Geologic Time

1. Geologic Time

2. Geologic Time • A major difference between geologists and most other scientists is their attitude about time. • A "long time” may not be important unless it is > 1 million years.

3. Two ways to date geologic events Relative dating (fossils, stratigraphy) Absolute dating (radioactive isotopes)

4. Amount of Time Required for Some Geologic Processes and Events BIG VARIATION! Fig. 10.1

5. Some geologic processes can be documented using historical records(brown is new land in Wellfleet Harbor (Cape Cod) from1887-1988) Fig. 10.2

6. Steno's Laws Nicolaus Steno (1669) • Principle of Superposition • Principle of Original Horizontality • Principle of Lateral Continuity Laws apply to both sedimentary and volcanic rocks.

7. Principle of Superposition In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom. YOUNGEST LAST OLDEST FIRST

8. Principle of Superposition Fig. 10.3

9. Principle ofOriginal Horizontality Layered strata are deposited horizontally or nearly horizontally (or nearly parallel to the Earth’s surface).

10. Principles ofOriginal Horizontalityand Superposition

11. Principle ofLateral Continuity Layered rocks are deposited in continuous contact. Map view

12. Stratigraphy Description and correlation of strata (layers) in sedimentary rocks.

13. Unconformity A surface between two layers that were not laid down in an unbroken sequence; a discontinuity in stratigraphy.

14. The Great Unconformity of the Grand Canyon Fig. 10.7

15. How can an unconformity form? Fig. 10.6

16. Types of unconformities • Disconformity: Lower set of beds is undeformed and still horizontal. • Nonconformity: Lower beds are metamorphic or igneous rocks. • Angular unconformity: Lower beds have been upended by tectonic processes, and then eroded so they have a horizontal surface.

17. Forming an angular unconformity Fig. 10.8

18. Angularunconfor-mity Fig. 10.7

19. Nonconformity in theGrand Canyon

20. Nonconformity in theGrand Canyon Tapeats Sandstone (~550 million years old) Vishnu Schist (~1700 million years old)

21. Cross-cutting relationships • What comes first? • Discordant dikes, intrusions, etc. must be YOUNGER than the strata they cut. • Faults must be YOUNGER than the strata they displace. • Provides a means of RELATIVE dating of strata and geological features.

22. Cross-Cutting Relationships

23. Cross-cutting relationships Fig. 10.9

24. Sediments deposited in flat, horizontal beds. Deposition of more sedimentary layers. Deformation and metamorphism. Igneous intrusion. Erosion results in angular unconformity. Erosion of surface to a plane. Creates an unconformity. Tectonic tilting. More erosion. Subsidence below sea level. Deposition of new sedimentary strata. Fig. 10.11

25. Paleontology • The study of life in the past based on fossilized plants and animals. • Fossil: Evidence of past life • Fossils preserved in sedimentary rocks are used to determine: 1) Relative age 2) Environment of deposition

26. William Smith- The Map that Changed the World • A surveyor in southern England late 1700s/early 1800s. • In each outcrop he identified distinct formations (a series of rock layers in a region with same physical properties). • Principal of faunal succession: Groups (assemblages) of types of fossils were formed at the same time • Same fossil assemblages in formations at different locations indicate that formations are of the same age. • 1815 he published his masterpiece: an 8.5- by 6-foot map "A Delineation of the Strata of England and Wales."

27. Using Fossils to Correlate Rocks Fig. 10.5

28. The Geologic Time Scale • Divisions in the worldwide stratigraphic column based on variations in preserved fossils • Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages

29. TheGeologicTimeScale Fig. 10.12

30. Fig. 10.17

31. Notable moments in geologic time • Oldest evidence of fossil cells is about 3.5 Ga • Multi-celled organisms appeared about 1 Ga • About 543 Ma, incredible expansion of diversity – caused by oxygen going above 1% in the atmosphere • 251 Ma, 90-95% of species become extinct • 65 Ma, about 80% of species become extinct

32. How do we know the dates?Absolute geochronology. Based on the regular radioactive decay of some chemical elements.

33. Geologic time scale is a RELATVE time scale. Need to know ABSOLUTE ages. “Like knowing that WWI preceded WWII but not knowing the specific years in which each conflict began and ended.”

34. Many methods have been used to determine the age of the Earth

36. Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons. i.e.: 235U, 238U 87Sr, 86Sr 14C, 12C

37. Isotopic dating • Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). • The rate at which this decay occurs is constant and knowable. • Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding. • Generally used on igneous and metamorphic rocks.

38. Radioactive Decay of Rubidium to Strontium

39. Half-life The half-life of a radioactive isotope is defined as the time required for half of the original number of radioactive atoms to decay.

40. Proportion of Parent Atoms Remaining as a Function of Time Fig. 10.15

41. number of number of number of D/P half-lives parents daughters 0 64 0 0 1 32 32 1 2 16 48 3 3 8 56 7 4 4 60 15 5 2 62 31

42. Geologically UsefulDecay Schemes

43. Oldest rocks on Earth? Slave Province, Northern Canada • Zircons in a metamorphosed granite dated at 3.96 Ga by the U-Pb method Yilgarn block, Western Australia • Detrital zircons in a sandstone dated at 4.10 Ga by U-Pb method. Several other regions dated at 3.8 Ga by various methods including Minnesota, Wyoming, Greenland, South Africa, and Antarctica.

44. Age of the Earth Although the oldest rocks found on Earth are 3.96 Ga (or even 4.1 Ga), we believe that the age of the Earth is approximately 4.6 Ga. All rocks of the age 4.6 to 4.0 Ga have been destroyed (the rock cycle) or are presently covered by younger rocks.

45. The geologic timescale and absolute ages Isotopic dating of interbedded volcanic rocks allows assignment of an absolute age for fossil transitions.

46. The big assumption • The half-lives of radioactive isotopes are the same as they were billions of years ago.

47. Test of the assumption Meteorites and Moon rocks (that are thought to have had a very simple history since they formed), have been dated by up to 10 independent isotopic systemsall of which havegiven the same answer. However, scientists continue to critically evaluate this data.

48. How does Carbon-14 dating work? • Cosmic rays from the sun strike nitrogen-14 atoms in the atmosphere and become carbon-14 in CO2. • C14 in CO2 is absorbed by plants—in the food chain. • Living things contain a constant ratio C14 to C12 (1 in a trillion). • At death, C14 exchange ceases. At that moment, C14 in the organism is the same as C14 in the atmosphere. • The change in the C14 to C12 ratio is the basis for dating (C14 steadily decreases as radioactive atoms decay). • The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. • Assumes that the rate of C14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).