MAGNETIC MINERALS IN ROCKS: SMALLER IS BETTER

As mentioned in Chapter 5, paleomagnetic studies suggest that remanent magnetization imprinted on volcanic rocks has been preserved for hundreds of millions of years without changing its direction and intensity. How can such remarkable stability be possible? To answer this question, we first need to introduce some basic concepts of rock magnetism. Magnetic materials such as iron, nickel, and iron oxide (typically magnetite) are said to be ferromagnetic, meaning that when subjected to an external magnetic field, they are magnetized strongly in parallel to the magnetic field. Even after the removal of the applied magnetic field they still retain a noticeable amount of magnetization, which is called remanent magnetization.

Ferromagnetism is retained only below the critical temperature known as the Curie temperature. This phenomenon was discovered by the French physicist Pierre Curie (the husband of Marie Curie). For example, the Curie temperatures of iron, nickel, and magnetite are 770 °C, 358 °C, and 585 °C, respectively. Above the Curie temperature, ferromagnetism disappears, and these materials no longer exhibit magnetization.

USEFULNESS OF THE EARTH’S HISTORY

The Earth’s history is in itself a very interesting research theme, but it is also essential if we want to understand fully what is happening in the present Earth. For example, let us consider the Earth’s magnetism. As said in earlier chapters, the fluid motion within the metallic core acts as a generator and gives rise to the geomagnetic field. Studying magnetohydrodynamics in the present-day core alone, however, will not lead to a total understanding of the origin of the geomagnetic field, because many of its characteristics, such as the reversals of its magnetic polarity, need to be examined on a time scale of tens of millions of years. In order to understand why and how the geomagnetic field emerged, we need to understand not only the evolution of the core, but also the evolution of other components in the Earth system. In fact, most topics in Earth sciences can be understood more deeply in the context of the evolving Earth. The Earth is steadily cooling down with declining internal heat production, and everything is changing with time, albeit very slowly.

Studying the Earth’s history also provides the most effective means to forecast the fate of this planet. We cannot, of course, predict everything by studying the past. For example, research into the Earth’s history does not give us a particularly useful way to predict the timing of major earthquakes, which may occur anytime. Also, studying the past does not instantly provide a clue to what we should do for the future. History does not repeat itself in exactly the same way. A careful reconstruction of what happened in the past must be followed by a theoretical study to understand why and how exactly it happened. Without this combination of observations and theory, we cannot extrapolate our understanding to the future Earth with confidence.

NON-RADIOGENIC STABLE ISOTOPES

In the preceding sections, we have shown how isotopes play an important role in tackling a variety of problems concerned with tracing the Earth’s evolution, with an emphasis on radioactive isotopes as a unique time marker. Let us recall, for example, that argon consists of three isotopes argon-36, argon-38, and argon-40, all of which are stable isotopes. Argon-40 is a radiogenic stable isotope, and its amount increases with time through the radioactive decay of potassium-40. The other two isotopes of argon are non-radiogenic stable isotopes, and their abundances have not changed since their birth in a star. We have discussed radiometric geochronology, in which the amount of a radiogenic stable isotope such as argon-40 yields an absolute time marker of rock or mineral formation age.

Here, we turn our attention to the use of non-radiogenic stable isotopes as a tracer of geochemical processes in nature. Thanks to the recent development of microchemical analysis of elements, systematic variations in the ratios of non-radiogenic stable isotopes, albeit extremely subtle, yield a unique means to entangle extremely complicated geochemical cycles of elements. Let us see how this becomes possible.

NUCLEOSYNTHESIS IN THE GALAXY

The colorful drama of Earth’s evolution begins with the formation of the Solar System. The Earth is, after all, merely one of several planets orbiting around the Sun, and the story of how the Earth formed cannot be told without describing how the entire Solar System was formed. Studying the formation of the Solar System in turn enables us to better understand the processes leading to the birth and death of stars, and the mechanisms by which all of the chemical elements in the universe were formed.

Thanks to nuclear physics, we now know how the chemical elements in the universe were made. Except for hydrogen, helium, and some lithium, which were created shortly after the Big Bang, all of the chemical elements were made by stars. Stars explode to end their life and eject the old and newly created elements into interstellar space. When a cloud of interstellar materials has high enough density, it will collapse under its own gravity to form a new star as well as a dusty disk around it, from which planets will emerge. A whole new cycle of chemical element synthesis (called nucleosynthesis) starts again within the central star, until the star consumes all the nuclear fuel and ends its life cycle explosively. In doing so, the universe becomes chemically more and more enriched with heavy elements.

