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Geologic Time

Geologic Time

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Geologic Time

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  1. Geologic Time Marble demo Some Index Fossils Coin Toss Sheet Color Copies of Expected Values for X2 in homework Time Scale.doc

  2. Determining geological ages • Relative age dates – placing rocks and geologic events in their proper sequence • Numerical dates – define the actual age of a particular geologic event (termed absolute age dating)

  3. Principles of relative datingDeveloped by Nicolaus Steno in 1669 • 1. Law of superposition • In an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at top Niels Steensen (Nicolas Steno) Steno recognized the organic origin of fossils and sketched a theory of geological strata, which he used in an attempt to reconstruct Tuscany's geological development

  4. Superposition illustrated by strata in the Grand Canyon

  5. Steno’s 2nd principle of relative dating • Principle of original horizontality • Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting)

  6. Steno’s 3rd principle of relative dating • Principle of cross-cutting relationships Chunks of this “country rock” have broken off and are visible in the intrusion

  7. 3rd principle of relative dating • Principle of cross-cutting relationships This fault shows the offset of the two sides. See the key beds? Notice this side is lower

  8. Unconformities (loss of rock record) • An unconformity is a break in the rock record produced by erosion and/or non-deposition • Types of unconformities • Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) with time lost • Angular unconformity – tilted rocks overlain by flat-lying rocks • Disconformity – strata on either side of the unconformity are parallel (but time is lost)

  9. Layered sedimentary rocks (a) 8_9 Nonconformity Igneous intrusive rock Metamorphic rock (b) Younger sedimentary rocks Angular unconformity Older, folded sedimentary rocks (c) Disconformity Trilobite (490 million years old) Brachiopod (290 million years old)

  10. Development of a Nonconformity Pennsylvanian sandstone over Precambrian granite is a nonconformity

  11. Nonconformity in the Grand Canyon - Sediments deposited over Schist

  12. Formation of an angular unconformity

  13. Horizontal younger sediments over tilted older sediments Cambrian Tapeats sandstone over Precambrian Unkar Group What type of unconformity is this? Grand Canyon in Arizona

  14. Cross Cutting Relationships in strata Zoroaster Granite across Vishnu Schist

  15. Correlation of rock layers • Matching strata of similar ages in different regions is called correlation

  16. Correlation of strata in southwestern United States Sections are incomplete Match with fossils Matching lithology is risky, discussion

  17. How impression fossils form (the most common type) 8_10 Shells settle on ocean floor Cast forms when mold is filled in with mineral water Rock broken to reveal fossil cast Rock broken to reveal external mold of shell Shells buried in sediment Mold, or cavity, forms when original shell material is dissolved

  18. Correlation of rock layers with fossils • Correlation often relies upon fossils • Principle of fossil succession (Wm. Smith) • fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it • Index Fossils - useful for correlation • Existed for a relatively brief time • Were widespread and common • Most fossils are just impressions. A few may have small amounts of some original tissue

  19. Determining the ages of rocks using overlap of fossils Overlap time span is shorter than that of any one fossil.

  20. Fossils allow correlation in spite of unconformities

  21. Geologic time scale • The geologic time scale – a “calendar” of Earth history • Subdivides geologic history into units • Originally created using relative dates • Structure of the geologic time scale • Eon, Era, Period, Epoch

  22. Geologic Timescale Homework Learn Timescale.doc less Epochs Divisions based on fossils Eon, Era, Period, Epoch

  23. Origin of Period Names

  24. Geologic time scale • Structure of the geologic time scale • Names of the eons • Phanerozoic (“visible life”) – the most recent eon, began about 545 million years ago • PreCambrian (Cryptozoic) • PreCambrian subdivisions: • Proterozoic – begins 2.5 billion years ago • Archean – begins 3.8 bya • Hadean – the oldest eon begins 4.6 bya Read from bottom to top – Oldest to Youngest

  25. Geologic time scale • Precambrian time • Nearly 4 billion years prior to the Cambrian period • Long time units because the events of Precambrian history are not know in detail – few fossils, most rock modified • Immense space of time (Earth is ~ 4.6 Ga) • PreCambrian spans about 88% of Earth’s history

  26. Geologic time scale • Structure of the geologic time scale • Era – subdivision of an eon • Eras of the Phanerozoic (visible life”) eon • Cenozoic (“recent life”) begins ~ 65 mya • Mesozoic (“middle life”) begins ~ 248 mya • Paleozoic (“ancient life”) begins ~ 540 mya • Eras are subdivided into periods • Periods are subdivided into epochs

  27. Using radioactivity in dating • Importance of radiometric dating • Allows us to calibrate geologic timescale • Determines geologic history • Confirms idea that geologic time is immense

  28. Radiometric Age Determinations show Earth not as old as Moon, Meteorites • Included in some sediment from NW Australia, detrital grains of the mineral Zircon that are 3.96 billion years old. The dates are based on datable Uranium in the Zircons. • Similar dates are known from Yellow Knife Lake, NWT, Canada • Claims of older zircons 4.4 by.

