Chapter 4 Measuring Geological Time
It’s one thing to know the facts about geological time—how long it is, how we measure it, how we divide it up, and what we call the various periods and epochs—but it is quite another to really understand geological time. The problem is that our lives are short and our memories are even shorter. Our experiences span only a few decades, so we really don’t have a way of knowing what 11,700 years means. What’s more, it’s hard for us to understand how 11,700 years differs from 65.5 million years, or even from 1.8 billion years. It’s not that we can’t comprehend what the numbers mean—we can all get that figured out with a bit of practice—but even if we do know the numerical meaning of 65.5 Ma, we can’t really appreciate how long ago it was.
You may be wondering why it’s so important to really “understand” geological time. There are some very good reasons. One is so that we can fully understand how geological processes that seem impossibly slow can produce anything of consequence. For example, we are familiar with the concept of driving from one major city to another: a journey of several hours at around 100 kilometres per hour. Continents move toward each other at rates of a fraction of a millimetre per day, or something in the order of 0.00000001 kilometres per hour, and yet, at this impossibly slow rate (try walking at that speed!), they can move thousands of kilometres. Sediments typically accumulate at even slower rates—less than a millimetre per year—but still they are thick enough to be thrust up into monumental mountains and carved into breathtaking canyons.
Another reason is that for our survival on this planet, we need to understand issues like extinction of endangered species and anthropogenic (human-caused) climate change. Some people, who don’t understand geological time, are quick to say that the climate has changed in the past, and that what is happening now is no different. And it certainly has changed in the past—many times. For example, from the Eocene (50 Ma) to the present day, Earth’s climate cooled by about 12°C. That’s a huge change that ranks up there with many of the important climate changes of the distant past, and yet the rate of change over that time was only 0.000024°C/century. Anthropogenic climate change has been 1.1°C over the past century,[1]; that is 45,800 times faster than the rate of natural climate change since the Eocene!
Recall the example at the beginning of this chapter that puts geological time into the perspective of single year, because we all know how long it is from one birthday to the next. At that rate, each hour of the year is equivalent to approximately 500,000 years, and each day is equivalent to 12.5 million years.
If all of geological time is compressed down to a single year, Earth formed on January 1, and the first life forms evolved in late March (roughly 3,500 Ma). The first large life forms appeared on November 13 (roughly 600 Ma), plants appeared on land around November 24, and amphibians on December 3. Reptiles evolved from amphibians during the first week of December and dinosaurs and early mammals evolved from reptiles by December 13, but the dinosaurs, which survived for 160 million years, were gone by Christmas Day (December 26). The Pleistocene Glaciation got started at around 6:30 p.m. on New Year’s Eve, and the last glacial ice left Yellowstone and the northern part of the U.S. by 11:59 p.m.
It’s worth repeating: on this time scale, the earliest ancestors of the animals and plants with which we are familiar did not appear on Earth until mid-November, the dinosaurs disappeared after Christmas, and most of Canada, the Northern U.S., and the Greater Yellowstone Ecosystem was periodically locked in ice from 6:30 to 11:59 p.m. on New Year’s Eve. As for people, the first to inhabit Yellowstone got here about one minute before midnight, and the first Europeans arrived about two seconds before midnight.
It is common for the popular press to refer to distant past events as being “prehistoric.” For example, dinosaurs are reported as being “prehistoric creatures,” even by the esteemed National Geographic Society.[2] The written records of our history date back to about 6,000 years ago, so anything prior to that can be considered “prehistoric.” But to call the dinosaurs prehistoric is equivalent to—and about as useful as—saying that Singapore is beyond the city limits of Pocatello! If we are going to become literate about geological time, we have to do better than calling dinosaurs, or early horses (54 Ma), or even early humans (2.8 Ma), “prehistoric.”
Exercise 4.5 What happened on your birthday?
Using the “all of geological time compressed to one year” concept, determine the geological date that is equivalent to your birthday. First go to Day Number of the Year Calculator to find out which day of the year your birth date is. Then divide that number by 365, and multiply that number by 4,570 to determine the time (in millions since the beginning of geological time). Finally subtract that number from 4,570 to determine the date back from the present.
For example, April Fool’s Day (April 1) is day 91 of the year: 91/365 = 0.2493. 0.2493 x 4,570 = 1,139 million years from the start of time, and 4,570 – 1,193 = 3,377 Ma is the geological date.
Finally, go to the Foundation for Global Community’s “Walk through Time” website to find out what was happening on your day. The nearest date to 3,377 Ma is 3,400 Ma. Bacteria ruled the world at 3,400 Ma, and there’s a discussion about their lifestyles.
See Appendix 3 for Exercise 4.5 answers.
Media Attributions
- Physical Geology-2nd Edition, by Steven Earle is licensed under CC BY 4.0, adaptions from the original by Ryan B. Anderson.
resulting from the influence of humans