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Grand Canyon in Arizona (stratification, bedding) stratification, bedding, stratum (strata) Rock Record and Geologic Time.

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Presentation on theme: "Grand Canyon in Arizona (stratification, bedding) stratification, bedding, stratum (strata) Rock Record and Geologic Time."— Presentation transcript:

1 Grand Canyon in Arizona (stratification, bedding) stratification, bedding, stratum (strata) Rock Record and Geologic Time

2 Timescale (Relative)

3 Timescale (Absolute)

4 How do we tell geologic time? Relative time Principles of stratigraphy - 17 th – 18 th century Fossils - 18 th century Paleomagnetism – 20 th century Absolute time Radiometric dating – late 19 th -20 th century

5 Relative dating Placing rocks and events in sequence Law of superposition – oldest rocks are on the bottom Principle of original horizontality – sediments are deposited in flat, horizontal layers

6 Grand Canyon in Arizona Glen Canyon Dam, Lake Powell Rio Colorado – Red River Red sediment now trapped in Lake Powell so the river is no longer red.

7 Superposition is well illustrated by the strata in the Grand Canyon

8 Original Horizontality

9 Relative dating Other geologic principles Cross-cutting relationships - fractures, faults and intrusions must be YOUNGER than rocks they cut Inclusions – one rock contained within another (rock containing the inclusions is younger)

10 Cross-cutting Relationships fractures, faults and intrusions must be YOUNGER than rocks they cut

11 Inclusions

12

13 (12) Youngest Event: Deposition of unit C (11) Erosion of unit G and intrusion A (10) Intrusion of A (9) Deposition of unit G (8) Erosion of folded unit and of intrusion B (7) Intrusion of B (or before 6) (6) Folding of all previously deposited layers (5) Deposition of layer E (4) Deposition of layer I (3) Deposition of layer F (2) Deposition of layer H (1) Oldest Event: Deposition of layer D

14 Telling Time with Fossils use first occurrence and last occurrence rapidly evolving (short-lived) organisms divide time into the finest divisions best index fossils have a wide geographic range (planktonic ocean organisms)

15 Index (Zone) Fossils trilobites – Cambrian ammonoids – Devonian to Cretaceous bivalves – Devonian to Cretaceous foraminifera - Cenozoic

16 Radioactivity and radiometric dating Radioactivity Spontaneous breaking apart (decay) of atomic nuclei Radioactive decay Parent – an unstable isotope Daughter products – isotopes formed from the decay of a parent

17 Radioactivity and radiometric dating Radiometric dating Half-life – the time for one-half of the radioactive nuclei to decay Requires a closed system Cross-checks are used for accuracy Complex procedure Yields numerical dates

18 The radioactive decay curve

19 Proportion of Parent Atoms Remaining as a Function of Time Half-lives and remaining parent isotope

20 Radiometric/Isotopic dating Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). The rate at which this decay occurs is constant and knowable (measurable). Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.

21 Radioactivity and radiometric dating Carbon-14 dating Half-life of only 5730 years Used to date very recent events Carbon-14 produced in upper atmosphere Incorporated into carbon dioxide Absorbed by living matter Useful tool for anthropologists, archeologists, historians, and geologists who study very recent Earth history

22 Parent IsotopeStable Daughter ProductCurrently Accepted Half-Life Values Uranium-238Lead billion years Uranium-235Lead million years Thorium-232Lead billion years Rubidium-87Strontium billion years Potassium-40Argon billion years Samarium-147Neodymium billion years


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