How old is your fossil? As we saw in chapter 4, this question is fundamental not only to identifying your fossil but also to knowing where to look for fossils. If you’re looking in beds of the wrong age, you won’t find the right kinds of fossils—or maybe no fossils at all.
Geologists and paleontologists determine the age of rocks and geologic events using two methods: relative dating and numerical dating. The first method, relative dating or relative age, tells us that geologic event A is younger or older than geologic event B. Four fundamental principles are used to help us determine the relative age of an event (fig. 5.1). The primary way geologists do this is by using the principle of superposition (fig. 5.1A), which was first proposed by the Danish scholar Nicholas Steno in 1669. In any layered sequence of rocks (usually layered sedimentary rocks, although it applies to layered lava flows as well), the oldest rocks are at the bottom of the stack, and each layer above it is progressively younger. In other words, the stack of rocks goes from older at the bottom to younger at the top. You can’t stack one layer on top of another if the lower layer isn’t already there first. A good analogy is the stack of papers on a messy desk or table. If they just keep accumulating through time without being turned over or sorted out, then the oldest papers will be at the bottom of the stack, and the most recent ones will be at the top. Thus, if you are looking at the impressive pile of layers in the Grand Canyon, the oldest ones are always at the bottom, and each layer above it is younger. They are like the pages in a book, with the first page at the bottom of the stack, and the last at the top.
Figure 5.1 ▲
Steno’s laws are used to determine the relative age of one rock body compared to another. (A) The principle of superposition says that the rocks near the top of a stack of layered sediments or lava flows are younger than those at the bottom of the stack. Thus Bed A is the youngest, and Bed F is the oldest. (B) The principle of original continuity says that rocks that match from one outcrop to another were once connected and have since been carved away by erosion. (C) The principle of original horizontality points out that rocks form in horizontal layers, so if you find them tilted or folded or faulted, the deformation is younger than the rocks it deforms. (D) The principle of cross-cutting relationship says that when a rock body (such as a dike of molten lava) or a fault cuts through another rock, the material that cuts through is younger than whatever it cuts. (Illustration by Mary Persis Williams)
When Steno looked at matching rock units exposed by erosion, he also realized that they were once continuous and only later were cut through by erosion (fig. 5.1B). This is the principle of original continuity. In the Apennine Mountains of Italy where Steno got his insights, many of the rocks are tilted, faulted, and folded. Steno realized that they had not been created that way but had been laid down horizontally, then later tilted and folded, so the deformation must be younger than the rocks it deforms (fig. 5.1C). This is the principle of original horizontality.
Another useful concept is the principle of cross-cutting relationships. If you have a molten igneous rock intruding into another rock (such an intrusion is usually called a “dike”), the rock that does the intruding must be younger that the rocks that it cuts through (fig. 5.1D). You can’t cut through something if it isn’t already there. Likewise, if a fault cuts through rocks, it must be younger than the rocks it cuts. These principles of relative dating first described by Steno in 1669 were widely in use by the time modern geology was born, about 1800 to 1830, and the geologic time scale was drawn. The various names for the eras and periods and epochs of the geologic time scale are relative ages (fig. 5.2).
Figure 5.2 ▲
The modern geologic time scale. (Courtesy of the International Commission on Stratigraphy)
The other way to date rocks is known as numerical dating (incorrectly called “absolute dating” in older books). This technique provides a date in number of years such as thousands of years or millions of years. Numerical dating is a young technique, developed in the early twentieth century, and the most popular method—potassium-argon dating—has only been around since the 1950s.
Numerical dating is done by measuring the ticks of the radioactive “clock” in certain minerals. As minerals crystallize out of a magma, they trap radioactive elements like uranium-238, uranium-235, rubidium-87, or potassium-40. These radioactive elements are naturally unstable, and they spontaneously decay into different elements. As this decay proceeds over millions of years, the unstable radioactive parent atoms decay into a stable form known as daughter atoms, such as lead-206, lead-207, strontium-87, and argon-40 (respectively, for each of the elements listed above). The rate of this decay is precisely known, so we can obtain the numerical date by measuring the ratio of parent atoms to daughter atoms in a crystal of a mineral such as feldspar or mica or zircon since that crystal was first formed.
This process only occurs in crystals that form from a molten rock, so we can only date igneous rocks directly. What about sedimentary rocks, which contain the fossils? We cannot directly date them by radioactive minerals. Instead, we need to find places where igneous rocks (such as lava flows or volcanic ash deposits) are interbedded with fossiliferous sedimentary rocks. If a bed has Oligocene fossils (“Oligocene” is a relative age term), and the bottom of the bed has an ash dated 34 million years old, and the top of the bed has a lava flow dated 23 million years old, then we bracket the age of the Oligocene between 23 and 34 million years ago. The entire geologic time scale was constructed this way: by finding fossiliferous sequences with fossils that gave well-determined relative ages, and then using any and all available igneous rocks that are in the right position to tell us the age.
There is one other radiometric system, known as radiocarbon dating, or carbon-14 dating. Unlike the other methods, you can date the fossil bones or shells or wood or any carbon-bearing substance directly because you are measuring the decay of unstable radioactive carbon-14 incorporated into the organism before it died. The main drawback of this technique is that radiocarbon decays very rapidly. Half of the original carbon-14 parent atoms are gone in just over 5,000 years, so the entire clock “runs down” in 60,000 to 80,000 years, and anything older than this cannot be dated by radiocarbon. This method is primarily used by archeologists who are interested in dating human bones and artifacts, and by paleontologists studying the last Ice Age, which spanned the interval from 80,000 to 10,000 years ago. It is useless to anyone studying older fossils because the clock is dead for them. For this reason, we could never date a Mesozoic dinosaur bone using radiocarbon.
The principles of numerical dating have told us that Earth is immensely old. Numerous meteorites and moon rocks are 4.6 billion years old, and that is how we estimate the age of the solar system. So far, the oldest Earth rocks are only 4.28 billion years old, and the oldest Earth minerals are 4.4 billion years old, so we have no Earth rocks as old as the moon rocks or meteorites. But this is not surprising because the earth’s crust is constantly being mobilized and remodeled by plate tectonics. We do not expect that any crust from the oldest earth could survive. The oldest known fossils are bacteria from South Africa and Australia, which are about 3.4 to 3.5 billion years old, and organic carbon has been found in rocks 3.8 billion years old, which most scientists think is evidence of ancient life.
From these principles, we can reconstruct the sequence of events in any part of the earth. To do this, we need to know the exact position in the sequence of rocks where each fossil was found. And, if we are using lava flows or volcanic ash beds, they must be interbedded with fossils whose positions are precisely known.
