9 Geologic History, Correlation and Radioactive Decay

9 GEOLOGIC HISTORY, CORRELATION AND RADIOACTIVE DECAY Relative Dating - any type of method that determines whether an event or object is older or younger than other events or objects Law of Superposition - in undisturbed rock layers, the oldest rocks are on the bottom and the youngest rocks are on the top Principle of Original Horizontality - layers of sediment that are generally deposited in a horizontal position Principle of Cross Cutting Relationships - a fault or intrusion is younger than the rock it cuts into Unconformities - gaps in the rock record created by erosion or non-deposition Inclusions - pieces of one rock unit that are contained within one another Fossil - a preserved remnant or impression of an organism that once lived in the past Radioactivity - a process that occurs when a nucleus decays and emits matter and energy Parent Isotope - the original radioactive particles, unstable Daughter Isotope - the stable isotope produced by the radioactive decay of the parent isotope Half-Life - length of time required for half of the radioactive atoms in one sample to decay Absolute Dating - any method of measuring the age of an event or object (in years) Radiocarbon Dating - a chemical analysis used to determine the age of organic materials based on their content of the radioisotope carbon-14 Geologic Time Scale - a record of the geologic events and life forms in Earth's history. Eon - the largest division of geologic time Geologic Era - a subdivision of geologic time that divides an eon into smaller units of time. The Phanerozoic Eon is divided into three time frames: The Paleozoic, Mesozoic, and Cenozoic represent the major stages in the macroscopic fossil record. Geologic Period - a segment of geologic time that subdivides the geologic eras into smaller chunks of time Geologic Epoch - a segment of geologic time that subdivides the geologic periods into smaller chunks of time. Only happen in Cenozoic Era Precambrian - the oldest Era, 88% of the timeline, little life, anaerobic bacteria, first photosynthesizing bacteria made Oxygen Mass Extinction - an event during which many species become extinct during a short period of time Paleozoic - an era occurring between 542 million and 251 million years ago, which is characterized by the advent of fish, insects, and amphibians Mesozoic - a middle era of Earth's history, during which Pangaea broke apart, dinosaurs appeared, and reptiles were the dominant life forms. Cenozoic - an era that began about 66 million years ago and continues today, known as the "Age of Mammals" Geologic Time Periods of Geological Time: 1. Cambrian (540-489 million years ago) - Phanerozoic Eon Paleozoic Era Included Burgess shale fauna, earliest fishes, earliest trilobites, and extinction of many primitive marine organisms. 2. - Ordovician (489-444 million years ago) Phanerozoic Eon Paleozoic Era Included invertebrates and Earth’s first coral reefs. 3. - Silurian (444-416 million years ago) Phanerozoic Eon Paleozoic Era Included earliest insects, earliest land plants and animals, and abundant fish. 4. - Devonian (416 – 360 million years ago) Phanerozoic Eon Paleozoic Era Included earliest amphibians and plant seeds, earliest forests, earliest ammonoids and shark, abundant fish, and the extinction of many marine organisms. 5. - Carboniferous (360 – 300 million years ago) Phanerozoic Eon Paleozoic Era Included abundant amphibians, large number of seed ferns and scale tees, and extensive coal-forming forests. 6. - Permian (300 – 250 million years ago) Phanerozoic Eon Paleozoic Era Included abundant reptiles, mammal-like reptiles, and mass extinction of many land animals and marine organisms (trilobites). 7. - Triassic (250 – 201.6 million years ago) Phanerozoic Eon Mesozoic Era Included earliest mammals, earliest dinosaurs, and mass extinction of many land animals and marine organisms (trilobites). 8. - Jurassic (201.6 – 145.5 million years ago) Phanerozoic Eon Mesozoic Era Included earliest birds and abundant dinosaurs and ammonoids. 9. - Cretaceous (145.5 – 65.5 million years ago) Phanerozoic Eon Mesozoic Era Included earliest flowering plants and diverse bony fish, and mass extinction of dinosaurs, ammonoids, and many land plants. 10. Tertiary (65.5 – 2.6 million years ago) - Phanerozoic Eon - Cenozoic Era - Large carnivorous mammals, abundant grass, earliest grasses, and modern groups of mammals. 11. Quaternary (2.6 million years ago – present) - Phanerozoic Eon - Cenozoic Era - Include humans, mastodons, and mammoths. Radioactive Dating ☆ Radioactive elements can be used to find out how long ago that a fossil organism lived. ☆ Radioactive dating is a process in which you use radioactive elements to derive the age of fossil organisms. ☆ Radioactive elements give off particles and energy as they decay. So the decay can be measured at a fixed rate. ☆ The decay is measured in a unit called half-life. ☆ Half-life is the amount of time needed for half of the radioactive element to decay. ☆ By measuring the amount of radioactive element in a fossil, the number of half-lives of the element is found. ☆ This is then used to calculate the actual age of the fossil. Common Radioactive Elements used for Radioactive Dating: Radioactive Element Half-life Rubidium-87 50 billion years Uranium-238 4.5 billion years Potassium-40 1.3 billion years Carbon-14 5,770 years Examples: 1. If a fossil contained one-half as much carbon-14 as when the organism first formed, how old is the fossil? 5,770 years old. {Remember, the fossil only contained half the original amount of carbon-14, so the other half was decayed. The amount of time it takes carbon-14 to decay half its amount is 5,770 years. Therefore, the fossil is 5,770 years old.} 2. If a rock contained one-half as much carbon-14 as when the organism first formed, how old is the rock? 2.6 billion years. {Remember, the fossil only contains one-fourth the amount of potassium-40, so it went through a half-life twice. When the first half decayed, it took 1.3 billion years. When the remaining half decayed (leaving 0ne-fourth), it took 1.3 billion years more. Therefore, in total it took 2.6 billion years.} 3. If 10g of uranium-238 are present now, how much will be left in 4.5 billion years? How much in 9 billion years? 5g in 4.5 billion years and 2.5g in 9 billion years. {Remember, 4.5 billion years is the half-life of uranium-238, which means half of the element has been decayed. Therefore, half of 10g is 5g. 9 billion years adds another 4.5 billion years, so another half-life has passed. The half of the remaining uranium-238 has been decayed. Therefore, the half of 5g is2.5g.}