May 2004

Image of the cover of 'Mysteries of Terra Firma' by James Lawrence Powell

This months book is "Mysteries of Terra Firma: The Age and Evolution of the Earth" by James Lawrence Powell. Powell is the president and director of the L.A. Museum of Natural History. He's also a former geology professor, and has written another book I have read called "Night Comes to the Cretaceous." This book is split up into three parts, and all deal with the changing science of geology, and its history. Each part takes a central aspect of geology and develops how we came to understand what we do. The first part deals with geologic time, the second with plate tectonics, and the third with impacts from extraterrestrial objects like asteroids and comets. Let's start with the first part, shall we?

Part I: Time

The Mill of Exquisite Workmanship

This first chapter of the book examines how the concept of the age of the Earth has changed over time. Up until relatively recently, the age for the Earth dealt in some fashion with some concept of creation. An example of this comes from Bishop Ussher, who calculated the age of the Earth by adding up the lineages listed in the book of Genesis in the Bible. Of course, with different cultures and creation beliefs came different ages. The question of age seemed, for at least the believers, to be something reasonable. Then, people were looking at what were determined to be the remains of once living organisms. Much of these had come from rock, so then came the question of how they came to be in rock? The first person to consider this question was Nicholas Steno (also known as Niels Stensen). Steno has come to be known for his principles of superposition, which are still used today. Soon after, people began looking for natural processes that could help calculate the age of the Earth. One of the first ways developed was loosely based on the hourglass. In an hourglass, the sand falls from top chamber to lower chamber in a fairly accurate and predictable manner; a certain quantity of sand is used to measure out a length of time that is about an hour long, hence the name. Applying this same concept to rocks, as Powell points out, if you know how much rock you have, and the rate at which it accumulated, you have an idea of how old it is. In principle, this sounds like a good thing, but it has many problems. The first is the assumption of thickness: are we sure we're looking at a complete sequence, or has some of it been removed by erosion? Another concerns the rates at which sediment accumulates; are they constant? These problems made this kind of dating method unreliable. Over time, many methods have been used to try and give an age of the Earth, but not many seemed to work.

Despite not having a reliable way of dating the Earth, early geologists and earth scientists were still able to arrange geologic time by oldest to youngest, using Steno's laws of superposition. They didn't have figures, but they could tell you how old one rock is in relation to another. With more study of rocks came more suggestions of how rocks formed, and how old they really are. Charles Lyell, one of the most prominent figures in the history of geology, came up with a claim that has since come to be known as "uniformitarianism." His claim states that natural processes are constant, and have always been constant. Given the immense size of the geologic column, and taking into account that processes have remained constant, one could suggest that the Earth's age was very old. This didn't sit too well with scientists in other fields, such as Physics. Lord Kelvin, the Einstein or Newton of his day, used reasonable calculations based in Physics to give an age of the Earth. He looked at it like this: the Earth is essentially a big ball of molten rock, which was cool on the outside, and hot on the inside. Logically, if you can measure how thick the cool crust is, and how fast it gets hot as you go deeper into the Earth, you can get a reasonable age. His assumptions were based on the rate of loss of heat from a solid object. His original calculations put the age of the Earth to be at about 100 million years, but as more data was collected from other parts of the world, his number kept on shrinking until finally reaching the age of 20 million years. This seemed impossible, even with Kelvin's sound mathematics. This simply wasn't enough time for geology. The remainder of this chapter talks about other methods used by geologists and other scientists to figure an age of the Earth. Although reasonable to the people working with them, none proved as accurate or as reasonable as...

Strange Rays

Right around the turn of the 19th century to the 20th, a peculiarity was being examined. Scientists were looking at the relationship between phosphorescent rocks and what could make them glow as they did. It had also been observed that some kinds of rocks were emitting some kind of energy that would affect things like photographic plates. This came to be known as radioactivity, and it was studied by Marie Curie, her husband, and Antoine-Henri Becquerel. This lead to studies done by Ernest Rutherford, the first scientist to record and measure the half-life of a radioactive element, thorium. This, then, lead to the study of radioactive decay, and opened up a whole new can of worms with which to fish an age for the Earth.

