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?