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Lecture 6. Formation of the Moon, Absolute Ages, Radiometric Dating

Lecture 6. Formation of the Moon, Absolute Ages, Radiometric Dating. reading: Chapter 4. Pre-Apollo Hypotheses for Formation of the Moon. 1. Fission Hypothesis Moon once part of Earth, somehow separated. Rapid spinning, cast off outer layers.

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Lecture 6. Formation of the Moon, Absolute Ages, Radiometric Dating

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  1. Lecture 6. Formation of the Moon, Absolute Ages, Radiometric Dating reading: Chapter 4

  2. Pre-Apollo Hypotheses for Formation of the Moon • 1. Fission Hypothesis Moon once part of Earth, somehow separated. Rapid spinning, cast off outer layers. Possibly separated from the Pacific Ocean basin. Composition resembles Earth’s mantle. • Capture Hypothesis Moon formed somewhere else in the solar system. Gravitational field of the Earth caught it. • Condensation Hypothesis Moon and Earth condensed in place individually from the solar nebula. • Giant Impact Hypothesis Planetesimal or planet struck the Earth just after it formed. Ejected large volumes of hot material into orbit. Disk of ejected matter formed, condensed into the Moon.

  3. Observations of the Moon What did Apollo find? • age of the moon is 4.5 Ga (same age as the solar system). • Moon rocks dry (contain very little water, or other volatiles). • Moon is less dense than the Earth • Moon 3.3 g/cc • Earth 5.5 g/cc • - Earth has a large iron core, Moon does not. • - Moon has same ratio of oxygen isotopes as the Earth. • - Moon is unusually large compared to other moons.

  4. Rejecting Hypotheses • 1. Fission Hypothesis Earth would have had to spun much faster - not likely. Moon lacks volatiles - water, Pb, Au. • Capture Hypothesis Moon would have to be traveling very slow - unlikely. Moon would have very different composition - not seen. Moon lacks volatiles - water, Pb, Au. Moon lacks a core. • Condensation Hypothesis Moon is too large - 1/4 Earth’s diameter. Most other moons thought to have formed this way are small. Moon lacks volatiles - water, Pb, Au. Moon lacks a core. • Giant Impact Hypothesis Explains lack of volatiles, size, composition, lack of core.

  5. Giant Impact Hypothesis Mars sized, grazing impactor. Melted the surface Ejected molten crust and mantle into space. Molten material formed a ring of asteroids. Asteroids accreted to form the Moon …..movie ….. simulation so… how old is the Moon and the Earth??

  6. Sedimentary Rocks Erosion (wind, water, chemical) produces sediment Sediment grains carried & deposited in different location Sediments: - often form flat layers - made of sand, silt, mud - can trap organisms or their remains - fossils Fossils can be: - tracks - physical traces - chemical traces - morphological traces - macroscopic or microscopic

  7. Igneous Rocks Molten rock that cools and solidifies - if it erupts on the surface: lava - if it is below the ground: magma basalt Mg and Fe rich Hawaii granite Si rich Cascades, Sierra Nevadas

  8. Metamorphic Rocks Sedimentary or igneous rocks Have been heated to high T or subjected to high P - not quite enough to melt - minerals in the rock change - often see original layering gneiss marble deformed shale

  9. Rocks Contain Minerals augite/clinopyroxene garnet & clinopyroxene calcite CaCO3 Atlas of Igneous & Metamorphic Rocks quartz SiO2

  10. Geological Record / Rock Record rise in oxygen oldest rocks on Earth - end of heavy bombardment plate tectonics? origin of the solar system first multi- cellular fossils Cambrian Explosion Phanerozoic Hadean Archean Proterozoic billions of years ago: 2.5 3.8 0.55 4.56 present • Hadean - Greek God of the underworld • - abundant impacts • - life?? • Archean - “ancient life” • minimal evidence of life • microbial life • - oceans and ‘continents’ different • Proterozoic - “earlier life” • - appearance of oxygen • - gradual increase in oxygen • - multi-cellular life (algae) • Phanerozoic - “visible life” • - multi-cellular animals • - plants • - fungi • colonization of land • most eons defined arbitrarily • except for the Phanerozoic

  11. The Phanerozoic • 3 eras in the Phanerozoic: • Paleozoic - “old life” • - 543-248 Ma • - origins of multi-cellular • animals • - origins of plants • - colonization of land • - C, O, S, D, C, P • Mesozoic - “middle life” • - 248-65 Ma • time of the dinosaurs • T, J, C • Cenozoic - “recent life” • - 65 Ma to present • - age of the mammals • (Ma = millions of years • ago)

  12. Absolute Ages Through Radiometric Dating Before the 1950’s ages were all relative. Radioactive isotopes are unstable, spontaneously decay to stabler elements. Parent Daughter Half-life 238U 206Pb 4.47 Ga 40K 40Ar 1.25 Ga 87Rb 87Sr 48.8 Ma 26Al 26Mg 700,000 years 14C 14N 5,730 years 238U time • Half-life = time for half of the starting • material to decay to the daughter element • Amount of 206Pb in the rock depends on: • - amount of starting 238U and 206Pb • - time 238U + 206Pb

  13. Radioactive Decay Is a probabilistic process. Any particular atom will have a 50% chance of decaying during the first half life. 1.25 Ga another 1.25 Ga 1 µg 40K (parent) 1/2 µg 40K 1/4 µg 40K 0 µg 40Ar (daughter) 1/2 µg 40Ar 3/4 µg 40Ar another 1.25 Ga 1/8 µg 40K 7/8 µg 40Ar

  14. Radiometric Dating of Rocks Measure the amount of 40K and 40Ar in the rock. Assume all 40Ar came from 40K (this is reasonable - Ar is a gas not normally found in rocks). Calculate the age, calculate the # of half lives: current amount 1 (amount of radioactive original amount 2 material in the rock) Can use different parent-daughter pairs depending on your sample. 14C only useful for objects <50,000 years old. 238U only useful for the oldest rocks on Earth. Chose the pair with the most parent and daughter contents. Note that the Age = date that the mineral in the rock formed. t/thalf ( ) =

  15. Radiometric Dating of Rocks, cont. Note that the Age = date that the mineral in the rock formed. Is this method useful for sedimentary rocks?

  16. Age of the Solar System • Determine radiometric age of the oldest rocks in the solar system. • 4.55 ± 0.02 Ga • 4.6 Ga Meteorites • - left from from accretion • carbonaceous chondrites (86% of stony • meteorites) • minerals formed during the • process of accretion • 4.5/4.4 Ga Moon • - oldest minerals on the moon • - when the moon formed; minerals crystalized from molten material

  17. Age of the Earth Oldest rocks are ~ 4.0 Ga. Heavily metamorphosed. Older mineral grains in sedimentary rock are 4.4 Ga (zircon mineral grains, ZrSiO4) Rock ground, grains separated. Put under SHRIMP = sensitive high-resolution ion micro-probe Zap with a ions to vaporize a bit of the mineral Separate out the atoms with a mass spectrometer bent magnet Count 238U and 208Pb (plus 204Pb, 206Pb, 207Pb) Estimate original Pb content, calculate how much has decayed (age). Is the Earth younger than the solar system?

  18. Earth is an Active Planet Water, erosion, plate tectonics.

  19. Lecture 7. The Hadean Earth, heavy bombardment, origin of the atmosphere and oceans reading: Chapter 4

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