Earth Revealed

Program 12

GEOLOGIC TIME

Oh, hi! I was just reading here, and I found an interesting quote that I'd like to share with you. Back in 1916, a geologist named Adolph Knopp wrote a passage about geologic time. Knopp wrote "If I were asked as a geologist, what's the single greatest contribution of the Science of Geology to modern civilized thought,the answer would be the realization of the immense length of time.

So vast is the span of time recorded in the history of the Earth that it's generally distinguished from the more modest kinds of time by being called "geologic time." Understanding the age of the Earth and universe ranks as one of the crowning achievements of the human intellect. Estimates of the age of the Earth have been expanding throughout time. The key to understanding this immensity of geologic time is really the Principal of Uniformitarianism.

You see, there's really no need for supernatural intervention if time can explain the existing geologic features on Earth. Some people would even argue that understanding the long stretch to geologic time requires a much less of a stretch of imagination than trying to understand supernatural intervention. See, science looks for simple explanation, which adequately explain the facts. It's a principal called "parsimony," which comes from a Latin word which means "to say" or "to skip."

Science also looks for natural causes, so it's as if the concept of geologic time is tailor made for us to understand the geologic processes of the Earth based upon natural causes without this need for supernatural intervention.

Ayer's Rock in central Australia is called "Yulara" by the central Australian aborigines, and it holds great spiritual significance in the aboriginal culture. It rises 1100 feet out of a nearly flat featureless desert, and it's pretty awesome. It shimmers in the distance, and as you approach it, it grows, larger, and larger, and larger until it's large beyond belief. How long must it have taken for Ayer's Rock to reach its present state?

Well, let's review briefly the sequence of events that had to happen for this rock to be in its present condition. First of all, there had to be layers of sand deposited nearly 3,000 feet thick. Then, the sand had to be cemented to form hard rock, and it had to be tilted and uplifted,and then eroded and subsided, and covered again by 1500 feet of sediment, and then uplifted again and eroded down to expose the 1100 feet or so that we now see. It's pretty hard to determine how long this may have taken in terms of years, but it's certain that it took more than just a few thousand years.

We can look back and see that at the present rate of sedimentation of sand, say on the Continental Shelf, would have taken about 10 million years just to deposit the red sand. That's not accounting for all the other changes that have taken place.

Today's lesson examines the evidence for the age of the Earth and the ways in which it's been organized and interpreted. Now, this is a fascinating story, and historically it set the stage for the revelations of plate tectonics.

Plate tectonics, of course, understands or helps us to understand geologic history, and the more we learn about geologic history, the more we can learn about plate tectonics,and the cycle goes on, and on, and on.

But before I actually introduce the lesson today,

• I want to remind you that you should have already read the text assignment. This is Chapter 8, pages 175 to 191 and be sure to carefully study the text diagrams, especially those showing cross cutting relationships and refer to Lesson 3 and
• reread the text on pages 20 and 22 and Chapter 1 on "Geologic Time." Also, be sure to read the sections on the birth of the solar system, that's Box 8.1 on page 185.
• As you're reading the text,
• make a special lookout for evidence that we have to support the age of the Earth, and, of course, you should
• study the key terms and concepts, and follow the study plan and the study guide, and when you're done,
• go back to the learning objectives to make sure you've learned each one.

I'll review the objectives with you very briefly. In this lesson we want to

• discuss the importance of the concept of geologic time. We also want to
• recognize the contributions of Hutton to our understanding of geologic time and discuss the concept of uniformitarianism. We also want to
• differentiate between "relative" time and "absolute" time and
• explain how the concepts of original horizontality, superposition, and cross cutting relationships are used to solve geologic problems. We also want to
• describe how rock units in separate areas are correlated through various methods, including physical continuity, stratographic position, similarity or rocks and comparisons of fossils, and, finally,
• explain why the evidence for the age of the Earth will never allow us to find the oldest rocks that we think that exist on the Earth.

Over the years, people have been curious about the age of the Earth and have tried many ingenious methods to figure out how old it is, and it seems that each estimate was older than the previous one. Now, our current estimate is based on the best evidence we have from many different areas, including radioactivity, and it seems to be a good one, but who knows, in the future we may find that the Earth is really even older yet than we think it is.

The first of these methods was tried by Bishop Usher back in the 1760s. He went back to the Bible Verses of Genesis, and looked at all the begat sections, and tried to figure out how long each person had lived to calculate back to the beginning, and determined that Earth was created in 4004 B. C. Not just in 4004 but on a particular day and at a particular time; in other words, the Earth is about 6,000 years old.

Today we may think this is kind of ridiculous or kind of absurd, but at the time it was considered to be a very scholarly effort and actually represented the best evidence or the best method we had of figuring out the Earth's age.