With this new science came a flurry of different methods and means for detecting radioactive decay, and different scientists were getting different numbers. The one thing they all had in common, however, were that their numbers far surpassed Kelvin's age of the Earth, and finally gave enough time for the geological processes of the Earth to work.

The End of the Debate

After the concept of radioactivity was discovered, next came the refinement of how it works. Part of the problems with early radioactive decay dating came with an incomplete understanding of parent and daughter isotopes. Rutherford, for example, thought that the uranium in the pitchblende radiated hydrogen, when we know now that the daughter of the uranium isotope is lead. This chapter pretty much just explores the refinement of radiometric dating. Once scientists figured out how to do radiometric dating, the next thing was to put it to practical application, which is explored in the next chapter.

The Age of Meteorites, the Moon and the Earth

Now that they had a method to obtain accurate ages, scientists now needed material with which to test their methods. Finding the right rock will get what you're looking for, but what you need is some way to verify the results independently. That means finding rocks that had radioactive material, but weren't from Earth. Since the beginning of recorded time, humans have noted that from time to time, rocks fell from the heavens. Only recently have we been able to identify where these heaven-sent particles came from. Some came from the asteroid belt located between Mars and Jupiter, some are chunks of Mars. Added to rocks from other planets, came moon rocks taken from our nearest companion. Samples obtained from the moon were also tested for radioactive isotopes, and along with the meteorites, dates were obtained that matched identically with those found on Earth. There are several radiometric dating methods; samples from the St. Severin meteorite were tested with five different radiometric techniques, and they all converge to a particular date: about 4.5 billion years old. The only problem with this is that having tested rocks on Earth, the oldest were only around 4 billion years old. The important find was that despite the missing 500 million years, samples from Earth fell right into a predictable curve made by radioactive material from other rock sources. What does this mean? Well, even though we don't have rocks that are 4.55 billion years old, we have samples that fall right into the same age range from rocks from meteors, Mars, and the Moon. Powell finishes this chapter and section discussing other methods and how they confirm the age of the Earth at around 4.55 billion years.

Part II: Drift

A Science Without a Theory

Part II discusses the development of Continental Drift and the Theory of Plate Tectonics. For much of human history, the Earth was considered a static, unmoving body of rock. All that exists does so because that's how God made it: in place and unchanging. With the discovery of the New World, and further exploration of the Western Hemisphere, better maps were reconstructed, and people were beginning to notice that, at least sometimes, the Earth moves, as in the case of earthquakes. There were other things like volcanoes, and hot springs. Geology didn't really have an explanation for them. Volcanoes spit out lava, sometimes flowing, sometimes blocky and chunky, other times ashy. The connection between volcanoes and the center of the Earth were easy to make, but scientists were finding volcanic rocks hundreds, even thousands of miles from the nearest volcanoes. Something needed to explain these anomalies. Another troubling problem came in the form of fossils. Fossils of the same species were being found on continents separated by oceans. The only way, it was thought, that this could happen is if there were some kind of land bridge, but nothing about the geology suggested that there was one. These issues were considered by Alfred Wegener, and after much study, he thought of the continents floating around the Earth's surface like huge sailboats on a sea of molten rock. If one could imagine in the mind's eye some distant time in the past, perhaps all the continents were all attached to each other. This was one way you could get fossils of the same species on now-separate continents. Wegener, a meteorologist, presented his idea at an American Association of Petroleum Geologist meeting, and no one accepted his ideas. How did Wegener suppose these ships of land sail across his fanciful molten rock oceans, they wondered. Wegener had no mechanism to drive his idea, so the notion of the continents moving was put on hold, for a little while at least.

