Image source: http://www.nasa.gov/mission_pages/stardust/main/index.html
Stardust
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First sample return mission in 34 years
Traveled ~ 3 billion miles
Retrieved samples of comet Wild-2
Flew through the comets tail
Collected dust particles in aerogel
Image source: http://www.nasa.gov/mission_pages/stardust/main/index.html
Image source: http://www.nasa.gov/mission_pages/stardust/main/index.html
Image source: http://www.nasa.gov/mission_pages/stardust/main/index.html
Image source: http://www.nasa.gov/mission_pages/stardust/main/index.html
Geologic Time
Image source: http://www.nasa.gov/mission_pages/stardust/main/index.html
Image source: http://www.nasa.gov/mission_pages/stardust/main/index.html
Geologic Time
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Fundamental geologic observations
Relative vs. absolute dating
Economic implications
Philosophical dimensions
Earth’s place in the Solar System
Meteorites
Relative dating techniques
• Nicolaus Steno (1669)
– Observations regarding sedimentary rocks
• Principle of Superposition
• Principle of Original Horizontality
• Principle of cross-cutting relationships
– Developed for dating igneous rocks
• Law of Faunal succession (~1800)
– Allows correlation of rocks of same age
Principle of Original Horizontality (Grand Canyon, USA)
Principle of Superposition (Grand Canyon, USA)
Source: © The McGraw-Hill Companies, Inc./Doug Sherman, photographer.
Schist and Granite
Source:Courtesy of Carla W. Montgomery.
Uniformitarianism
• Still many unanswered questions…
– Absolute age, length of geologic processes
“The Present is the Key to the Past”
James Hutton, late 18th century
Source: © The McGraw-Hill Companies, Inc./Doug Sherman, photographer.
Age of the Earth
Age of the Earth
• How old is the Earth?
Bishop Usher (16th century)
Bible…..~6,000 years old (4004 BC)
Lord Kelvin (19th century)
Cooling…20-40 million years old
John Joly (1899)
Salinity of oceans…100 million
Radioactivity
Age of the Earth
• Radiometric Dating
Radioactive isotopes in rocks decay at a known rate
U, Rb, K, Th, C, others….
• Counting atoms in the rocks tells time
Parent-daughter ratio
• Concept of half-life
How long it takes for half the parent isotope to decay
Parent-daughter ratios become smaller in time
Isotope clocks are useful for ~ 7 ½ lives
Absolute dating: isotopic clocks
• Carbon
 14C in the atmosphere, then we eat the carbon
14C isotopic half-life = ~6,000 years
14N
• U-Pb
 206Pb, and 235U  207Pb
238U isotopic half-life = ~4,500,000,000 years
4.5 billion years old (4.5 byo)
238U
• Other ‘old’ isotopic clocks
Rb  Sr, K  Ar, Th  Pb
A very old Earth…
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Meteorites ~ 4.57 byo
Earth ~ 4.56 byo
Oldest rocks on Earth ~ 4.0 byo
Oldest minerals ~ 4.4 byo
Oldest sediments ~ 3.8 byo
Oldest fossils ~ 3.5 byo
Billions, not millions!
The Oldest Minerals on Earth: Zircon (ZrSiO4)
50 um
Geologic time scale
• Subdivisions of geologic time
– Eon, Era, Period, Epoch
– Eons
• Precambrian: 4.5 b.y. to ~0.5 b.y.
• Phanerozoic: ~0.5 b.y. to today
Geologic time scale
• Subdivisions of geologic time
– Eon, Era, Period, Epoch
– Eras
• Paleozoic: ~560 m.y. to ~250 m.y.
• Mesozoic: ~250 m.y. to ~65 m.y.
• Cenozoic: ~65 m.y. to today
Geologic time scale
• Subdivisions of geologic time
– Eon, Era, Period, Epoch
– Today (your lifetime)
• Eon: Phanerozoic (~550 m.y. now
• Era: Cenozoic (65 m.y.  now
• Period: Quaternary (2 m.y.  now
• Epoch: Holocene (10 k.y  now
Image source: http://www.gly.fsu.edu/~salters/GLY1000/12Rock_record_time/Slide27.jpg