A few years later a Frenchman named Clerk suggested that the Earth was older than that, about 75,000 years. James Hutton, who was as we'll see today is really the father of modern geological thought, saw "no vestige of a beginning and no prospect of an end"; that's a quote from Hutton, but he accepted that it must have taken a long time for these various geologic features to form; in fact, much longer than previously thought. Not just longer, but much longer.

Charles Lyell in the early 1800s estimated the age of Mount Etna, a volcano in Italy, to be about 10 million years old simply based upon the distribution of cinder cones around the flank of the volcano, and he concluded that if Mount Etna is 10 million years old,that Earth must be much much older than this because Mount Etna is such a small and insignificant feature.

Lyell is significant in the history of geologic thought for several reasons. Number one is that it was he who planted the idea of the long time periods that were available in geologic time in Darwin's mind, so that Darwin had this model when he took his voyage on the Beagle, and Darwin recognized that in order for evolution to occur, long periods of time were needed, and here, again tailor made, was the concept of these long geologic times as implanted by Lyell.

We should also note that it was Lyell who encouraged Darwin to publish the material on the origin of the species upon Darwin's return from his trip around the world. Darwin is one of the few people whose ideas of geologic time is actually longer than what we think today; in fact, Darwin thought there must have been about 300 million years have elapsed since the Mesozoic.

We now think this is about three times longer than that what we accept now; in other words, we think the Mesozoic ended probably a hundred million years ago. There were several other methods that were used by other people who were not geologists to estimate the age of the Earth.

Two of these were physicists and one was a chemist. A physicist named Helmholtz estimated the sun to be about 20 to 40 million years old, and he did this by calculating how much heat would have been released in the gravitational collapse which we think formed the sun from the solar nebula. Lord Kelvin, another physicist. used a different approach; he looked at the Earth and considered the Earth to have once been a molten object and simply calculated using thermodynamics how much heat would have been lost and how long it would have taken the Earth to cool to its present age, but Kelvin came up with a number of about a hundred million years for the age of the Earth.

The problem was that Kelvin was not aware of the radioactivity which we know today contributes extra heat to the heating and would give a longer age for the Earth.

The chemist was a man named Jolly, who estimated that it would take about a hundred million years for the salts in the ocean to accumulate to their present levels looking at the amount of salt input by streams. Well, normally we think of streams being freshwater, but they actually contain quite a bit of dissolved material which accumulates in the ocean. The problem with Jolly's estimate was that he wasn't aware that there are certain regulatory processes which tend to keep the salinity of the oceans more or less constant, and, in fact, we still accept today that it would have taken the oceans about a hundred million years to attain their present salinity; it's just that they've been at that salinity for maybe as much as two billion years.

With all of these different methods and different people that were applying the methods to try and understand the age of the earth, no one, and I mean no one, ever really conceived of the billions of years that we now accept the age of the Earth, and it wasn't really until we started understanding radioactivity in the early part of the century that we began to get a good sense that the Earth is really even much older than some of these earliest estimates thought.

One of the objectives of the lesson is to distinguish between what we might call "relative" time and "absolute" time, so I want to go over with you a little bit about relative time now, and after the video we'll come back and discuss more about absolute time. You see, in the process of trying to figure out the age of the Earth, we also learn about its history, and it's very much like a pile of old newspapers. If they're undisturbed, the oldest is on the bottom, and reading them from the bottom up gives us clues to how history progressed, but first we have to be able to read, and learning to read the rock record is one of the major aspects of geology, but, like the newspapers, the sequences of rock layers give us an incomplete picture of the details of history even if we could piece them together completely.

The basic principles that we use to interpret Earth's history follow from uniformitarianism; in other words, that processes have operated in the past the same way that they operate today and common sense, and I always tell my geology students that geology is about 90 percent common sense. A lot of the rest of it is memorizing terms and vocabulary, which you've probably learned by now, but there are basically three laws or three principles that geologists use to help them unravel the history of Earth, which when used with uniformitarianism form a very powerful technique for understanding Earth's history.

• One of the these is the law of horizontality and continuity. That's kind of a mouthful, but basically it just means that sediments are deposited in layers that are generally horizontally, and they don't end abruptly. The sediments are deposited in horizontal layers because they settle through water by gravity, and as we saw in an earlier demonstration, they tend to form horizontal layers. They don't end abruptly because most sediments are deposited in rather large sedimentary basins, which don't have steep edges, so that, if anything, the sediments simply thin out at the edges, as they would if they were deposited in a lake, for example, rather than having nice steep edges, and we understand that the sedimentation takes place or took place in the past about the same rates and about the same manner as it does today.
• The second of these principles is the law of "superposition." "Superposition" simply means that sediments are deposited in a particular age sequence with young sediments are deposited on top of existing sediments or rocks, so that the youngest are always on the top, and the oldest are always on the bottom. This makes a lot a sense if you think about it. If sediments are going to be deposited, they must be deposited on something, and that something must have been there at the time the sediments were deposited, and so by definition it's older than the sediments that are being deposited on it, so in general the youngest rocks are at the top unless they've been overturned by tectonic forces, but most sedimentary beds are not overturned, and even if they are, it's usually easy to determine that they've been overturned either on a local scale or on a regional scale.
• Okay, the third of these principles is called the law of cross cutting relationships, and that is in many ways very similar to the law of superposition. It simply says that any geologic feature which cuts across another one must be younger; in other words, if a fault shows displacement of layers of sedimentary rock, it's obvious that the sedimentary rock must have been there before the fault occurred, so it applies not only to faults but also to intrusions of igneous rock, that if you find a dyke of basaltic rock cutting across a particular sedimentary bed, the sedimentary bed was disturbed by the intrusion and not the other way around, so the intrusion is younger.