The Dream of a Great Poet

This chapter discusses what made Wegener conclude what he did, and why. It begins with a slight biography of how he came to study climates and the subsequent adventures that would lead to his discovery. Wegener was already considering continental drift; ever since South America and Africa became well-mapped, there has always been the suggestion that they fit together like pieces of a puzzle. He needed more than just an anecdotal reference to how nice Africa and South America fit together, he needed to show that the other views didn't adequately explain what geologists were finding. He also needed to show that what he proposed was happening now, and could be tested by others so that they could see for themselves. Despite Wegener's lines of evidence, he was missing a major, crucial point: he had no mechanism for his sailing continents.

The Rejection of Drift

The most unfortunate aspect of Wegener's drift theory is his timing. He presented his idea when he didn't have much in the way of geology to support it (Well, he did, but not enough that would convince the prevailing geologists. Wegener, after all, was a meteorologist, what could he possibly know about geology?). Wegener's drift wasn't very well accepted, and many geologists spent a lot of time and effort showing how what he proposed went against knowledge and information of the day. It almost became a requirement, in fact, to reject drift at all costs, and to consider drift seriously was to potentially commit career suicide. Powell explores the major opponents to Wegener's drift theory in this chapter, showing how adamantly it was argued against.

A Plausible Mechanism

Despite Wegener's drift theory not having a mechanism, it really did fit the evidence well. People that rejected drift opted for other explanations, but each of these lacked mechanism as well. One prevailing notion was the concept of land bridges connecting continents, such as Beringia, the Bering Strait Land Bridge. Bridges were imagined to cross spans of oceans, sometimes thousands of miles wide, but no one knew how they would have formed, or where they went. Students of geology, while told to reject drift by their experienced predecessors, couldn't honestly disregard it, however. Right around the time that drift theory was presented, the notion of radioactivity and heat in the Earth's core were being addressed within the scientific community. Wegener doesn't seem to have made the connection between the two, but other people did. If radioactivity could keep rocks hot, they could also likely melt them. Molten rock would behave like any other hot liquid, and would have convection. Convection, then, could be a very likely mechanism that would be strong enough to move lighter continental crust around the surface of the Earth. This provided such a good explanation, that Arthur Holmes created a model of Sea Floor Spreading, decade before it was discovered by marine geologists. Despite what Holmes suggested, drift was still rejected by geologists. That is until WWII.

Data from the Abyss

The invention of the submarine had tremendous implications on how wars would be fought in the 20th century. It also had a tremendous impact on the science of geology within the 20th century as well. Before the second World War, submarines were being used to explore the depths of the oceans. Since the war, they have been used as weapons on the high seas. Submarines need some sort of navigation, some kind of map to get from one place to another in secrecy, and as a result for this need, maps of the sea floor were being created. Along with these maps, data were being collected, and analyzed, showing things that shocked many geologists. For example, there was evidence of polar reversals, not just once, but many times. Another thing that was discovered was a massive ridge that extended the entire length of the Atlantic Ocean, and that it lay right in the middle of Africa and South America. Seismic data showed earthquakes occurring along this rift, so something had to be going on. This information caused some to reconsider drift theory.

Perhaps the most influential factor in the argument for continental drift came from the evidence in support of seafloor spreading. Paleomagnetism showed a peculiar trend in the rocks gathered from the floor of the Atlantic Ocean. They formed in rough bands, and they alternated polarity. What's more, these bands seemed to form in exactly the same pattern on either side of a mid-ocean ridge. These patterns matched so closely, they seemed mirror images of each other. It looked like molten rock was pushing up through the thin crust, pushing material to the sides, and cooling off, keeping record of the Earth's magnetism. It also seemed that the Earth's magnetic field had swapped many times in the past. Despite the mounting evidence that suggested that the seafloor was in fact being split apart by molten rock from below, the notion was met head on with much opposition.

Rejection and Priority

This last chapter discusses the arguments made against seafloor spreading, and why people would stick to their guns even when their arguments were incorrect or unfounded. I think the idea here is that, with many paradigm shifts in science, the old views tend to die only when its last adherent does. Eventually, however, plate tectonics, by way of seafloor spreading, was accepted within the geological community. At present, it represents the best explanation for many geologic processes, such as volcanoes, earthquakes, and mid-oceanic ridges.

