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Lecture 14 The Age of the Earth. How old is the Earth…. ….and how do we know?. Estimates of the Age of the Earth. Hindu chronology, ~140 BC 2 Ga (Billion years) old archbishop James Ussher, 1654 October 23, 4004 B.C. age of the Earth = length of human history James Hutton, 1785
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How old is the Earth…. ….and how do we know?
Estimates of the Age of the Earth • Hindu chronology, ~140 BC • 2 Ga (Billion years) old • archbishop James Ussher, 1654 • October 23, 4004 B.C. • age of the Earth = length of human history • James Hutton, 1785 • Very very old • Uniformitarianism (“the present is the key to the past”) • William Thomson, Lord Kelvin, late 1800’s • 20-40 Ma • Cooling planet from molten rock • Modern estimates • 4.6 Ga • Radiometric dating
The time to cool the Earth Assumptions: the Earth has not gained any heat since its formation The Earth is solid rock • Heat moves from hot places to cold places • The surface of the Earth loses heat at about 100-400 W/m2 • Kelvin estimates 24- 400 Ma to cool Earth from a molten body to the present Earth temperature Cold . HOT
The time to cool the Earth Assumptions: • The Earth is solid rock • the Earth has not gained any heat since its formation Problem: • The inner core is still molten • The mantle convects • The surface of the Earth is heated by the Sun • Radioactivity! .
Why does radioactivity matter? 234U = 92Kr + 141Ba + 3 Neutrons + Energy • Radioactive decay: some elements have naturally occurring isotopes that are unstable. • Decay produces heat: the Earth is far older than Kelvin calculated.
Radioactive Decay Energy = Mass Loss = Mass(Starting particles) – Mass(Ending particles) E=mc Time can be determined by their rate of decay. • Alpha decay • emits an alpha particle (helium-4) • E.g., 238U = 234Th + a + energy • Beta decay • Emission or capture of electrons • E.g., energy + 40K + electron = 40Ar + neutrino • E.g., 40K = 40Ca + electron + neutrino + energy
Radioactive Decay occurs in half-lives Parent (Starting) Isotope Number of half-lives
K-Ar age dating • Potassium (K) is abundant in many rock-forming minerals, including: • mica (biotite and muscovite) • feldspar • amphibole • Argon (Ar) is a gas, so the initial concentration of the daughter product is 0.
Potassium-Argon System:half-life 1.26 x 109 years electron capture 40Ar 22 E.C. electron emission 40K 21 Neutron number N (number of neutrons) 40Ca 20 18 19 20 Atomic number Z (number of protons)
Radioactive Decay occurs in half-lives 40Ar Parent = 40K 40K Number of half-lives
If you measure 40Ar and 40K in a rock and find equal quantities, how many half lives have occurred since the rock cooled? • 1 • 2 • 3 • 4 • 5 40Ar Parent = 40K 40K Number of half-lives
If the half-life of the K-Ar system is 1.26 x 109 years, how old is the rock with equal quantities of 40Ar and 40K ? • 0.63 x 109 years • 1.26 x 109 years • 2.52 x 109 years • 5.04 x 109 years • 10.1 x 109 years 40Ar Parent = 40K 40K Number of half-lives
If a rock start with 160 moles of 40K, how many moles does it have after 3 half-lives ? • 80 • 40 • 20 • 10 • 5 40Ar Parent = 40K 40K Number of half-lives
Radioactive decay involves the decay of one isotope of an element into an isotope of another element. It also • Creates mass. • Produces energy. • Decreases temperatures.
K-Ar dating can be used to date • Any rocks. • Rocks that contain Ar. • Rocks that contain K. • Rocks that contain both.
The half-life of a radioactive isotope is • The time it takes all atoms of the isotope to decay. • The time it takes half the atoms of the isotope to decay. • The time it takes one quarter of the atoms of the isotope to decay. • Half the time a particular atom of the isotope will take to decay.
