HISTORICAL NOTES. Catastrophism. James Usher, mid-1600s, concluded Earth was only a few thousand years old

Transcription

1 1 GEOLOGIC TIME

2 HISTORICAL NOTES Catastrophism James Usher, mid-1600s, concluded Earth was only a few thousand years old

3 Uniformitarianism Charles Lyell published Principles of Geology

4 HOW DO WE KNOW HOW OLD THE EARTH IS? Two methods to determine the age of the Earth: RELATIVE dating ABSOLUTE dating What s the difference? 4

5 GEOLOGIC TIME 1. Relative age dating order of features and events Not actual age in years. 2. Absolute age dating the actual age (in thousands, millions, or billions of years) of a rock or other geologic feature. 5

6 RELATIVE DATING PLACES ROCKS AND EVENTS IN SEQUENCE Age refers to the sequence in which events took place, not an actual age (in years). 6

7 VOCABULARY: STRATA STRATA simply, layers of rock! 7

8 RELATIVE DATING TECHNIQUES Principles used to study stratigraphy (layered rocks). Law of Superposition oldest rocks at the bottom Principle of Original Horizontality sediment is deposited horizontally Principle of Cross Cutting Relationshipsyounger feature cuts through older feature Principle of Lateral Continuity rock layers of the same type and layers of the same sequence it is assumed they were once continuous Principle of Inclusions Object enclosed in the rock is older than the time of rock formation 8

9 LAW OF SUPERPOSITION The oldest is at the bottom and successively higher layers are successively younger. 9

10 SUPERPOSITION IS WELL ILLUSTRATED IN THE GRAND CANYON

11 PRINCIPLE OF ORIGINAL HORIZONTALITY Due to gravity; sediments deposited horizontally and parallel to the surface 11

12 VOCABULARY: FOLDING OF LAYERS Anticline Syncline 12

13 PRINCIPLE OF ORIGINAL HORIZONTALITY 13

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15 PRINCIPLE OF CROSS CUTTING RELATIONSHIPS Younger feature cuts through older feature 15

16 CROSS-CUTTING RELATIONSHIPS

17 Older Younger 17

18 PRINCIPLE OF LATERAL CONTINUITY Sediments are deposited layers on top of each other. If you find the layers of rock are the same type of rock and sequence of layers here and over there, we can assume they were once continuous. HOW DOES THIS HAPPEN? A fault can disrupt once continuous layers of rock. Streams can erode a valley making rock layers discontinuous. 18

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20 PRINCIPLE OF INCLUSIONS Older Younger Older Younger Fragments of one rock type can be included into a totally different rock type. In order to be included into another rock you must exist first. 20

21 THE FOSSIL RECORD William Smith s Principle of Faunal Succession Principle of faunal succession 21

22 INDEX FOSSILS Easy to identify. Abundant. Short life span for the species Geographically Widespread Certain index fossils are keys to matching and dating sedimentary strata in widely separated outcrops 22

23 DETERMINING THE AGES OF ROCKS USING FOSSILS

24 RELATIVE DATING - UNCONFORMITIES An unconformity is a break in the rock record: sediment is not deposited erosion is removing previously deposited sediment.

25 3 Types of unconformities 1. Angular unconformity 2. Disconformity 3. Nonconformity 25

26 Angular unconformity tilted rocks are overlain by flatlying rocks 26

27 The Great Unconformity The Grand Canyon 27

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29 Disconformity an irregular surface of erosion between parallel, sedimentary, strata. No tilting occurs 29

30 DISCONFORMITY MISSING STRATA DUE TO EROSION OR UPLIFT OF ROCKS 30

31 Nonconformity» Metamorphic or igneous rocks below» Younger sedimentary rocks above» Boundary exists between igneous or metamorphic and sedimentary rock 31

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33 Layer #1 is younger than the other layers because it is on top. Layer #5 is older than Layers#1-#4, because it is below them. The fault is younger than all the layers, because it cuts through all the layers. The blue layer is the same age in both places, because it contains the same fossils. Relative Age Dating - an example 33

34 YOU TRY IT 34

35 ABSOLUTE DATING Absolute ages of rocks/inerals can be determined using radiometric dating. Used only for rocks and minerals that contain radioactive elements. HOW? 35

36 Absolute Age Dating Radioactive decay: when an unstable element changes (decays) into another element, or a new variation of itself. This change occurs at a precise rate that can be determined by experimentation. 36

37 ABSOLUTE DATING Absolute dating involves atoms. An atom is made of electrons, protons and neutrons. 37

38 RADIOACTIVITY AND RADIOMETRIC DATING Atomic structure Atomic number Number of protons in the atom s nucleus Mass number (atomic mass) Number of protons plus (added to) the number of neutrons in an atom s nucleus Radioactive decay looks at the number of NEUTRONS

39 ISOTOPES An element with differing number of neutrons in the nucleus Different number of neutrons, therefore a different mass number, but still the same element Mass # = protons + neutrons Atomic # = # of protons 39

