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Historical Geology

Historical Geology. Introduction. What is Historical Geology?. Immensity of time span difficult to appreciate. Earth’s History based on evidence preserved in rocks. Changes occur over millions to billions of years. Human life span = 70-80 years. Reconstruct history through extrapolation.

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Historical Geology

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  1. Historical Geology Introduction

  2. What is Historical Geology? • Immensity of time span difficult to appreciate • Earth’s History based on evidence preserved in rocks • Changes occur over millions to billions of years • Human life span = 70-80 years • Reconstruct history through extrapolation

  3. Modern Ripples Fossilized Ripples Fossilized Mudcracks Modern Mudcracks Uniformitarianism • Very important to Historical Geology • Defined as: Geologic processes operating today, operated in past • Not inviolable • Geologic events last k’s to m.y. • Past conditions not similar to today

  4. Grand Canyon The Rock Record • Read rock record to reconstruct past

  5. Law of Superposition • Crucial to establishing older/younger rocks, therefore history of area • Law defined as: In undisturbed section – youngest rock on top • Map formations, beds, etc.

  6. Unconformities • Rock record not continuous. Have sections missing • Must identify the unconformities: Gaps in rock record • All geologic history not evident in rock • Have uplift and erosion, i.e. book with pages missing • Part of rock record always remains

  7. Halite Coal Interpretation from Rocks • Need to read sedimentary rocks • Non-marine, coal, marine siltstone, marine limestone

  8. Fossils • Remains of organisms—Very Important!! • Important for • time, • environments, • evolution of life • Record depth, temp, relative ages for correlation, and event timing

  9. J. Lightfoot Bishop Ussher Thales deBuffon Early Concepts of Geologic Time Attempts to determine the Earth’s age • Greeks • Thales (636 BC) • knew of fossils, coastal erosion, sed. • But couldn’t tell time • Medieval times • J. Lightfoot (1644) • Relied on biblical accounts • Earth formed (:OOAm, 17 Sept 3929 B.C • Bishop Ussher (1581-1665) • Lightfoot wrong • 23 Oct 4004 B.C • deBuffon (1707-1788) • Cooling rates of spheres • Earth 74,047 yrs

  10. Early Concepts of Geologic Time Attempts to determine the Earth’s age • Catastrophists • Believed in catastrophes • Changes in fossils + changes in creation • Fit biblical accounts of Noah’s flood • Therefore, Earth old, 5-6k years • Support from floods, earthquakes & volcanoes –common catastrophes

  11. Early Concepts of Geologic Time Attempts to determine the Earth’s age • Neptunists—Attempt to put rocks in sequence—Important! • Werner (1750-1817 • Controversial geologist (mineralogist/metallurgist) • Rocks precipitated from H2O • Divided Earth into formations • Oldest =primary = granite/gneiss • Transition+ greywacke, sedimentary rocks • Secondary rocks = flat=lying LS, SS, basalt • Tertiary rocks + alluvial sands & gravels

  12. Early Concepts of Geologic Time Attempts to determine the Earth’s age • Plutonists • Hutton (1726-1797) • Theory of Uniformitarianism • Earth formed by processes acting today • Saw no act of Creation or Catastrophe • Wrote Theory of the Earth (1795) • Severely Criticized, Ignored biblical accounts • Hutton’s ideas popularized by Charles Lyell • Hutton not a good writer

  13. Early Concepts of Geologic Time Attempts to determine the Earth’s age • Plutonists • Lyell (1797-1875) • Supported Uniformitarianism • Published Principles of Geology (1830-1833) • Local gaps in record • Erosion, not catastrophies • Influenced Darwin

  14. Age of Earth • Darwin • Observable geol. Record • Small part of Earth history • Jolly • Studied rate of ocean salt add’n • Earth is 100 m.y. • Thomson (Lord Kelvin) • Rate of Earth cooling • Earth is 20-30 m.y. • Sody and Rutherford • Studied radioactivity • Boltword • Using radioactivity • Earth is 1640 m.y. • Uniformitarianism followers – very old Earth

  15. Actualism in Uniformitariansim • Today follow “actualistic” model of Uniformitariansim • Follow Uniformitarianism • But, consider natural laws: • Erosion, climate, deposition  not same rates through time • Earth’s surface  different today • Conditions not the same  no epeiric seas today • Cataclastic events have happened  dinosaur extinction (meteor)

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