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Impact Craters and Age - Review

Impact Craters and Age - Review. Finding ages of objects. Absolute ages are assessed through radioactive decay (1/2 lives) Carbon 14, half-life of 5700 years Uranium 238, half-life of 700 million years! Requires lab analysis of physical samples. How it works.

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Impact Craters and Age - Review

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  1. Impact Craters and Age - Review

  2. Finding ages of objects • Absolute ages are assessed through radioactive decay (1/2 lives) • Carbon 14, half-life of 5700 years • Uranium 238, half-life of 700 million years! • Requires lab analysis of physical samples

  3. How it works • Each half life reduces the amount of the radioactive element by 1/2 • 2 half lives = 1/4 the amount left • 3 half lives = 1/8 the amount left • If a half life is 700 million years, and there is 1/8 the original amount left… • 1/8 amount = 3 half-lives x 700 million years = ??? • 2100 million (2.1 billion) years.

  4. No lab samples, no problem! • Craters can provide a RELATIVE age • Formation of craters (and other features) will not always happen in a fresh area. • They overlap each other! A B C

  5. Crosscutting and Superposition • Older craters/features are broken up and overlaid by newer features • Superposition: craters overlapping others • Crosscutting: features sliced through by channels • Try it again:

  6. C • Try again A B

  7. Crater Classification • Erosion of a crater can happen *anywhere* • (it is just faster on some astronomical bodies that others…) • The appearance of a crater can reveal its (relative) age • 3 main classes • Preserved (A) • Modified (B) • Destroyed (C)

  8. Life of a Crater • here

  9. Describe the main feature- HYPOTHESIZE!

  10. Which is the older surface? • How do you know?

  11. A second method • Crater Density Method! • It’s pretty simple: • More craters = older! • This assumes relatively stable rates of impacts over time. • Also, assumes relatively even distribution.

  12. Famous impacts • Many on earth, but hard to see sometimes… • Not always though!

  13. Barringer Crater • Arizona, 50,000 years ago • 1.2 km diameter, 0.2 km deep • Created by 300,000 ton iron meteor, 50 m across @ 20 km/s

  14. Tunguska Event • 1908, Siberia • Blue column of light • Bright as the sun! • Object airburst • Broke apart before hitting the ground • Blast = 1000x hiroshima • Knocked down 80 million trees over 2,150 km2 • Blew out windows several hundred km away

  15. Peekskill Meteorite • 1992, Eastern US • 12 kilo stony meteor • First major meteor captured on video from multiple angles/areas • Recovered and sold for $70,000

  16. Chicxulub! • Dino Killer! • PDX sized object! • 200 km crater! • Global effects!

  17. k-t

  18. Shoemaker-Levy 9 • 20+ fragments • Each left an impact scar the size of earth.

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