FEU High School Earth Science Study Guide (2023-2024) PDF

Summary

This is a study guide for Lesson 13 on the Formation and Dating of Rocks in an earth science course, likely for high school students. The guide covers relative and absolute dating, stratification of rocks, and related concepts. It includes learning objectives, prerequisite skills, comprehension checks, examples, and links to learning resources.

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Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 Unit 4: History of th...

Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 Unit 4: History of the Earth Lesson 13: Formation and Dating of Rocks Introduction The immensity of Earth history and the importance of time as a component in all geologic processes. In the nineteenth century, others effectively demonstrated that Earth had experienced many episodes of mountain building and erosion, which must have required great spans of geologic time. Although these pioneering scientists understood that Earth was very old, they had no way of knowing its true age. Was it tens of millions, hundreds of millions, or even billions of years old? Because they didn’t know exactly how old Earth was, they developed a geologic time scale that showed the sequence of events based on relative dating principles. What were these principles? With the discovery of radioactivity and the development of radiometric dating techniques, geologists can now assign fairly accurate dates to many of the events in Earth history. What is radioactivity? Why is it a good “clock” for dating the geologic past? This module will answer these questions. Learning Objectives After studying this module, you should be able to: 1. Describe how layers of stratified rocks are formed; and 2. Describe the different methods (relative and absolute dating) of determining the age of stratified rocks. Pre-requisite Skills / Review Instructions: Students will be given a reading assignment entitled, “Explainer: Understanding Geologic Time” which is about how scientists have organized time throughout Earth’s 4.6-billion-year history. Comprehension Checks: 1. How do scientist/geologist track such a long, complex history on Earths’ geologic time? 2. What is the difference between relative and absolute dating? 3. Why is it important for us to understand the geologic time scale? What will be its implication to our life? Below is the link where students could read the article: https://www.sciencenewsforstudents.org/article/explainer-understanding-geologic-time Lesson 13.1 – Stratification Stratigraphy is the classification of different layers or layering of sedimentary deposits, and in sedimentary or layered volcanic rocks. Stratification General term for layering of sedimentary rocks. It serves as a reflection of changing conditions during deposition where every strata or layer represents an interval of time where conditions have remained uniform. A change in color can reflect differences in grain size and/or composition. Beds: stratification which are greater than 1 cm thick. Lamination: stratification which are less than 1 cm thick. Figure 1. Stratification in Sedimentary Rocks Lesson 13.2 – Relative Dating Page 1 of 9 Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 Relative Dating When we place rocks in their proper sequence of formation—which formed first, second, third, and so on—we are establishing relative dates. Such dates cannot tell us how long ago something took place, only that it followed one event and preceded another. The relative dating techniques that were developed are valuable and still widely used. Numerical dating methods did not replace these techniques; rather, they supplemented them. To establish a relative time scale, a few basic principles or rules had to be discovered and applied. Although they may seem obvious to us today, they were major breakthroughs in thinking at the time, and their discovery was an important scientific achievement. A. Principle of Superposition Nicolas Steno, a Danish anatomist, geologist, and priest (1638–1686), is credited with being the first to recognize a sequence of historical events in an outcrop of sedimentary rock layers. Working in the mountains of western Italy, Steno applied a very simple rule that has come to be the most basic principle of relative dating—the principle of superposition (super = above; positum = to place). The principle simply states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below. Although it may seem obvious that a rock layer could not be deposited with nothing beneath it for support, it was not until 1669 that Steno clearly stated this principle. This rule also applies to other surface-deposited Figure 2. Grand Canyon Sedimentary Strata. materials, such as lava flows and beds of ash from volcanic eruptions. Applying superposition to the beds exposed in the upper portion of the Grand Canyon, we can easily place the layers in their proper order. The sedimentary rocks in the Supai Group are the oldest, followed in order by the Hermit Shale, Coconino Sandstone, Toroweap Formation, and Kaibab Limestone. B. Principle of Original Horizontality Steno is also credited with recognizing the importance of another basic principle, called the principle of original horizontality. It