reading: Chapter 4Lecture 6. Formation of the Moon, Absolute Ages, Radiometric Dating
Pre-Apollo Hypotheses for Formation of the Moon1.Fission HypothesisMoon once part of Earth, somehow separated.Rapid spinning, cast off outer layers.Possibly separated from the Pacific Ocean basin.Composition resembles Earths mantle.
Capture HypothesisMoon formed somewhere else in the solar system.Gravitational field of the Earth caught it.
Condensation HypothesisMoon and Earth condensed in place individually fromthe solar nebula.
Giant Impact HypothesisPlanetesimal 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.
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/ccEarth 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.Observations of the Moon
1.Fission HypothesisEarth would have had to spun much faster - not likely.Moon lacks volatiles - water, Pb, Au.
Capture HypothesisMoon 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 HypothesisMoon is too large - 1/4 Earths diameter.Most other moons thought to have formed this way are small.Moon lacks volatiles - water, Pb, Au.Moon lacks a core.
Giant Impact HypothesisExplains lack of volatiles, size, composition, lack of core.Rejecting Hypotheses
Giant Impact HypothesisMars sized, grazing impactor.Melted the surfaceEjected 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??
Sedimentary RocksErosion (wind, water, chemical) produces sedimentSediment 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
Igneous RocksMolten rock that cools and solidifies- if it erupts on the surface: lava- if it is below the ground: magmabasaltMg and Fe richHawaiigraniteSi richCascades, Sierra Nevadas
Metamorphic RocksSedimentary or igneous rocksHave been heated to high T or subjected to high P- not quite enough to melt- minerals in the rock change- often see original layeringdeformed shalegneissmarble
Rocks Contain Mineralsaugite/clinopyroxene calcite CaCO3garnet & clinopyroxenequartz SiO2Atlas of Igneous & Metamorphic Rocks
Geological Record / Rock RecordHadean - 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 arbitrarilyexcept for the Phanerozoic
The Phanerozoic3 eras in the Phanerozoic:
Paleozoic - old life- 543-248 Ma- origins of multi-cellularanimals- 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 yearsago)
Absolute Ages Through Radiometric DatingBefore the 1950s ages were all relative.Radioactive isotopes are unstable, spontaneously decay to stablerelements.
ParentDaughterHalf-life238U206Pb4.47 Ga40K40Ar1.25 Ga87Rb87Sr48.8 Ma26Al26Mg700,000 years14C14N5,730 years
238U238U + 206PbtimeHalf-life = time for half of the starting material to decay to the daughter elementAmount of 206Pb in the rock depends on:- amount of starting 238U and 206Pb- time
Radioactive DecayIs a probabilistic process.Any particular atom will have a 50% chance of decaying duringthe first half life.
1 g 40K (parent)1/2 g 40K1/4 g 40K0 g 40Ar (daughter)1/2 g 40Ar3/4 g 40Ar1.25 Gaanother 1.25 Ga1/8 g 40K7/8 g 40Aranother 1.25 Ga
Radiometric Dating of RocksMeasure 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 radioactiveoriginal amount 2 material in the rock)
Can use different parent-daughter pairs depending on your sample.14C only useful for objects
Radiometric Dating of Rocks, cont.Note that the Age = date that the mineral in the rock formed.
Is this method useful for sedimentary rocks?
Age of the Solar SystemDetermine radiometric age of the oldest rocks in the solar system.4.55 0.02 Ga
4.6 GaMeteorites- left from from accretion carbonaceous chondrites (86% of stonymeteorites) 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
Age of the EarthOldest rocks are ~ 4.0 Ga.Heavily metamorphosed.Older mineral grains in sedimentary rockare 4.4 Ga (zircon mineral grains, ZrSiO4)
Rock ground, grains separated.Put under SHRIMP = sensitive high-resolutionion micro-probeZap with a ions to vaporize a bit of the mineralSeparate out the atoms with a mass spectrometerbent magnetCount 238U and 208Pb (plus 204Pb, 206Pb, 207Pb)Estimate original Pb content, calculate how muchhas decayed (age).
Is the Earth younger than the solar system?
Earth is an Active PlanetWater, erosion, plate tectonics.
reading: Chapter 4Lecture 7. The Hadean Earth, heavy bombardment, origin of the atmosphere and oceans