1 Chapter 8 Time and Geology GEOL 101 Introductory Geology

2 Determining Geological Ages Relative age dates – placing rocks and events in their proper sequence of formation Numerical (absolute age) dates – specifying the actual number of years that have passed since an event occurred

3 Principles of Relative Dating Law of superposition Undeformed section of sedimentary or layered igneous rocks Oldest rocks are on the bottom Principle of original horizontality Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed (deformed) Principle of cross-cutting relationships Younger features cut across older features

4 Superposition Strata in the Grand Canyon

5 Horizontality

6 Cross-cutting Relationship

7 Which crater is youngest?

8 Cross-cutting Relationships

9 Principles of Relative Dating Inclusions A piece of rock that is enclosed within another rock Rock containing the inclusion is younger Unconformity Break in rock record produced by erosion and/or nondeposition of rock Represents period of geologic time

10 Principles of Relative Dating Types of unconformities Angular unconformity – tilted rocks (disturbed) are overlain by flat-lying rocks Disconformity – strata on either side of the unconformity are parallel Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata

11 Angular Unconformity

12 Angular Unconformity

13 Angular Unconformity Animation

14 Disconformity

15 Disconformity

16 Nonconformity

17 Nonconformity

18 Nonconformity Inclusions

19 Grand Canyon Unconformities

20 Interpret Geologic History Animation

21 Correlation of Rock Layers Matching of rocks of similar ages in different regions is known as correlation Correlation often relies upon fossils similar sedimentary strata in widely separated areas identified and correlated by distinctive fossil content Principle of fossil succession –fossil organisms succeed one another in a definite and determinable order –time period recognized by its fossil content

22 Correlation of Rock Layers

23 Fossils Ages of Rocks

24 Radioactivity: spontaneous changes (decay) in the structure of atomic nuclei Basic atomic structure Types of radioactive decay Parent, daughter product, half-life Principle of radioactive dating Sources of error Carbon-14 (radiocarbon) dating Importance of radiometric dating Radiometric Dating

25 Basic Atomic Structure Nucleus –Protons: positively charged particles with mass –Neutrons: neutral particles with mass –Electrons: negatively charged particles, orbit nucleus Atomic number –An element’s identifying number –Number of protons in the atom’s nucleus Mass number –Sum of protons & neutrons in atom’s nucleus Isotope –Variant of the same parent atom –Differs in the number of neutrons –Different mass number than the parent atom

26 Types of radioactive decay Alpha emission –Emission of 2 protons & 2 neutrons (alpha particle) –Mass number is reduced by 4 and the atomic number is lowered by 2 Beta emission –Electron (beta particle) is ejected from the nucleus –Mass number remains unchanged and the atomic number increases by 1 Electron capture –Electron is captured by the nucleus –Electron combines with a proton to form a neutron –Mass number remains unchanged and the atomic number decreases by 1 Radiometric Dating

27 Radiometric Decay Electron Capture Alpha Emission Beta Emission

28 Parent – an unstable radioactive isotope Daughter product – the isotopes resulting from the decay of a parent Half-life – the time required for one-half of the radioactive nuclei in a sample to decay Radiometric Decay

29 Uranium Isotopes Parent U 238 Daughter PB 206 Atomic # - 92 Mass # Atomic # - 82 Mass # - 206

30 Principle of radioactive dating The percentage of radioactive toms that decay during one half-life is always the same (50 percent) However, the actual number of atoms that decay continually decreases Comparing the ratio of parent to daughter yields the age of the sample Radiometric Dating

31 Radioactive Decay Curve

32 Radioactive Decay Curve

33 Radiometric Dating

34 Sources of error A closed system is required To avoid potential problems, only fresh, unweathered rock samples should be used Carbon-14 (radiocarbon) dating Half-life of only 5730 years Used to date very recent events C 14 is produced in the upper atmosphere Radiometric Dating

35 C 14 Production and Decay Neutron Capture Beta Emission

36 Importance of radiometric dating Radiometric dating is a complex procedure that requires precise measurement Rocks from several localities have been dated at more than 3 billion years Confirms the idea that geologic time is immense Radiometric Dating

37 Other Dating Methods Shell Growth Rings Tree Rings Rhythmic Layering in Glaciers

38 Cross-Dating Principle in Dendrochonology YoungestOlder Oldest

39 Geologic Time Scale Geologic time scale – Earth history calendar Subdivides geologic history into units Originally created using relative dates Structure of the geologic time scale Eon – the greatest expanse of time –Phanerozoic – “visible life”, most recent eon –Proterozoic – before life –Archean – ancient –Hadean – mythical subterraneon world of departed spirits, oldest eon –Precambrian: Proterozoic, Archean, Hadean

40 Geologic Time Scale Era – subdivision of an eon Precambrian – no era subdivisions Eras of the Phanerozoic eon Cenozoic (“recent life”) Mesozoic (“middle life”) Paleozoic (“ancient life”) Eras are subdivided into periods Periods are subdivided into epochs

41 Geologic Time Divisions

42 Geologic Time Scale

43 Geologic Time Scale Precambrian time Nearly 4 billion years prior to the Cambrian period Not divided into smaller time units because the events of Precambrian history are not know in great enough detail –First abundant fossil evidence does not appear until the beginning of the Cambrian Visualizing vast period of geologic time

44 Evolution vs Geologic Time

45 Visualizing Geologic Time Twice around the equator

46 Geologic Time Scale Difficulties Not all rocks can be dated by radiometric methods –Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed –The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages

47 Sedimentary Ages using Radiometric Dating

48 Stratigraphic Dating