way the seismics waves pass through the core tells us that it consists of two parts, a liquid outer core and a solid inner core. Study of Earth's interior suggests that this core is composed mostly of iron combined with smaller amounts of lighter elements.
Seismic studies aren't the only way of unlocking secrets of the deep Earth. Geophysics provides other techniques as well.
Gravity is another type of indirect evidence that geophysicists use to learn about the interior of the Earth. The force of gravity between two objects depends on the mass of the object and the distance between them. The greater the mass of either object or the closer they are together, the stronger the gravitational attraction. For example, the ocean tides are produced by the gravitational pull of the sun and the moon.
However, even though the sun is 25 million times more massive than the moon, the effect of the moon on tides is more than twice as great because it's so much closer to the Earth. The instrument that geophysicists use to measure the force of gravity is called a "gravimeter." This device is incredibly sensitive, capable of detecting variations in the force of gravity as tiny as one part in 100 million. On Earth, changes in gravity are often due to variations in the mass of the rock in the Earth's interior.
For example, the Earth's mantle rock is denser and, therefore, more massive than the crustal rock of the Earth, so in areas where the mantle is unusually thick, there are unusually high gravity readings.
Gravimeters have also proven useful in detecting high density iron ore deposits and low density bodies of salt hidden from view. At the heart of a gravimeter is a spring with a weight attached to its end. The stronger the pull of gravity, the more the spring is extended. This extension of function of gravity is accurately measured by the gravimeter.
Magnetism provides more indirect information about the interior of the Earth. The Earth is a giant magnet. Its magnetic field is generated within the core and acts exactly as if a giant bar magnet were present at the center of the Earth. All magnets are surrounded by magnetic lines of force. If I cover this magnet with a piece of paper and sprinkle iron filings on top, the magnetic lines of force become easily visible. The magnetic lines of force that surround the Earth are capable of aligning the magnetic fields in rocks in the same way that they align the needle of a compass.
Geophysicists continue to learn a great deal about the characteristics of the Earth's magnetic field and the processes that generate it. But the magnetic field of the Earth has been observed for centuries, ever since the Greeks discovered lodestone, a natural magnet, some 25 hundred years ago.
Mariners discovered early on how useful this curious metal can be. When a sliver of lodestone is placed on a piece of wood floating on water, it always turns to point approximately north. The simple magnetic compass was to guide navigators to every corner of the globe.
In the Nineteenth Century it was discovered that a link exists between electricity and magnetism. English Scientist, Michael Faraday, invented a device that illustrated this principle. As an electric current is passed through a wire, a magnetic field is generated. The wire moves in a circle following the shape of the field. The same principle can be observed using iron filings. An electric current generates a magnetic field causing the filings to orient themselves toward invisible north and south poles. Faraday went on to discover that moving a magnet through a coil generates an electric current. Faraday's observations are the bases of our modern views about Earth's magnetic field.
We know the Earth's magnetic field originates deep in the Earth. We think it originates in the outer core of the Earth. The basic idea is that the outer part of the Earth's core is a fluid, molten iron, with some light element alloyed in, possibly silicon or oxygen. Because it's a liquid metal it can move easily, and it can conduct electricity.
Heat coming from deeper in the Earth's core produces convection currents or fluid motions, which then has the effect of moving an electrical conductor, the metallic fluid through a magnetic field. When you move a conducting substance through a magnetic field, you generate electrical currents, which then generate magnetic fields. So, a self-sustained electromagnetic cycle, though far from steady, is active deep inside Earth.
On occasion, an astonishing light show triggered by the sun reveals parts of the magnetic field surrounding our planet. From the surface of the sun, a constant flow of electrically charged particles, known as the solar wind, enters space. The solar wind is mostly deflected from Earth by our planet's magnetic field, but near the poles, these particles are drawn magnetically toward Earth's surface. As they hit the atmosphere, they causes gases to glow and form shimmering curtains of red, white and green lights, which we call the "aurora." When solar flares occur, the aurora are especially spectacular because charged particles bombard the Earth's atmosphere across the higher latitudes.
Although the Earth's magnetic field is similar to that of a bar magnet, there is one important difference. The strength of the Earth's field varies considerably over time. In fact, the Earth's field has decreased in strength about five percent over the last 150 years.
Rocks can also have a magnetic field, but they're magnetism is permanent. Iron rich rocks that have been strongly heated become magnetized by the Earth's magnetic field. When the rock cools, that magnetism is locked in, becoming a permanent part of the rock, and because it's permanent, magnetized rocks of different ages are actually records of the Earth's magnetism throughout geologic time.
When a rock is magnetized, its magnetic field is aligned to the magnetic North Pole of the Earth . This is similar to the way a compass needle points to magnetic north. This alignment, called "magnetic polarity" is also permanent in magnetized rocks.