These principles seem simple, and they are, but simple doesn't necessarily mean trivial. You see, using these very simple principles, it's very easy to determine the relative ages in a single exposure of rock. We can simply look at the exposure of rock and know that the oldest ones are on the bottom, or that the cross cutting rocks are younger. The problem is that tens of thousands of meters of sedimentary rock have been deposited over geologic time, and there's no one place on Earth or no one exposure which shows all of them.

Even the Grand Canyon, which probably is the greatest single sequence of rocks anywhere on the Earth's surface and spans a period of time of two billion years has gaps. These gaps are unconformities, so we find, you see,that sediments are deposited at different rates and different places at different times, and so rocks exposed in one location may either be older, or younger, or the same age as those in another place,and what's really needed is to find out how rocks around the world fit into the same relative time frame.

We'd like to be able to find out about rocks that were deposited in the central part of Europe, as opposed to those which were deposited, say, in the central part of the United States and to compare their ages. Because we can't find sedimentary rocks that represent all of geologic time in any one place, we have to piece together rock sequences from one locality to another, and this is a process that geologists call "correlation."

There are several ways in which correlations can be done. The easiest of these is what we might call "physical continuity." If a particular layer of sedimentary rocks or a particular group of rocks covers a rather large area, then we can simply follow them throughout their contact. Rocks do tend to cover relatively large areas because most rocks are deposited in large sedimentary basins like on the continental shelf or in large lakes. The problem we have here is that sediments may be deposited in different areas at the same time under different conditions or under different environments, and we'll study those environments a little later on, so, you see, rocks that are deposited at the same time may have no physical connection.

Again, rocks that are deposited in a shallow sea in the interior United States and in Europe at the same time. The physical continuity is not generally useful to correlate relative ages of two sequences of rocks on different continents; for that we need other methods.

One method is the method used by Wegener in his original theory of continental drift. He looked at the structures of rocks in South America and Africa and found that the rocks had similar characteristics. Now, this may work, but the geologist has to be very certain that if he sees a rock in South America and sees another rock in Africa that these rocks are really similar enough, so that the conclusion can be made that they really were deposited at the same time or are a part of the same rock unit, so physical continuity and rock structures require a sense of the rock requires the geologist to understand the rocks.

Probably the main way in which geologists have learned to correlate rock units from one place to another is by the use of fossils. We can state a general principle here generally known as the principle of faunal succession, which simple says that groups of fossil animals and plants have succeeded one another in a definite and discernible order, and that the older the fossil, the more it differs from modern organisms, so that each geological time period was first identified and can now be recognized by the particular group of fossils that are found in rocks of that age more or less worldwide.

This idea first arose from William Smith, who was an English engineer involved in the building of canals. Smith found that many sedimentary rocks contained fossils, and he noticed that certain strata or certain layers in widely separated locations in England had identical fossils in them; whereas, the rock layers above and below those layers might have different fossils. It's rumored that Smith became so proficient that if you threw him a rock, he could look at the fossil and tell you the particular layer of rock from which that came within the region where he was building the canals. He could, in other words, use a particular index fossil, or a particular kind of fossil, to correlate one rock structure to another.

At about the same time, a Swiss geologist and a French geologist were working outside of Paris and finding similar results. There, instead of just memorizing the rocks that they found in various sequences, they actually arranged the rocks, or I should say, the fossils in the same order as the rocks from which they had been dug. They put the oldest fossils on the bottom and arranged them successively from old to young. They found that the groups of fossils varied in a systematic way with the chronological positions of rocks. They found that fossils from the higher layers bore closer resemblance to modern forms than did the fossils from the older layers. It became evident to them that the relative age of layers of sedimentary rock could be determined by the assemblage or groups of fossils that it contained.

What we see here is that there are two general principles as far as fossil correlations go, that fossils are a key in determining relative ages, and rocks containing the same fossil assemblages are similar in age.