  29. Radiometric Age Determinations of the Earth • However, the age of the Earth is thought to be about 4.5 - 4.6 billion years • Based on the dates obtained from meteorites and samples collected on the moon, assumed to have formed at the same time.

  30. Recall Isotopes • The number of protons in an atom's nucleus is called its atomic number –defines “element” • Protons + neutrons called atomic weight • The number of neutrons can vary • Atoms of the same element with different numbers of neutrons are called isotopes.Some are radioactive

  31. Radioactive parent nucleus Decay process Daughter nucleus p p p p p p p Atomic mass decreases by 4; atomic number decreases by 2 p p p p p p p Proton Neutron p Alpha particle Emission of 2 protons and 2 neutrons (alpha particle) (a) Alpha decay p p p p p p Atomic mass not changed much; atomic number increases by 1 because Neutron becomes proton p p p p p p p p p p p p p Beta particle An electron (beta particle) is ejected from the nucleus (b) Beta decay p p p Atomic mass not changed much; atomic number decreases by 1 p p p p p p p p p p p p p p p p p electron combines with a proton to form a neutron Beta particle (c) Electron capture

  32. Using radioactivity in dating • Parent – an unstable radioactive isotope • Daughter product – stable isotopes resulting from decay of parent • Half-life – time required for one-half of the parent isotope in a sample to decay into stable daughter product

  33. A radioactive decay curve 1/2 = 50% parent: 1 half-life has passed 1/2x1/2 = 1/4 = 25% parent: 2 half-lives have passed 1/2x1/2x1/2 = 1/8 = 12.5% parent: 3-half-lives have passed MARBLE DEMO

  34. Uranium to Lead used for granites; Potassium to Argon used for basalts

  35. How do we actually “date” a rock? • Collect sample • Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids • Measure parent/daughter ratio of target isotopes - mass spectrometer • Substance heated – Ions – move in Electrical Field, curved in Magnetic

  36. Mineral Dating a crystal crystal 1 Mineral crystal formed in igneous rock Parent atoms Igneous rock 2 buried beneath younger rocks; daughter atoms formed by normal decay (3) We calculate age based on half-life Daughter atoms Usual Case

  37. 8_22b But IF: Deep burial and 3 metamorphism during mountain building causes daughter atoms to escape from crystal Heat Resets the clock Rock looks as if it just formed: it looks young Age found dates from metamorphic event After mountain 4 building ends, accumulation of daughter atoms in crystal resumes Easily recognized, useful in studying metamorphism Metamorphism Case

  38. Dating sediments without fossils: Superposition, Cross-cutting Wasatch Fm. younger than 66 my Mancos Shale and Mesa Verde Fm. older than 66 my Rule of Cross-cutting Morrison Fm older than 160 my (superposition) Radiometric Dating with Igneous Rocks Or Bracket between fossiliferous layers

  39. Basalt Lava flow 2 200 mya We can bracket this limestone’s age between 209 and 200 mya Lava flow 1 209 mya Even better: we get lucky. A layer we need to date is between two datable beds So we have and upper and lower bound on the age of this limestone:

  40. Dating with carbon-14 (Carbon Dating) • Half-life only 5730 years • Used to date very young rocks • Carbon-14 is produced in the upper atmosphere • Useful tool for geologists who study very recent Earth history

  41. Cosmic rays bombard atmospheric atoms Atoms split into smaller particles, among them neutrons Carbon-14 Neutrons strike nitrogen atoms Nitrogen atom gains a neutron and loses a Proton; becomes carbon-14 C-14 mixes with atmospheric oxygen to produce CO2 C-14 absorbed by living organisms CO2 taken up by plants, water CO2 dissolved in water C-14 intake ceases when organism dies; C-14 concentration decreases

  42. Tree Rings both modern and past 2000 years Years of age 500 Annual-ring similarities show correlation 400 Current year 200 150 100 50 Tree growth rings A D C B A B Sediment layers with tree logs to be collected for dendrochronology C D Buried tree logs

  43. Southern lakes track glaciation 8_28 Dating with Lake Varves Very little or no runoff Heavy runoff into lake Ice Winter Summer Clear water Turbid water Winter layer (fine, thin, and dark-colored) Summer layer (coarse, thick, and light-colored) Lake deposits, fossil plants C14. Fossil tree pollen track climate.


  45. Hanneke Bos

  46. End of Geologic Time Lecture