states that layers of sediment are generally deposited in a horizontal position. Thus, if we observe rock layers that are flat, it means they have not been disturbed and still have their original horizontality. But if they are folded or inclined at a steep Figure 3. Deformed Strata due to Differential Stress angle, they must have been moved into that position by crustal disturbances sometime after their deposition. C. Principle of Lateral Continuity The principle of lateral continuity refers to the fact that sedimentary beds originate as continuous layers that extend in all directions until they eventually grade into a different type of sediment or until they thin out at the edge of the basin of deposition. Figure 4. Principle of Lateral Continuity Page 2 of 9 Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 For example, when a river creates a canyon, we can assume that identical or similar strata on opposite sides once spanned the canyon. Although rock outcrops may be separated by a considerable distance, the principle of lateral continuity tells us those outcrops once formed a continuous layer. This principle allows geologists to relate rocks in isolated outcrops to one another. D. Principle of Cross-Cutting Relationship It is clear that the rocks must be older than the fault that broke them. The principle of cross-cutting relationships states that geologic features that cut across rocks must form after the rocks they cut through. Igneous intrusions provide another example. The dike is a tabular mass of igneous rock that cuts through the surrounding rocks. The magmatic heat from igneous intrusions often creates a narrow “baked” zone of contact Figure 5. Dike cutting through the surrounding Igneous Rocks metamorphism on the adjacent rock, also indicating that the intrusion occurred after the surrounding rocks were in place. E. Principle of Inclusions Sometimes inclusions can aid in the relative dating process. Inclusions are fragments of one rock unit that have been enclosed within another. The principle of inclusions is logical and straightforward: The rock mass adjacent to the one containing the inclusions must have been there first in order to provide the rock fragments. Therefore, the rock mass that contains the inclusions is the younger of the two. For example, when magma intrudes into surrounding rock, blocks of the surrounding rock may become dislodged and incorporated into the magma. If these pieces do not melt, they remain as inclusions, known as xenoliths. In another example, when sediment is deposited atop a weathered mass of bedrock, pieces of the weathered rock become incorporated into the Figure 6. Principle of Inclusions younger sedimentary layer F. Unconformities When we observe layers of rock that have been deposited essentially without interruption, we call them conformable. Particular sites exhibit conformable beds representing certain spans of geologic time. However, no place on Earth has a complete set of conformable strata. Throughout Earth history, the deposition of sediment has been interrupted over and over again. All such breaks in the rock record are termed unconformities. An unconformity represents a long period during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed. In each case, uplift and erosion are followed by subsidence and renewed sedimentation. Unconformities are important features because they represent significant geologic events in Earth history. Moreover, their recognition helps us identify what intervals of time are not represented by strata and thus are missing from the geologic record. There are three basic types of unconformities: angular unconformities, disconformities, and nonconformities. a. Disconformity A disconformity is a gap in the rock record that represents a period during which erosion rather than deposition occurred. Imagine that a series of sedimentary layers are deposited in a shallow marine setting. Following this period of deposition, sea level falls or the land rises, exposing some the sedimentary layers. During this span, when the Page 3 of 9 Figure 7. Disconformity Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 sedimentary beds are above sea level, no new sediment accumulates, and some of the existing layers are eroded away. Later sea level rises or the land subsides, submerging the landscape. Now the surface is again below sea level, and a new series of sedimentary beds is deposited. The boundary separating the two sets of beds is a disconformity—a span for which there is no rock record. Because the layers above and below a disconformity are parallel, these features are sometimes difficult to identify unless you notice evidence of erosion such as a buried stream channel. b. Angular Unconformity Perhaps the most easily recognized unconformity is an angular unconformity. It consists of tilted or folded sedimentary rocks that are overlain by younger, more flat- lying strata. An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred. Figure 8. Angular Unconformity c. Nonconformity The third basic type of unconformity is a nonconformity, in which younger sedimentary strata overlie older metamorphic or intrusive igneous rocks. Just as angular unconformities and some disconformities imply crustal movements, so too do