Summary

This document covers various approaches to determining absolute time—a critical concept in geology. It explores religious, scientific, and radioactive dating methods to establish Earth's age and track geologic events. The use of natural archives like tree rings for understanding longer timescales is also discussed.

Full Transcript

4 - Absolute Time Notes Status Done Lab Notes Status Done Systematics L4 - Trilobites Files & media ABSOLUTE CHRONOLOGY ⌛ How old is Earth? How long ago did the trilobite...

4 - Absolute Time Notes Status Done Lab Notes Status Done Systematics L4 - Trilobites Files & media ABSOLUTE CHRONOLOGY ⌛ How old is Earth? How long ago did the trilobites go extinct? How much time has passed since the end of Cambrian period? When did humans appear? How long ago did dinosaurs rule the Earth? → we require an ABSOLUTE (=NUMERICAL) GEOLOGIC TIME SCALE! 💡 Lyell & co.: Earth’s geological processes and organic evolution are really slow processes! But still missing precise estimates, Earth perceived as very young 1. 🛐 Attempt n°1: the religious approach Archbishop J. Ussher calculated the Earth’s age based on generations cited in the Bible: created on the vigil of sunday october 22, 4004 BC → then reported in all the popular editions of the Bible throughout XVIII century 2. ⚛️ Attempt(s) n°2: the scientific approach → since the renaissance, estimates were based on the arrow of time (remember ⚠️we can’t just look at the beginning and the end like anything happened in the middle, this leads to wrong estimates) Decreasing sea level - B. de Maillet, 2 Ga (wrong estimate) Tidal dissipation - G. Darwin, >56 Ma (wrong estimate) Ocean salinity - J. Joly, 80-100 Ma (good idea, wrong estimate) Erosion-sedimentation - A. Geikie (wrong estimate) Cooling of the Earth - Isaac Newton, Comte de Buffon, W.T Kelvin → physicist in contrast with most Earth Scientists (whom he criticized): 1) time has a direction and cannot be infinite! 2) Earth passed to solid state 98 Ma - later changed to 20-40 Ma (widely accepted but still criticized) → still, WRONG estimates, as he was unaware of the effects 4 - Absolute Time 1 of radioactivity and never trusted J. Perry, who insisted on the existence of convective plumes within the Earth’s mantle 3. ☢️ Attempt(s) n°3: the “radioactive approach” Arthur Holmes (1890-1965), english physicist → first “modern results”, pioneer of radiometric dating (exploiting the decay of natural radioactive isotopes as a clock for deriving the age of rocks) C.C. Patterson (1922-1995) → calculated the age of very ancient rocks using U/Pb decay chain to obtain an age of ca. 4.6 Ga, STILL VALID TODAY NON-RADIOMETRIC ABSOLUTE CHRONOLOGY Two different ways to describe GT: When = absolute age: Ga, Ma, Ka For how long = duration: Gyr, Myr, Kyr → Absolute ages for blind samples can only be obtained via radiometry but only works for igneous rocks and their byproducts! → Still, absolute ages can be the result of cyclical processes of know duration (days, years, Kyr..) - the first day of class che be defined bot as a “when” (sept. 30, 2024) or “how long ago” (94 days ago as of now, counting the days) Natural “archives” holding a long continuous record → tree rings (can overlap sections to go back in time - like wood from buildings), varves (well layered lacustrine sediments), coral bands, layers in ice and sedimentary rocks → CYCLESTRATIGRAPHY (exploits recursive, cyclical signals preserved in natural archives for constructing absolute chronology) 4 - Absolute Time 2

Use Quizgecko on...
Browser
Browser