University:
University of Colorado Boulder
Course:
GEOL 1020 | Dodos, Dinos, and Deinococcus: The History of a Habitable Planet
Academic year:

2022
Views:

472

Pages:

15
Author:

Niko Short

Geologic Laws How to decode the secrets of time within rock layers...dun, dun, dah! Relative Age Age of rock, fossil, geologic features or events, RELATIVE to other rock, fossil, geologic features or events. Not an exact age in years! Example of relative age: { The formation of the Earth’s core is older than the rise of oxygen in the atmosphere. Uniformitarianism “The present is the key to the past.” All of the processes happening on Earth today also happened in the past. Law of Original Horizontality Sediments deposited form flat, level layers If layers are currently angled, then the rock layer has moved. Principle of Superposition The bottom layer is the oldest, with layers on top being younger Layers C and F are the oldest layers Law of Cross Cutting Igneous rock (intrusions) or faults/folds are younger than the rocks it has intruded or cut through Layer H is younger than layers: R, M, F, B and I Unconformities Buried erosional surfaces Cause gaps in the geologic time record Fossil Evidence and Correlation: Found exclusively in sedimentary rocks Provide clues to the environments in which the rocks formed Example: Trilobites = marine/ocean environment Index fossils –organism that lived over a large area (preferably over the entire Earth) for a relatively short period of time. Volcanic Time Markers Violent volcanic eruption can deposit a thin layer of volcanic ash over the entire Earth. This layer is very distinguishable and is similar to index fossils (cover entire earth and represent a very short period of time) Which layer is older: • Foster City or Larsonton? • Lutgrad or Birkland? • Granite or Birkland? • The unconformity or Skinner Gulch Limestone? Radiometric dating Radiometric dating uses the ratio of radioactive isotopes in a sample to determine the age of the sample. So, how does Radiometric dating work? Radioactivity- the nuclei of some elements break apart or decay Isotope- an atom with the same number of protons but different number of neutrons. Radioactive elements decay from parent isotopes to daughter isotopes. The rate of decay is expressed in half-lives. One half-life is the amount of time necessary for one half of the parent isotopes to decay to the daughter isotopes 1 half-life = 1/2 or 50% parent material remains 2 half-lives = 1/4 or 25% parent material remains 3 half lives = 1/8 or 12.5% parent material remains 4 half lives = 1/16 or 6.25% parent material remains To determine the age of the sample use the formula: 1. Number of parent isotopes Total number of isotopes = % of parent isotopes remaining 2. Use the percentage to determine the number of half lives passed. 3. Use the known half life times the number of half-lives passed to find the age of the sample Example: A sample of rock has 500 parent atoms and 1500 daughter atoms. The element has a half-life of 1000 years. What is the age of the sample? 1. 500 parent atoms/2000 total atoms = 25% parent atoms remaining. Using the chart: 2. You can see 2 half-lives have passed 1 half-life = 1/2 or 50% parent material remains 2 half-lives = 1/4 or 25% parent material remains 3 half lives = 1/8 or 12.5% parent material remains 4 half lives = 1/16 or 6.25% parent material remains 3. The half-life of the element is 1000 years times 2 half-lives passed = a 2000 year old sample Example 2: A sample of rock has 2000 parent atoms and 14000 daughter atoms. The element has a half-life of 3 million years. What is the age of the sample? 1. 2000 parent atoms/16000 total atoms = 12.5% parent atoms remaining. Using the chart: 2. You can see 3 half-lives have passed 1 half-life = 1/2 or 50% parent material remains 2 half-lives = 1/4 or 25% parent material remains 3 half lives = 1/8 or 12.5% parent material remains 4 half lives = 1/16 or 6.25% parent material remains 3. The half-life of the element is 3 million years times 3 half-lives passed = a 9 million year old sample. Commonly used isotopes for geologic samples: Radioactive parent Stable daughter Half-life Uranium-238 Lead-206 4.5 billion years Uranium-235 Lead-207 713 million years Potassium-40 Argon-40 1.3 billion years

Geologic Laws and Radiometric Dating

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