13 1.6 Geological Time — Physical Geology – 2nd Edition
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. Hutton didn’t even try to assign an age to Earth, but we now know that it is approximately 4,570 million years old. Using the scientific notation for geological time, that is 4,570 Ma (for mega annum or “millions of years”) or 4.57 Ga (for gigaannum or billions of years). More recent dates can be expressed in ka (kiloannum); 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.
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 1.3 answers.
Unfortunately, knowing how to express geological time doesn’t really help us understand or appreciate its extent. A version of the geological time scale is included as Figure 1.6.1. Unlike time scales you’ll see in other places, or even later in this book, this time scale is linear throughout its length, meaning that 50 Ma during the Cenozoic is the same thickness as 50 Ma during the Hadean—in each case about the height of the “M” in Ma. The Pleistocene glacial epoch began at about 2.6 Ma, which is equivalent to half the thickness of the thin grey line at the top of the yellow bar marked “Cenozoic.” Most other time scales have earlier parts of Earth’s history compressed so that more detail can be shown for the more recent parts. That makes it difficult to appreciate the extent of geological time.
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.
Geologists (and geology students) need to 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 New York City and northwestern Africa 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. And yet, since it 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)!
A useful mechanism for understanding geological time is to scale it all down into one year. The origin of the solar system and Earth at 4.57 Ga would be represented by January 1, and the present year would be represented by the last tiny fraction of a second on New Year’s Eve. 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 1.1.
| [Skip Table] | ||
| Event | Approximate Date | Calendar Equivalent |
|---|---|---|
| Formation of oceans and continents | 4.5 to 4.4 Ga | January |
| Evolution of the first primitive life forms | 3.8 Ga | early March |
| Formation of Colorado’s oldest rocks | 2.0 Ga | July |
| 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 Present-Day Rocky Mountains started to form | 90 Ma | December 25 |
| Extinction of the non-avian dinosaurs | 65 Ma | December 26 |
| Beginning of the Pleistocene ice age | 2 Ma or 2000 ka | 8 p.m., December 31 |
| Retreat of the most recent glacial ice | 14 ka | 11:58 p.m., December 31 |
| Arrival of the first people in North America | 10 ka | 11:59 p.m., December 31 |
| Arrival of the first Europeans on the west coast of what is now Canada | 250 years ago | 2 seconds before midnight, December 31 |
Along with being one of the seven natural wonders of the world, the Grand Canyon serves as one of the ultimate natural laboratories in which vast amounts of geological time can be studied. Now, imagine yourself starting at the top of the canyon and you are able to embark on a 12.6 km trek to the bottom of the canyon where the Colorado River is. When you start the hike, you’ll start at sedimentary rocks that are dated to be 270 Ma. When you reach the bottom, the oldest rocks are an age of 2 Ga (they are metamorphic rocks). With this distance and age data, see if you can answer the following:
- As you hike, how far back in time would you go with each meter along the trail? Give your answer in yr/m (years per meter).
- The elevation at the top of the canyon is 2085 m, with the elevation being 734 m at the river. In knowing that the Colorado River has carved the Grand Canyon through time, at an average rate of 0.5 mm/yr (millimeters per year), how many years has it taken for the canyon to form?
Helpful hints: 1 km = 1,000 m and 1 m = 1,000 mm
See Appendix 3 for Exercise 1.4 answers.
Image descriptions
Figure 1.6.1 image description: The Hadean eon (3800 Ma to 4570 Ma), Archean eon (2500 Ma to 3800 Ma), and Proterozoic eon (542 Ma to 2500 Ma) make up 88% of geological time. The Phanerozoic eon makes up the last 12% of geological time. The Phanerozoic eon (0 Ma to 542 Ma) contains the Paleozoic, Mesozoic, and Cenozoic eras. [Return to Figure 1.6.1]
Media Attributions
- Figure 1.6.1: © Steven Earle. 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. ↵
