Telling Geologic Time. Putting it in perspective

Transcription

1 Telling Geologic Time Putting it in perspective

2 Determining geological ages Relative age dates placing rocks and events in their proper sequence of formation Numerical dates specifying the actual number of years that have passed since an event occurred (known as absolute age dating)

3 Figure 1-15 (p. 16) Geologic Time Scale.

4 Uniformitarianism The physical, biological and chemical principles acting on the Earth in the present also acted on the Earth in the past.

5 Getting Dates Relative Dating

6 Law of Superposition Developed by Nicolaus Steno in 1669 In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom

7 Superposition in the Grand Canyon

8 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

9 Original Horizontality (d) Lateral Continuity (e)

10 Figure 1-6 (p. 5) Illustration of original lateral continuity.

11 Law of Cross-cutting Relationships

12 Inclusions An inclusion is a piece of rock that is enclosed within another rock Which layer is younger?

13 Inclusions

14 Baked Contacts Contact Metamorphism Lava flow

15 Geologic History A cross-section through the earth reveals the variety of geologic features. View 1 of this animation identifies a variety of geologic features; View 2 animates the sequence of events that produced these features, and demonstrates how geologists apply established principles to deduce geologic history. [by Stephen Marshak] Play Animation Windows version >> Play Animation Macintosh OS9 version >> Play Animation Macintosh OSX version >>

16 Gaps in the Record: Unconformities Angular unconformity, Siccar Point, Scotland

17 Erosional Surfaces Unconformities represent gaps in the geologic record because of surface erosion

18 Figure 1-8 (p. 8) Siccar Point, Eastern Scotland. It was here that James Hutton first realized the historical significance of an unconformity.

19 Formation of an angular unconformity

20 Development of a nonconformity

21 Development of a disconformity

22 Unconformities Four types of erosional unconformities. (A) Angular unconformity. (B) Nonconformity. (C) Disconformity. (D) Paraconformity.

23 Sub-bottom seismic profile

24 Gaps in Time in the Grand Canyon

25 Types of Unconformity This animation shows the stages in the development of three main types of unconformity in cross-section, and explains how an incomplete succession of strata provides a record of Earth history. View 1 shows a disconformity, View 2 shows a nonconformity and View 3 shows an angular unconformity. [by Stephen Marshak] Play Animation Windows version >> Play Animation Macintosh OS9 version >> Play Animation Macintosh OSX version >>

26 Top: Block diagram Bottom: Geologic map

27 Figure 1-11 (p. 11) An example of how the sequence of geologic events can be determined from cross-cutting relationships and superposition. From first to last, the sequence indicated in the cross-section is first deposition of D, then faulting to produce fault B, then intrusion of igneous rock mass C, and finally erosion followed by deposition of E. Strata labeled D are oldest, and strata labeled E are youngest.

28 Interpreting the Sequence of Events

29 Principle of Faunal Succession Correlation often relies upon fossils Principle of fossil succession fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content

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31 Determining the ages of rocks using fossils

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33 Getting Dates By the numbers: Absolute Dating

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35 Using radioactivity in dating Radioactivity Spontaneous changes (decay) in the structure of atomic nuclei

36 Radioactive Decay Isotopes Elements with the same number of protons and electrons but with different numbers of neutrons in their nucleus Radioactive (radiogenic) Parent Original unstable radioactive isotope Daughter The isotopes resulting from the decay of a parent Half-life Time it takes for half of the parent isotope to decay into the daughter isotope Given as an average time

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39 Figure 1-20 (p. 23) Radioactive decay series of uranium-238 ( 238 U) to lead- 206 ( 206 Pb).

40 Figure 1-25 (p. 26) Rate of radioactive decay of uranium-238 to lead-206.

41 Figure 1-27 (p. 28) Decay curve for potassium-40.

42 Using radioactive C 14 in dating Dating with carbon 14 (radiocarbon dating) Half-life of only 5730 years Used to date very recent events Carbon-14 is produced in the upper atmosphere Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history

43 Carbon-14 Dating Figure 1-29 (p. 29) Carbon-14 is formed from nitrogen in the atmosphere. It combines with oxygen to form radioactive carbon dioxide and is then incorporated into all living things.

44 Difficulties in dating the geologic time scale 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

45 Dating sedimentary strata using radiometric dating

46 Dating sedimentary strata using radiometric dating Figure 1-23 (p. 25) Igneous rocks that have provided absolute radiogenic ages can often be used to date sedimentary layers. (A) The shale is bracketed by two lava flows. (B) The shale lies above the older flow and is intruded by a younger igneous body.

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