nonconformities. Intrusive igneous masses and metamorphic rocks originate far below the surface. Thus, for a nonconformity to develop, there must be a period of uplift and the erosion of overlying rocks. Once exposed at the surface, the igneous or metamorphic rocks are subjected to weathering and erosion prior to subsidence and the renewal of sedimentation. Figure 9. Nonconformity d. Paraconformity Disconformities are characterized by subaerial erosion features. This type of erosion may be left in the rock record channels and paleosols. A paraconformity is a type of disconformity where separation is a simple bedding plane with no apparent buried erosional surface. There is no evidence of a gap in time because the planes above and below the gap are parallel, and there is no evidence of deposition. Lesson 13.3 – Absolute Dating Absolute Dating It is also possible to obtain reliable numerical dates for events in the geologic past. We know that Earth is about 4.6 billion years old and hat the dinosaurs became extinct about 65.5 million years ago. Dates that are expressed in millions and billions of years truly stretch our imagination because our personal calendars involve time measured in hours, weeks, and years. Nevertheless, the vast expanse of geologic time is a reality, and absolute dating allows us to measure it accurately. Atomic Structure Each atom has a nucleus that contains protons and neutrons and that the nucleus is orbited by electrons. An electron has a negative electrical charge, and a proton has a positive charge. Page 4 of 9 Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 An element’s atomic number (an element’s identifying number) is the number of protons in the nucleus. Every element has a different number of protons in the nucleus and thus a different atomic number (hydrogen = 1, oxygen = 8, uranium = 92, etc.). Atoms of the same element always have the same number of protons, so the atomic number is constant. Practically all (99.9 percent) of an atom’s mass is found in the nucleus, indicating that electrons have practically no mass at all. By adding together, the number of protons and neutrons in the nucleus, the mass number of the atom is determined. The number of neutrons in the nucleus can vary. These variants, called isotopes, have different mass numbers. We can summarize with an example. Uranium’s nucleus always has 92 protons, so its atomic number always is 92. But its neutron population varies, and uranium has three isotopes: uranium-234 (number of protons + neutrons = 234), uranium-235, and uranium-238. All three isotopes are found together in nature. They look the same and behave the same in chemical reactions. Radioactivity The forces that bind protons and neutrons together in the nucleus are usually strong. However, in some isotopes, the nuclei are unstable because the forces binding protons and neutrons together are not strong enough. As a result, the nuclei spontaneously break apart (decay) in a process called radioactivity. Alpha emission Alpha particles (α particles) may be emitted from the nucleus. An alpha particle consists of 2 protons and 2 neutrons. Consequently, the emission of an alpha particle means (a) the mass number of the isotope is reduced by 4, and (b) the atomic number is decreased by 2. Beta emission When a beta particle (β particle), or an electron, is given off from a nucleus, the mass number remains unchanged because electrons have practically no mass. However, because the electron has come from a neutron (remember that a neutron is a combination of a proton and an electron), the nucleus contains 1 more proton than before. Therefore, the atomic Figure 10. Alpha and Beta Emission number increases by 1. Half-Life The time required for one-half of the nuclei in a sample to decay is called the half-life of the isotope. Half-life is a common way of expressing the rate of radioactive disintegration. Methods of Absolute Dating A. Radiocarbon Dating This is used to find the age of once living materials between 100 and 50,000 years old. It is usually used to determine ages of human fossils and habitation sites. B. Potassium-Argon Dating Potassium is common in many minerals, such as feldspar, mica, and amphibole. With its half-life, the technique is used to date rocks from 100,000 years to over a billion years old. Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Argon is a gas, allowing it to escape from molten magma. Thus, any argon that is found in an igneous crystal probably formed as a result of the decay of potassium-40. Measuring the ratio of potassium-40 to argon-40 yields a good estimate of the age of that crystal. C. Uranium-Lead Dating Two uranium isotopes are used for radiometric dating: Page 5 of 9 Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 Uranium-238 decays to lead-206 with a half-life of 4.47 billion years. Uranium-235 decays to form lead-207 with a half-life of 704 million years. Uranium-lead dating is usually performed on zircon crystals. When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. If any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium. Radioactive Isotopes used in Proving Radiometric Dating in Rocks Rubidium-87 Thorium-232 Uranium-235 Uranium-234 Potassium-40 GENERAL FORMULA: SAMPLE PROBLEM: If the amount of carbon-14 in the original sample is 48 grams, about how much carbon-14 would be left after 22,800 years? Note: C-14’s half-life is 5,700 years A basalt rock sample was analyzed using potassium-40. The half-life of potassium-40 is 1.3 billion years. If the basalt rock sample had only 75 grams of the original potassium-40 in 300 grams, how long ago was the basalt rock was formed? Page 6 of 9 Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 Learning Activity Seatwork 13.1: Picture Analysis Directions: Refer to the picture and answer the following questions: 1. Which is older between Muav Limestone and Tonto group? 