You see, species of animals, like individual creatures, have finite lifetimes. Species come and go on the Earth. Some species live longer than others. There are some species which have lived very long periods of time with very little change. Examples are sharks and cockroaches. We find shark teeth dating 400 million years ago that look very much like shark teeth today, and we find cockroaches that fossilized, which look very much like the cockroaches you find crawling around your kitchen today.

These long lived species are not particular useful for correlations. Index fossils in order to be useful need to be species which have relatively short lifetimes but also wide distributions over the whole earth. Many of these are marine organisms, such as formanifera,which lived in the oceans, so that they could be carried worldwide. What's most useful is if we find two species whose time periods overlap.

If one species has this particular time sequence, and another one has this particular time sequence, it's the overlap between the two that helps us to use these index fossils, so we can define then "index fossils" as simply species which had short lifetimes and wide distributions. Fossil assemblages, on the other hand, (The word "assemblage" means "groupings.") fossil assemblages are groups of index fossils which all lived at the same time, and they're usually found together in the same rock strata.

Superposition and faunal succession go hand and hand, and using the two of them together we can actually work out the geologic history of any region with sedimentary rocks and make correlations between that region and another one. Superposition is basic to faunal succession because when we find fossils in a particular layer, we know that if that layer was near the bottom of a sequence, then it's older than those on top, so it also shows us, then, based on superposition that life has changed with the passage of time, and that life has evolved, if I can use the word.

One last thing before we look at the video. There's no known place on Earth, as I mentioned earlier, where sedimentation has been continuous throughout geologic time. The periods of quiet are disrupted by tectonic processes, such as uplift, downwarping, or subsidence. These activities or tectonic activities produce gaps in the sedimentary record as erosion predominates temporarily in one area, but eventually sedimentation will once again ensue, leaving a buried erosional surface with the younger rocks on top of much older rocks. An interface of this type is called an "unconformity." The time represented by the absence of sedimentation in a particular area may be thousands or even billions of years, so at this point you may want to go back to the chapter on geologic structures to review the types and sequences of unconformities to get a sense of how unconformities are used along with sequences of rocks to help interpret Earth history.

The video today shows many examples of unconformities and details Hutton's work and, I think, gives us a pretty good sense of how Earth's history is unraveled, but you can't just watch the video, remember. You need to look at the textbook and the study guide. So with all these things in mind, let's watch the video.

Major funding for "Earth Revealed" was provided by the Annenberg CPB Project.

More than 300 years ago in Ireland, Archbishop Usher calculated the age of Earth based on the Book of Genesis He determined that our planet was created 4,004 years before the Birth of Christ on the 26th of October at 9 o'clock in the morning. Widely praised by scholars and theologians, Usher's calculation was not seriously challenged in the Western World for almost a century.

Today scientific evidence indicates that Earth is much older than biblical texts suggest. By counting growth rings, botanists have found trees as much as 8,000 years old, and with the development of radioactive age dating, geologists have concluded that our planet came into being not thousands but billions of years ago.

Humorist Mark Twain once remarked "Nothing hurries geology." He was referring, of course, to the almost imperceptible geologic change that one sees in a human lifetime. No aspect of this science is more difficult for most people to grasp than the concept of geologic time. Our lives are governed events that are measured by clocks in seconds, or minutes, or hours. At most we may plan months or years in advance, but to understand the nature of Earth, we need to stretch our concept of time beyond our human experience and into a geologic frame of reference.

Events in Earth's history, such as the development of mountain ranges, the rifting apart of continents, and the creation of new ocean basins have been repeated over and over, and each of these geologic events takes place over many millions of years.

The estimated age of Earth itself is over 4.6 billion years, an incomprehensibly vast period of time. One way to illustrate the immense panorama of geologic time is to draw an analogy using the time period that we can easily relate to, so let's condense the entire age of the Earth into one calendar year.

On this scale the Earth and it's neighboring planets were formed on January 1. By sometime in early March, the planet had cooled sufficiently to allow water to stand in pools on its surface. The very earliest forms of life appeared by late March, and oxygen gas became an important part of Earth's atmosphere beginning sometime in late July. Multicellular organisms appeared by October 25th, but it was not until late November that plants and animals were abundant on land. On about December 15th, the dinosaurs appeared but disappeared in a sudden mass extinction on December 25th. The first humans appeared at about 11:20 p.m. on December 31, and the Industrial Revolution began less than a second and a half before midnight.

The first person to realize the immensity of geologic time and begin to investigate it in a systematic way was an Eighteenth Century Scottsman named James Hutton. His observations and ideas about the origin of rocks began as a pasttime that he shared with other wealthy intellectuals of Edinburgh society, but Hutton's interpretations and conclusions ignited a major controversy throughout Europe and survive today as the foundation for modern geologic thought.

James Hutton was born in Edinburgh, Scotland in 1726. Like most intellectuals of his time, he explored many disciplines from law and medicine to chemistry and agriculture, and it was this last pursuit that was to lead him into the field of geology.

