Time. How we achieved a modern sense of time. Yearly Calendars are Ancient

Transcription

1 Time How we achieved a modern sense of time. Yearly Calendars are Ancient Stonehenge is BC and indicates that ancient cultures counted days and knew precisely the repeat cycle of the seasons. 1

2 Renaissance Italy was marked by a passion for measurement Art Sculpture (Michelangelo) Painting (Michelangelo, Raphael) Science (Astronomy, Optics, Military) Galileo, davinci Copernicus Raphael 2

3 Michelangelo Clocks Sundials are relative (solar) clocks. Mechanical clocks developed late 13th century (pre Renaissance). Clocks spread through Europe in 14-16th Centuries. Renaissance Italy was obsessed with measurement, for painting, sculpture, as well a practical matters (commerce and war). 3

4 Sundial: Solar Clock Clocks Mechanical Clocks spread through Europe in the Centuries. This elaborate one is in Prague. 4

5 Relative Age of Rocks Original Horizontality Sediments were originally flat-lying Superposition The oldest ones are on the bottom Cross-cutting The disturbed rocks are older than disturbing rocks Superposition 5

6 Correlation of Layers Physical Continuity Horizontal tracing Similarity of rock types and sequences Correlation of Fossils Faunal succession Correlation of Layers 6

7 Host rocks (red) are older than the intruding rocks (black). Crosscutting 7

8 Grand Canyon 8

9 Event Sequence 1. Sedimentation of layers a-k. 2. Intrusion of basaltic sill. 3. Intrusion of granite. 4. Uplift, tilting, and erosion. 5. Subsidence and deposition of layers l - m. 6. Intrusion of basaltic dike 7. Uplift and erosion. 8. Subsidence and deposition of layers o, p, q, r. 9. Uplift and erosion. 9

10 Relative Age of Rocks By the mid 19th century a relative time scale had been worked out for the sedimentary rocks of Europe (Phanerozoic). They lacked an absolute time scale. Kelvin and classical physicists advocated 40 million max. Darwin and evolutionary biologists advocated billions of years. Discovery of radioactivity at about 1900 confirmed billions. The Age of the Earth Bishop Ussher 17th Cent. (biblical): 4004BC Buffon 18th Cent. (Cooling of spheres): ~50000 Y Hutton late 18th Cent. (Geological cycles):infinite Darwin late 19th Cent. (Biological changes): Billions Kelvin late 19th C (Sun s energy): 40 Million Max Modern (Radiometric): 4.55 Billion 10

11 Radiometric Dating: Establishing an absolute time scale Minerals contain naturally radioactive elements K, U, Th, Rb, Sm These elements decay to stable daughter elements When minerals crystallize, they contain parent only. If we measure the concentration of daughter element in a mineral and we know the decay rate, we can calculate when the mineral crystallized. Radiometric Dating Example: 40 K - 40 Ar A K-feldspar (KAlSi 3 O 8 ) crystallizes in a granite and initially contains no Ar. Natural K is 0.012% 40 K 40 K decays to 40 Ar with a half-life of 1.31 x 10 9 years (1.3 billion years). If we measure the 40 Ar content of the feldspar, we can get a crystallization date of the mineral. Isotope measurements are made with a mass spectrometer. 11

12 Exponential Decay Types of Radioactive Decay Particle composed of: Mass# Atomic # Example alpha 2 neutrons+ 4 2 U, Th, 2 protons beta- electron K beta+ positron K gamma photon 0 0 all nuclear reactions neutron neutron U 12

13 Naturally Radioactive Isotopes Parent DaughterHalf life Decay 40K 40Ar 1.3 x 10 9 y b + 87Rb 87Sr 4.9 x y b - 238U 206Pb 4.5 x 10 9 y 8a, g 235U 207Pb 7.1 x 10 8 y 7a, g 232 Th 208Pb 1.4 x y 6a, g 14 C 14 N 5.7 x 10 3 y b - Geologic Time Scale Eon Era Period Began (My ago) Phanerozoic Cenozoic Quaternary 0.01 Pleistocene 1.6 Tertiary Pliocene 5.3 Miocene 23.7 Oligocene 36.6 Eocene 57.8 Paleocene 66.4 Mesozoic Cretaceous 144 Jurassic 208 Triassic 245 Paleozoic Permian 286 Pennsylvanian 320 Mississippian

14 Geologic Time Scale Phanerozoic Cenozoic Mesozoic Paleozoic Permian 286 Pennsylvanian 320 Mississippian 360 Devonian 408 Silurian 438 Ordovician 505 Cambrian 550 Proterozoic 2500 Archean 4000 Hadean 4550 Some Major Events Latest warming 7000y Ice age ~1.8 MY Dinosaur extinction 66 MY Dinosaurs ~245 MY Vertebrates ~400 MY Multi-cell life forms ~550 Cambrian Explosion Snowball earth 600 MY Free O 2 ~ 2.5 GY (CH 4 and NH 3 decline) Single cell life forms ~3.7 GY Oceans: at least by 4.3 GY Accretion: 4.55 GY 14

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