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

reading: Chapter 4

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

Pre-Apollo Hypotheses for Formation of the Moon

1. Fission HypothesisMoon once part of Earth, somehow separated.Rapid spinning, cast off outer layers.Possibly separated from the Pacific Ocean basin.Composition resembles Earth’s mantle.

2. Capture HypothesisMoon formed somewhere else in the solar system.Gravitational field of the Earth caught it.

3. Condensation HypothesisMoon and Earth condensed in place individually fromthe solar nebula.

4. 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.

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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.

2. 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.

3. Condensation HypothesisMoon 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.

4. Giant Impact HypothesisExplains lack of volatiles, size, composition, lack of core.

Rejecting Hypotheses

Giant Impact Hypothesis

Mars 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 Rocks

Erosion (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 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 richHawaii

graniteSi richCascades, Sierra Nevadas

Metamorphic Rocks

Sedimentary 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 layering





deformed shale

gneiss

marble

Rocks Contain Minerals

augite/clinopyroxene

calcite CaCO3

garnet & clinopyroxene

quartz SiO2 Atlas of Igneous & Metamorphic Rocks

Geological Record / Rock Record

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 arbitrarilyexcept for the Phanerozoic

Hadean Archean Proterozoic Phanerozoic

4.56 3.8 2.5 0.55 presentbillions of years ago:

origin of thesolar system

oldest rockson Earth - end ofheavy bombardment

rise inoxygen first multi-

cellular fossilsCambrianExplosion

platetectonics?

The Phanerozoic3 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 yearsago)

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-life238U 206Pb 4.47 Ga40K 40Ar 1.25 Ga87Rb 87Sr 48.8 Ma26Al 26Mg 700,000 years14C 14N 5,730 years

238U

238U + 206Pb

time

Half-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 Decay

Is a probabilistic process.Any particular atom will have a 50% chance of decaying during

the first half life.

1 µg 40K (parent) 1/2 µg 40K 1/4 µg 40K0 µg 40Ar (daughter) 1/2 µg 40Ar 3/4 µg 40Ar

1.25 Ga another 1.25 Ga

1/8 µg 40K7/8 µg 40Ar

another 1.25 Ga

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

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 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



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-probeZap with a ions to vaporize a bit of the mineralSeparate out the atoms with a mass spectrometer

bent magnetCount 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?

Earth is an Active Planet

Water, erosion, plate tectonics.







reading: Chapter 4

Lecture 7. The Hadean Earth, heavy bombardment, origin of the atmosphere and oceans

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