Summary: Radiometric Dating • Decay of parent element (isotope) to daughter element (isotope) at constant rate • Half-life (t1/2 ): time required for half of parent element to decay to daughter element • Assume closed system • All daughter products must be from parent only • Measure parent + daughter in sample • Parent + daughter = amount original parent • Present parent /original parent = % decayed
What does the date actually date?: Blocking temperature Bottom line: Radiometric dates can tell you - when an igneous rock cooled below its blocking temperature - when regional or thermal metamorphism has ceased and cooled
The Earth’s oldest rocks • •Imataca Complex, Venezuela: 3.77 Ga • Rhodesian Craton, Zimbabwe: 3.52 Ga • Mount Stones, Antarctica: 3.53 Ga • Northwest Territories, Canada: 3.96 Ga • Detrital zircons: 4.15 Ga • Etc....... • …..So, problem solved?
Not quite, we have rocks from the moon!
Apollo 17 (30 samples) 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Rb-Sr 40Ar/39Ar Radiometric age (Ga) Sm-Nd U-Pb
Irons (11 meteorites), Stony irons (1 meteorites) Achondrites (20 meteorites) Chondrites (37 meteorites) 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Rb-Sr Radiometric age (Ga) 40Ar/39Ar Sm-Nd
So…. Why are moon rocks and meteorites so much older than the oldest Earth rocks?
The Hadean Eon: The first 600 Ma (million years). -Separation of Core from Mantle. -So hot: oceans of magma, no permanent crust? -No rock record
With radiometric dating, we can now put “numerical” ages on the “relative” geological timescale. Relative age: a sequence of events listed ‘relative’ to each other. Ex. a ‘younger’ brother Numerical age: a specific date, given in units. Ex. a 2-year-old brother…
Radiometric dating – years since the parent isotope started decaying
Magnetic polarity – on land and in oceans – can correlate sequences – but only relative dates
Why isn’t magnetic polarity an absolute dating method? • Magnetic polarity is either negative or positive. • Sequences of polarity can be matched. • Polarity changes happen at irregular intervals. • Sedimentation rates vary from place to place. • Knowing the total sequence doesn’t tell you the start.
Tree Rings – annual growth – can yeild absolute dates if correlate with living tree!
Review of Absolute Dating Methods • Radiometric • K-Ar/Ar-Ar (~50 ka – ~4.6 Ga) • U-Pb (~10 Ma – ~4.6 Ga) • C14 (< 70 ka) • Others (including fission tracks) • Seasonally controlled growth (<100 ka) • Growth rings of trees and shells (< 10 ka) • Annual mud layers in lakes and seas • Annual snow/ice accumulations in glaciers k=thousand, M=million, G=billion, a=years ago
Review of Relative Dating Methods • Fossils thru faunal succession • Paleomagnetism and magnetic polarity • Principles of • Cross-cutting relations • Superposition • Baked contacts • Inclusions • Horizontality • Continuity
Dating of geological events… Big problem: you can’t date (absolutely) sedimentary rocks!
What does a radiometric date actually date?: Blocking temperature Bottom line: Radiometric dates can tell you - when an igneous rock cooled below its blocking temperature - when regional or thermal metamorphism has ceased and cooled Exception: 14C dates time of death of a living system – so works for fossils, charcoal, etc. younger than 70 ka.
How old is the Cretaceous sandstone? • < 50 million years old • 50 - 80 million years old • 80 – 125 million years old • > 125 million years old
Olduvai Gorge, Tanzania Volcanic Ash Fossils
Stratigraphic placement of Olduvai hominid 65 Tuff IF Ng’eju Tuff Figure 2 from Blumenschine et al., 2003
Summary • Earth is ~4.5 billion years old (Ga), but the oldest rocks are only 3-4 Ga due to the early Earth being too hot for a rock record to survive. • Relative dating places strata (or events) in an order. • Absolute dating gives the number of years ago that a fossil existed or an volcanic ash layer was deposited. • The geologic time scale has both a relative ages (based on the fossil succession and magnetic reversals) and absolute ages (based on radiometric dates).