Themes of the Day:

• Absolute Age Dating
• Geologic Time
• Geologic History of the Earth
• A Brief History of the Geologic Assembly of North America

Absolute Age Dating

• Measures Quantifiable Spans of Time
• Calendars, Tree Rings, Ice Cores, Varved Sediments
• Radiometric Dating
  • radioactive isotope (parent) undergoes one or more nuclear decays to form stable (daughter) isotope
  • Decay rate is exponential - not linear
  • Half-life - time that it takes for half of the parent to decay to daughter
  • Visualize exponential decay
  • Accurate dating requires:
    • material that forms with parent isotope, but no daughter isotope
    • closed system - no gain or loss of parent or daughter
    • known decay rate
    • accurate and precise measurements of parent and daughter isotopes
  • Examples:
    • radiocarbon dating - ¹⁴C --> ¹⁴N - Half-life of 5730 years - useful for archeology
    • Potassium-Argon dating - ⁴⁰K --> ⁴⁰Ar - argon is a noble gas, does not bond to mineral structures
    • Rubidium-Strontium dating - ⁸⁷Rb --> ⁸⁷Sr - useful in many geologic contexts
    • Uranium-Lead - ²³⁸U --> ²⁰⁶Pb and ²³⁵U --> ²⁰⁷Pb - twice as nice, two independent clocks in the same mineral (zircon)
  • Read more about using the U/Pb decay system in zircons to date very old geologic events.

Geologic Time

• Correlation - linking distant outcrops - Try it yourself.
• The use of Fossils - index fossils and fossil assemblages
• The use of Fossils and Correlation
• The Geologic Time Scale

Geologic History of the Earth

• Age of the Earth ~4.6 to 4.5 Ga (billion years old) - equivalent to formation of our solar system
• Oldest terrestrial mineral ~4.4 Ga (zircon, western Australia); Oldest surviving rocks on Earth ~4.0 Ga (Acasta gneiss - northern Canada)
• Outgassing and evolution of the atmosphere and oceans
• Earliest fossil evidence for life (single celled organisms) ~3.5 Ga - stromatolites (algal mounds) common throughout Precambrian
• Formation of continental cratons (shields) - Plate Tectonics operational
• Cambrian explosion (rapid diversification of multicelled organisms) ~570 Ma (million years ago)
• Geologic time scale - many subdivisions since ~500 Ma reflect a preservational bias in the geologic record
• Paleozoic Era
  • Life - Ordovician, Devonian, Permian, Mississippian
  • Fossils - End Permian Extinction
  • Paleogeography - Silurian, Late Paleozoic
• Mesozoic Era
  • Reptile Evolution
  • Life in Kansas
  • Dinosaurs and the K/T boundary (extinction) event
  • Paleogeography - Breakup of Pangea
• Cenozoic Era

A Brief History of the Geologic Assembly of North America

• Oldest rocks in NA - Slave Craton, 4.0 Ga (4000 Ma)
• Superior Province (Craton) - granite-greenstone belts 3.5 Ga to 2.5 Ga - Archean Greenstone near Wawa, ON
• Huronian and Marquette Range Supergroups - sedimentary rocks (including BIF) on the southern margin of the Superior Province - 2.3 Ga to 2.1 Ga
• Penokean Orogeny - 1.9 Ga to 1.75 Ga - accretion and deformation at southern edge of Superior Province
• Yavapai and Mazatal Orogenies - 1.8 Ga to 1.6 Ga - more accretion to southern NA - Kansas' basement rocks - metamorphic rocks in the Colorado Rockies
• Pikes Peak granite batholith - 1.4 Ga - anorogenic magmatism
• Mid-Continent Rift - Keweenawan volcanics and sediments in Lake Superior Basin - 1.1 Ga, Jacobsville Sandstone - extends to eastern Kansas
• Grenville Orogeny - Continent-continent collision - Eastern NA - 1.0 Ga (1000 Ma) - Rodinia Supercontinent
• Rodinia breakup, shallow sea covers much of NA - Late Cambrian, 500 Ma - sandstones at Tahquamenon Falls, Pictured Rocks
• Ordovician-Silurian-Devonian coral reefs cover much of Michigan (1, 2, 3), equatorial latitude - oil bearing formations of Kansas deposited in tropical marine environments - Appalachian Orogenies begin in Eastern NA
• Mississippian-Pennsylvanian-Permian - coal swamps in PA, WV, KY, Mississippi basin, Appalachian Orogenies culminate in closing of Iapetus Ocean, formation of Pangea Supercontinent - Ancestral Rockies uplifted
• Mesozoic Era - Triassic-Jurassic-Cretaceous - dinosaurs in northern Great Plains, Cretaceous Seaway in Kansas, Sierran volcanic arc along West coast, Atlantic ocean begins opening in East
• Cenozoic Era - Tertiary-Quaternary - Atlantic and Gulf Coastal plains, Rocky Mountains - Ogalalla Formation deposited, Basin & Range, and San Andreas Fault form in West, Uplift and Erosion in Great Plains (see below), Ice Ages, Humans Arrive, FHSU formed, class dismissed.

This concludes the material that will be covered on Exam #1.