Grotzinger Jordan Understanding Earth Sixth Edition Chapter 8

Grotzinger • Jordan Understanding Earth Sixth Edition Chapter 8: CLOCKS IN ROCKS Timing the Geologic Record © 2011 by W. H. Freeman and Company

Chapter 8: Clocks in Rocks: Timing the Geologic Record

About the Clocks in Rocks • Geologists have uncovered deep time by studying the “clocks in rocks. ” • These clocks can be used to measure the duration of geologic processes and cycles of the Earth system. • Geologists refer to both relative age and absolute age.

Lecture Outline 1. Reconstructing geologic history from the stratigraphic record 2. 2. Geologic time scale: relative ages 3. 3. Measuring absolute time with isotopic clocks 4. 4. Geologic time scale: absolute ages 5. 5. Advances in timing the Earth system

1. Reconstructing Geologic History from 2. the Stratigraphic Record ● Principles of stratigraphy ● original horizontality ● superposition ● faunal succession

Original Horizontality and Superposition Younger Older Example: Marble Canyon, Arizona

Original Horizontality and Superposition Younger Older Left undisturbed, the youngest layers always remain above the oldest.

Original Horizontality and Superposition sedimentation in lake or sea

Original Horizontality and Superposition Sediments are deposited in horizontal layers and slowly change into rock.

Original Horizontality and Superposition

Fossils as Recorders of Time: The Principle of Faunal Succession Outcrop A I Outcrop B II II III

Outcrop A I Outcrop B II II III

Outcrop A I Outcrop B II II III

Some of the fossils found in outcrop A are the same as fossils found in outcrop B, some distance away. Outcrop A I Outcrop B II II III

Some of the fossils found in outcrop A are the same as fossils found in outcrop B, some distance away. Outcrop A Outcrop B I II II Layers with the same fossils are the same age. III

Some of the fossils found in outcrop A are the same as fossils found in outcrop B, some distance away. Outcrop A Outcrop B I II II Layers with the same fossils are the same age. III

1. Reconstructing Geologic History from 2. the Stratigraphic Record ● Unconformities – gaps in the record ● disconformity ● angular unconformity

Disconformity

Disconformity

Disconformity

Disconformity

Disconformity

Angular unconformity

Angular unconformity

Angular unconformity

Angular unconformity

Angular unconformity

Angular unconformity The ‘great unconformity’ in the Grand Canyon, Arizona.

1. Reconstructing Geologic History from 2. the Stratigraphic Record ● Cross-cutting relationships ● faults ● intrusions

Cross-cutting relationships

Cross-cutting relationships

Cross-cutting relationships

Cross-cutting relationships

Cross-cutting relationships

Relative age relationships

Thought questions for this chapter As you pass by an excavation in the street, you see a cross section showing paving at the top, soil below the paving, and bedrock at the base. You also notice that a vertical water pipe extends through a hole in the street into a sewer in the soil. What can you say about the relative ages of the various layers and the water pipe? Why did 19 th-century geologists who constructed the geologic time scale find sedimentary strata deposited in the sea more useful than strata deposited on land?

Thought questions for this chapter In studying an area of tectonic compression, a geologist discovers a sequence of older, more deformed sedimentary rocks on top of a younger, less deformed sequence, separated by an angular unconformity. What tectonic process might have created the angular unconformity? A geologist documents a distinctive chemical signature caused by Proterozoic organisms. Would you consider this chemical signature to be a fossil?

2. Geologic Time Scale: Relative Ages ● Divisions of geologic time ● eras ● periods ● epochs

3. Measuring Absolute Time with Isotopic Clocks ● Isotopic dating theory ● radioactive atoms ● parent-daughter ● half-life

3. Measuring Absolute Time with Isotopic Clocks ● Isotopic dating methods ● uranium-lead ● potassium-argon ● rubidium-strontium ● carbon-nitrogen

Example: Rubidium-Strontium Neutrons Protons Electron

Rubidium-87 nucleus Neutrons Protons Electron A neutron decays, ejecting an electron…

Rubidium-87 nucleus Neutrons Strontium-87 nucleus Protons Electron A neutron decays, ejecting an electron…

Rubidium-87 nucleus Neutrons Strontium-87 nucleus Protons Electron A neutron decays, ejecting an electron… …and producing a proton, which changes the atom.

Thought questions for this chapter Is carbon-14 a suitable isotope for dating geologic events in the Pliocene Epoch? How does determining the ages of igneous rocks help to date fossils?

4. Geologic Time Scale: Absolute Ages ● Four Eons of geologic time ● Hadean ● Archean ● Proterozoic ● Phanerozoic

5. Advances in Timing the Earth System ● Sequence stratigraphy ● Chemical stratigraphy ● Paleomagnetic stratigraphy ● Clocking the climate system

Key terms and concepts Absolute age Eon Epoch Era Geologic time scale Half-life Isotopic dating Mass extinction Period Principle of faunal succession Principle of original horizontality Principle of superposition Relative age Stratigraphic succession Unconformity