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Learning Goals Clocks Learning Goals • Relative Geologic Time – Superposition (oldest on bottom) Time – Crosscutting and inclusion • Intrusions younger than host • Cobbles are older than host How we achieved a modern sense • Radiometric dating (absolute time) of time. – Half lives and exponential decay • Geologic Time – Hadean, Archean, Proterozoic, Phanerozoic Yearly Calendars are Ancient 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’ • Stonehenge is 2000+ BC and with measurement, for painting, indicates that ancient cultures sculpture, as well a practical counted days and knew precisely the matters (commerce and war). repeat cycle of the seasons. Sundial: Solar Clock Clocks • Mechanical Clocks spread through Europe in the 14-16 Centuries. • This elaborate one is in Prague. 1 Deep Time Relative Age of Rocks (Clocks in the rocks) • Bishop Ussher 17th Cent. (biblical): 4004BC • Original Horizontality • Buffon 18th Cent. (Cooling of spheres): – Sediments were originally flat-lying ~50000 Y • Hutton late 18th Cent. (Geological cycles): • Superposition Infinite – The oldest ones are on the bottom • Darwin late 19th Cent. (Biological changes): Billions • Cross-cutting • Kelvin late 19th C (Sun’s energy): 40 Million – The disturbed (host) rocks are older Max (Kelvin was wrong!) than disturbing rocks • Modern (Radiometric): 4.55 Billion Superposition: Oldest on bottom Correlation of Layers • Physical Continuity – Horizontal tracing • Similarity of rock types and sequences • Correlation of Fossils – Faunal succession Crosscutting: Host rocks (red) are older than Correlation the intruding rocks (black). of Layers 2 Principle of Crosscutting Cross-cutting Relationships Grand Canyon Angular Unconformity, GCNP 3 Angular Unconformity, GCNP Angular Unconformity, GCNP 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 4 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 Event Sequence 1. Original horizontality 2. Superposition (oldest on the bottom) 3. Crosscutting (Intruding igneous rocks are younger than their hosts) Radiometric Dating: Radiometric Dating Establishing an absolute time scale Example: 40K - 40Ar • Minerals contain naturally radioactive • A K-feldspar (KAlSi3O8) crystallizes in elements a granite and initially contains no Ar. – K, U, Th, Rb, Sm • Natural K is 0.012% 40K • These elements decay to stable daughter elements • Atmospheric Ar has 3 stable isotopes 36 38 40 • When minerals crystallize from melt, they – K, K, K contain parent only. – No 36K, or 38K in feldspar. • If we measure the concentration of daughter element in a mineral and we know the decay rate, we can calculate when the mineral crystallized. 5 Radiometric Dating Radiometric Dating Example: 40K - 40Ar • Igneous and metamorphic rocks can • A K-feldspar (KAlSi3O8) crystallizes in be dated directly by radiometric a granite and initially contains no Ar. methods. • Natural K is 0.012% 40K • Sediments cannot be dated directly. • 40K decays to 40Ar with a half-life of • Igneous rock fragments in sediments 9 1.31 x 10 years (1.3 billion years). can be dated. (Sed must be younger) • If we measure the 40Ar content of the • Igneous rock intruding sediments can feldspar, we can get a crystallization be dated. (sed must be older) date of the mineral. • 14C can be used to date organic • Isotope measurements are made matter less than ~50000 yrs old. with a mass spectrometer. Inclusion: Intrusion: Host rocks are younger than Host rocks (red) are older than the included rocks (cobbles). the intruding rocks (black). 