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# Lecture5 lawsofgeology

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### Lecture5 lawsofgeology

1. 1. Announcements I will be writing PS #1 over the weekend, and posting it by Monday… It will be due Feb 9 th .
2. 2. Clarification: The angle of Earth’s tilt does not change during the year! Views of the Earth from the Sun’s perspective It only looks that way from the sun’s frame of reference .
3. 3. The axis is always parallel to itself.
4. 4. Quiz 2!
5. 5. 2. Sun Angle: insolation curves
6. 6. 2. Sun Angle: insolation curves Winter Summer Autumn Spring Winter
7. 7. Insolation 23.5 o axis tilt 0 o axis tilt
8. 8. An Equation for the Sun Angle On an equinox Declination = 0 °
9. 9. An Equation for the Sun Angle On an equinox Declination = 0 ° At the equator Lat = Sun angle = 0 ° 90 °
10. 10. An Equation for the Sun Angle On an equinox Declination = 0 ° At the north pole Lat = Sun angle = 90 °N 0 °
11. 11. An Equation for the Sun Angle On an equinox Declination = 0 ° At some latitude x Lat = Sun angle = x °N 90 °- x° x ° 90 °- x°
12. 12. But then we have to adjust for declination! Equator is sub-solar point North pole tilted toward the sun North pole tilted away from the sun Equator is sub-solar point Summer Winter Fall Spring
13. 13. But then we have to adjust for declination! Sun Angle = 90 °-x° Sun Angle = 90 °-x° Sun Angle = 90 °-x°-23.5° Sun Angle = 90 °-x°+23.5° Summer Winter Fall Spring
14. 14. Final Formula Sun Angle at Noon = 90 ° - Latitude° + declination° Summer Note: this formula is for the angle with the southern horizon, so it makes the most sense to only use it in the Northern Hemisphere
15. 15. Lithosphere: solid Earth
16. 16. Geologic time: dating <ul><li>2 types </li></ul><ul><li>Relative Dating = sequential age </li></ul><ul><ul><li>based on positions of rocks relative to each other </li></ul></ul><ul><ul><li>e.g., rock layer B is older than A and younger than C </li></ul></ul><ul><li>2. Absolute Dating = actual age </li></ul><ul><ul><li>based on radiometric dating </li></ul></ul><ul><ul><li>e.g., rock layer B is 50 million years old </li></ul></ul>
17. 17. Dating using Geological Laws <ul><li>Law of superposition </li></ul><ul><ul><li>- younger rocks are deposited on top of older rocks </li></ul></ul>younger young oldest older old
18. 18. Grand Canyon Layers
19. 19. <ul><li>Law of original horizontality </li></ul><ul><ul><li>- sedimentary rocks & lava are originally laid down horizontally </li></ul></ul>Roughly horizontal Dating using Geological Laws
20. 20. A roadcut photo that I stole… WV maybe?
21. 21. <ul><li>Law of cross-cutting relations </li></ul><ul><ul><li>rocks that intrude into other rocks are younger than their host rocks </li></ul></ul><ul><ul><li>faults are younger than the rocks they cut through </li></ul></ul><ul><ul><li>folds are younger than the rocks they bend </li></ul></ul>Dating using Geological Laws
22. 22. Roadcut near Moab, UT
23. 23. A B C D Events: A deposited B deposited C deposited D deposited Faulting Erosion of Surface X Erosion of Canyon But in what order? Order of Events: D deposited C deposited B deposited Faulting Erosion of Surface X A deposited Erosion of Canyon X
24. 24. Events: A deposited B deposited C deposited D deposited E deposited F deposited G deposited Folding Erosion of Surface X Order of Events: G deposited F deposited E deposited D deposited C deposited Folding Erosion of Surface X B deposited A deposited A B C D X E F G
25. 25. Events: A deposited B deposited C deposited D deposited E deposited F deposited Folding Faulting Erosion of Surface X Erosion of Surface Y Order of Events: F deposited E deposited D deposited C deposited Folding Erosion of Surface Y B deposited Faulting Erosion of Surface X A deposited A B C D X E F Y
26. 26. Events: Cooling of Granite A Deposition of Sandstones B, E, & I Deposition of Shales C & H Deposition of Limestone D & F Cooling of Basalt G Folding (Tilting) Faulting Erosion of surfaces X, Y, Z Formation of mesa Order of Events: 1 . A 9 . E 2 . X 10 . F 3 . B 11 . Z 4 . C 12 . G 5 . D 13 . H 6 . fold/tilt 14 . I 7 . Y 15 . mesa 8 . fault
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