Geologic Time

1. Geologic Time

2. Determining geological ages • Relative dating – placing rocks and events in their proper sequence of formation, without actual dates. • Numerical dating – specifying the actual number of years that have passed since an event occurred (also known as absolute dating)

3. Principles of Relative Dating: Law of Superposition In an undeformed sequence of surface-deposited rocks, the oldest rocks are on the bottom. • Includes sedimentary rocks, lava flows, ash deposits and pyroclastic strata. • Does not include intrusive rocks, which intrude from below.

4. Law of Superposition – Grand Canyon

5. Principles of Relative Dating • Principle of original horizontality • Layers of sediment are generally deposited in a horizontal position • Rock layers that are flat have not been disturbed • Principle of cross-cutting relationships • Younger features cut across older features (faults, intrusions etc)

6. Figure 18.3

7. Figure 18.4, #4 • Is Fault A o/y than the ss layer? • Is Dike A o/y than the ss? • Was the conglom. deposited b/a fault A? • Was the cong. deposited b/a fault B? • Which fault is older, A or B? • Is dike A o/y than the batholith?

8. Figure 18.4 - a • Is Fault A o/y than the ss layer? -Y • Is Dike A o/y than the ss? • Was the conglom. deposited b/a fault A? • Was the cong. deposited b/a fault B? • Which fault is older, A or B? • Is dike A o/y than the batholith?

9. Figure 18.4 - b • Is Fault A o/y than the ss layer? -Y • Is Dike A o/y than the ss? - Y • Was the conglom. deposited b/a fault A? • Was the cong. deposited b/a fault B? • Which fault is older, A or B? • Is dike A o/y than the batholith?

10. Figure 18.4 - c • Is Fault A o/y than the ss layer? -Y • Is Dike A o/y than the ss? - Y • Was the conglom. deposited b/a fault A? - After • Was the cong. deposited b/a fault B? • Which fault is older, A or B? • Is dike A o/y than the batholith?

11. Figure 18.4 -d • Is Fault A o/y than the ss layer? -Y • Is Dike A o/y than the ss? - Y • Was the conglom. deposited b/a fault A? - After • Was the cong. deposited b/a fault B? - Before • Which fault is older, A or B? • Is dike A o/y than the batholith?

12. Figure 18.4 - e • Is Fault A o/y than the ss layer? -Y • Is Dike A o/y than the ss? - Y • Was the conglom. deposited b/a fault A? - After • Was the cong. deposited b/a fault B? - Before • Which fault is older, A or B? - A • Is dike A o/y than the batholith?

13. Figure 18.4 - f • Is Fault A o/y than the ss layer? -Y • Is Dike A o/y than the ss? - Y • Was the conglom. deposited b/a fault A? - After • Was the cong. deposited b/a fault B? - Before • Which fault is older, A or B? - A • Is dike A o/y than the batholith? - Y

14. Figure 18.4 - Answers • Is fault A o/y than the ss? – Y – fault cuts the ss • Is dike A o/y than the ss? – Y – dike cuts ss • Was the conglom. deposited b/a fault A? – after – conglom not cut • Was the conglom deposited b/a fault B? –before – fault cuts it • Which fault is older?-A – conglom older than B but younger than A • Is dike A o/y than the batholith? – Y – Dike A cuts Dike B, which in turn cuts the batholith.

15. Inclusions • An inclusion is a piece of rock that is enclosed within another rock. • Principle of cross-cutting relationships tells us rock containing the inclusion is younger than the inclusion itself. • The presence of inclusions allow us to determine whether a intrusive igneous rock is older or younger than the rock above it. • Let’s see how

16. Inclusions • Magma intrudes into an existing rock formation, surrounding small pieces of it. • The magma becomes an intrusive igneous rock (e.g. granite). • Even though it is underneath the pink rock, it is younger • The contact between the two layers is not an unconformity, because it was never exposed at the surface.

17. Inclusions • First the “country rock” (the pink stuff) weathers away, exposing the granite (gray) at the surface. • The granite also weathers away, leaving an erosional surface.

18. Inclusions • Conditions change and the erosional surface becomes a depositional environment. • The lower layers of the sedimentary formation contain inclusions of granite. • This shows the granite is older than the sedimentary layers. • The contact between the older igneous and younger sedimentary rocks is a type of unconformity, because it was at one time exposed at the surface.