14 Radiometric Dating C system is different from • Igneous and metamorphic rocks can other radiometric dating be dated directly by radiometric systems. methods. • 14C in the atmosphere comes from 14N • Sediments cannot be dated directly. • Plants take 14C from atmosphere • Igneous rock fragments in sediments • 14C has half-life of 5730 years can be dated. (Sed must be younger) • There is no 14C in rocks. • Igneous rock intruding sediments can • 14C can be used to date plant and animal be dated. (sed must be older) matter that is younger than about 50,000 • 14C can be used to date organic years. matter less than ~50000 yrs old. 6 Exponential Decay Types of Radioactive Decay • Particle composed of: Mass# Atomic # Example • alpha 2 neutrons+ 4 2 U, Th, 2 protons • beta- electron 0 -1 40K • beta+ positron 0 +1 40K • gamma photon 0 0 all nuclear reactions • neutron neutron 1 0 235U Naturally Radioactive Isotopes Naturally Radioactive Isotopes Parent Daughter Half life Decay • 40K 40Ar 1.3 Gy + • 87Rb 87Sr 49 Gy - • 238U 206Pb 4.5 Gy 8, • 235U 207Pb 0.7 Gy 7, • 232Th 208Pb 14 Gy 6, • 14C 14N 5700 y - Geologic Time Scale Geologic Time Scale • Eon Era Period Began (My ago) • Phanerozoic Cenozoic • Phanerozoic Cenozoic Quaternary 0.01 • Mesozoic • Pleistocene 1.6 • Paleozoic Permian 286 • Tertiary Pliocene 5.3 • Pennsylvanian 320 • Miocene 23.7 • Mississippian 360 • Oligocene 36.6 • Devonian 408 • Eocene 57.8 • Silurian 438 • Paleocene 66.4 • Ordovician 505 • Mesozoic Cretaceous 144 • Cambrian 550 • Jurassic 208 • Proterozoic 2500 • Triassic 245 • Archean 4000 • Paleozoic Permian 286 • Hadean 4550 • Pennsylvanian 320 • Mississippian 360 7 Some Major Events Tree of Life • 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 Ice Ages Mammals and Flowering Plants Dinosaurs Fish Trilobites Geologic Time Terms A conglomerate contains some granite cobbles. K-Ar date on • Hadean • Half-life the granite gives 180 MY. • Archean • Alpha particle • Proterozoic • Beta particle • Phanerozoic • Gamma ray • Paleozoic • Neutron • Mesozoic • Cenozoic(Tertiary) • Cambrian • Unconformity • Angular unconformity 8 A conglomerate contains some granite cobbles. K-Ar date on the granite gives 180 MY. It is only rarely possible to use radiometric methods to directly date sedimentary rocks. But igneous and metamorphic rocks can be dated directly. A conglomerate contains some Host rocks (red) are older than granite cobbles. K-Ar date on the intruding rocks (black). the granite gives 180 MY. • A. The conglomerate is older than 180 MY • B. The conglomerate is younger than 180 MY • C. The conglomerate is 6000 y old • D. No age inference can be made. A shale is intruded by a basalt dike that Clicker has an age of 1100 MY. A shale is intruded by a basalt dike that has an age of 1100 MY. • A. The shale is older than 1100 MY • B. The shale is younger than 1100 MY • C. The shale is less than 1 million years old. • D. No relative age inference can be made. 9 Exponential Decay 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 The era of dinosaurs is Why can’t 14C be used to subdivided into Triassic, date limestones? Jurasssic, and Cretaceous. Together these are known • A. No carbon in limestone as the: • B. No 14C in limestone • C. 14C half-life too long • A. Archean • D. 14C half-life too short • B. Proterozoic • E. Daughter 14N not retained by limestone • C. Paleozoic • D. Mesozoic • E. Cenozoic Half-lives: If the amount of Half-lives: If the amount of radioactive isotope is ¼ radioactive isotope is ¼ the amount originally the amount originally present, how many half- present, how many half- lives have gone by? lives have gone by? • A. 1 • A. 1 • B. 2 • B. 2 • C. 3 • C. 3 • D. 4 • D. 4 10 Event Sequence Event Sequence 1. The basaltic dike is older than the granite 2. The basaltic sill is older than the granite. A. True A. True B. False B. False Event Sequence Event Sequence 3. Sediment layer ‘a’ is older than the granite 4. Sediment layer ‘q’ is older than the granite A. True A. True B. False B. False 11.
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