He had been left a little piece of land, and he decided to become a farmer, so he went to Norfolk, England and lived with a farmer for a couple of years to learn how to farm and farmed for 15 years, but while he was farming and all of his neighbors believed that he was a perfectly good farmer, he was spending a great deal of his time looking and becoming more and more fascinated with the land, and what was under the soil that he was tilling.

An important question intrigued Hutton. He saw that land is worn down in many places by the destructive forces of erosion. it possible any land was left to support life? He believed there must be an undiscovered force building up the land, which counteracts the processes wearing it away. He began searching for evidence of this force and soon found it.

Hutton observed that streams washed sand and gravel into the seawhere the sediment accumulated in offshore deposits. He found similar material in the form of ancient sedimentary strata, tilted and raised above sea level. Surely, Hutton thought, these layers must also have formed under water, then risen to become new land.

Of Earth history, James Hutton wrote "We find no vestige of a beginning; no prospect of an end." Hutton was the first scientist to show that the world must be far older than most people believed. One of the most astonishing things to him, and one of the most important elements of his work was to try to establish that, in fact, geologic time was a very long period rather than the biblical creation time by Archbishop Usher sometime later 4004 B. C., but Hutton was convinced that geologic time was very extensive. Hutton reasoned that if it takes about a year for a river to deposit a millimeter thick layer of sediment on the sea floor, that it must take at least a million years to lay down the thousand meter thick sequences of sedimentary rock that he had observed in some parts of Scotland.

Many church authorities were stunned by Hutton's research, and not only because he claimed the world is much older than the Bible implied. Hutton also showed that supernatural events like the Biblical Flood aren't needed to explain why the world's surface looks the way it does. Ordinary processes seen everyday could produce the same landscape given enough time. Hutton concluded that these processes have operated in the same manner throughout Earth's history.

This idea became known as the Theory of Uniformity, or later as Uniformitarianism. The principle of uniformity underlies modern geology; for example, the presence of vast amounts of layered rock in the Grand Canyon can be explained as the result of ordinary sedimentation operating over millions of years. Such vast spans of history have been distinguished from more modest everyday experience by a special designation, "geologic time."

Although arguments about the age of the Earth raged among the intellectual community throughout the Nineteenth Century, most scientists have accepted the premise that the Earth must be millions of years old,instead of just a few thousand, and although they were not yet able to measure the age of rocks, geologists realized they could determine the sequence of events in Earth's history by combining careful observation with some simple logic.

They first reasoned that any layered sequence of rocks is formed with the oldest layer at the bottom and successively younger layers on top. This principal called "superposition" allows us to place rock units in a historic sequence according to their relative age. They also observed that layered rocks, such as lava flows, volcanic ash layers, and sedimentary rocks are deposited under the influence of gravity. For this reason, the rock layers themselves are horizontal. According to this principle known as "original horizontality," rock layers that are folded or tilted must have been tectonically deformed, and this had to have happened after they were deposited, and, finally, any geologic feature that cuts across or truncates a group of rocks have to be younger than the rocks that are cut, so features such as faults, intrusions of molten rock, and erosional surfaces can also be placed in a relative age sequence using this principle called "cross cutting relationships."

Although these geologic principles are based on nothing more than common sense, they are absolutely fundamental to the interpretation of Earth history. According to the principle or original horizontality, beds of sediment form as horizontal layers as the sediment settles under the force of gravity. The principle of superposition maintains that the oldest layers of undeformed sedimentary strata are on the bottom and the youngest on top.

Finally, the principle of cross cutting relationships states that a rock is younger than any rock it cuts across as in the case of veins of igneous rock cutting across strata or unconformities, such as Hutton studied.

The principles of relative age dating are simple and yet so powerful that they were used by early geologists in the Eighteenth and Nineteenth Centuries to construct a complete historic sequence of all of the rocks in Europe, and because rock formations in a sequence like this often extend over a large area, they can be used to correlate rocks from one sequence with historically equivalent strata in other regions. This is easy to see in a place like the Grand Canyon.

This formation, the kind of limestone can clearly be recognized in the sequence of sedimentary rocks on the opposite side of the Canyon,and because it's the same formation, it marks the same period of geologic time in both places. Some rock layers persist laterally for thousands of kilometers and can be recognized in different places by their thickness, mineral composition, and other characteristics; for example, volcanic ash from the 1980 eruption of Mount Saint Helens has been identified in sedimentary sequences as far away as Minnesota and Arizona.

Rock formations can be valuable time markers, but correlation is difficult over great distances because physical continuity and rock characteristics can be ambiguous. Fossils like these shells of marine organisms can be extremely valuable time markers. Each fossil species has its own life span, and rocks that contain fossils with overlapping life spans are actual time markers that can be recognized wherever they are found.

