11 MEASURING GEOLOGIC TIME ETextbook Chapter 8 1

11. MEASURING GEOLOGIC TIME E-Textbook: Chapter 8 1. Relative age dating - Order of events are known, but not dates 2. Radiometric (Absolute) age dating - Dates are known DATING Means determining when geologic events happened GEOLOGIC EVENT (1) The formation of rocks (4) Folding (2) Faulting (5) Metamorphism (3) Tilting (6) Erosion etc…

1. RELATIVE AGE DATING – determined by spatial relationships between adjacent rock bodies WHEN DID THE KOREAN WAR HAPPEN? 1950 -52 What came first. . . chicken or the egg?

WHEN DID THE DIFFERENT ROCKS IN THIS AREA FORM? IN WHAT SEQUENCE? HOW CAN WE KNOW? “Principle of Original horizontality” “Principle of Cross-cutting relationships” “Principle of Inclusions” “Principle of Superposition” “Principle of Unconformities”

PRINCIPLE OF ORIGINAL HORIZONTALITY • Sediments are always deposited horizontally • If layers are inclined, then some tectonic forces must have tilted them - they didn’t form that way

PRINCIPLE OF SUPERPOSITION n If one layer is on top of another, then the top layer came later (it’s younger) Youngest Oldest

PRINCIPLE OF CROSS CUTTING RELATIONSHIPS If a rock unit is cut or disrupted by a fault or a dike, then the rock must have been there first. That is, the rock is older, the fault or the dike are younger! FA U LT dike Sandstone shale ke i d

PRINCIPLE OF INCLUSIONS If a rock unit contains pieces of another rock, it must be younger than the pieces of rocks. UNCONFORMITY

PRINCIPLE OF UNCONFORMITIE S Unconformities represent gaps in the geologic record that formed because layers were not deposited for a time or else layers were removed by erosion Unconformity: is like the place where pages are

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FOSSILS AND THE RELATIVE TIME SCALE Dinosaur Ammonite Trilobite

What is the possible age of a rock that contains fossils of both: a) Shark and Mucrospirifer? b) Shark and Dinosaurs? Estimate the age of the fossiliferous limestone

RADIOMETRIC (Numerical) AGE DATING Ø Provides actual # of years of rocks (minerals) Ø Places rocks and events in Geologic Time Scale Ø Makes use of Radiometric techniques such as K-Ar, C-14, UPb etc.

WHICH OF THE FOLLOWING STATEMENTS COMES CLOSEST TO YOUR VIEWS ON THE ORIGIN OF HUMAN BEINGS? A) Man developed over millions of years from less advanced life forms B) Man developed over millions of years, but God guided the process C) God created human beings in their present form within the last 10, 000 years

HOW OLD IS THE EARTH? Archbishop James Ussher (1654) à the world was created in 4004 (B. C. E. ) Dr. John Lightfoot (biblical scholar) à Was more specific: the Earth was created on October 26, 4004 at 9: 00 a. m. (B. C. E) à Age of the earth: 4004 + 2018 = 6, 022 years https: //www. youtube. com/watch? v=zs. Bp 4 D 9 d. MU&feature=youtu. be Michio Kaku on Scientific Evidence of Geological Time [start at 2. 47 min. ] https: //youtu. be/rx. RDb. CTCJb 8 Radiometric Dating - General (8 min)

RADIOMETRIC DATING: BASICS n n Radioactivity: Spontaneous nuclear transformations that change the number of protons & neutrons in the nucleus of an isotope. n Forces that bind p & n may be unusually weak, thus the nucleus becomes unstable! Isotope: One of two or more species of the same chemical element having the same number of protons but differing from one another by having a different number of neutrons. n U has always 92 protons, but varying # of neutrons; U 234, U 235, U 238

HALF LIFE (λ) Time it takes for half of the parent isotope to decay into daughter isotope C-N 5730 years

INSTRUMENT FOR RADIOMETRIC DATING Mass Spectrometer

Half-life % of Parent Isotope (element) remaining in sample Starting amount of parent isotope (at time=zero): 100 % EXAMPLE Q 1. Suppose the half-life value of a radioactive element is 50 m. y. How long will it take for 50% of the parent isotope to change (decay) to the daughter product? Q 2. If the % of parent isotope found in a sample is 6. 25%, how old is the sample?

WHAT CAN BE DATED? DATING METHOD MINERALS/MATERIALS THAT CAN BE DATED U-Pb Zircon, apatite K-Ar Muscovite, biotite, hornblende Rb-Sr Muscovite, biotite, K-feldspar C-N Wood, coal, peat, bone, shell, water Organic Material (fossils) Metamorphic Rock Igneous Rock

C-14 METHOD • Cosmic particle bombardment of Nitrogen (N-15) emits a proton and becomes C-14. This is radioactive element with a λ of about 5, 730 yrs. N atomic # = 7 C atomic # = 6 • Plants and animals ingest this radioactive C-14 while they are alive. When they die, the ingestion stops, and the radioactive C-14 clock begins to count down. https: //youtu. be/Kcuz 1 Ji. Mk 9 k Carbon (C-14) dating (4 min)

AGE OF THE EARTH Oldest Rocks on Earth (Acasta Gneiss, N Canada) Ca. 3. 96 b. y. old 4. 56 b. y. Oldest Mineral on Earth (Zircon, Jack Hills Conglomerate, W Australia) : ~ 4. 4 b. y. old

THE SOLAR SYSTEM • It is believed that all planets of our solar system, including the asteroid belt, originated at about the same time. Therefore, the age of one is the age for all! • Age determined for Earth from rocks and minerals that originate on Earth does not give the best estimate for the age of the Earth because most of these rocks have been recycled during subduction or by metamorphic processes.