1. Dating the Rock Record Soo, do you come here often?

2. Relative Age Uniformitarianism – a principle that geologic processes that occurred in the past can be explained by current geologic processes. Put this idea into your own words. James Hutton was an 18 th century physician. He noticed the landscape of his farm changing and theorized that the same slow forces that were shaping his farm had changed Earth’s surface in the past. He argued that the Earth had to be millions of years old instead of only a 6000 years old, which is what most scientists thought at this time.

3. Relative Age Relative age – the age of an object in relation to the ages of other objects. For example, arrange the objects according their relative ages:

4. Reconstruction of Geologic Time No one location has a complete set of all the rock layers that exist on Earth. Geologists and paleontologists have to record, compare, and correlate information from all over the world. Correlate - have a mutual relationship or connection, in which one thing affects or depends on another. Relative age indicates that one layer is older or younger than another layer but does NOT indicate the rock’s age in years. So, you know the disco ball is younger than the cell phone, but you do not know the year the disco ball is from.

5. Relative Age Strata – layers in rock or soil that show the order of events in the past. Sedimentary rocks form when sediments accumulate and are compressed and hardened into rock layers called BEDS. The boundary between beds is called the BEDDING PLANE. Law of Superposition – the law that a sedimentary rock layer is older than the layers above it and younger than the layers below it IF the layers are not disturbed.

6. Principal of Horizontality – the principal which states that sedimentary rocks left undisturbed will remain in horizontal layers. Why does that matter? What might cause problems for this principal? Rock structures = rock clues Graded bedding – a clue to the original position of rock layers because the size of the particles in the layers. Cross-beds – when sand is deposited, sandy sediment forms curved beds at an angle to the bedding plane. Ripple marks – the crests of the ripple marks point upward.

7. Unconformities A break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long time. Nonconformity – unstratified igneous or metamorphic rock may be uplifted to Earth’s surface by crustal movements. Angular nonconformity – forms when rock deposited in horizontal layers is folded or tilted and then eroded. Disconformity – layers of sediment are uplifted without folding or tilting and are eroded.

8. Crosscutting relationships Law of crosscutting relationships – the principle that a fault or body of rock is younger than any other body of rock that it cuts through.

9. Absolute Dating Techniques Absolute age – the numeric age of an object or event, often stated in years before the present. Established by absolute dating processes. Rates of erosion – one way to estimate the absolute age. If scientists measure the rate at which a stream erodes its bed, they can estimate the age of the stream. For example, the ledge above the Niagara Falls erodes at a rate of 1.3 m per year for nearly 9,900 years. How many km has the ledge eroded in the last 9,900 years?

10. Absolute Dating Techniques Rates of deposition – calculating the rate of sediment deposition. If we know the constant rate of any geologic process, we can use this process to date objects and events. Varve – a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers, and that can be used to determine absolute age. Sort of like how we use tree rings to find a tree’s age.

11. Radiometric dating A method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope. Read about the radioactive decay of Uranium. (pg 193) Half-life – the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope. Penny half-life activity: Graph your results.

12. Radiometric dating methods Review the chart and answer the following questions: Identify the parent isotope and daughter isotope. Which method should be used for dating the oldest rocks on Earth? What about the youngest rocks? Why is it important to know the composition of rocks before choosing a dating method?

13. Carbon dating Younger rock layers may be dated indirectly by dating the organic material found within the layers. Bone, shell, and wood that are in layers less than 70,000 years old can be dated using C-14, or radiocarbon dating. The half-life of carbon-14 is 5,730 years. The carbon-14 decays to become non-radioactive nitrogen-14.

15. Quiz What are the four “pillars” of relative dating? What do each of these mean? Would you use carbon-14 dating or uranium-lead dating in order to date a fossilized tooth of a Smilodon (saber-toothed cat), which lived in Antarctica about 12,000 years ago? Explain.