At the beginning of this unit you agreed or disagreed with statements about geologic time and the Earth's past. Not all of the statements were true. Now that you have completed the unit, carefully think about each statement again then decide whether you agree or disagree with the statement based on what you have learned. How many of your responses changed?

1. The Earth is approximately 4.6 billion years old. (Agree / Disagree)
   
   See "G1 - Age of the Earth / Age of the Earth" for more information.
2. The underlying principle of all of geology is the belief that geologic forces at work on the Earth today were also at work in the past. (Agree / Disagree)
   
   See "G1 - Age of the Earth / Age of the Earth" for more information.
3. In a sequence of sedimentary rocks, the lowest sedimentary layer is always the oldest layer. (Agree / Disagree)
   
   The Principle of Original Horizontality states that sedimentary rock forms in horizontal layers. Any bending or folding of the rocks must occur after the original formation of the sedimentary rocks. See "G1 - Age of the Earth / Relative Age" for more information.
4. Radioactive decay is a widely used method for determining the ages of rocks. (Agree / Disagree)
   
   See "G1 - Age of the Earth / Absolute Age: Radioactive Decay" for more information.
5. The first supercontinent was known as Pangaea. (Agree / Disagree)
   
   The world's first supercontinent was known as Rodinia. See "G2 - The Geologic Timescale / Precambrian Time" for more information.
6. Dinosaurs flourished in the time period known as the Paleozoic Era. (Agree / Disagree)
   
   Dinosaurs flourished in the Mesozoic Era, particularly the Jurassic and Cretaceous Periods. See "G2 - The Geologic Timescale / The Mesozoic Era" for more information.
7. The death of the dinosaurs represents the largest mass extinction that the Earth has ever known. (Agree / Disagree)
   
   The world's largest mass extinction of life occurred in the late Permian Period, about 251 million years ago, and is known as the Great Dying. See "G3 - Catastrophic Events and Mass Extinction / The Great Dying" for more information.
8. The world's largest known volcanic eruptions occurred 251 million years ago in Siberia, Russia. (Agree / Disagree)
   
   See "G3 - Catastrophic Events and Mass Extinction / The Great Dying" for more information about the Siberian Traps.