Geologic Time Relative Age Absolute Age DETERMINING THE
Geologic Time, Relative Age & Absolute Age DETERMINING THE RELATIVE AND ABSOLUTE AGE OF ROCK LAYERS IN THE GEOLOGIC RECORD
The Geologic Time Scale - How Geologists Describe the RATE of geologic change CATASTROPHISM (pg 120 -121) UNIFORMITARIANISM • Changes occur suddenly over large areas, short-lived, violent events. • Examples of geologic catastrophes are earthquakes, tsunamis, floods, mass movements, etc. • Changes occur gradually, slow • Same geologic processes that shape Earth today have happened throughout Earth’s history **catastrophism and uniformitarianism both occurred throughout history to shape Earth**
How Do Geologist Use the Geologic Time Scale? (pg 122) Geologic Time Scale: divides Earth’s history into intervals of time defined by major events or changes on Earth ■ helps makes sense of time and organize Earth’s history ■ use the fossil record to divide geologic time into segments (ex: Cenozoic era, Jurassic period) ■ use Radiometric Dating techniques to estimate ABSOLUTE age of Earth (pg. 113) Earth is about 4. 6 billion years old
Geologic Time Scale - How do we know Ages? What is the difference between relative and absolute age? RELATIVE AGE/ RELATIVE DATING (pg. 94) ➔ Age of the rock is determined by comparing rock layers. ➔ Does NOT give rock’s age in years, only allows scientists to find out what rock layer is older or younger than another rock layer. ➔ Methods: law of superposition, law of crosscutting relationships ABSOLUTE AGE/ABSOLUTE DATING) (pg. 108 -109) ➔ The exact age of the rock layer or fossil in years. ➔ Method: Radiometric dating is a method used to allow us to find absolute age of rock layers or fossils.
Law of Superposition (pg. 95) Law of Superposition: older rock layers end up on the bottom layer and newer rock layers are towards the top. (when looking at the profile of layers. . . aka - geologic column; cross section of Earth). **This is true for any undisturbed rock exposure**
Law of Crosscutting Relationships (pg 98) Law of Crosscutting Relationship: a fault or body of rock, such as an intrusion, must be younger than any feature or layer of rock that the fault or rock body cuts through. Example Below: if the fault (4) has broken a rock layer, the fault is younger than the rock layer it has broken.
Radiometric Dating (pg 113) Radiometric Dating: determining the absolute age of a rock by comparing the percentage of radioactive isotopes to stable isotopes.
How are rock layers disturbed? (pg 96)
Unconformities in Layers (pg. 97) Unconformity: a break in the geologic record when rock layers are eroded or when sediment is not deposited for a long period of time. • When scientists find an unconformity, they must question if the “missing layer” was simply never present or removed.
Review / Overview: How can scientists analyze geologic time? ➢Comparing fossil and/or rock layers ➢Radiometric dating ➢Observing rock sequences (Law of Superposition, Law of Crosscutting)
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