1 Time How we achieved a modern sense of time. Learning Goals • Relative Geologic Time – Superposition (oldest on bottom) – Crosscutting and inclusion • Intrusions younger than host • Cobbles are older than host • Radiometric dating (absolute time) – Half lives and exponential decay • Geologic Time – Hadean, Archean, Proterozoic, Phanerozoic Yearly Calendars are Ancient • Stonehenge is 2000+ BC and indicates that ancient cultures counted days and knew precisely the repeat cycle of the seasons. Clocks • Sundials are relative (solar) clocks. • Mechanical clocks developed late 13th century (pre Renaissance). • Clocks spread through Europe in 14- 16th Centuries. • Renaissance Italy was ‘obsessed’ with measurement, for painting, sculpture, as well a practical matters (commerce and war). Sundial: Solar Clock Clocks • Mechanical Clocks spread through Europe in the 14-16 Centuries. • This elaborate one is in Prague.
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TimeHow we achieved a modern sense
of time.
Learning Goals• Relative Geologic Time
– Superposition (oldest on bottom)– Crosscutting and inclusion
• Intrusions younger than host• Cobbles are older than host
• Radiometric dating (absolute time)– Half lives and exponential decay
• Geologic Time – Hadean, Archean, Proterozoic,
Phanerozoic
Yearly Calendars are Ancient
• Stonehenge is 2000+ BC and indicates that ancient cultures counted days and knew precisely the repeat cycle of the seasons.
Clocks
• Sundials are relative (solar) clocks.• Mechanical clocks developed late
13th century (pre Renaissance).• Clocks spread through Europe in 14-
16th Centuries.• Renaissance Italy was ‘obsessed’
with measurement, for painting, sculpture, as well a practical matters (commerce and war).
Sundial: Solar ClockClocks
• Mechanical Clocks spread through Europe in the 14-16 Centuries.
Angular Unconformity, GCNP The Age of the Earth• Bishop Ussher 17th Cent. (biblical): 4004BC• Buffon 18th Cent. (Cooling of spheres):
~50000 Y• Hutton late 18th Cent. (Geological cycles):
Infinite• Darwin late 19th Cent. (Biological changes):
Billions• Kelvin late 19th C (Sun’s energy): 40 Million
Max (He was wrong!)• Modern (Radiometric): 4.55 Billion
Ice Ages
Mammals and Flowering Plants
Dinosaurs
Fish
Trilobites
Time scale is NOT linear
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Relative Age of Rocks• By the mid 19th century a relative time
scale had been worked out for the sedimentary rocks of Europe (Phanerozoic).
• They lacked an absolute time scale.• Kelvin and classical physicists advocated
40 million max. • Darwin and evolutionary biologists
advocated billions of years.• Discovery of radioactivity at about 1900
confirmed billions.
Event Sequence
1.Original horizontality2.Superposition (oldest
on the bottom)3.Crosscutting
(Intruding igneous rocks are younger than their hosts)
Event Sequence
Radiometric Dating:Establishing an absolute time scale
• Minerals contain naturally radioactive elements– K, U, Th, Rb, Sm
• These elements decay to stable daughter elements
• When minerals crystallize from melt, they contain parent only.
• If we measure the concentration of daughter element in a mineral and we know the decay rate, we can calculate when the mineral crystallized.
Radiometric DatingExample: 40K - 40Ar
• A K-feldspar (KAlSi3O8) crystallizes in a granite and initially contains no Ar.
• Natural K is 0.012% 40K• Atmospheric Ar has 3 stable isotopes
– 36K, 38K, 40K– No 36K, or 38K in feldspar.
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Radiometric DatingExample: 40K - 40Ar
• A K-feldspar (KAlSi3O8) crystallizes in a granite and initially contains no Ar.
• Natural K is 0.012% 40K• 40K decays to 40Ar with a half-life of
1.31 x 109 years (1.3 billion years).• If we measure the 40Ar content of the
feldspar, we can get a crystallization date of the mineral.
• Isotope measurements are made with a mass spectrometer.
Radiometric Dating• Igneous and metamorphic rocks can
be dated directly by radiometric methods.
• Sediments cannot be dated directly.• Igneous rock fragments in sediments
can be dated. (Sed must be younger)• Igneous rock intruding sediments can
be dated. (sed must be older)• 14C can be used to date organic
matter less than ~50000 yrs old.
Inclusion:Host rocks are younger than the included rocks (cobbles).
Intrusion:Host rocks (red) are older than
the intruding rocks (black).
Radiometric Dating• Igneous and metamorphic rocks can
be dated directly by radiometric methods.
• Sediments cannot be dated directly.• Igneous rock fragments in sediments
can be dated. (Sed must be younger)• Igneous rock intruding sediments can
be dated. (sed must be older)• 14C can be used to date organic
matter less than ~50000 yrs old.
14C system is different from other radiometric dating
systems.• 14C in the atmosphere comes from 14N• Plants take 14C from atmosphere• 14C has half-life of 5730 years• There is no 14C in rocks.• 14C can be used to date plant and animal
Geologic Time Scale• Eon Era Period Began (My ago)
• Phanerozoic Cenozoic Quaternary 0.01
• Pleistocene 1.6
• Tertiary Pliocene 5.3
• Miocene 23.7
• Oligocene 36.6
• Eocene 57.8
• Paleocene 66.4
• Mesozoic Cretaceous 144
• Jurassic 208
• Triassic 245
• Paleozoic Permian 286
• Pennsylvanian 320
• Mississippian 360
Geologic Time Scale• Phanerozoic Cenozoic
• Mesozoic
• Paleozoic Permian 286
• Pennsylvanian 320
• Mississippian 360
• Devonian 408
• Silurian 438
• Ordovician 505
• Cambrian 550
• Proterozoic 2500
• Archean 4000
• Hadean 4550
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Some Major Events• Latest warming 7000y • Ice ages ~1.8 MY to 7000 years ago • Dinosaur extinction 66 MY• Dinosaurs ~245 MY• Vertebrates ~400 MY• Multi-cell life forms ~550 ‘Cambrian Explosion’• ‘Snowball earth’ 600 MY• Free O2 ~ 2.5 GY (CH4 and NH3 decline)• Single cell life forms ~3.7 GY• Oceans: at least by 4.3 GY• Accretion: 4.55 GY