Special Lesson In Geologic Time And Dating PDF

GEOLOGIC TIME AND DATING JAFFY F. BUSTAMANTE Lecturer, Department of Environmental Science OBJECTIVES 1 2 3 4 ANALYZE THE EXPLORE THE METHODS DISCUSS HOW DIFFERENTIATE RELATIVE...

GEOLOGIC TIME AND DATING JAFFY F. BUSTAMANTE Lecturer, Department of Environmental Science OBJECTIVES 1 2 3 4 ANALYZE THE EXPLORE THE METHODS DISCUSS HOW DIFFERENTIATE RELATIVE SIGNIFICANCE OF THE AND PRINCIPLES BEHIND SCIENTISTS DETERMINE AND ABSOLUTE GEOLOGIC TIME SCALE AS SCIENTIFIC TECHNIQUES THE AGES OF EVENTS IN GEOLOGIC DATING A FRAMEWORK FOR USED TO ESTABLISH THE THE PAST. TECHNIQUES. UNDERSTANDING THE CHRONOLOGICAL ORDER SEQUENCE OF MAJOR OF PAST EVENTS. GEOLOGICAL EVENTS. HOW OLD IS THE EARTH? HOW OLD IS LIFE ON EARTH?  GEOLOGIC TIME SCALE The geologic time scale divides up the history of the earth based on life-forms that have existed during specific times since the creation of the planet. These divisions are called geochronologic units (geo: rock, chronology: time). Most of these life-forms are found as fossils, which are the remains or traces of an organism from the geologic past that has been preserved in sediment or rock. Without fossils, scientists may not have concluded that the earth has a history that long precedes mankind. The Geologic Time Scale is divided by the following divisions: ✓ Eons: Longest subdivision; based on the abundance of certain fossils ✓ Eras: Next to longest subdivision; marked by major changes in the fossil record ✓ Periods: Based on types of life existing at the time ✓ Epochs: Shortest subdivision; marked by differences in life forms and can vary from continent to continent. Paleontology Studies the remains of ancient life to determine the age of the fossil and enclosing rock. Through years of correlating and compiling data on fossils, almost all species have been assigned an age from 3.8 billion years ago (conception of crust) to ten thousand years ago (last glacial period). To simplify the designation of ages, the Geologic time scale is used “The present is the key to the past.” Uniformitarianism James Hutton: “the present is the key to the past.” Same physical laws have operated throughout the earth’s history Uniformitarianism does not allow numerical answers to questions about geologic process rates in the past. RELATIVE DATING Process by which a specimens are dated relative to one another (for instance, earlier, later, more recent) instead of in absolute terms. Placing a sequence of events in the proper order Nicolas Steno (1669): Two Laws (Sedimentary Rocks) Superposition Original Horizontality Other laws for igneous rocks William Smith (1800): Law of Faunal Succession Principle of Superposition In an undisturbed pile of sediments, those at the bottom were deposited first, followed in succession by the layers above them, ending with the youngest on top. Oldest layer are at the bottom. Principle of Original Horizontality Sediments are deposited in horizontal, flat-lying layers. Rocks with layers folded or dipping steeply, must have been displaced or deformed after deposition and solidification into rock. Deformation comes after deposition. Principle of Lateral Continuity The law or principle of lateral continuity states that rock layers or strata will continuously extend outwards in all directions until they meet a barrier, thin out, or grade into a different rock layer. This law implies sedimentary rock layers were originally deposited as continuous lateral layers or sheets only terminated by thinning, barriers, or grading to other rocks. However, erosion and Earth Movements, including faulting, tectonic drift, etcetera, tore or displaced them. Principle (or Law ) of Cross- cutting Relationships If an igneous rock cuts across layers of sedimentary rocks, the sedimentary rocks must have been there first, the igneous rock introduced later. The rock that cuts is younger. Principle of Inclusion If an igneous rock contains pieces of other rock, those pieces must have been picked up as solid chunks by the invading magma, so the rocks making up the inclusions must predate the host rock. The rock included in another rock is older. Law of Faunal Succession The same life form is never exactly duplicated at two different times in history. Same type of fossil preserved in two rocks, should be the same age. Correlation of sedimentary rock units Index or Marker Fossil Any animal or plant preserved in the rock record of the Earth that is characteristic of a particular span of geologic time or environment. Viviparus glacialis, a mollusk that serves as an index fossil for the Early Pleistocene in Europe, collected from Rosmalen Borehole, The Netherlands. (Photo by Tom Meijer)  Major interruptions in the geologic record. Break in deposition Unconformities The rock below the unconformity is older than those above the unconformity. Angular Unconformity Nonconformity Disconformity  Consists of tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata. Angular Indicates that during the pause in deposition, a period of deformation (folding or tilting) and Unconformity erosion occurred. Surface separates strata tilted differently  Gap in the rock record that represents a period during which erosion rather than deposition occurred. Disconformity Surface separates parallel strata on either side.  