Chapter 4 Measuring Geological Time
The Vast Expanse of Geologic Time
In 1788, after many years of geological study, James Hutton, one of the great pioneers of geology, wrote the following about the age of Earth: The result, therefore, of our present enquiry is, that we find no vestige of a beginning — no prospect of an end.[1] Of course he wasn’t exactly correct, there was a beginning and there will be an end to Earth, but what he was trying to express is that geological time is so vast that we humans, who typically live for less than a century, have no means of appreciating how much geological time there is. We now know the Earth is is approximately 4,570 million years old, based primarily on isotopic dating of meteorites that have fallen to Earth’s surface, such as the Diablo Canyon meteorite (Figure 4.1.1). Using the scientific notation for geological time, that is 4,570 Ma (for mega annum or “millions of years”) or 4.57 Ga (for giga annum or billions of years). More recent dates can be expressed in ka (kilo annum); for example, the last cycle of glaciation ended at approximately 11.7 ka or 11,700 years ago. This notation will be used for geological dates throughout this book.
Exercise 1.3 Using geological time notation
To help you understand the scientific notation for geological time—which is used extensively in this book—write the following out in numbers (for example, 3.23 Ma = 3,230,000 years).
- 2.75 ka
- 0.93 Ga
- 14.2 Ma
We use this notation to describe geological events in the same way that we might say “they arrived at 2 pm.” For example, we can say “this rock formed at 45 Ma.” But this notation is not used to express elapsed time. We don’t say: “I studied for 4 pm for that test.” And we don’t say: “The dinosaurs lived for 160 Ma.” Instead, we could say: “The dinosaurs lived from 225 Ma to 65 Ma, which is 160 million years.”
See Appendix 3 for Exercise 4.1 answers.
Despite his recognition of the vast expanse of geologic time, Hutton didn’t even try to assign an age to Earth. He wouldn’t have been able to even conceive of the idea of dating a meteorite with radioactive isotopes, or that the age of the Earth could be BILLIONS of years old. The concept of radioactive decay wasn’t even discovered until 1896, and our ability in the geoscience community to assign such precise numerical ages to events in Earth history was not well refined until the 1950’s and 60’s. Still, knowing the exact number of years events happened in the geologic past, and how to express geological time doesn’t really help us understand or appreciate its extent.
To create some context, the Phanerozoic Eon (the last 542 million years) is named for the time during which visible (phaneros) life (zoi) is present in the geological record. In fact, large organisms—those that leave fossils visible to the naked eye—have existed for a little longer than that, first appearing around 600 Ma, or a span of just over 13% of geological time. Animals have been on land for 360 milli