In the early 1900s, geophysicists studying lava flows in France made a startling discovery. The lavas and the big soils underneath them were both permanently magnetized, and their magnetic polarity was identical in orientation. But that orientation was exactly opposite that of the Earth; that is,the magnetic north pole of the rocks pointed toward the magnetic south pole of the Earth. This observation was the first clear evidence that the magnetic field of the Earth completely reverses itself.
During a time of normal polarity, magnetic lines of force leave the Earth near the South Pole and reenter the Earth near the North Pole. During reversed polarity, the lines of force run the other way, going from north to south. In other words, the magnetic poles switch places.
Many rocks contain a record of the strength and direction of the Earth's magnetic field that existed when the rocks were formed. Here in the Mojave Desert of California, Geologist Scott Bogue drills for cores of lava, which are keys to our magnetic past. Lava flows of differing ages preserve evidence of a number of previous magnetic reversals. They are like tape recordings of prehistoric magnetic behavior. The study of these ancient magnetic fields is called paleomagnetism.
We can say something about ancient magnetic fields of the Earth because of the record they leave in rocks, and many rocks become magnetized when they are formed or some time after they are formed. When they become magnetized, they become magnetized parallel to the first magnetic field at that time, so if you can collect a rock sample and measure which way it's magnetized and determine when it was magnetized, then you can learn about the orientation of the magnetic field at that time in the past.
Scientists generally agree that the Earth's magnetic field reverses itself on the average of once every 500 thousand years. Widespread evidence suggests that the last reversal occurred 700 thousand years ago, but there is also some evidence that as many as a dozen reversals may have occurred since that time.
We know that in general terms during a magnetic reversal, the field intensity decreases to10 or 20 percent of its normal value, so at some point as the Earth were entering a reversal, there would be a decrease in field intensity. We know by comparing the magnetic field strength today to measurements of the magnetic field 160 years ago that the Earth's magnetic field is currently decreasing in strength. It's decreased about five percent since 1832, so some have claimed that the Earth's magnetic field may be entering a reversal right now.
The discovery that the Earth's magnetic field is turned off briefly during a reversal raises some profound questions. How long is the Earth without a magnetic field? How does a reversal affect animals who depend on the Earth's magnetic field for navigation during their migrations?
Does the loss of field strength cause an increase in potentially deadly solar radiation? Linking magnetism, gravity, and seismicity with the geologic history of the Earth are goals of geophysical research.
By creatively using indirect evidence, these Earth scientists probe the Earth's interior for knowledge of structure, composition, and to locate valuable stores of natural resources as well. Geophysics has forever changed our simple model of a three-layered Earth.
Without having actually seen it, we know that the interior of the Earth is remarkably complex in structure, in composition, in behavior. Understanding that complexity will ultimately yield explanations for the origin of the Earth and for the tectonic forces that continuously change in space.
Major funding for Earth Revealed was provided by the Annenberg CPB Project.
Those animations of wave motion are really excellent. I like to watch those. I suggest that if you're taping the program to go back and watch those animations and to get a sense of how the individual particles move in relation to one another.
I also want to note that the videos don't cover everything from the text and the study guide. There are things that you are left to make your own connections, all part of the game of learning.
I also want to note that as far as we know, Earth's magnetism is generated in the molten part of the iron core, the outer core. I might also note that Earth is one of the few planets in the Solar System that has a magnetic field. Both of our nearest neighbors, Venus and Mars, have essentially no magnetic field, which many people interpret to mean that they're geologically dead.
Within the iron core, the actual movements of the fluid are quite chaotic. There are convection currents moving the iron around, and each one of these little whirling convection currents basically generates its own magnetic field, so if one of the currents is going in the opposite direction, for example, it generates an opposite magnetic field.
The total field due to the Earth at any given time is really the combination or the sum of all these individual fields, and many people think that magnetic reversals occur when, for one reason for another, the motions in one direction become greater than motions in the other direction.
Lodestone is a particular variety of the mineral called "magnetite", which has its own magnetism. As I mentioned before, lodestone was basically the method by which we learned about magnetism in the first place. Now, this magnetism in lodestone is not generated by the Earth's magnetic field. It's generated by the natural crystal structure of the magnetite mineral, but the magnetism in the lodestone is affected by the Earth's magnetic field. Mangetite's a common mineral in most volcanic rocks, and because it's a common mineral, it often occurs in fragments in beach sand.
In fact, I have some sand here from a beach on Maui. I'm going to sprinkle a little bit of this onto the paper. You can see some of the dark fragments in here, but I can separate these dark fragments out with a magnet. You don't really see much happening here as I run the magnet through the sand. Don't think you can really see that the dark particles are disappearing, but there a bunch of them stuck to the magnet.
I can get a better view of that if I had a clean piece of paper in here. I'm going to brush these articles off onto the paper. You can see all the dark fragments here, quite a few of them, actually, that were attracted to the magnet from the sand.
