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Geologic time. Geologic Time Geologic time scale Relative dating principles Absolute dating w/ radioactivity Correlation of rock layers.

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Presentation on theme: "Geologic time. Geologic Time Geologic time scale Relative dating principles Absolute dating w/ radioactivity Correlation of rock layers."— Presentation transcript:

1 Geologic time

2 Geologic Time Geologic time scale Relative dating principles Absolute dating w/ radioactivity Correlation of rock layers

3 The relative geologic time scale has a sequence of eons eras periods epochs –but no numbers indicating how long ago each of these times occurred Relative Geologic Time Scale

4 Geologic time scale Eons Phanerozoic - last 570 million years Cenozoic - recent life Mesozoic - age of middle life Paleozoic - ancient life Precambrian - from birth of Earth up to before complex life forms developed Eras m.y.a m.y.a m.y.a (mammals, humans) (dinosaurs, 1st scrawny mammals) (fish, trilobites, clams, corals, ferns ) (algae, bacteria, some fossils without shells like jellyfish)

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6 Telling time geologically v Earths history concealed in rocks v Goal of geology: unraveling Earths history v Principle time keeping devices: Relative dating - putting rocks/events in proper order Absolute dating - determining events actual time Relative Dating: Key principles

7 Law of superposition Principle of original horizontality Principle of cross-cutting relationships InclusionsUnconformities A bed of rock is older than that above, younger than that below

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9 Law of superposition Principle of original horizontality Principle of cross-cutting relationships InclusionsUnconformities A bed of rock is older than that above, younger than that below Relative Dating: Key principles Sedimentary layers deposited flat

10 Law of superposition Principle of original horizontality Principle of cross-cutting relationships InclusionsUnconformities A bed of rock is older than that above, younger than that below Sedimentary layers deposited flat Faults & dikes are younger than beds they cross Relative Dating: Key principles

11 InclusionsUnconformities Fragments of one rock enclosed in another Relative Dating: Key principles

12 Fragments of one rock enclosed in another

13 InclusionsUnconformities Break in time in deposition (Conformable layers: deposited w/out interruption) Relative Dating: Key principles An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

14 Relative Dating: Key principles Unconformities 3 types v Angular unconformity v Disconformity v Nonconformity A disconformity is when two sedimentary rock layers are separated by an erosional surface. A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks. An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.

15 Relative Dating: Key principles Unconformities 3 types v Angular unconformity v Disconformity v Nonconformity Tilted/folded sedimentary rocks overlain by younger, more flat layers

16 Deposition Folding/Uplift Erosion Subsidence/more erosion Recipe for an angular unconformity

17 Relative Dating: Key principles Unconformities 3 types v Angular unconformity v Disconformity v Nonconformity Layers on either side of unconformity are parallel

18 Relative Dating: Key principles Unconformities 3 types v Angular unconformity v Disconformity v Nonconformity Interface between sedimentary layers and metamorphic or igneous rock

19 Unconformity Types

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21 Relative Dating Principles

22 Correlation of rock layers Matching up rocks from different places that are similar in age u Physical criteria u Fossils

23 Correlation of rock layers u Physical criteria (OK for short distances) è position of a bed in sequence of beds è distinct/uncommon minerals

24 Correlation of rock layers u Fossils è fossil organisms succeed each other in systematic fashion è any time period can be determined by fossil content principle of fossil succession

25 Correlation of rock layers Index fossils ègeographically wide-spread èshort span of time T&L Figure 8.10

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27 Overlapping Ranges of Fossils

28 correlated sequence Mostly nonmarine limestone 1 Kansas 2 Indiana 3 Ohio 4 Pennsylvania

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30 Absolute dating w/ radioactivity Atom model u Chemistry review nucleus electrons protrons neutrons # protrons = atomic #, defines the element # neutrons can vary: isotopes

31 Absolute dating w/ radioactivity ParentDaughter Now Next year yrs from now yrs from now yrs from now1 999 Example: an isotope with a half life of 1 year # of atoms

32 Radioactive Decay

33 Absolute dating w/ radioactivity


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