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 million years, or 8% of geological time. Mammals have dominated since the demise of the dinosaurs around 65 Ma, or 1.5% of geological time, and the genus Homo has existed since approximately 2.8 Ma, or 0.06% (1/1,600th) of geological time.
A useful mechanism for understanding geological time is to scale it all down into one year, let’s say the year 2022. The origin of the solar system and Earth at 4.57 Ga would be represented by the clock striking midnight on December 31st, 2021, and the present day would be represented by the last tiny fraction of a second on New Year’s Eve 2022. At this scale, each day of the year represents 12.5 million years; each hour represents about 500,000 years; each minute represents 8,694 years; and each second represents 145 years. Some significant events in Earth’s history, as expressed on this time scale, are summarized on Table 4.1.
| [Skip Table] | ||
| Event | Approximate Date | Calendar Equivalent |
|---|---|---|
| Formation of Earth | 4.57 Ga | January 1st, 12:00 AM |
| Formation of oceans and continents | 4.5 to 4.4 Ga | January 1st to January 8th |
| Evolution of the first primitive life forms | 3.8 Ga | early March |
| Formation of the oldest rocks in Yellowstone and Teton National Parks | 2.7 Ga | Memorial Day Weekend |
| Evolution of the first multi-celled animals | 0.6 Ga or 600 Ma | November 15 |
| Animals first crawled onto land | 360 Ma | December 1 |
| The Rocky Mountains were formed | 90 Ma | December 25 |
| Extinction of the non-avian dinosaurs | 65 Ma | December 26 |
| Beginning of the Pleistocene ice age, and Yellowstone super eruption that produced the Huckleberry Ridge Tuff. | 2 Ma or 2000 ka | 8 p.m., December 31 |
| Retreat of the most recent glacial ice from Yellowstone | 14 ka | 11:58 p.m., December 31 |
| Arrival of the first people in the Yellowstone region | 10 ka | 11:59 p.m., December 31 |
| Establishment of Yellowstone as a National Park | 150 years ago | less than 1 second before midnight, December 31 |
Geologists, and the broader public should understand geological time. That doesn’t mean memorizing the geological time scale; instead, it means getting your mind around the concept that although most geological processes are extremely slow, very large and important things can happen if such processes continue for enough time.
For example, the Atlantic Ocean between eastern coast of the U.S. and continental Europe has been getting wider at a rate of about 2.5 centimeters (cm) per year. Imagine yourself taking a journey at that rate—it would be impossibly and ridiculously slow. It is also approximately the length your fingernails would grow in one year. But, over the long expanse of geologic time, such miniscule rates add up. Since North America and Europe broke apart and the Atlantic Ocean started to form at around 200 Ma (just 4% of geological time), the Atlantic Ocean has grown to a width of over 5,000 kilometers (km). Just think, if you were to grow your fingernails out for 200 million years, they would be 5,000 kilometers long! Gross!
As you read through the rest of this chapter, you will learn how geoscientists have pieced together the geologic history of Earth, both in the order that events happened (relative age dating), and the numerical time that the events occurred (absolute or radiometric age dating).
Media Attributions
- Physical Geology-2nd Edition, by Steven Earle is licensed under CC BY 4.0, adaptions from the original by Ryan B. Anderson.
- Figure 4.1.1: © John St. James. CC BY.
- Hutton, J, 1788. Theory of the Earth; or an investigation of the laws observable in the composition, dissolution, and restoration of land upon the Globe. Transactions of the Royal Society of Edinburgh. ↵
(Mega annum) millions of years before the present.
(Giga annum) billions of years before the present.
(Kilo annum) thousands of years before the present.
The most resent eon of geological time, encompassing the Paleozoic, Mesozoic and Cenozoic.