The use of fossils to make geologic time correlations was first discovered in the Fifteenth Century, and by the beginning of this century, geologists had used correlation to extend the historic rock sequence from Europe to every continent on Earth. The word "fossil" refers to any evidence of a prehistoric plant or animal that has been preserved in rock. Sometimes part of the organism itself remains intact, in some instances recrystallized or petrified like the trees in a petrified forest. In other cases, the organism may have been completely dissolved leaving only a hollow mold of its own image.

Throughout geologic time, fossil species succeed each other in a distinct recognizable order. In general, progressively older fossils are increasingly different from life forms today. This observation came to be known as the principle of faunal succession, one of the strongest lines of evidence supporting Darwin's Theory of Evolution.

The Nineteenth and early Twentieth Centuries were the heyday of the amateur paleontologist or student of old life. These fossil hunters combing the cliff faces and quarries of the globe identified thousands of fossil species and painstakingly established their relative ages from the positions of the rock layers in which the fossils were entombed.

Regardless of where on Earth fossils are found, they always occur in the same sequence relative to one another. Some of the lower rock strata in the Grand Canyon contain fossils of the same general groupings as those in certain varied ancient rock layers found in whales. Since the ancient name for whales is "cambria," these rocks are called "Cambrian" rocks,and the geologic period in which they were formed, the "Cambrian" Period.

Nineteenth Century geologists used the names of places and ancient peoples to label other times in geologic history as well; for example, the Ordivician Period, the Silurian Period, and the Devonian Period after Devon in Southern England. Characteristic rocks and relative positions of strata were also used to designate new time units, and gradually the geologic time scale was constructed. Of the standard geologic time scale is based really on the law of superposition; the oldest rocks are on the bottom and the youngest rocks on the top.

Looking at rocks in various parts of Western Europe, it was recognized that certain formations could be correlated from one region to another, and the older rocks were very distinctive with a distinct group of fossils in them. The younger rocks overlay the older ones, they of different colors, different characteristics,different fossils. By arranging them sequentially, which ones were the oldest and which ones are the youngest, they were able to establish the chronology just on the basis of superposition. Each of the rock units took a name from the organisms that were in it or the region where it was first studied giving us the modern geologic time scale.

The relative age relationships that we see in rocks allow us to recognize geologic events in a historic sequence through time, but knowing only the order of events is a bit unsatisfying. Without the ability to measure the absolute age of these rocks, it's impossible to fully understand Earth history? And without a calibration of geologic time, we can't study the rates of geologic change that are used to make forecasts about future events.

Scientific discovery is often born of chance events coupled with keen perception. In the late Nineteenth Century, Madame Marie Curie discovered that when certain types of rocks were stored in the same drawer overnight with undeveloped photographic film, the film was exposed. This chance event led to the discovery of radioactivity. In this process certain types of atoms spontaneously disintegrate or decay. The decay is random, but it proceeds at a constant rate through time for each particular type of atom or isotope.

Because many types of rocks contain radioactive isotopes, Ernest Rutherford reasoned in 1905 that the uniform rate of radioactive decay could be used to measure geologic time. From this realization came the eventual development of techniques to measure the absolute age of geologic materials. Now, for example, not only do we know that the dinosaurs disappeared in a global mass extinction, we know it happened 64 million years ago. Each element can be separated into isotopes, groups of atoms with similar atomic structure, though slightly different mass. Some isotopes are radioactive and, therefore, unstable. This instability causes the atom to radiate energy, much of it in the form of electrically charged particles. The radiation gradually causes the element to disintegrate or decay transforming the original radioactive isotope or "parent" into a stable isotope known as a "daughter" product. It is this process of decay that we call "radioactivity."

Grains of sand trickling through the narrow opening of an hourglass are somewhat analogous to isotopes decaying in a radioactive element. The sand in the upper half of the hourglass represents a supply of parent isotope. The sand in the lower half is its daughter product. The proportion of parent to daughter depends upon how much time has elapsed. This can be determined simply by measuring the ratio of parent to daughter sand. This is how radioactive dating works.

The most familiar example is radiocarbon dating. The carbon in air and water is made up of three different isotopes. One of these, carbon 14, is radioactive. As part of the atmosphere carbon 14 is absorbed by living plants and animals. In this way, the carbon in all living organisms contains the same proportion of carbon 14 to nonradioactive carbon. When the organism dies, it stops taking in any new carbon, and the amount of carbon 14 inside it gradually decreases as it decays. It is this shrinking proportion of carbon 14 that tells us how long ago the organism died. The rate of decay of any radioisotope is measured in terms of its half life,how long it takes for half of its atoms to decay.