2. Which is younger between Supai group and Redwall limestone? 3. Why do you think is the Unkar group lying diagonal not horizontal? 4. What occurs phenomena occurs before the formation of Redwall limestone? 5. Which is the youngest rock formation? Tutorial Videos For more information, refer to the link below: Methods of Dating the Earth Part 1: Relative Dating – YouTube Methods of Dating the Earth Part 2: Absolute Dating (Radiometric Dating) – YouTube Radiometric Dating: Carbon-14 and Uranium-238 – YouTube Key Concepts ❖ Stratification is the general term for layering of sedimentary rocks. It serves as a reflection of changing conditions during deposition where every strata or layer represents an interval of time where conditions have remained uniform. A change in color can reflect differences in grain size and/or composition. ❖ Geological specimens that are unearthed need to be assigned an appropriate age. To find their age, two major geological dating methods are used. These are called relative and absolute dating techniques. Absolute dating, also called numerical dating, arranges the historical remains in order of their ages. Whereas, relative dating arranges them in the geological order of their formation. ❖ To establish a relative time scale, a few basic principles or rules had to be discovered and applied. These are: Principle of Superposition, Principle of Original Horizontality, Principle of Lateral Continuity, Principle of Cross-Cutting Relationship, Principle of Inclusions, and Unconformities. ❖ Absolute Dating determines the age of a rock/object using radiometric techniques and is quantitative. The dating is based on atomic structure, radioactivity, and half-life. ❖ The different methods used in absolute dating are Radiocarbon Dating, Potassium-Argon Page 7Dating of 9 and Uranium-Lead Dating. Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 Enrichment Activity Directions: Read each item carefully. Choose the best answer among the choices. 1. Where can oldest fossils be found according to the law of superposition? A. somewhat near the surface B. at the top of the rock layers C. in the middle of the rock layers D. near the bottom of the rock layers 2. Which statement best describes a sedimentary rock? A. It forms when cooling takes place slowly beneath the earth’s surface. B. Rocks that came from the cooling or crystallization of magma or molten volcanic materials within the Earth’s surface or at the surface. C. All rock that are formed by the deposition and subsequent cementation of that material at the Earth's surface and within bodies of water. D. Rocks that forms when pre-existing rocks are subjected to high temperatures and/or pressure and interaction with chemically active fluids. 3. Which rock layer in figure seems to be the oldest? A. A B. B C. E D. F 4. Which is TRUE about the stratification of rocks? A. Sediments are transformed into solid sedimentary rocks. B. It involves the pushing together of the particles by the weight of overlying sediment that is subsequently deposited. C. It encompasses the precipitation (crystallization) of minerals that are in solution in waters flowing through the sediment. D. Water and wind sort sediments according to size, weight, and shape of particles, and these sediments settle in layers of relative homogeneity. 5. Which is used to interpret rock strata? A. relative age of layers B. slippage in tectonic boundaries C. location where magma hardened D. how fast you are driving when you pass them 6. What principle states that a stratum must always be older than any feature that cuts or disrupts it? A. principle of horizontality B. principle of cross-cutting C. principle of lateral continuity D. principle of horizontal continuity 7. Which law states that new rock layers are always deposited on top of existing rock layers? A. principle of horizontality B. principle of superposition C. principle of lateral continuity D. principle of original horizontality 8. What principle states that deposited sediments tend to form flat layers? A. principle of horizontality B. principle of superposition C. principle of lateral continuity D. principle of original horizontality Page 8 of 9 Science, Technology, Engineering and Mathematics Earth Science SY 2023 – 2024 Enrichment Activity For numbers, 9-10, refer to the picture below: 9. Which rock type is the youngest? A. granite B. limestone C. shale D. volcanic ash 10. Which statement is TRUE regarding the figure? A. siltstone is the oldest B. granite is older than basalt dike C. basalt dike is younger than shale D. sandstone is younger than limestone. Page 9 of 9

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