G eologists commonly study events that occurred in

the past. They observe rocks and landforms and ask questions such as “What geologic processes shaped that mountain range?” “When did the mountains rise and erode?” For example,compare the Appalachians,a low, rounded mountain range,with the Tetons,whose rocky peaks rise precipitously from the valley floor. We might ask, “Do the two ranges seem so different because the Appalachians are older and have been eroding for a longer time? Were the Appalachians once as steep as the Tetons are today? If so,when did their rocky summits rise,and when did they become rounded?”

Fossil trilobites. Trilobites dominated the seas during Cambrian time and survived for about 300 million years, until the end of the Paleozoic Era. (© John Cancalosi/OKAPIA 1991)

140 CHAPTER 9 G E O L O G I C T I M E : A S TO RY I N T H E RO C K S

Figure 9–1 Limestone was deposited in a shallow sea and then uplifted and folded in the Canadian Rockies,Alberta.

䊳 9.1 GEOLOGIC TIME

geologic features formed can almost always be inter- preted by observation and logic.

While most of us think of time in terms of days or years, Absolute age is age in years. Dinosaurs became ex- geologists commonly refer to events that happened mil-

tinct 65 million years ago. The Teton Range in Wyoming lions or billions of years ago. In Chapter 1 you learned

began rising 6 million years ago. Absolute age tells us that the Earth is approximately 4.6 billion years old. Yet

both the order in which events occurred and the amount humans and our human-like ancestors have existed for 4

of time that has passed since they occurred. million years, and recorded history is only a few thou- sand years old. How do geologists measure the ages of rocks and events that occurred millions or billions of

䊳 9.2 RELATIVE GEOLOGIC TIME

years ago? Geologists measure geologic time in two different

Absolute age measurements have become common only ways. Relative age lists the order in which events oc-

in the second half of this century. Prior to that time, ge- curred. Determination of relative age is based on a sim-

ologists used field observations to determine relative ple principle: In order for an event to affect a rock, the

ages. Even today, with sophisticated laboratory processes rock must exist first. Thus, the rock must be older than

available, most field geologists routinely use relative the event. This principle seems obvious, yet it is the ba-

ages. Geologists use a combination of common sense sis of much geologic work. For example, consider the

and a few simple principles to determine the order in rocks shown in Figure 9–1. Sediment normally accumu-

which rocks formed and changed over time. lates in horizontal layers. If you observe a fold in the lay-

The principle of original horizontality is based on ers, you can deduce that the folding occurred after the

observation that sediment usually accumulates in hori- sediment was deposited. The order in which rocks and

zontal layers (Fig. 9–2a). If sedimentary rocks lie at an

Relative Geologic Time 141

(a)

(b)

Figure 9–2 (a) The principle of original horizontality tells us that most sedimentary rocks are deposited with horizontal bedding (San Juan River,Utah). (b) When we see tilted rocks,we infer that they were tilted after they were deposited (Connecticut).

Older

D Figure 9–3 In a sequence of sedimentary E beds,the oldest bed is the lowest,and the

youngest is on top. These beds become older in the order A,B,C,D,E.

angle, as in Figure 9–2b, we can infer that tectonic forces tilted them after they formed.

The principle of superposition states that sedimentary rocks become younger from bottom to top (as long as tectonic forces have not turned them upside down). This is because younger layers of sediment always ac- cumulate on top of older layers. In Figure 9–3, the sedimentary layers become progressively younger in the order E, D, C, B, and A.

The principle of crosscutting relationships is based on the obvious fact that a rock must first exist before anything can happen to it. Figure 9–4 shows light granite dikes cutting through older country rock. Clearly, the country rock must be older than the dikes. Figure 9–5 shows sedimentary rocks intruded by three granite dikes. Dike B cuts dike C, and dike A cuts dike B, so dike C is older than B, and dike A is the youngest. The sedimen-

Figure 9–4 Light granitic dikes cutting across older country tary rocks must be older than all of the dikes.

rock along the coast in southeast Alaska.

Dike A

Dike B

Dike C

Figure 9–5 Three granite dikes cutting sedimentary rocks. The dikes become younger in the order C,B,A. The sedimentary rocks must be older than all three dikes.

G eologists commonly study events that occurred in