time is critical for geologic processes!

Transcription

1 geologic time!

2 time is critical for geologic processes! Rockies and Alps are ~3000 m tall! -- mountains grow at ~1 meter per 5000 yrs (0.2 mm/yr)! m x 5000 yr/m = 15,000,000 (yrs necessary)! Atlantic Ocean is ~5500 km across! -- today, seafloor spreading in Atlantic is ~4 cm/yr! km = 6000 km x 1000 m/km x 100 cm/m! " = 600,000,000 cm! ,000,000 cm / 4 cm/yr = 150,000,000 years! for comparison: fingernail grows at 1 cm/yr!

3 age of the Earth! prior to 19th century, accepted age from religious beliefs! -- 6,000 years for Western culture (Christian)! " Bishop Usher from geneology in the Bible! -- old beyond comprehension (Hindu/Buddhist/Chinese)! -- age not certain (Islam)! during 19th century, length of time required for! "geologic processes to occur was recognized! -- fundamental contribution of geology! to scientific knowledge!

4 historical developments! James Hutton ( ) Father of Modern Geology! native of Edinburgh, Scotland! educated as a medical doctor in Leiden (1749)! passionate about scientific inquiry! Theory of the Earth -- processes are slow; take a long time! " "! Charles Lyell ( )! Scotsman who attended Oxford University! father was an avid naturalist! rebelled against prevailing thought of catastrophism.! " "! Principles of Geology -- popularized Hutton s views! idea of uniformitarianism --! same processes operating today occurred in the past!.the present is the key to the past.!

5 the key to the past! relative time vs. absolute time! relative time! order of events or objects from first (oldest) to last (youngest)! she is older than he is; she was born first and he was born last! absolute time! age of events or objects expressed numerically! she is twenty-one and he is nineteen! study of timing of geologic events and processes is geochronology!

6 relative time and relative order! apply simple concepts to determine! original horizontality! superposition! lateral continuity! cross-cutting relationships! inclusions! unconformities!

7 relative age dating concepts! original horizontality! all beds originally deposited in water formed in horizontal layers! sediments will settle! to bottom! and blanket! the sea floor!

8 relative age dating concepts! superposition! youngest! within a sequence of undisturbed! sedimentary or volcanic rocks,! oldest rocks are at the bottom! and youngest at the top! ".young upward! lateral continuity! oldest! original sedimentary layers extend! laterally until they thin at edges! continue! continue!

9 relative age dating concepts! cross-cutting relationships! a disrupted pattern is older than! the cause of the disruption! e.g. an intrusion is younger! than the rocks it intrudes!

10 relative age dating concepts! inclusions! fragments of other rocks contained in a body of rock! must be older than the! host rock! e.g.! 1) xenoliths in granite are older! than granite and! 2) pieces of rock in! conglomerate are older! than conglomerate!

11 relative age dating concepts! unconformities! a contact between sedimentary formations that represents a gap! in the geologic record -- gap represented is variable (i.e. amount of time or the amount of missing section)! different types of unconformities! conformity! relatively continuous deposition! deposition of a sequence of parallel layers! contacts between formations do not represent significant amounts of time!

12 conformity! from:

13 relative age dating concepts! different types of unconformities! angular unconformity! contact separates overlying younger layers from tilted older layers! sequence of layers! is not parallel! contacts between formations! may represent significant! amounts of time! angular unconformity!

14 from: angular unconformity!

15 angular unconformity!

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17 relative age dating concepts! different types of unconformities! disconformity! contact separates beds (formations) that are parallel! sequence of layers! is parallel! contacts between! formations! may represent significant! amounts of time! missing time is difficult to recognize (may need other! information--paleosol?)!

18 relative age dating concepts! different types of unconformities! nonconformity! strata deposited on older crystalline (metamorphic/igneous) rock! erosion surface on igneous/metamorphic rock covered by! sedimentary rocks! large gap in! geologic record! nonconformity!

19 what events occur?! angular unconformity!

20 what events occur?! nonconformity!

21 now that we know all this what happened?!

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23 deposition!

24 intrusion!

25 tilting and! erosion!

26 subsidence! and! renewed! deposition!