40 STABILITY OF ISOTOPES Some isotopes want to become stable, Stability is different for each element They will decay by gaining or loosing particles, turning into a different version of itself or a different element 40

41 RADIOACTIVITY AND RADIOMETRIC DATING Radioactivity Spontaneous breaking apart (decay) of unstable, atomic nuclei atoms wants to become stable When the atom breaks apart, it changes into a different element Radioactive decay Parent an unstable isotope Daughter products isotopes formed from the decay of a parent Parent breaks down (decays) to form daughter products

42 RADIOACTIVITY AND RADIOMETRIC DATING Radioactivity Radioactive decay Types of radioactive decay (3 ways parent isotopes break down to form daughter products) Alpha decay Beta decay Gamma decay

43 .. ALPHA DECAY Alpha Particle (α) is a particle made up of 2 protons and 2 neutrons. An alpha particle has the same nuclear composition as helium 43

44 BETA DECAY A beta particle has the same characteristics (mass = 0 and charge = -1) as an electron, but the beta particle is ejected from the nucleus. 44

45 GAMMA DECAY Unlike alpha and beta particles, gamma rays are not particles. Gamma decay is a form of electromagnetic energy like visible, UV, or X-rays, but more energetic. Gamma radiation is defined to have a mass = 0 and a charge = 0 Daughter is stable because it has less energy 45

46 HALF- LIFE time for one-half of the radioactive nuclei to decay to a stable form the time for half of the nucleus to breakdown time for half of the parent to breakdown into daughter products Yields numerical dates

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48 HALF-LIFE Unlike alpha and beta particles, gamma rays are not particles. Gamma decay is a form of electromagnetic energy like visible, UV, or X-rays, but more energetic. Gamma radiation is defined to have a mass = 0 and a charge = 0 48

49 Radioactive Element Atomic Number Atomic Mass Number Decay Type Half-Life Hydrogen (H) 1 3 Beta Decay (β - ) years Beryllium (Be) 4 8 Alpha 7 x sec Carbon (C) 6 14 Beta Decay (β - ) 5,730 years Krypton (Kr) Beta Decay (β - ) years Rubidium (Rb) Beta Decay (β - ) 4.88 x years Yttrium (Y) Beta Decay (β - ), Gamma 2.67 days Yttrium (Y) Beta Decay (β - ), Gamma 58.5 days Zirconium (Zr) Beta Decay (β - ) 1.53 x 10 6 years Polonium (Po) Alpha 138 days Radon (Rn) Alpha, Beta Decay (β + ) 1 min Radon (Rn) Alpha 4 days Radium (Ra) Alpha 4 days Radium (Ra) Alpha 1,622 years Thorium (Th) Alpha 2 years Thorium (Th) Beta Decay (β - ) 24 days Proactinium (Pa) Beta Decay (β - ) 6.75 hours Uranium (U) Alpha 159,200 years 49 Uranium (U) Alpha billion years

50 ABSOLUTE DATING Bottom Line: To determine the age when a rock formed, you need to know two things: - the percentage of parent and daughter atoms in the rock - the half-life (rate of decay) 50

51 CALCULATING HALF-LIFE Suppose you obtain 400 mg of Cobalt-60. Cobalt-60 has a half-life of 5.25 years. Now suppose you want to know how much of the cobalt-60 sample remains after years. How many half lives? How much cobalt will be left? So, in 5.25 years, one half life will have occurred. In years, 3 half lives will have occurred (15.75/5.25 = 3) So, ½ of the cobalt will remain after 1 half life, 200 mg ½ of the cobalt will remain after the 2 nd half life, 100 mg ½ of the cobalt will remain after the 3 rd half life, 50 mg After years. 50 mg of Cobalt will remain 51

52 ABSOLUTE DATING Different radioactive elements decay at different rates. C-14 = 5730 yrs; U-238 = 4.5byrs. 52

53 All living things absorb C-14 and C-12 in equal amounts as they live. After death, Carbon-14 undergoes beta decay, losing an electron. What remains is Nitrogen-14. Measuring C-14 vs. C-12 amounts, we can tell how old the fossil is. Carbon-14 radiometric dating can be used to determine ages of organic remains up about 50,000 years old. 53

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55 GEOLOGIC TIME SCALE Divides geologic history into units Originally created using relative dates Subdivisions Eon Greatest expanse of time Four eons Phanerozoic ( visible life ) the most recent eon Proterozoic Archean Hadean the oldest eon

56 The Geologic Time Scale

57 GEOLOGIC TIME SCALE Era Subdivision of an eon Eras of the Phanerozoic eon Cenozoic ( recent life ) Mesozoic ( middle life ) Paleozoic ( ancient life )

58 The Geologic Time Scale

59 GEOLOGIC TIME SCALE Subdivisions Eras are subdivided into periods Periods are subdivided into epochs

60 The Geologic Time Scale

61 GEOLOGIC TIME SCALE Difficulties in dating the time scale Not all rocks are datable (sedimentary ages are rarely reliable) Materials are often used to bracket events and arrive at different ages

62 YOU TRY IT RADIOACTIVE DATING 62