Relative / Absolute Dating Fossils and Relative / Absolute Dating

THE FOSSIL RECORD

THE FOSSIL RECORD Importance Provides the best proof of the history of life showing how extinct species have led to today’s species “Lucy”, 3.2 MY Archaeopteryx, 150 MY

Next: Read and summarize Becoming a Fossil by Bill Bryson

NO Becoming a Fossil Formation of Fossils (Summary) Organism is buried quickly in sediment under water (sometimes: ice, amber or tar) Hard parts (bone& shell) are slowly replaced by minerals dissolved in water Over millions of years, must maintain identifiable shape that someone will recognize as worth keeping Will you ever find fossils in metamorphic or igneous rock? NO

Completeness of the Fossil Record?

The Story Fossils Tell What type of rock is it? Provide an observation & an inference about this fossilized animal. If this fossil was found in central Cumberland County, PA, how old is it? (Use the Geologic Map on next slide)

What type of rocks are most common in Bucks County? What age fossils might you find in Bucks County?

Next Complete the Mystery Fossil Activity

Mystery Fossil Activity DIRECTIONS: 1. Every group member should help cut out the mystery fossil bones. 2. Work as a group to put together the skeleton of the animal. 3. Once you have agreed, glue the bones on a large piece of paper. 4. Work together to decide what the outside of the animal looked like. Was it covered with skin, fur, scales, something else? How large was it? What did it eat? 5. Once finished with re-creating the fossil – work as a group to complete the 10 Summary Questions.

Summary Questions 1. Did you make any assumptions at the beginning of the activity that slowed you down in putting the bones together? ______ Explain: 2. Did any of your group members resist the group’s ideas? _____ Explain: 3. Did any information from another group influence your construction? _____ If so, what? 4. What did your group you say about how and where this animal lived? 5. What did this animal eat? 6. How big was it? 7. Where did it live and when? 8. This animal was a(n) _____________________________________. We know this because: 9. How close to reality was your construction? 10. How is it possible for scientists to do studies about things that happened millions of years ago?

Mystery Revealed FACTS: 1. The Archaeopteryx may have eaten insects and small animals. It lived during the Jurassic Period, about 150 million years ago. 2. Its name means “Ancient Wing.” It’s pronounced: ark-ee-OP-ter-iks 3. The Archaeopteryx seemed to be part-bird and part-dinosaur. Unlike modern-day birds, it had teeth, three claws on each wing, and a long, bony tail. Like modern-day birds, it had feathers and a very light body with hollow bones. 4. Length - 1 foot long from beak to tail Wingspan - 1.5 feet Weight - 11 to 18 ounces 5. This is the size of the Archaeopteryx next to an adult male: 6. Archaeopteryx is one of the oldest-known birds.

RELATIVE DATING

WARM-UP

ANSWER

Uniformitarianism  Earth’s geologic processes acted in the same manner and with essentially the same intensity in the past as they do in the present and that such uniformity is sufficient to account for all geologic change MEANING- things that change earth’s geology today are the same things that have changed it for millions of years

TWO Ways to Date Fossils & Rocks Relative Dating Determines the sequence of events by comparing rock layers, but cannot tell how long go the events occurred. Radiometric Dating AKA – Absolute Dating Precise dating by measuring radioactive decay of elements in rock Radiometric Dating

Relative Dating Rules to Follow: Law of Superposition - older rocks are found under younger rocks Principle of Original Horizontality – sediment is deposited in flat, horizontal layers

Rules Continued Principle of Cross-Cutting Relationships – A fault or igneous intrusion must be younger than the layers it cuts through. Igneous Intrusions – solidified magma that is contained within another rock layer

Rules Continued Unconformities A break in the rock record. Erosion removes rock before deposition continues. Types: Disconformity Nonconformity Angular Unconformity

Disconformity Only with Sedimentary Rock - Erosion occurs before new sedimentary layers are formed

Nonconformity Erosion occurs to metamorphic or igneous rocks and then younger sedimentary layers form on top

Angular Unconformity New layers of sedimentary rock are deposited over older layers that have been tilted or folded and then eroded.

Practice – Which type of Unconformity? Angular Unconformity Disconformity Nonconformity

Rules Continued Inclusions – Pieces of rock contained within another rock layer. The rock layer containing the inclusions must be younger.

Practice – Place layers in order from youngest to oldest.

Answers

Practice Youngest ______ Oldest

Practice Youngest C B A D E Oldest

PRACTICE

ANSWER

Correlation & Index Fossils Correlation - Rock layers formed at the same time and by the same processes are compared at different locations to get a more complete picture of the rock record Index Fossil – a fossil that is geographically widespread and abundant in the fossil record, but that existed for a limited span of time (so usually in one layer). The presence of an index fossil in rock layers at different locations means that the layers are roughly the same age and likely formed in similar environments.

Correlation Example:

Radiometric Dating (Absolute Dating) Radioactive Elements (or Isotopes): unstable atoms giving off radiation to become stable protons & neutrons are given off as alpha, beta and gamma particles – this is called radioactive decay Ex: Uranium-238 & Carbon-14 Radioactive dating: Radioactive decay (going from unstable to stable) occurs at a constant rate called a half life. Each radioactive element has its own half life.

Half Life - Radioactive Decay Half life: the amount of time it takes for half the radioactive atoms in a substance to become stable. Example: Red Dots are radioactive elements Green Dots are stable elements

Examples of Radioactive Elements Uranium-238 has a half-life of 4.5 billion yrs (becomes Lead) Potassium-40 has a half-life of 1.3 billion yrs (becomes Argon-40) Carbon-14 has a half-life of 5730 yrs (becomes Nitrogen) – AKA – Radiocarbon Dating – used to date once living material Which would you use to date a wooly mammoth bone? Which would you use to date igneous rock that may be the oldest sample on North America?

Carbon 14- It’s in You!!!! Carbon atom is in every living thing Some are radioactive Carbon-14 When an organism dies Carbon-14 begins to decay to stable Nitrogen

Carbon-14 Rate of Decay Every 5,730 years is the half life of Carbon-14

Example of Radiocarbon Dating Scientists were able to determine the age of the Iceman. 5,300 years old Used the decay of radioactive carbon

Practice What percentage of a radioactive element will be left after: 1 half-life ______ 2 half-lives _____ 3 half-lives ______ One isotope of Iodine has a half-life of 8 days. That is, after 8 days, ½ of all the atoms in a given sample of the isotope will have undergone radioactive decay. Using a 1 gram sample. How much will remain after 8, 16, and 32 days? 8 days ___________ 16 days ___________ 32 days __________