Relative Age Dating Geologic Time PDF

SH1632 Relative Age Dating they formed. Sediment normally accumulates in horizontal...

SH1632 Relative Age Dating they formed. Sediment normally accumulates in horizontal layers. However, the sedimentary rocks in Figure 2 and 3 are Geologic Time folded and tilted. We deduce that the folding and tilting occurred Scientists measure geologic time in two (2) ways. after the sediment accumulated. The order in which rocks and Relative age measurement refers only to the order in which the geologic features formed can usually be interpreted by such events occurred. Absolute age is age in years. You probably observation and logic. know relative ages for many things in your life. For example, if a friend tells you she has an older brother and a younger brother, B. Principle of Superposition you know the relative ages of her brothers even if you don’t This states that the sedimentary rocks become younger from know their exact ages. bottom to top (as long as tectonic forces have not turned them upside down or thrust an older layer over a younger one). This Before absolute dating of rocks was developed in the is because younger layers of sediment always accumulate on top 20th century, geologists had to rely on relative age dating, which of older layers. places geologic events in their order of occurrence. Prior to that time, geologists used field observations to determine relative C. Principle of Crosscutting Relationships ages. Even today, with sophisticated laboratory processes This is based on the obvious fact that a rock must first exist available, most field geologists routinely use relative age before anything can happen to it. Figure 4 shows Colorado’s measurement. highest cliff, Painted Wall, where an igneous intrusion- called a dike- cuts across gneiss. Clearly, the dike must be younger than I. Relative Geologic Time the gneiss. It means that geologic features, such as faults, and Determination of relative age is based on a simple principle: igneous intrusions are younger than the rocks they cut. Try In order for an event to affect a rock, the rock must exist first. drawing a sedimentary rocks intruded by three granite dikes. Thus, the rock must be older than the event. This principle Dike 2 cuts dike 1, and dike 3 cuts dike 2. Which dike is the seems obvious, yet it is the basis of much geologic work. youngest? Which is the oldest? Figure 5 shows that sedimentary rocks must be older than the dike. Geologists use a combination of common sense and a few simple principles to determine the order in which rocks formed D. Principle of Inclusions and Components and changed over time. A rock body that contains inclusions of preexisting rocks is younger than the rocks from which the inclusions came from. If A. Principle of Original Horizontality inclusions are found in a formation of sedimentary rocks, it This is based on the observation that sediment usually means that the inclusions are older than the formation that accumulates in horizontal layers (Figure 1). If sedimentary rocks comprises them. As such, it is common for a gravel to be torn lie at an angle, as in Figure 2, or if they are folded as in Figure and eventually be part of a new formation (see Figure 6). 3, we can infer that tectonic forces tilted or folded them after 11 Handout 1 *Property of STI Page 1 of 5  SH1632 E. Principle of Faunal Succession particular cross section are older than the upper layers in that Fossils are clues in the story of Earth’s past. But for the story cross section (“the law of superposition”) and that an object to make sense, the clues need to be arranged in order. cannot be older than the materials of which it is composed. (www.classzone.com, n.d.) Igneous rocks are dated according to whether they caused Groups of fossil plants and animals occur in the geologic metamorphism in the surrounding rock (proof that they intruded record in a definite and determinable order. A period of geologic into the preexisting rock), whether they cross cut preexisting time can be recognized by its respective fossils (Figure 7). rocks, or whether sediments were deposited on them after they Fossils are useful in determining relative ages of rocks were formed. The profile from one location is then compared because different animals and plants lived at different times in with profiles from surrounding sites to determine the geologic Earth history. For example, trilobites lived from 543 million to history of a larger area. In short, in order to do study the rocks about 248 million years ago, and the first dinosaurs appeared and events in a geologic cross section and put them in the correct about 248 million years ago. The principle of superposition tells order from oldest to youngest, you must understand the basic us that sedimentary rock layers become younger from bottom to principles that are applicable to relative age relationships top. If the rocks formed over a long time, different fossils appear between rocks which were discussed earlier. and then vanish from bottom to top in the same order in which organisms evolved and then became extinct. Rocks containing Here are some additional hints that will help you with your dinosaur bones must be younger than those containing trilobite diagrams (see Figure 11): remains. · If rocks are folded, the folding is younger than the Fossils allow geologists to interpret ancient environments youngest rock affected. because most organisms thrive in specific environments and do · If they are folded into a syncline (a U-shaped fold) the not survive in others. For example, corals live in clear, warm, youngest rocks are in the core of the fold. The opposite shallow seas. Yet today, fossil corals are abundant in rocks of is true for an anticline (a big dome-shaped fold). many cold mountain environments such as the Mount