1. Rock Dating Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, in which it is found.

2. Two fundamentally different ways of telling geological dates: relative and absolute. Methods of Dating Relative dating places events or rocks in their chronologic sequence or order of occurrence. e.g. If you claim to be younger than your friend, that is a relative age. Absolute dating places events or rocks at a specific time. e.g. If you claim to be 17 years old, that is an absolute age.

3. Radiometric dating The nuclear decay of radioactive isotopes behaves in a clock-like fashion. Absolute Dating Radioactive decay is the process by which a "parent" isotope changes into a "daughter" isotope. Rates of radioactive decay are constant and measured in terms of half-life, the time it takes half of a parent isotope to decay into a stable daughter isotope.

5. Some rock-forming minerals contain naturally occurring radioactive isotopes with very long half- lives. Half-lives of these isotopes and the parent-to- daughter ratio in a given rock sample can be measured, then a calculation based on the total number of elapsed half lives gives the absolute (radiometric) date at which the parent began to decay, i.e., the age of the rock.

6. Of the three basic rock types, igneous rocks are most suited for radiometric dating. These rocks have not changed since they were formed from cooling magma containing the parent isotope. Metamorphic rocks may also be radiometrically dated. However, radiometric dating generally gives the age of metamorphism, not the original rock. Most ancient sedimentary rocks cannot be dated radiometrically, so other methods will need to be used to give an estimation of the absolute age.

7. Radiometric Methods: Uranium–lead (U–Pb) Detects an age range of about 1 million years to over 4.5 billion years, precise to 0.1–1%. The method relies on two separate decay chains where U-238 decays to Pb-206, with a half-life of 4.47 billion years and U-235 decays to Pb-207, with a half- life of 704 million years. This is the most commonly used method as it can detect the absolute age of very old and quite young rocks.

8. Potassium-argon (K–Ar) This involves decay of potassium-40 to argon-40. Potassium-40 has a half-life of 1.3 billion years, and so this method is used to date the oldest rocks.

9. Radiocarbon dating Carbon-14 is a radioactive isotope of carbon, with a half-life of 5,730 years. Carbon-14 is continuously created in the upper atmosphere and ends up as a trace component in atmospheric carbon dioxide. An organism acquires carbon during its lifetime. Plants through photosynthesis and animals through consumption. When an organism dies, it stops acquiring new carbon-14 and the existing isotope decays. The proportion of carbon-14 left when the remains of the organism are examined provides the time elapsed since its death. The carbon-14 dating limit is around 58,000 to 62,000 years. Useful for dating sedimentary rocks with organic material trapped inside.

10. Relative Dating As it can’t give an exact age for the rocks, relative dating methods use the rocks in the formation to give clues as to the ages of rock. By comparing the different types of rocks in a formation with each other, the sequence of rock building can be determined.

11. Relative Dating Methods: Stratigraphy Using rock layers and layering (stratification) to determine the sequence of rock formation. It is mainly used in the study of sedimentary and layered volcanic rocks. These rocks are built up layer upon layer to form strata. The Law of Superposition states that the oldest strata will be the ones at the bottom of an undisturbed rock formation.

13. If a stratum has other rocks surrounding it that can be accurately dated, then the relative age can be determined. Strata may not stay undisturbed. Geological processes such as uplifting, faulting, folding and weathering & erosion can shift or remove the layers.

14. Folded strata Faulted strata

16. Cross cutting relationships A feature which cuts another is the younger of the two features. These can also be used in conjunction with radiometric age dating to produce an age bracket for rocks that cannot be directly dated by radiometric techniques.

17. D<E<F<G<H due to Law of Superposition. X-Y is a fault that occurred before A and C but after D. Igneous intrusion A crosscuts D-H and the fault so it is younger. C crosscuts A so is younger. B is superpositioned on all so it is the youngest.

18. Unconformities Buried erosional or non-depositional surface separating two strata of different ages, indicating that sediment deposition was not continuous. The rocks above an unconformity are younger than the rocks beneath.

19. An unconformity represents time during which no sediments were preserved in a region. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus.

20. Video on stratigraphy

21. Relative Dating Methods: Biostratigraphy Using fossil evidence in rock strata. Strata from widespread locations containing the same fossils were created at the same time in history even if the type of rock that the fossils are found in are different. The principle of faunal succession states that the fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances e.g. you will never see a human fossil at the same level as a dinosaur fossil.

22. Unconformity shown by the irregular strata boundary and the lack of Fossil #5. This rock layer has been eroded away.

23. This timescale is was relative until radiometric dating could put absolute dates on key indicator fossil species. Fossilisation is the preserving of the remains of an organism. There are different methods depending on the local conditions at the time of formation. The fossil is made of rock, not any living tissue. Fossilisation only occurs in correct conditions and then only hard parts to the body (shells, bones, teeth, pollen) are usually preserved. Consequently the fossil record is incomplete.

24. There are fossils that are used as index fossils – these are most useful for determining ages. These include ammonites and trilobites.

25. Microfossils are very important, especially foraminifera – single-celled algae that have a shell. These are extremely useful in dating sedimentary rocks as they are quite commonly found in them. They evolve quickly so the types found in rocks will be easily identifiable by their age.

26. Geological Ages of Earth By using all of the techniques available to geologists, a timeline for the age of the Earth has been produced. The ages of rock are cross-correlated by different techniques to get the best estimate of a definite age. Ages are separated by mass-extinction events as the fossil record was the first to be used in relative dating.