27 missing formation (time)?!

28 dike intrusion!

29 erosion and exposure!

30 subsidence and deposition!

31 uplift/sea level fall and river deposition!

32 relative ages of the formations!

33 relative age: correlation! correlation -- determining time equivalency of rocks within a region, between continents, etc.! how is this done?! physical continuity! physically following a continuous exposure of a rock unit! --most direct; easily done in some locations, not in others! e.g. within the Grand Canyon! lithologic similarity! assuming similar sequences of rocks formed at same time! -- inaccurate if common rocks are involved! " "e.g. the Grand Canyon and Zion National Parks!

34 physical continuity -- Coconino Sandstone in Grand Canyon! Coconino Sandstone!

35 lithologic similarity -- Coconino and Navajo Sandstones!

36 lithologic similarity -- Coconino and Navajo Sandstones! Navajo is much younger!

37 relative age: correlation! how is this done?! faunal succession (correlation by fossils)! fossil species succeed one another through the layers! in a predictable order! index fossil! short-lived organism;! points to narrow range! of geologic time! fossil assemblage! group of fossils! associated! together!

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40 use of index fossils/fossil assemblages! permits global correlation! similar units found in India, Africa, S. America, Australia, Antarctica.!

41 established initially as a! relative scale using! sedimentary rocks! and fossils! absolute ages! were determined later! with! radiometric dating!

42 absolute time! natural clock is necessary! -- radiometric dating! "(nuclear clock: decay of radioactive isotopes)! -- dendrochrolonology! -- astronomical methods!

43 age of the Earth! early methods: long debated! 1625: Archbishop Usher determined Earth was created in 4004 B.C.! by counting generations in the Bible! Hindus regarded Earth as old: 2000 A.D. is 1.97 million years! according to Hindu calendar! 1866: Lord Kelvin calculated age by assuming that Earth was! molten and cooled to a solid; age between million years old.! - did not know about radioactive decay (makes heat)! - assumed all heat dissipated by conduction! early isotopic methods (radioactivity known in 1896)! 1905: first crude estimates yielded 2 billion year age! meteorites gave dates of 4.5 to 4.6 billion years old! modern uranium/lead methods yield values of 4.55 billion years!

44 radioactive isotopes! have nuclei that spontaneously decay! -- emit or capture subatomic particles! parent: decaying radioactive isotope! daughter: product as a result of decay! parent! loss or gain! daughter! loss or gain of neutron converts parent to daughter of same element! loss or gain of proton changes parent into entirely new daughter!

45 3 primary ways of decay! alpha decay (Z 58) particle has 2 neutrons and 2 protons! U 238 " "Th 234! 92 protons "90 protons! beta decay (n 0 = p + + e - ) breakdown of neutron into an! electron and a proton and loss! of the electron to leave a proton! (result is gain of one proton)! K 40 " "Ca 40! 19 protons "20 protons! electron capture (e - + p + = n 0 ) capture of an electron by a proton! and change of proton to neutron! (result is loss of proton)! K 40 " "Ar 40! 19 protons "18 protons!

46 radiometric dating! uses continuous decay to measure time since rock formed! only possible since late 1890 s -- radioactivity discovered in 1896! as minerals crystallize in magma;! they trap atoms of radioactive isotopes in their crystal structures! radioactive isotopes will decay immediately and continuously! emitted particle and energy! parent! daughter! as time passes, rock contains less parent and more daughter!

47 half-life! amount of time it takes for half the atoms of the! parent isotope to decay! different radioactive isotopes have different and! distinct half-lives! if rock has 12 parents and 12 daughters today -- ratio of 1:1! original rock had 24 parents and one half-life has elapsed! after another half life, rock will have 6 parents and 18 daughters! ratio of 1:3---note that total number (24) remains the same! regardless of isotope, the ratio of parent to daughter atoms! is predictable at each half-life!

48 predictable ratios at each half-life! exponential decay (half always remains)!

49 exponential decay: never goes to zero! exponential! linear!