19. Unconformity • a break in the rock record produced by erosion of rock units and/or nondeposition of sediments • Between sedimentary rocks and crystalline (non-layered) bedrock • Between two sets of layered sedimentary rocks deposited at two different times • Angular unconformity – tilted rocks are overlain by flat-lying rocks

20. Formation of an angular unconformity Figure 18.7

21. Unconformity Types

22. Unconformities in the Grand Canyon Unconformities, especially between sedimentary strata, are hard to distinguish.

23. Figure 18.6

24. Fossils: the remains or traces of living organisms • Conditions favoring preservation • Rapid burial • Possession of hard parts (shells or bones • Correlation: Matching of rocks of similar ages in different regions • Correlation often relies upon fossils

25. Principle of Fossil Succession: Fossil organisms succeed one another in a definite and determinable order, so any time period can be recognized by its fossil content.

26. Principle of Fossil Succession: • Although developed over 50 years before Darwin’s work, it is now known that the reason this principle is valid is due to evolution. • Fossil organisms become more similar to modern organisms with geologic time • Extinct fossils organisms never reappear in the fossil record

27. Index Fossils • Widespread geographically • Limited to short span of geologic time • Valuable for correlation: use of index fossils can often provide numerical dates for rock units and events • Similar accuracy to radiometric dating techniques.

28. Using fossil groups to determine the age of rock strata

29. Geologic time scale: a “calendar” of Earth history • Subdivides geologic history into units based on appearance and disappearance of fossils from the geologic record • Structure of the geologic time scale • Eon– the greatest expanse of time • Era– subdivision of an eon • Eras are subdivided into periods • Periods are subdivided into epochs

30. The “Precambrian” • Used to refer to all geologic time before the Phanerozoic (Visible Life) Eon • Represents almost 88% of geologic time • Originally it was thought that no life existed before the Phanerozoic Eon • Now we know that the lack of fossil evidence in the Precambrian rocks is partially due to the lack of organisms with exoskeletons

31. Eras of the Phanerozoic eon • Cenozoic (“recent life”) • Mesozoic (“middle life”) • Paleozoic (“ancient life”)

32. Notable divisions between the Eras • Paleozoic-Mesozoic – 248 mya • Mass extinction of trilobites and many other marine organisms • Possibly due to climate change that occurred with the formation of Pangaea • Mesozoic-Cenozoic – 65 mya • Mass extinction of dinosaurs and many other species • Probably caused by meteor impact • Made way for the domination of mammals • Cenozoic- ????

33. Figure 18.16

34. Correlation #1 U

35. Figure 18.18 Correlation #2 U Assume volcano F occurred before Fault G E occurred last D and K are plutons M is metamorphic

37. Radioactivity activity 2 - rhyolite

38. Radioactivity 3 – felsic ash

42. Basic atomic structure • Proton– positively charged particle found in nucleus. • Neutron– neutral particle, which is a combination of a proton and an electron, found in nucleus. • Electrons– very small, negatively charged particle that orbits the nucleus. Also, an elementary charged particle that can be be absorbed by a proton or emitted by a neutron to change one into the other.

43. Basic atomic structure • Atomic number • An element’s identifying number • Equal to the number of protons in the atom’s nucleus • Carbon’s atomic number is always 6. • Mass number (formerly “atomic weight”) • Sum of the number of protons and neutrons in an atom’s nucleus • Indicates the isotope of the element (e.g. C-12, C-14).

44. Periodic Table

45. Isotopes and Radioactivity • Isotope: Variety of an atom with a different number of neutrons and mass number • Some isotopes (not all!) are inherently unstable, which means the forces binding nuclear particles together are not enough to hold the nucleus together. These are called radioactive isotopes. • Examples of isotopes include O-16, O-18, C-12, C-13, and C-14. Only the last is radioactive.

46. Comparison of C-12 with C-14

47. Radioactivity • Many common radioactive isotopes are naturally occurring. • Most radioactive processes release energy; formation of C-14 by neutron capture is an exception. It requires cosmic (solar) radiation. • They also often release energy and sometimes eject atomic particles as they “decay” or change into a more stable substance.

48. From Parent to Daughter • In many cases atomic particle are ejected during radioactive decay • Protons and/or neutrons ejected from nucleus • Protons become neutrons or vice verse • Atomic number changes so a new daughter element results. • How long does a radioactive parent take to turn into a stable daughter?

49. Figure 2.4

50. Half-life • the time required for one-half of the radioactive nuclei in a sample to change from parent isotope to daughter isotope. • Decay occurs at random. Can’t predict when an individual atom will decay. • However, decay is statistically predictable. • Comparison with coin toss