11.2. Relative Ages of Rocks www.ck12.org 11.2 Relative Ages of Rocks Lesson Objectives • Explain Steno#8217;s laws of superposition and original horizontality. • Based on a geological cross-section, identify the oldest and youngest formations. • Explain what an unconformity represents. • Know how to use fossils to correlate rock layers. Vocabulary • biozone • cross-cutting relationships • geologic time scale • key bed • lateral continuity • microfossil • original horizontality • relative age • superposition • unconformity • uniformitarianism Introduction Something that we hope you have learned from these lessons and from your own life experience is that the laws of nature never change. They are the same today as they were billions of years ago. Water freezes at 0o C at 1 atmosphere pressure; this is always true. Knowing that natural laws never change helps scientists understand Earth#8217;s past because it allows them to interpret clues about how things happened long ago. Geologists always use present-day processes to interpret the past. If you find a fossil of a fish in a dry terrestrial environment did the fish flop around on land? Did the rock form in water and then move? Since fish do not flop around on land today, the explanation that adheres to the philosophy that natural laws do not change is that the rock moved. Fossils were Living Organisms In 1666, a young doctor named Nicholas Steno dissected the head of an enormous great white shark that had been caught by fisherman near Florence, Italy. Steno was struck by the resemblance of the shark#8217;s teeth to fossils found in inland mountains and hills (Figure 11.14). Most people at the time did not believe that fossils were once part of living creatures. Authors in that day thought that the fossils of marine animals found in tall mountains, miles from any ocean could be explained in one of two ways: • The shells were washed up during the Biblical flood. (This explanation could not account for the fact that 328 www.ck12.org Chapter 11. HS Evidence About Earth’s Past FIGURE 11.14 Fossil Shark Tooth (left) and Modern Shark Tooth (right). fossils were not only found on mountains, but also within mountains, in rocks that had been quarried from deep below Earth#8217;s surface.) • The fossils formed within the rocks as a result of mysterious forces. But for Steno, the close resemblance between fossils and modern organisms was impossible to ignore. Instead of invoking supernatural forces, Steno concluded that fossils were once parts of living creatures. He then sought to explain how fossil seashells could be found in rocks and mountains far from any ocean. This led him to the ideas that are discussed below. Superposition of Rock Layers Steno proposed that if a rock contained the fossils of marine animals, the rock formed from sediments that were deposited on the seafloor. These rocks were then uplifted to become mountains. Based on these assumptions, Steno made a remarkable series of conjectures that are now known as Steno#8217;s Laws. These laws are illustrated below in (Figure 11.15). Other scientists observed rock layers and formulated other principles. Geologist William Smith (1769-1839) identi- fied the principle of faunal succession, which recognizes that: • Some fossil types are never found with certain other fossil types (e.g. human ancestors are never found with dinosaurs) meaning that fossils in a rock layer represent what lived during the period the rock was deposited. • Older features are replaced by more modern features in fossil organisms as species change through time; e.g. feathered dinosaurs precede birds in the fossil record. • Fossil species with features that change distinctly and quickly can be used to determine the age of rock layers quite precisely. Scottish geologist, James Hutton (1726-1797) recognized the principle of cross-cutting relationships. This helps geologists to determine the older and younger of two rock units (Figure 11.16). The Grand Canyon provides an excellent illustration of the principles above. The many horizontal layers of sedi- mentary rock illustrate the principle of original horizontality (Figure 11.17). • The youngest rock layers are at the top and the oldest are at the bottom, which is described by the law of superposition. • Distinctive rock layers, such as the Kaibab Limestone, are matched across the broad expanse of the canyon. These rock layers were once connected, as stated by the rule of lateral continuity. 