How old is it

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How old is it?
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4.5 Billion years old

• 4,500,000,000 years

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But - how do we
know that?
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Archbishop Ussher of Ireland1664

• Heaven and Earth, Centre and substance were made
  in the same instant of time and clouds full of
  water and man were created by the Trinity on the
  26th day of October, 4004 B.C at 900 in the
  morning.

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1750 -Molten Earth modelGeorges de Buffon

• Earth was originally molten and cooled to present
  surface temperature
• 75,000 years.

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This seemed too young

• Darwin - Beagle voyage in 1830s
• Hutton - Uniformitarianism
• The present is the Key to the past.

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Late 1800sLord Kelvin

• Reworked the cooling model using rock properties
  instead of iron.
• 20-40 million years

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1893 - sedimentary rock recordCharles Walcott.
USGS

• Used typical sedimentation rates
• Tried to calculate thickness of all rocks
• 75 million years

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1899 - ocean salinityJohn Joly, physicist

• Assumed all oceans were originally pure
• How long would it take to get to present
  salinity?
• 100 million years

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1900s - radioactive elements discovered

• Uranium - Henri Becquerel
• Marie and Pierre Curie - radium

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Radioactivity
A radioactive isotope is one with an unstable
nuclear configuration
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The structure of an Atom

• All atoms are built from the same basic
  components.
• Protons and neutrons are found in the nucleus.
• Electrons are found in orbits around the nucleus.

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Radioactive decay involves the emission of
radiation.

• Alpha decay - loss of 2 protons and 2 neutrons
• Beta decay - loss of electrons or positrons
• Gamma decay - electromagnetic radiation - similar
  to X-rays

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The unstable parent nucleus decays to a stable
daughter nucleus

• This decay is not random, but is a statistical
  phenomenon that follows the laws of probability.
• (like a coin toss)

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Half Life

• The length of time required for half of a certain
  number of parent atoms to decay to daughter
  atoms.

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Half life
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Ratio of parent / daughter

• Uranium 235 decays to Lead 207
• Half life is 704 million years.
• If the ratio of U235 Pb 207 is 13
• Age 2 704 ma 1408 ma

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• Uranium 238 to Lead 206. 4.5 billion years
• Uranium235 to Lead 207 704 million years
• Potassium 40 to Argon 40 1.3 billion
  years
• Carbon 14 to Nitrogen 14 5730 years

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Potassium-Argon Dating

• Potassium-Argon dating is the only viable
  technique for dating very old archaeological
  materials.
• Why?
• Geologists have used this method to date rocks as
  much as 4 billion years old.

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Problems with Potassium-Argon Dating

• Argon 14 is a gas
• When rocks are heated to the melting point, any
  Ar-40 contained in them is released into the
  atmosphere.
• When the rock re-crystallizes it becomes
  impermeable to gasses again.
• As the remaining K-40 in the rock decays into
  Ar-40, the gas is again trapped in the rock.
• Which age are we recording?

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Other methods typically used for working out dates

• Dendrochronology
• Magnetic Dating
• Thermoluminescence
• Fission Track Dating

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Dendrochronology

• Tree Ring Dating

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• Trees show annual growth rings that can be easily
  counted.
• This technique can give dates to around 10,000
  years ago.

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Bristle Cone Pines

• The oldest Bristle Cone discovered so far was
  4,900 years old
• Many are 4,000 years old.
• These trees were already 2000 years old during
  the time of the Cesars.

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Magnetic Dating

• The direction and strength of the Earths
  magnetic field is constantly changing.
• Field fixed in molten stone or fired clay.

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Paleomagnetism

• Correspondence between the locked magnetic
  polarities of different layers of lava flows and
  the magnetic-reveral time scale

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Fission Track Dating

• fission track analysis is widely used as a means
  of dating minerals
• Observing the tracks made in objects by the
  fission fragments of the uranium nuclei that they
  contain.
• Objects are irradiated with neutrons to induce
  fission then the density and number of the tracks
  before and after irradiation is noted to estimate
  the time that has elapsed since the object
  solidified.
• Objects can be dated to 10,000,000 years old

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Fission Track Dating
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Carbon 14 in the Atmosphere

• Carbon 14 undergoes constant decay.
• C14 ratios remain relatively stable in the
  atmosphere.
• C14 is constantly formed in the upper atmosphere.

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Carbon 14 Decay to Nitrogen 14

• Carbon 14, an unstable isotope of Carbon, loses
  an electron and decays into Nitrogen.

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Carbon 14 and Living Things

• Organisms are constantly replenishing their
  carbon and so while living maintain the same C14
  ratios as found in the atmosphere.

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Exponential Decay of Carbon 14

• ½ of the Carbon 14 decays every 5730 years

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Practical use of Carbon 14

• Bison from Ornee Caves in Niaux, France
• Dated by Carbon 14 technique to be 13,000 years
  old
• Objects can be dated as old as 70,000 years old

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Cited Figures

• Magnetic dating and spreading plates
  http//www.kie.berkeley.edu/ned/data/E01-971212-00
  8/E01-971212-008.html
• Bristle Cone Pine http//web.utk.edu/grissino/i
  mages/bcp.jpg
• Growth Rings http//emuseum.mnsu.edu/archaeology/
  dating/images/tree_ring.gif
• Magnetic dating by period http//geomag.gly.fsu.e
  du/parker/1000/seis/1000seis.htm
• Exponential Decay of C14 http//carbon14.univ-lyo
  n1.fr/courb2.gif
• C14 in the atmosphere http//www.cnde.iastate.edu
  /ncce/RT_CC/Sec.2.4/c-14b.gif
• Carbon 14 Decay to Nitrogen 14
  http//sol.sci.uop.edu/jfalward/nuclearphysics/ca
  rbon14decay.JPG
• Bison from Ornee Caves in Niaux, France
  http//www.laradioactivite.com/pages/05_laboratoir
  e/01_methodes_d.htm
• Thermoluminescence test http//www.yang-guifei.co
  m/Welcome.html
• Fission Track Dating http//bullard.esc.cam.ac.uk
  /hobro/fission_image_hires.gif
• Potassium-Argon Dating http//anthro.palomar.edu/
  time/images/potassium_argon_dating.gif
• Radioactivity http//www.wps.com/archives/FilmStr
  ips/Effects-of-Fallout/atoms-are-unstable.GIF
• The structure of an Atom http//info.fuw.edu.pl/f
  estiwal/2001/struny/czastki/atom.gif
• Radioactive decay http//earth.leeds.ac.uk/dynami
  cearth/dating/decay.gif
• Carbon 14 and Living Things http//jan.ucc.nau.ed
  u/wittke/GLG100/Carbon14.jpg