11/5/2015. Creating a Time Scale - Relative Dating Principles. Creating a Time Scale - Relative Dating Principles. The Geologic Time Scale

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1 GEOL 110: PHYSICAL GEOLOGY Why is the Geologic Time Scale important? Rocks record geologic and evolutionary changes throughout Earth s history Without a time perspective, events have little meaning Chapter 9: Geologic Not for Distribution. For study purposes only for related class. Time Scale The Geologic Time Scale The geologic time scale encompasses all of Earth history Subdivides geologic history into units Originally created using relative dates Eon, Eras, Periods, Epochs Most epochs are termed early, middle, and late, except for the epochs in the Cenozoic. How does it relate to rocks and fossils? Numerical dates number of years since formation. Relative dates place rocks in a sequence of formation Numerical: Jaime is 13 years old. Liz is 10 years old. Relative: Jaime is older than Liz How do we determine the age and sequence of geologic events? 4 primary principles and clues. 1) Principle of Superposition Undeformed sedimentary layers are in order from oldest to youngest. This principle also applies to surface features like lava flows and beds of ash Youngest above, oldest below 1

2 2) Principle of Original Horizontality Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed Lateral Continuity in the Grand Canyon 3) Principle of Lateral Continuity Beds originate as continuous layers that extend in all directions 4) Principle of Cross-Cutting Relationships Younger features cut across older features Clues: Inclusions Inclusions are fragments of one rock unit that are enclosed within another rock unit The rock containing the inclusion is younger Clues: Conformable Rock layers Layers of rock that have been deposited without interruption are called conformable layers Are all rocks layers conformable? Unconformities An unconformity indicates time missing within the rock record; a result of nondeposition and erosion of rock units 3 basic types: angular unconformity, disconformity, nonconformity 2

3 1) Angular unconformity Tilted rocks are overlain by flat-lying rocks 2) Disconformity Sedimentary strata on either side of the unconformity are parallel 3) Nonconformity Sedimentary strata overlay metamorphic or igneous rocks All 3 types of unconformities can be seen in the Grand Canyon Applying Principles Sequence of Events: Applying Principles Sequence of Events exercise: 3

4 Fossils and geologic time Fossils are traces or remains of prehistoric life preserved in rock Paleontology is the study of fossils Helps researchers understand past environmental conditions Fossils and Geologic Time Quaternary: evolution of humans Cretaceous: first primates Jurassic: first birds Triassic: first dinosaurs Carboniferous: first reptiles Devonian: first amphibians Cambrian: first fishes Late proterozoic trilobites earliest ~521 mya Best conditions for preservation and development of fossils? Rapid burial Possession of hard parts Most organisms are not preserved; soft bodies 5 1) Permineralization Groundwater containing Silica flows through porous tissue and replace the wood molecules. Example: petrified wood 2) Molds and casts A mold is created when a shell is buried and then dissolved by underground water, leaving a cavity. A cast is created when the hollow spaces of a mold are filled with sediments. Generally not actual organism. 3) Carbonization and Impressions Carbonization occurs when an organism is buried, then compressed, squeezing out gases and liquids leaving a thin film of carbon eg: leaves and fragile animals Impressions remain in the rock when the carbon film is lost 4

5 4) Amber Amber is the hardened resin of ancient trees Eg: preserving insects 5) Trace fossils Indirect evidence of prehistoric life Eg: tracks, burrows, coprolites, How can we date the ages of rocks? How can we date the ages of rocks? Radioactivity: spontaneous decay in the structure of an atom s nucleus 238 U to 226 Ra to 206 Pb Radiometric dating: uses the decay of isotopes in rocks to calculate the age of that rock. 238 U to 234 U indicates 100,000 years and 1.2 mya. Capital Reef, UT Isotopes of Hydrogen H has 1 proton Isotopes of H have 0, 1, 2 neutrons Dating with Carbon-14 Radiocarbon dating uses the radioactive 14 C to date recent events Can be used to date events as old as 70,000 years 14 C is only useful in dating organic matter Living organisms intake equal amounts of 14 C and 12 C Dead organisms: carbon intake stops, 14 C begins to decay 5

6 Correlation of Rock Layers Correlating Rock layers Correlation matching rocks of similar ages from different regions Correlation provides a more comprehensive view of the rock record The Geologic Time Scale Why don t we know much about Precambrian time? The Geologic Time Scale Why don t we know much about Precambrian time? 88% of Earth s history Less is known about Earth further back in geologic time. During the Precambrian, simple life-forms that lacked a hard parts. Algae, bacteria, worms, fungi dominated. Many Precambrian rocks are highly deformed metamorphic rocks. End of Chapter 9 6