Everest. Sedimentary rocks that contain fragments of another The fossil corals tell us that the rocks that lie exposed in these rock are younger than the rocks that the fragments came mountains were once submerged beneath clear, warm, shallow from. seas. Today, geologists understand that tectonic processes raised · Remember that granites can intrude into other rocks, these marine rocks and their fossils to form some of the world’s even though they may be on the bottom of a geologic highest peaks (Figure 8 and 9). diagram. Look carefully for the granitic pattern and for irregular contacts between the granite and the country II. Relative Age Dating (preexisting) rock. The granite may also metamorphose The method of relative age dating begins with the careful the country rocks. drawing (Figure 10) and description of strata (the geologic cross · If an igneous body crosscuts another rock, the igneous section or profile). It assumes that the lower layers in any rock is younger than that rock. If a body of granite 11 Handout 1 *Property of STI Page 2 of 5  SH1632 contains unmelted inclusions of another rock, the granite below it, but without more information we do not know how is the younger rock. much younger. · Intrusive rocks produce contact metamorphism along their contacts with the older rocks they intrude into. A. Unconformities · Lava flows may cause contact-metamorphism with the Layers of sedimentary rocks are conformable if they were older rocks they lie upon. deposited without detectable interruption in deposition, usually · Metamorphic rocks are preexisting rocks that have been of long duration. During the interval when no sediment was metamorphosed (changed into different rocks) by large deposited, some rock layers may have been eroded. Thus, an amounts of heat and pressure in a region. These rocks unconformity represents a long time interval for which no have usually been deformed by large, mountain forming geologic record exists in place. The lost record may involve events, and therefore if they are in contact with layered hundreds of millions of years. or unmetamorphosed rocks, they are usually the oldest An unconformity, to put it simply, is a missing rock layer; A rocks in the sequence (considering that if those rocks had missing page in the history book of a rock column (canyon). We been in place when the metamorphism occurred, they have to figure out why the layer is missing, so we can piece also would be metamorphosed!). Always look for the together the history of a canyon. Unconformities help us put the metamorphic pattern to determine if there is a puzzle of a canyon's history together by ruling out what metamorphic rock in your sequence. prehistoric environments didn't exist and what puzzle pieces we · Metamorphic rocks are older than sedimentary rocks don't need. Unconformities are caused by a couple of things. deposited above them or with igneous rocks that may Different prehistoric environments produce different kinds of intrude them. rock, so if a site didn't have that certain environment, it wouldn't have that rock layer. III. Unconformities and Correlation Look at Figure 13 for an example. Suppose layer D is a type The two-kilometer-high walls of the Grand Canyon (Figure of limestone that formed in a shallow sea. We can infer that 12) are composed of sedimentary rocks lying on older igneous Canyon B is missing Layer D because it was never under a and metamorphic rocks. The principle of superposition tells us shallow sea. Unconformities are also sometimes caused by mass that the deepest sedimentary rocks are the oldest, and the rocks erosion. Say Layer F was deposited in Canyon B. A river, become younger as we climb up the canyon walls. However, no glacier, or some other massive force might have eroded layer F principle assures us that the rocks formed continuously from before the next layer deposited. There are unconformities in the two (2) billion to 200 million years ago. The rock record may be Grand Canyon and Zion Canyon. Neither of the canyons have incomplete. Suppose that no sediment were deposited for a been in the environments that produced the rock layers in the period of time, or that erosion removed some sedimentary layers Bryce Canyon. before younger layers accumulated. In either case a gap would exist in the rock record. We know that any rock layer is younger 11 Handout 1 *Property of STI Page 3 of 5  SH1632 Several types of unconformities exist. In a disconformity, the geological evidence from many localities and match rocks of the sedimentary layers above and below the unconformity are same age from different localities in a process called correlation parallel. Figure 14 shows that A) Sediment is deposited below (Figure 20). But how do we correlate rocks of the same age over sea level; B) Rocks are exposed above sea level and layers 4 and great distances? 3 are removed by erosion; C) Rocks subside below sea level and The chart in Figure 20 shows the rock layer correlations layers 5 and 6 are deposited on the eroded surface. A between the Grand, Zion, and Bryce canyons. All three (3) are disconformity may be difficult to recognize unless an obvious located in the south-west United States. The word correlation soil layer or erosional surface developed. However, geologists means a mutual similarity between two things. In this case, it is can identify disconformities by determining the ages of rocks, three things. The three (3) canyons all share one (1) or two (2) using methods based on fossils and absolute dating. rock layers. The Grand Canyons top two (2) rock layers, Kaibab (Google Sites, n.d.) and Moenkopi, are also Zion's bottom two (2) layers. The Zion In an angular unconformity, tectonic activity tilted older Canyon's top two (2) layers, Navajo and Carmel, are the sedimentary rock layers and erosion planed them off