Preface to the first edition

It is thought that the Earth was born as a planet about 4500 million years ago. Throughout the long years since then it has continually evolved, and has undergone a transformation into its present form.

Tracing the evolution of the Earth is a central topic in Earth science, and has been dealt with by many writers. However, most previous histories of the Earth have been concerned with the past 600 million years, since fossils have been found in abundance from this period, and only touch very briefly on the Precambrian period, which is equivalent to roughly seven-eighths of the Earth’s history. But those basic qualities of the Earth with which we are so well acquainted – the magnetic field, the layered structure of the core and mantle, the atmosphere and oceans, were all formed in the very early stages of the Earth’s history.

THE ENERGY HIDDEN IN ROCKS

Granite is one of the most common and well-recognized rocks to occur at the surface of the Earth. Let us suppose that we put a fragment of granite in a small container, which is then completely sealed. We will assume that the container is made of an ideal thermally insulated material, and that heat can neither escape from within nor enter from outside. What changes will occur in the granite inside the box?

If the contents of the box were examined after one or two years, probably no changes at all would be observed. However, if it were examined after the passage of several hundreds or thousands of years, a careful observer would no doubt realize that the temperature within the container was rising very slightly. After a few hundred thousand years, this rise in temperature would be apparent to any observer. If the calculation described later is carried out, it is clear that the granite in the sealed container would melt completely after several tens of millions of years owing to the rise in temperature.

MEASURING THOUSANDS OF MILLIONS OF YEARS

How do we determine the time scale that forms the backbone of the Earth’s evolution? This chapter will focus on the method of measuring incredibly long “geological ages”, far exceeding the bounds of human experience.

Our sense of time is usually connected to some kind of change in geometrical or physical quantities. Taking a watch as an example, the angle of the hand in its revolution corresponds to the time. Similarly, when measuring geological ages of thousands of millions of years, it is necessary to find some appropriate quantity for the transition in time, such as the length, angle, or weight of an object.

Though the principle in measuring geological time is the same as in measuring time in everyday life, several important problems arise from the extraordinarily long time involved. A particularly important aspect is that there must be a guarantee that a “geological clock” hasmoved at the same pace for thousands of millions of years. At best, a human lifespan is no more than 100 years or so, and the history of the entire human race covers less than amillion years. Even if we are successful in finding a “geological clock” and can ascertain that it has been ticking away at an extremely regular pace since we have been on Earth, how can we be sure that it has moved at the same pace over a period hundreds of thousands of times longer than our existence?

Preface to the English edition

To cover the 4500 million years of the history of the Earth in one book is certainly a formidable task. As my particular field lies in isotope geochronology and rock magnetism, which are the most effective means of clarifying the Earth’s evolutionary history, I have been able in this book to present my own view of the Earth’s evolution mainly on the basis of results obtained by these two approaches.

In preparing the English edition, I have made a few changes following comments by my colleagues on the original Japanese edition. I have now realised that to prepare the English edition involved far more than mere translation. I have had to admit that the Japanese language is more suited to literature than it is to being a scientific medium. So for Mrs Judy Wakabayashi the task was to convert a language suited to the heart into a language suited to the mind. And as far as the English edition is concerned, I feel that she is almost entitled to be a co-author, and I would like to express my very deep appreciation of her work and for all the “blood, sweat and tears” which she has endured during the past six months.

SECONDARY ORIGIN OF THE ATMOSPHERE

The seemingly simple conclusion that the Earth was not born in a space filled with air, but that the air was formed after the birth of the Earth, is the starting point in considering the origin of our atmosphere. This was pointed out clearly for the first time in 1947 by Harrison Brown of Caltech.[1] When considering the origin of the Earth’s atmosphere, Brown turned his attention to the abundance of rare gases in the atmosphere. There are five types of rare gases. In order from the lightest to the heaviest atomic weight, they are helium, neon, argon, krypton, and xenon. Since none has any chemical affinity, they hardly ever combine with other atoms and are extremely inactive. Because of this, a rare gas is also called a noble gas (i.e. too “noble” to bind with other elements), but the term “rare gas” actually hints at the origin of the atmosphere. As this term implies, the atmosphere contains only an extremely small amount of these (argon-40 is the only exception, which will be discussed later). In terms of volume, xenon constitutes no more than about 0.000 01 percent, or 0.1 ppm (parts per million), of the atmosphere, and even neon, which is the most abundant rare gas (excluding argon-40), constitutes only about 20 ppm.

Looking at the whole Solar System, however, rare gases are by no means rare, compared with other elements with similar atomic weights. Recall the solar abundance of chemical elements discussed in Chapter 2; neon (atomic number 10) is as abundant as nitrogen (atomic number 7) and magnesium (atomic number 12).