50 example: Uranium 238 decay to Lead 206 (stable)! several steps! (each has its own half-life)! beta decay alpha decay

51 most common dating systems! uranium-thorium-lead dating (previous example)! U-238, U-235, Th-232! "each of these decays through a series of steps to Pb! potassium-argon dating! U-238 to Pb-206 "half-life = 4.5 by! U-235 to Pb-207 "half-life = 713 my! Th-232 to Pb-208 "half-life = 14.1 my! argon is a gas--may escape! " "(ages too young--daughter missing)! K-40 to Ar-40 " "half-life = 1.3 by! rubidium-strontium dating! Rb-87 to Sr-87 "half-life = 47 by!

52 basic geochronological assumptions! decay constants are constant through geological time! -- good reasons to believe this is correct from nuclear physics! -- measurements of decay sequences in ancient supernovae! "yield the same values as modern lab measurements! system closed to adding or subtracting of parent/daughter! -- isotopic system and type of mineral (rock) are important! -- careful procedure is essential to correct analysis! igneous rocks are most reliable for dating! metamorphism may cause loss of daughter products! sedimentary rocks will give ages of source rocks!

53 Instruments and Techniques Mass Spectrometry: measure different abundances of specific nuclides based solely on atomic mass.! Basic technique requires ionization of the atomic species of interest and acceleration through a strong magnetic field to cause separation between closely similar masses (e.g. 87 Sr and 86 Sr).! Count individual particles using electronic detectors.! TIMS: thermal ionization mass spectrometry! SIMS: secondary ionization mass spectrometry - bombard target with heavy ions or use a laser! Sample Preparation: TIMS requires doing chemical separation using chromatographic columns.

54 Clean Lab - Chemical Preparation

55 Thermal Ionization Mass Spectrometer From:

56 Schematic of Sector MS

57 Zircon Laser Ablation Pit

58 Rate Law for Radioactive Decay P t = P o exp - (t o t) 1st order rate law Where P t quantity of the parent isotope (i.e. 87 Rb) at time t; P o quantity of the parent isotope at some earlier time t o, when the isotopic system was closed to any additional isotopic exchange; λ is the characteristic decay constant for the system of interest, which is related to the half-life, t 1/2, by the equation below: λ = ln 2 / t 1/2 t 1/2 is defined as the half-life, which is the amount of time required for 1/2 of the original parent to decay and is a constant.

59 Rb/Sr Age Dating Equation 87 Rb t = 87 -λ (to t) Rb o e (Assume that t = 0, for the present) 87 Rb o + 87 Sr o = 87 Rb t + 87 Sr t (Conservation of Mass, with 87 Sr o as the initial concentration and 87 Sr t as the concentration today) 87 Sr t - 87 Sr o = 87 Rb t (e λ to 1) 87 Sr 86 Sr t = 87 Sr 86 Sr o + 87 Rb 86 Sr t (e λt 1) y = b + x m

60 Rb/Sr Isochron Systematics M 1 M 2 M 3

61 Independent Checks on Radiometric Ages Correlation of erosion with age on Hawaiian Island Chain: Dates increase in age to the NW as does erosion.! Annual growth bands in Devonian corals: 400/yr yields date that is similar to radiometric date. Consistent with slowing of Earth rotation with time.! Independent determination of Pacific plate motion yields age progression that is consistent with K/Ar dates of the island chains formed by hotspots.! Agreement between magnetic age from deep marine sediments and radiometric ages of tuffs in East African Rift

62 Other dating methods: dendrochronology! annual growth of trees produces concentric rings! dating back to 9000 years is possible! - rings need to be calibrated! against C-14 dates to yield! true numerical age! - other information may also! be obtained from rings,! including rainfall and temperature! - can develop composite! chronologies for specific regions! of interest for climate studies! photo H.D. Grissino-Mayer

63 relative and absolute dates combined! same example! as in! relative age!

64 geological time scale! eons, eras, periods, epochs! Oldest rocks: Greenland gneisses Oldest rock fragments: W. Australia detrital zircons

65 earliest life! cyanobacteria: primitive single-celled organisms! found in Australia and dated at 3.7 billion years old! modern equivalents in! Shark s Bay, Australia!

66 proportional time scale!

67 combine relative and absolute time for geologic time scale!

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