One of the activities suggested in the study guide is to take a magnet to the beach and drag it through the sand and see what you collect, and here in the Hawaiian Islands most of the beaches contain significant amounts of magnetite. There are several reasons why the magnetite is significant in the study of the Earth because when volcanic rocks form the magnetite in the rock becomes magnetic when the rock cools below a certain temperature.
These tiny grains of magnetite then align themselves with the Earth's magnetic field, much in the same way that the compass does, so that when the rock cools, those tiny grains of magnetite retain that degree of magnetization, so that we can go back and look at them later to find out what direction and what strength the Earth's magnetic field was at the time these rocks were formed. We might also note that when magnetite grains settle through water, as they're settling, they also orient themselves as they float down through the water, orient them themselves in the direction of the magnetic field, so when the sediment accumulates on the ocean floor, these tiny grains of magnetite are also oriented much like the needle on the compass, and, once again, we can decipher what the direction of the Earth's magnetic field was at the time that those rocks were formed.
One of the important concepts that, I think, the video didn't cover very well is the concept of "Isostasy." Basically, isostasy refers to the floating of an object in a fluid that's more dense than itself.
This is one of those physical models that we can use to great benefit to understand why, for example, continents exist on the Earth. The video noted that if you were to level all of the land on the Earth to one level, that the oceans would still cover about 2 thousand meters.
In other words, these different types of rocks congregate on the continents and keep the continents higher than they would be otherwise. We can explain this with the concept of isostasy.
The best model for understanding isostasy is simply to think about ice cubes floating in water. You can try this at home for yourself. Take an ice cube. Put it into a glass of water and note how the ice floats in the water. You'll note that the ice floats with most of its volume below the surface of the water. How much of the material is below the surface and how much is above the surface depends upon the density of the ice and the water.
Ice has a density about 90 percent that of water, which means that ice float with about 90 percent of its volume below the surface. You might also note that a large ice cube next to a small ice cube will have the same percentage below the surface, 90 percent of its volume, but that 90 percent extends both further below the surface and also further above the surface, so that the larger block of ice both stands higher in the water and also its bottom is deeper in the water.
It's also fairly obvious, I think, that if you add weight to the ice; for example, if you push down on top of the ice, the extra force that you exert will force the ice down into the water, and by the same line of reasoning if you were to push up from the ice from below, it would raise the ice out of the water.
So what's the point of all this? When the ice is floating by itself, we say it's in isostatic equilibrium. Again, the word "isostatic" means floating. The word "equilibrium" means "balance," so what's really happening here is that the forces are balanced in such a way that the upward force of the water exactly balances the downward weight of the ice, and the ice remains in one place.
What's the connection here? Well, we know that the continental crust is less dense than the mantle. We also know that the oceanic crust is less dense than the mantle, and the continental crust is less dense than oceanic crust. When we look at the depth of the continental crust, we see that beneath tall mountain ranges, the roots of the continent are deeper.
In other words, it seems as if the lighter continental rocks are actually floating on the more dense oceanic crust on the mantle. We usually don't think of rock as being a fluid. We usually don't think of rock as something which has the ability to flow, but as we'll see in the next lesson, rock actually is very much like a fluid only over a long time span.
So the point of all this is, for example, during the last Ice Age, much of what's now continental North America and Canada was covered with a sheet of ice up to 25 thousand feet thick. When that ice melted, the extra weight that it placed on the continent was no longer there, and the continent began to rise, and we can still measure this upward rebound.
A similar effect happened in the Scandinavian part of Europe. One of the ways we can test whether or not something is in isostatic equilibrium is by the use the gravity meter. If, for some reason, there's a high concentration of mass below the surface, gravity will be a little bit stronger in that place, and the gravimeter is stretched.
One thing that might cause extra mass to be concentrated below the surface of the Earth is a large iron ore body because iron's a fairly dense material, so miners use gravity explorations to locate large ore bodies simply by the effect on gravity. There are other things that can cause changes in this icesthetic equilibrium. One of these is simply when a new mountain range is formed, the extra weight causes it to press. When erosion takes place removing surface material, it causes the roots of the mountains to rise.
Most importantly, I think, for our purposes is at subduction zones, where lithospheric plates are descending into the mantle below, the force that's being used to jam the plates down into the mantle causes extra mass.
In other words, it's the same effect as pushing down on top of the ice cube, so we would expect to find positive gravity anomalies at the subduction zones, and, in fact, we do. The opposite happens at the spreading centers on mid-ocean ridges where the material is being held up, and, in fact, there it's like pushing up on the ice from below, and we do, indeed, find negative gravity anomalies at that point.
Well, next time we'll take a look at the ocean floor. The ocean floor, as it turns out, is one of the most important missing pieces in understanding how the Earth operates, so I'll remind you for next time to
Well, that's it for this time. Study hard but enjoy yourself, so I'll see you next time.