before the Bryce Canyon's bottom two (2) layers. Scientist put together younger sediment accumulated. Figure 15 shows that A) these Canyon Correlations by studying the index fossils in the Sediment is deposited below sea level; B) Rocks are uplifted and rock layers. Say a geologist found an index fossil in the Kaibab tilted; C) Erosion exposes folded rocks; D) Rocks sink below of the Grand Canyon. After doing some research, he finds that sea level and new rocks are deposited. An angular unconformity the same fossil was found in the Zion Canyon. He can infer that in Figures 16 and 17 is evident between the eroded limb of the the Zion Canyon fossil was found in a Kaibab rock layer. syncline and the overlying horizontal younger layers. Because the fossil was an index fossil, he'll also know that the A nonconformity (Figure 18) is an unconformity in Kaibab from both canyons was deposited in the Permian Period, which sedimentary rocks lie on igneous or metamorphic rocks. because that's when the fossil lived. The same process was used The nonconformity shown in Figure 19 represents a time gap of to find correlating rock layers between the Zion and Bryce about 1 billion years. Canyon. The Bryce Canyon rock layers are the youngest. You can tell by the index fossils found in it and because of the law of B. Correlation superposition. The Grand Canyon is the oldest of the three (3) Ideally geologists would like to develop a continuous because it is the deepest, and contains the oldest fossils. history for each region of Earth by interpreting rocks that formed If you follow a single sedimentary bed from one place to in that place throughout geologic time. However, there is no another, then it is clearly the same layer in both places. But this single place where rocks formed and where preserved approach is impractical over long distances or where rocks are continuously. Erosion removed some layers, and at times no not exposed. Another problem arises when time correlation is rocks formed. Consequently, the rock record in any one place is based on continuity of a sedimentary layer of a particular rock full of unconformities, or historical gaps. To assemble as type, such as sandstone. When rocks are correlated for the complete and continuous a record as possible, geologists gather purpose of building a geologic time scale, geologist want to 11 Handout 1 *Property of STI Page 4 of 5  SH1632 show that certain rocks all formed at the same time. Suppose you volcanic ash into the atmosphere. Wind carries the ash over great are attempting to trace a beach sandstone that formed as sea level distances before it settles. Some ash clouds have encircled the rose. The beach would have migrated inland over time, and as a globe. When the ash settles, the glass rapidly crystallizes to form result, the sandstone becomes younger in a landward direction. a pale clay layer that is incorporated into sedimentary rocks. Over the distance short enough to be covered in an hour Such volcanic eruptions occur at a precise point in time, so the or so of walking, the age difference may be unimportant. But if ash is the same age everywhere. The thin, sooty, iridium-rich you traced a similar beach sand for hundreds of kilometers, its clay layer deposited 65 million years ago from debris of a giant age may vary by millions of years. Thus, there are two (2) kinds meteorite impact in Figure 23 is a classic example of a key bed. of correlation: time correlation refers to age equivalence, and lithologic correlation refers to continuity of a rock unit such as the sandstone. The two (2) are not always the same because References: some rock units, such as sandstone, were deposited at different Google Sites. (n.d.). Fossils & Time. Retrieved from times in different places. To construct a record of Earth history https://sites.google.com/a/ecsd.us: and a geologic time scale, Earth scientists use several types of https://sites.google.com/a/ecsd.us/vutci_stemsciencepor evidence to correlate rocks of the same age. tfolio/unit-7--fossils-time One source is index fossil (Figure 21). Index fossils can Jenkins, J. (2003, December 1). The essential guide to black accurately indicate the ages of the sedimentary rocks because canyon of the gunnison national park. Retrieved from they were produced by an organism that (1) is abundantly https://books.google.com.ph: preserved in rocks, (2) was geographically widespread, (3) https://books.google.com.ph/books?id=W31yWpLZMx existed as a species or genus for only relatively short time, and UC&pg=PA46&lpg=PA46&dq=black+canyon+of+gun (4) is easily identified in the field. Floating or swimming marine nison+national+park+gneiss+and+dikes&source=bl&ot mammals make the best index fossils because they spread s=xDthw4qD9m&sig=6SsPEa04XqluSlqzZWlsT5AsY rapidly and widely throughout the seas. The shorter the time vY&hl=en&sa=X&ved=0ahUKEwiG6Nql3JbKAhXB span that species existed, the more precisely the index fossil GZQKHceUBA0Q6AEIHzAB#v=onepage&q=bla reflects the age of a rock. STI Academic Center. (n.d.). Earth and Life Science Instructor's In many cases, the presence of a single type of index Guide. fossil is sufficient to establish the age of the rock. More Turk, & Thompson. (2012). Introduction to earth science. Pasig commonly, an assemblage of several fossils is used to date and City,Philippines: Cengage Learning Asia Pte Ltd. correlate rocks. www.classzone.com. (n.d.). Rocks provide a timeline for Earth. Another source used in correlation is the key bed – a thin, Retrieved from www.classzone.com: widespread, easily recognized sedimentary layer that was http://www.classzone.com/science_book/mls_grade7_F deposited rapidly and simultaneously over a wide area (Figure L/465_471.pdf 22). Many volcanic eruptions eject great volumes of fine, glassy 11 Handout 1 *Property of STI Page 5 of 5