The half life of carbon 14 is 5,730 years. Carbon 14 is most commonly preserved in the form of charred wood that is buried by sediment, lava, or volcanic ash. If a sample contains only half the level of carbon 14 found in living organisms, geologists conclude that it must be 5,730 years old. If the ratio of carbon 14 is only a quarter of that of a living organism, another half life has elapsed, and it must be 11,460 years old, and so on. This method is only reliable for dating things back about 50,000 years. Beyond that, the amount of radiocarbon remaining is too small to permit accurate measurement.

To date rocks that are millions or even billions of years old, geologists work with radioisotopes that take much longer to decay, such as uranium 235. This element makes an excellent geologic clock because it takes 713 million years for half of it to decay into lead 207. By measuring the relative amounts of these two isotopes in a rock, it's possible to figure out the rock's age even if it is several billion years old.

The first step in determining the age of the rock is finding the proper rock in the field and determining its mineralogy. Once we've determined what it is we want to date, then we sample it in the field, bring it back into the laboratory, separate out the minerals that we want to date that tell us the age of the rock. Once we have a clean mineral separate, then we take it into the laboratory, and we separate out chemically the elements that are within the rock that have the isotopic system that we're interested in, and then once we have the elements separated, we bring the elements into the mass spectrometer and with the mass spectrometer we analyze the isotopes of the elements that we're interested in, and if the isotopic ratios between the parent and the daughter isotopes and between the various daughter isotopes, they give us the age of the rock.

The mass spectrometer accelerates the ions or charged atoms of an element, so that they form a beam which is then spread out into a succession of points, a mass spectrum. From the position and intensity of the points, the mass and relative abundance of individual isotopes can be determined.

Once the element that we're going to measure is loaded in the mass spectrometer it's ionized at a temperature of between a thousand and two thousand degrees centigrade depending on what element it is. The ions are then accelerated by eight to ten thousand volt drop down the tube which is a high vacuum tube, and as they're being accelerated, they're focused into a very fine beam.

Once they enter the tube as a fine beam, they move through a magnetic field. The magnetic field puts a drag on the ion beam, which causes the heavy isotopes to bend along the outer part of the tube and the lighter ones into the inner part of the tube. By the time the beam comes out of the magnetic part of the tube, it's broken into its various isotopes, and at the far end of the tube we count the ions of each isotope, which ultimately gives us our isotopic ratio.

The complete process of collecting rock samples, preparing them for analysis, and doing the mass spectrometry requires a great deal of time and skill. Then you finally get to the point where you put the final bit of data into your computer and calculate your age, and at that point it all comes to somewhat of a crescendo, and you get very excited, and there it is finally, you have your results, and sometimes you're totally surprised.

Sometimes you're disappointed, and sometimes you're elated with your results, and then the final course, aspect of it is really applying the age data to your actual geological relations.

Minerals which contain radioactive isotopes act like tiny clocks inside of rocks, clocks which are reset when a rock is formed or metamorphosed. Before geologists learned to measure the absolute ages of rocks using radioactive isotopes, they could only establish relative ages using the common sense logic of superposition, original horizontality, and cross cutting relationships. They knew that one rock formed before another, but never how old the rocks actually were.

The reliability and accuracy of radiometric age dating is truly impressive. The results of over half a century of radiometric age dating studies are entirely consistent with the relative age relationships developed by early geologists, so the science of geology has given us two ways to understand Earth history.

• First, we determine the relative age of rock formations and geologic structures and then use these ages to construct a historic sequence of events through time.
• Then, by applying radiometric age dating techniques, we determine the absolute timing of these events and measure the rates of geologic change as well, so not only does our study of geologic time allow us to understand Earth's past, it gives us a historic framework and the tools for predicting the planet's future.

Major funding for "Earth Revealed" was provided by the Annenberg CPB Project.

I like that video a lot. It shows us a lot of things that we haven't been really seeing in the other videos, and for the first time in this video, we have a chance to actually see some of those features that are being talked about.

It's very hard to show actual pictures of the ocean floor or to show pictures of tectonic plates moving, so we're beginning to get to the point where we can actually see some of the features that are described.

I also noted in this video that they showed the geologic arrows with the oldest at the top. Shame on them. We always show rock layers and their names with the oldest at the bottom. Don't get confused by that if you go back and rewatch the video.

You might want to study Figures 8.1 to 8.10 in the textbook to see how the geologic history of an area is inferred by using these block diagrams.

Before we go on and mention absolute age, I want to say one more thing about relative age. That has to do with what we call the "geologic column." See, using laws of superposition, faunal succession, horizontal continuity, and cross cutting, geologists have made a chronological arrangement of sedimentary rocks from all over the world, which we call the "geologic column." It's the results of the correlations of all the sedimentary rocks that we find.

You might want to pay special attention to Box 8.4 on page 190, and also go back and look at Table 1.1 on page 22, both of which show the geologic column in slightly different ways. You'll notice as you look at these columns, there are a lot of names involved, and it's not necessary to memorize all of them, but there are certain things that you should know.