Younger sedimentary strata overlie older metamorphic or intrusive igneous rocks. Nonconformity For a nonconformity to develop, there must be a period of uplift and the erosion of overlying rocks. Surface cut into crystalline (igneous and/or metamorphic) rocks, then covered by sedimentary rocks  Let’s Practice A B C Nonconformity Angular Unconformity Disconformity Let’s Practice Youngest Oldest Youngest Oldest Let’s Practice Youngest Oldest Let’s Practice Youngest Oldest Youngest Oldest Let’s Practice Youngest Oldest Youngest Oldest Let’s Practice Youngest Oldest  1. Deposition of E ; Principle of Superposition F 2. Unconformity B 3. Deposition of G K 4. Deposition of L N 5. Deposition of C A 6. Tilting and Erosion J 7. Faulting (H) of layers E-C D 8. Unconformity (erosion) M 9. Deposition of M, then D, then J H 10. Intrusion of A ; Cross-cutting Relationship C 11. Unconformity L 12. Deposition of N, then K, then B G 13. Tilting and Erosion E 14. Deposition of F 15. Active Unconformity ANSWER KEY Let’s Practices Absolute Dating Process by which a specimen is dated in specific units of scientific measurement of time. years, centuries, or millennia Absolute determinations attempting to pinpoint a discrete, known interval of time. Chronometric or Calendar dating. Techniques/disciplines: Radiometric dating Example Suppose that parent isotope A decays to daughter B with a half-life of 1 million years.  Half-life The amount of time required for half of the radioactive material to decay in an item or in our case the rock. Rock 1st forms 100% of K40 1 half-life 50% of K40, 50% of Ar40 2 half-life 25% of K40, 75% of Ar40 3 half-life 12.5% of K40, 87.5 of Ar40 4 half-life 6.25% of K40, 93.75 of Ar40 5 half-life 3.125% of K40, 96.875 of Ar40 Half-Life Questions 1. If you have 10g of K40 and 2 half-life’s have passed how many grams of K40 do you have left? 10g of K40 1 half-life; 5g of K40 2 half-life; 2.5g of K40  Complete this table for Rb-87 and Sr-87 decay Let’s Practice until the 8th half-life if there were initially 10,000 atoms of the said radioactive isotope. Half lives Atoms of Rb- 87 Atoms of Sr- 87 0 10000 0 1 ? ? 2 ? ? 3 ? ? 4 ? ? 5 ? ? 6 ? ? 7 ? ? 8 ? ? Half lives Atoms of Rb- 87 Atoms of Sr- 87 0 10000 0 1 5000 5000 2 2500 7500 3 1250 8750 4 625 9375 5 312.5 9687.5 6 156.25 9843.75 7 78.125 9921.875 8 39.0625 9960.9375 Generic Radioactive Decay Curve Rb-87 Decay Curve / Sr-87 Rb-87 Sr-87 12000 10000 9843.75 9921.86 9960.94 9687.5 10000 9375 8750 7500 NUMBER OF ATOMS 8000 6000 5000 4000 2500 2000 1250 625 312.5 156.25 78.125 0 39.0625 0 0 1 2 3 4 5 6 7 8 9 HALF-LIVES  Element Parent Isotope Daughter Isotope Half-life Carbon C-14 N-14 (Nitrogen) 5,730 years Potassium K-40 Ar-40 (Argon) 1.3 billion years Uranium U-238 Pb-206 (Lead) 4.5 billion years U-235 Pb-207 (Lead) 710 million years U-234 Th-230 (Thorium) 80,000 years A clam shell fossil is found in a quarry. Radiometric dating with U-234 was used. It was determined that the clam shell has undergone five half-lives. What is the percent sample of U-234 remaining in the sample? What is the age of the clam shell fossil? a) What is the percent sample of U-234 remaining in the sample? y= [½](5) y=1/32 y=0.03125 x 100= 3.125% of Ur-234 b) What is the age of the clam shell fossil? Half-life = 80,000 years so 5 x 80,000 years = 400,000 years old If only 7500 atoms of Rb-87 remain, how old would the amphibole be? P = 7500 D = 2500 λ = 1.419 x 10-11 𝑙𝑛2 λ= 𝑡 1 Τ2 𝑙𝑛2 λ= 48,800,000,000 λ = 1.42 𝑥 10−11  𝑛 1 7,500 = 𝑋 10,000 2 𝑛 1 𝑙𝑛 7,500Τ10,000 = 𝑙𝑛 2 7,500 1 𝑙𝑛 = 𝑛 𝑥 ln 10,000 2 0.415 𝑥 48.8 𝑏𝑖𝑙𝑙𝑖𝑜𝑛 𝑦𝑒𝑎𝑟𝑠 = 20.252 𝑏𝑖𝑙𝑙𝑖𝑜𝑛 𝑦𝑒𝑎𝑟𝑠 𝑙𝑛 0.75 =𝑛 𝑙𝑛0.5 0.415 = 𝑛  Rb-87 Decay Curve / Sr-87 Rb-87 Sr-87 12000 10000 9843.75 9921.86 9960.94 10000 9687.5 9375 8750 8000 7500 NUMBER OF ATOMS 6000 5000 4000 2500 2000 1250 625 312.5 156.25 0 78.125 39.0625 0 0 1 2 3 4 5 6 7 8 9 HALF-LIVES Sample Problem Measurements of zircon crystals containing trace amounts of uranium from a specimen of granite yield parent/daughter ratios of 25 percent parent (uranium235) and 75 percent daughter (lead-206). The half-life of uranium-235 is 704 million years. How old is the granite?  END OF THE PRESENTATION