329 11.2. Relative Ages of Rocks www.ck12.org FIGURE 11.15 (a) Original Horizontality: Sediments are deposited in fairly flat, horizontal layers. If a sedimentary rock is found tilted, the layer was tilted after it was formed. (b) Lateral continuity: Sediments are deposited in continuous sheets that span the body of water that they are deposited in. When a valley cuts through sedimentary layers, it is assumed that the rocks on either side of the valley were originally continuous. (c) Superposition: Sedimentary rocks are deposited one on top of another. The youngest layers are found at the top of the sequence, and the oldest layers are found at the bottom. • The Colorado River cuts through all the layers of rock to form the canyon. Based on the principle of cross- cutting relationships, the river must be younger than all of the rock layers that it cuts through. Determining the Relative Ages of Rocks Steno#8217;s and Smith#8217;s principles are essential for determining the relative ages of rocks and rock layers. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. The relative age of a rock then is its age in comparison with other rocks. If you know the relative ages of two rock layers, (1) Do you know which is older and which is younger? (2) Do you know how old the layers are in years? An interactive website on relative ages and geologic time is found here: http://www.ucmp.berkeley.edu/education/e xplorations/tours/geotime/gtpage1.html In some cases, it is very tricky to determine the sequence of events that leads to a certain formation. Can you figure out what happened in what order in (Figure 11.18)? Write it down and then check the following paragraphs. The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. The fault cuts through all three sedimentary rock layers (A, B, and C) and also the intrusion (D). So the 330 www.ck12.org Chapter 11. HS Evidence About Earth’s Past FIGURE 11.16 If an igneous dike (B) cuts a series of metamorphic rocks (A), which is older and which is younger? In this image, A must have existed first for B to cut across it. FIGURE 11.17 The Grand Canyon, with the Kaibab Lime- stone marked with arrows. fault must be the youngest feature. The intrusion (D) cuts through the three sedimentary rock layers, so it must be younger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is still younger. The full sequence of events is: 1. Layer C formed. 2. Layer B formed. 331 11.2. Relative Ages of Rocks www.ck12.org FIGURE 11.18 A geologic cross section: Sedimentary rocks (A-C), igneous intrusion (D), fault (E). 3. Layer A formed. 4. After layers A-B-C were present, intrusion D cut across all three. 5. Fault E formed, shifting rocks A through C and intrusion D. 6. Weathering and erosion created a layer of soil on top of layer A. Earth#8217;s Age During Steno#8217;s time, most Europeans believed that the Earth was around 6,000 years old, a figure that was based on the amount of time estimated for the events described in the Bible. One of the first scientists to question this assumption and to understand geologic time was James Hutton. Hutton traveled around Great Britain in the late 1700s, studying sedimentary rocks and their fossils (Figure 11.19). Often described as the founder of modern geology, Hutton formulated uniformitarianism: The present is the key to the past. According to uniformitarianism, the same processes that operate on Earth today operated in the past as well. Why is an acceptance of this principle absolutely essential for us to be able to decipher Earth history? Hutton questioned the age of the Earth when he looked at rock sequences like the one below. On his travels, he discovered places where sedimentary rock beds lie on an eroded surface. At this gap in rock layers, or unconformity, some rocks were eroded away. For example, consider the famous unconformity at Siccar Point, on the coast of Scotland (Figure 11.20). 1. A series of sedimentary beds was deposited on an ocean floor. 2. The sediments hardened into sedimentary rock. 3. The sedimentary rocks are uplifted and tilted, exposing them above sea level. 4. The tilted beds were eroded to form an irregular surface. 5. A sea covered the eroded sedimentary rock layers. 6. New sedimentary layers were deposited. 7. The new layers hardened into sedimentary rock. 332 www.ck12.org Chapter 11. HS Evidence About Earth’s Past FIGURE 11.19 A drawing by James Hutton. "Theory of the Earth, FIGURE 11.20 Hutton 8. The whole rock sequence was tilted. 9. Uplift occurred, exposing the new sedimentary rocks above the ocean surface. Since he thought that the same processes at work on Earth today worked at the same rate in the past, he had to account for all of these events and the unknown amount of missing time represented by the unconformity, Hutton realized that this rock sequence alone represented a great deal of time.