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Geologic Time Marble demo Some Index Fossils Coin Toss Sheet Color Copies of Expected Values for X 2 in homework Time Scale.doc.

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Presentation on theme: "Geologic Time Marble demo Some Index Fossils Coin Toss Sheet Color Copies of Expected Values for X 2 in homework Time Scale.doc."— Presentation transcript:

1 Geologic Time Marble demo Some Index Fossils Coin Toss Sheet Color Copies of Expected Values for X 2 in homework Time Scale.doc

2 Determining geological ages 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 dating Developed by Nicolaus Steno in 1669 Principles of relative dating Developed by Nicolaus Steno in 1669 1. Law of superposition1. Law of superposition In an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at topIn an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at top 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 Niels Steensen (Nicolas Steno)

4 Superposition illustrated by strata in the Grand Canyon

5 Stenos 2nd principle of relative dating Stenos 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 Stenos 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 8_9 (a) (b) (c) Layered sedimentary rocks Nonconformity Metamorphic rock Igneous intrusive rock Younger sedimentary rocks Angular unconformity Older, folded sedimentary rocks Disconformity Brachiopod (290 million years old) Trilobite (490 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 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 8_10 Rock broken to reveal external mold of shell Rock broken to reveal fossil cast Shells buried in sediment Mold, or cavity, forms when original shell material is dissolved Cast forms when mold is filled in with mineral water Shells settle on ocean floor How impression fossils form (the most common type)

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 http://www.csun.ed u/~psk17793/ES9 CP/ES9%20fossils.htm 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 Divisions based on fossils Eon, Era, Period, Epoch Homework Learn Timescale.doc less Epochs

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 Earths 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 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. Radiometric Age Determinations show Earth not as old as Moon, Meteorites

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

32 Using radioactivity in dating 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 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? 1.Collect sample 2.Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids 3.Measure parent/daughter ratio of target isotopes - mass spectrometer 4.Substance heated – Ions – move in Electrical Field, curved in Magnetic

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

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

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

39 Basalt Lava flow 2 200 mya Lava flow 1 209 mya We can bracket this limestones age between 209 and 200 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 Atoms split into smaller particles, among them neutrons Neutrons strike nitrogen atoms Nitrogen atom gains a neutron and loses a Proton; becomes carbon-14 C-14 mixes with atmospheric oxygen to produce CO 2 CO 2 taken up by plants, water C-14 absorbed by living organisms CO 2 dissolved in water C-14 intake ceases when organism dies; C-14 concentration decreases Cosmic rays bombard atmospheric atoms Carbon-14

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

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



46 Hanneke Bos

47 End of Geologic Time Lecture

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