Relative & Absolute Dating

1. Relative & Absolute Dating Unit 4 Lesson 3 

2. Rocks Record Earth History Discovering Earth’s History  Rocks record geological events and changing life forms of the past.  We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today.

3. A Brief History of Geology Discovering Earth’s History  Uniformitarianism means that the forces and processes that we observe today have been at work for a very long time.

4. Relative Dating Discovering Earth’s History  Relative dating tells us the sequence in which events occurred, not how long ago they occurred.  Law of Superposition • The law of superposition states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.

5. Ordering the Grand Canyon’s History

6. Relative Dating Discovering Earth’s History  Principle of Original Horizontality • The principle of original horizontality means that layers of sediment are generally deposited in a horizontal position.

7. Disturbed Rock Layers

8. Ways rocks can be disturbed….. Faults Intrusions Unconformity

9. Relative Dating Discovering Earth’s History  Principle of Cross-Cutting Relationships • The principle of cross-cutting relationships states that when a fault cuts through rock layers, or when magma intrudes other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected.

10. Relative Dating Discovering Earth’s History  Unconformities • An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

11. Formation of an unconformity

12. Fossil Formation Fossils: Evidence of Past Life  Fossils are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks.  Index fossils are over a large geographical area, are limited to a short span of geologic time, and occur in large numbers.

13. Overlapping Ranges of Fossils

14. Absolute Dating • Determining the actual age of an event or object in years is called absolute dating. • Scientists often use radioactive isotopes to find the absolute age of rocks and other materials. • Atoms with the same number of protons but different numbers of neutrons are called isotopes. • Isotopes are formed from nuclear reactions.

15. How a nuclear reaction can change an atom into another one. - Beryllium Atom 4 protons 4 electrons 5 neutrons Lithium Atom 3 protons 3 electrons 5 neutrons + - Nuclear Reaction Source N + N + N + - N N - Because the radiation decreased the number of protons and electrons from 4 to 3 in the atom, it changed from an atom of Beryllium to Lithium.

16. How a nuclear reaction can change an atom into an isotope. - Beryllium Atom 4 protons 4 electrons 5 neutrons Beryllium-8 Isotope 4 protons 4 electrons 4 neutrons Nuclear Reaction Source - + N N + N + + - N N - Because the radiation decreased the number of neutrons from 5 to 4 in the atom, it changed from an atom of Beryllium to the Beryllium-8 isotope.

17. Absolute Dating How can the absolute age of rock be determined? • Radioactive isotopes, also called radioisotopes, are isotopes that are unstable and break down into other, stable isotopes by a process called radioactive decay. As they break down, they release excess energy by emitting radiation in the form of alpha, beta and gamma rays. • The radioactive isotope is called the parent isotope, and the stable isotope formed by its breakdown is called the daughter isotope. • Half-lifeis the time needed for half of a sample of a radioactive element to undergo radioactive decay and form daughter isotopes.

18. Absolute Dating • After one half-life has passed, one-half of the parent isotope has changed into daughter isotopes. 2865 years Time Elapsed: 5730 years = Half-life = Radiation = Parent Isotope = Daughter Isotope

19. Absolute Dating How can the absolute age of rock be determined? • Scientists study the amounts of parent and daughter isotopes to date samples. • Finding the absolute age of a sample by determining the relative percentages of a radioactive parent isotope and a stable daughter isotope is called radiometric dating.

20. Absolute Dating What is the best rock for radiometric dating? • Igneous rocks are the best types of rock samples to use for radiometric dating. • When igneous rocks form, minerals in them often contain only a parent isotope and none of the daughter isotope. • This makes the isotope percentages easier to interpret and helps dating to be more accurate.

21. Absolute Dating How can fossils help to determine the age of sedimentary rock? • Sedimentary rock layers and the fossils within them cannot be dated directly. • But igneous rock layers on either side of a fossil layer can be dated radiometrically. • Once the older and younger rock layers are dated, scientists can assign an absolute age range to the sedimentary rock layer containing the fossils.

22. Absolute Dating How can fossils help to determine the age of sedimentary rock? • Index fossils are fossils used to estimate the absolute age of the rock layers in which they are found. • Once the absolute age of an index fossil is known, it can be used to determine the age of rock layers containing the same fossil anywhere on Earth.

23. Absolute Dating How can fossils help to determine the age of sedimentary rock? • To be an index fossil, the organism from which it formed must have: • lived during a short geologic time span • Must be a large amount of them • must be found over a large area • Index fossils must also have features that make them different from other fossils

24. Radioactive Decay The process….

25. Radioactive Decay • Rocks are a form of matter • Atoms- tiny particles that make up all matter • Element- any substance that cannot be broken down into simpler substances (examples: carbon, oxygen, iron, lead) • Most elements are stable- do not change under normal conditions

26. Radioactive Decay • Radioactive Decay- Over time unstable elements break down, or decay, by releasing particles and energy- these unstable elements are said to be radioactive • Key Concept- During the process of radioactive decay, the atoms of one element break down to form atoms of another element.

27. Radioactive Decay During radioactive decay, a “parent” atom releases energy and particles as it changes to a new kind of atom, a “daughter” atom.

28. Radioactive Decay • Radioactive elements happen naturally in igneous rocks • Scientists use the rate of decay to calculate the rock’s age • “Birthday” for an igneous rock is when it first hardens to become rock

29. Radioactive Decay • As the radioactive element in the rock decays it changes into another element • Composition changes slowly over time • Amount of radioactive element goes down and amount of new element goes up

30. Radioactive Decay • Rate of decay is constant for each radioactive element • Scientists can measure the rate of decay experimentally • Half-life- the time it takes for half of the radioactive atoms to decay

31. Determining Absolute Ages Key Concept- Radioactive dating is used to determine the absolute ages of rocks.

32. Determining Absolute Ages • Table lists several common radioactive elements and their half-lives. • Carbon-14 has a half-life of 5,730 years and is useful in dating plants and animals that lived up to about 50,000 years ago. • Potassium-40 decays to a stable argon-40 and has a half-life of 1.3 billion years. It is useful in dating most ancient rocks because of its long half-life. Example is the decay of Potassium-40 to Argon-40

33. Determining Absolute Ages • In radioactive dating first measure the amount of radioactive element in a rock (“parent” element) • Then measure the amount of stable element that the unstable radioactive element decays into (“daughter” element) • By calculating the ratio of radioactive element to the stable element you can determine the age of the rock

34. How is radiometric dating used to determine the age of Earth? • Radiometric dating can be used to find the age of Earth. But there are no Earth rocks which can be directly studied that are as old as our planet because they have been eroded, buried, or melted. • Therefore we can study meteorites and moon rocks • The absolute age of meteorites • and other rocks in the solar system • is about 4.6 billion years.