Part III: Chance

Glimpses of the Moon

This last section explores the idea of large bolides (comets or asteroids) hitting the Earth. Just about everyone on Earth has seen the Moon, and may have even wondered about how it got where it is. Outside of religious belief, there were, at least in 1964, three general ideas of how it formed. The first held that it was a captured body. The second held that it had formed at the same time and place as the Earth (as a "sibling" planet). The third held that under some force, the mass of the Moon broke apart from the Earth and then cooled off, forming the familiar night companion we all know. Powell spends a little time on each of the three, discussing strong points and weaknesses. After all of the pros and cons had been examined, there still wasn't a satisfying explanation for the Moon. That is, until 1984. At a conference in Hawaii, a new mechanism was proposed, that was a slight variation of the third notion, that is, the Moon spinning off of the Earth and forming a separated body. This idea held that sometime in the early Earth's history, a large Mars-sized object struck the Earth, and massive amounts of material were injected into orbit. This event also knocked the Earth on its side. This model has held out fairly well, and it seems to fit in with what we know about the moon, such as the relative density of Moon material to that of the Earth. This suggestion of impact met with criticism (as all good ideas seem to have), and one of the prevailing notions was that the Earth simply isn't struck by any large objects. This idea came from the earlier 20th century when impacts were considered, and subsequently dismissed. The Earth just didn't have anything to indicate that it'd been hit by rocks from space. What they needed was a smoking gun...

Moonlighting

This next chapter simply continues the story of how the story of the Moon unfolded. It is rather short (only about 7 pages) so there isn't much that I can mention that wouldn't give what it covers away, so I guess you'll have to get the book to see what is going on in here.

Voyages

This chapter explores the Space Race, and how it helped us understand the Moon in a much clearer sense. It details the history of the Space Race, and how finally getting men on the Moon helped settle the controversies about the Moon;s origins, and misconceptions on what was expected to be there. This is another short chapter, and I won't spoil it any more.

A Most Difficult Birth

Even after we went to the Moon, it took astronomers a while to figure out its origins. This chapter again continues the story of how the Moon formed, this time really focusing on the people who made a major contribution to the current notion of it forming after an impact event. Scientists, using computer models, showed that it was possible for a Mars-sized object to hit the Earth in just such a way that the resulting ejecta could form a satellite body. The problem was, how likely is it that this could happen?

Impact Revolution

Once scientists came to terms with impact events, they were looking for evidence of them everywhere they could. The inner planets of our solar system are a hodgepodge mixture of planets, all terrestrial. Powell writes that many of the unique features of the inner four planets can be attributed to some kind of impact event. In the wake of Shoemaker-Levy 9 (the comet that struck Jupiter in 1994), astronomers are beginning to see the importance of asteroids and comets, and the impact (pun intended) they've had in the development of the solar system, in general, and our Earth in particular. Powell touches on the end-Cretaceous impact event (sometimes called Chixculub), which is the subject of an entire book by him. This impact event brought an end to the non-avian dinosaurs, and about 75% of all other living things, according to the Alvarez's (the father and son team who recognized the event and discovered it off the Yucatan Peninsula off the coast of Mexico).

The Tapestry

This last chapter is more of an epilogue than anything else. He uses an analogy of a tapestry to see how all of these topics (radioactivity and the age of the Earth, plate tectonics, and impact events) can be woven together to form the fabric of Earth history. Not much more to say, really.

Summary

As a geology major, this is probably one of my favorite books. Science is somewhat lacking in the history of scientific ideas, and this book is an excellent step in the right direction. Powell takes his experience as an instructor and writes a book in an easy to understand format, even for someone not as interested in geology as I am, personally. The field of geology really depends on all three of these topics, and Powell just does a wonderful job of putting them into perspective, really detailing the 'how we know what we know' for people that don't get the chance to use expensive scientific equipment. If you run across this book in a bookstore, and have an extra bunch of cash, I would suggest that you purchase it. This would make an excellent contribution to any library.