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.

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.

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.