Geologic Time Notes

1. Geologic Time Notes May 1, 2013

2. Relative Dating • Evidence from geologic layers and radioactive dating indicates Earth is approximately 4.6 billion years old and that life on this planet has existed for more than 3 billion years. • Geologists use a process called relative dating to determine the age of fossils and rocks

3. Relative Dating (continued) • Relative datingis a method of sequencing • events in the order they happened. • When you use relative dating, you are not trying to determine the exact age of an object. Instead, you use clues to sequence • the order of events that occurred around it.

4. Paleontologists, scientists who study fossils, • use relative dating to determine the sequence • of fossils (ex. Index fossils) in the order that each species existed.

5. Determining the Age of Rocks • Relativeage: the age of rock compared to the ages of other rocks above or below it in a sequence of rocklayers

6. Principles to follow to determine relative age 1. Principle of OriginalHorizontality: When sedimentary rock layers are being deposited, gravity forces them to be deposited as flat, horizontal layers. Once the sediment has solidified and become rock, they can be tilted or folded by a geologic event.

7. B) Principles to follow to determine relative age 2. PrincipleofSuperposition a. Sedimentary rocks form as horizontal layers b. This principle states that if rock layers are undisturbed, younger rocks lie aboveolder rocks, and the oldest rocks are at the bottom

8. (Principle of Superposition cont.)c. Geologists use the principle of superposition to determine the relative ages of rocks from the sequence of rock layers and the fossils within each layer. d. Examine sedimentary rocks from locations around the world to develop a relative time scale

9. Principle of Superposition

10. 3. Principle of Cross-cutting relations: An igneous rock unit that cuts across another rock unit must be younger than the unit it cuts across. In other words, the other rock unit must have already been there for the igneous rock to cut across it.

11. Cross Cutting Sometimes magma pushes, or intrudes, into cracks in existing rocks. When the melted rock cools and solidifies, the resulting feature is called an igneous intrusion A dark-colored igneous intrusion cuts across metamorphic rock in Death Valley, California.

12. 4. Principle of Inclusion: An igneous rock unit that spreads into one layer of preexisting rocks must be younger than the rock unit it has spread into.

13. Sometimes rock pieces called inclusions (older rock) are found inside another (younger) rock. • During the formation of • a rock with inclusions, • sediments or melted • rock surrounded the • inclusion and then • solidified. • Therefore, the inclusions • are older than the • surrounding rock.

14. A rock with inclusions is like a chocolatechip cookie. • The chocolate chips(inclusions) are made • first. Then they are added to the batter • (melted rock or sediment) • before being baked (hardened) • into a cookie (rock).

15. C) Index Fossils and Relative Dating 1. Can determine relative ages of rocks by examining fossils, most of which are extinct, that are found in them 2. Index fossils: fossils of organisms that are easily identified, occurred over a large area and lived for a welldefined period of time 3. Use these index fossils to determine relative age of rocks because the rocks must have formed at the same time that those organisms lived

16. D) Radioactive Dating 1. Geologists use radioactivedating to determine the absolute ages of rocks a. Absoluteage: the time that has passed since the rock formed 2. Measure the radioactive isotopes that are being emitted from the rock and use the half-life of this isotope to determine the absolute age