You should know the major geologic periods. The Precambrian, which is about 8/9 or 22 out of 25 of all geologic time. It encompasses the great majority of geologic time.

The rest of geologic time, recent geologic time, if you like, the last 550 million years, is called the Phanerozoic. "Phanerozoic" means visible life and consists of the Paleozoic or Ancient Life, the Mesozoic, middle life, and Cenozoic, recent life, so you certainly should know those four terms and be able to refer to them in the diagrams when the appropriate time comes.

Until the middle of the Twentieth Century, the geologic column was all that known about the ages of rocks, and there was no way to know exactly when, for example, the boundary between the Cambrian and Precambrian occurred. We simply knew it was ancient, but we didn't really know how ancient. What was needed was some way to absolutely date those times; in other words, to find the number of years. You see, our human units of time are based on the rotation of the Earth on its axis, a day, and the revolution of the Earth around the sun, or a year.

One of the important goals of geology is to determine how many of these units have passed in the distant past when there was no ones around to observe or record them, so the way we do this is by radioactive dating. The video was fairly good, I think, on describing radioactive dating, but I want to elaborate on that a little bit.

Rocks, like all substances, are made of atoms. Atoms are the tiniest building blocks on matter. There are 92 different types of atoms of which 88 occur naturally on Earth. These correspond to the various chemical elements. All of these are found in varying degrees in rocks, but only a few of them are abundant, and we'll discuss the abundant ones specifically when we discuss minerals in a later lesson.

You see, the key here is that atoms of a given chemical element are basically all alike--well, almost all alike. They have certain properties that define that particular chemical element, so atoms of oxygen are called "atoms of oxygen," for example because they all react in a certain way with other atoms. The difference is that some atoms are different weights than others. Now, the different weight of the atom doesn't affect its chemical property, but it does affect certain physical properties, so that sometimes if we can separate the isotopes of a particular atom from one another, by counting the number of atoms, we can learn a lot of information.

Specifically, we learn about the age of the Earth by looking at radioactive isotopes. Radioactivity's kind of a complicated topic in itself, but it works something like this. Certain isotopes of certain atoms are radioactive. What that means is they decay. When they decay they eject electrical particles from the nucleus, and that basically changes them into others kinds of atoms, so that if we start out, for example, with uranium, some of the uranium atoms in the rock will spontaneously decay and change into a different kind of atom, so if we can count the number of uranium atoms and count the number of atoms that we know uranium atoms change into, then we have a way of at least knowing how many uranium atoms have decayed.

The other key that we need here is to understand that different isotopes or different radioactive isotopes decay at different rates. The term we use to describe this is called "half life," but when we talk about the "half life" of a particular element like uranium 238, we simply mean that in a certain specified amount of time, half of the atoms of that substance will have decayed to have become a new substance. Although it's impossible to predict exactly which atom will decay and when, we do know from laboratory studies how long it takes a particular sample to decay. So each isotope has a particular unique decay rate and a decay product.

We call the products "daughters," so we find, for example, that uranium 238 goes through a series of decays producing different daughter products in the process, and some of these daughters are also radioactive. The end result of this is a particular isotope of the element lead called "lead 206," so we find that starting with uranium 238, we go through a series of steps and wind up with lead 206. Now the key here is that we know how long it takes the uranium to decay into lead.

It turns out in this case it's about the age the Earth, about 4-1/2 billion years, so of all the uranium that was present on the Earth at the time of its formation, about half of it has decayed. The other thing we need to know here is that lead 206 is only formed by the decay of uranium 238, so whenever we find lead 206 in the rocks, we know that that lead 206 was formed by the decayed uranium.

There are other isotopes of lead that are formed in other ways, but lead 206 we understand was formed from uranium, so basically by counting atoms of different types and knowing the sequences and the half lives, we can use these to estimate the age of the Earth.

It turns out that there are several radioactive isotopes which have the right properties to be useful, and generally any one parent-daughter pair has a large margin of error, so we use several pairs to check the dates independently, and we find that usually when we do this that all of them give ages within the same range, and the probability of the chance that any two methods will agree randomly is very small, so we generally accept the fact that dates determined from two or more sets of isotopes give us relatively good dates.

Okay, radiocarbon dating was covered in the video and the text. Unfortunately, the concept of mass spectroscopy was not covered very well in the video, but all we really need to understand about that is it's a way of separating isotopes from one another so that we can count them.

Okay, we'll find that understanding the evidence for geologic time will help us to understand future lessons and keep in mind as we go into the future lessons that small processes over long time periods can produce large results.

Okay, I also want to note that the next lesson does not have a separate chapter in the textbook, so we'll use this chapter, and you need to check in the study guide for the assignment for this particular lesson which involves several readings, so I'll leave you with that until the next program.

I'll see you next time.