John McPhee, a New Yorker writer who wrote several popular books on geology, attempted to capture geologic time by calling it “deep time.” In this excerpt from the book Deep Time, McPhee describes a conversation with Princeton University geologist Ken Deffeyes:
“In geologists’ own lives, the least effect of time is that they think in two languages, function on two different scales…A sense of geologic time is the most important thing to suggest to the nongeologist: the slow rate of geologic processes, centimeters per year, with huge effects, if continued for enough years. A million years is a short time—the shortest worth messing with for most problems. You begin tuning your mind to a timescale that is the planet’s time scale.”
What is Old?
Geologic time is no different from “normal time.” Geologic time consists of seconds, years, decades, and millennia. The only difference is how geologists view time. If you ask the normal person what “really old is,” they’ll probably say a few hundred years, maybe a few thousand years ago. He or she may point to something built in the last 200 years, like the county courthouse built in 1830, as being really old. To a geologist, “old” means millions of years old. To the normal person, a fast event probably lasts a few minutes to a few hours. To a geologist, a geologic event that happens over a period of 10 million years can be “really fast.”
Look at the example to the right. Imagine simply asking people at different stages in their life what “old means.” To a grandparent, old may mean the Depression or World War II. To an 8th grader, the era before the Internet seems “really old.” To a child in 2nd grade, Pre-school seems like a pretty long time ago.
Take a look at the geologic time scale in your textbook (Table 1.2, p 11). This scale is somewhat skewed: the table doesn’t visually show the size of each time period. Look at the far right-hand part of the scale under the column “True Scale.” This gives you a sense of how massive geologic time is. Of the 4.6 billion years Earth has been around, 88% of the time was absent of any significant (complex) life. The scale in the text exaggerates the past 550 million years because most Earth events of interest (to us) have only happened in this time frame.
How do we know?
Estimates of geologic time were initially based on observations by early geologists that noted Earth processes happen extremely slowly, the Earth's rock record could be correlated across different countries and continents, and that fossils could provide a general sequence to the age of materials. Advances in chemistry and physics allowed geologists to use radiometric dating, or to measure small amounts of radioactive material present in all Earth material (you contain radioactive material too, by the way). Using radiometric dating, geologists were able to determine the approximate ages when certain types of Earth material formed. To geologists surprise, their "very rough" estimates and sequence of events predicted by simply looking at rocks were shown to be correct. This is an example of how observations create a hypothesis that can then be supported or confirmed by new methods of measuring Earth phenomenon.
More detail on radiometric dating (click the link to read this required article) is found in an explanation from Dr. Weins, where a scientist (who has published research on the topic) has taken the complicated concepts (even complicated in introductory textbooks) and explained them. He's published the information under a society that links together religious philosophy with scientific discovery, with consideration that certain religious sects believe geologic time violates their written beliefs. You don’t have to read this entire article! See the Reading and Articles page for information on which sections to pay close attention.