Dating the age of the Earth

Transcription

1 Dating the age of the Earth What is the age of the Earth? A million years B. 1 million years C billion years D years Discuss this with your neighbor: How do we know the age of the Earth? (I will call on one of you.) 1

2 Absolute Age = Number age Based on abundance of radiogenic isotopes Atoms produce by nuclear decay Calculated from measured Parent/Daughter Are radioactive elements naturally occurring? A. Yes B. No So, let s try to understand how this works as a clock Half-life & Parent-Daughter ratio As time proceeds, number of Daughters grow, parents diminish Thus the ratio of parents to daughters (P/D) is a function of time! animation Animate Be-11, F-23 2

3 What happens to the Parent/Daughter ratio after many half-lives? A. It becomes very large B. It becomes very small So, can very young rocks be easily age dated? A. Yes B. No Why can t all rocks be dated? b/c not all rocks are attractive Not all rocks contain appropriate isotopes (composition) Age must be within magnitude of half-life or there won t be enough parent or daughter isotope present to measure. Many rocks are Open Systems - they loose parent or daughter during their long history. Dating Sedimentary rocks = age of sediment NOT age of the rock. 3

4 Age of the Earth: Patterson Slope = function of time = 4.55 billion years old Some cool results of absolute age dating Geologic Time Scale Oxygen atmosphere at ~ Ga Constructed using Absolute age of rocks Radiometric date Relative age dating Sequence of events First continental crust and oceans at 4.4 Ga Planets form in 1st 0.2% of SS history Principles of Relative Age Dating 4

5 Which is older, A or B? Discuss it with your neighbor. I may call on you A B Principle of Original Horizontality Steno: 1600s Sediment (& lava) deposited horizontally in sheets. Thus, inclined/folded beds are disturbed. Principle of Superposition: Steno s If sediment is deposited in layers, then Oldest layers are on bottom provided they have not been disrupted (e.g. faulted) 5

6 Principle of Continuity The layers of sediments (and lavas) are continuous over great distances. Thus, can infer that they continue across eroded distances. Principle Cross-cutting relationships Faults & intrusions cut older rocks (i.e., faults and igneous intrusions are younger than the rocks they cut) Principle of Fossil Succession Fossils are remnants of previously living organisms preserved in rock or sediment Smith - late 1700s noticed: Fossils occur in groups of species (assemblage) Fossil organisms succeed one another in definite order vertically through a sediment column. He correctly reasoned that one could group/correlate rock units over distance based on their common fossil assemblage Loss of a fossil organism from an assemblage = extinction (Curvier, early 1800s) i.e., Fossil assemblages are distinct in time 6

7 Fossil succession & relative time Given that fossil assemblages change over time due to evolution and extinctions, And, fossils assemblages correlate over distances Then, distinct fossil assemblages can be used to assign relative age of the rocks in which they are preserved. Fossil Succession and Correlation of Strata Use fossil assemblage to correlate fossils over regional scales Combining Absolute and Relative age dating with correlation allows us to place rocks within the geologic time scale. 7

8 First Mammals ~ 220 Ma First Marine Mammals ~ 60 Ma Earth History Extinction of ~85% of life Vertebrates on land ~354 Ma Fish radiate in Devonian time Plants invade land, fish develop jaws ~440 Ma Cephalopods are main marine predator until ~360 Ma Fish begin to evolve before 500 Ma First fossil ~3.5 Ga - likely Marine Time Scale compressed to 1 year More fun examples of geologic time Register roll: if 1 Ma = 1 inch, then earth history = 379 feet of paper or 126 yards. Driving time: if 1 year = inch then earth history = 71,811 miles At 75 mph, it would take 957 hours or 40 days of continuous driving. (driving at 1320 inches/second). Counting: 1 yr = 1 sec 4.55e9 sec = 7.6E7 min = 1.3E6 hr = 5.3E4 days = 144 years 8

9 Review Questions How did each of the following scientists contribute to our understanding of radioactivity: Bacquerel, Curie, Rutherford? What are the products of radioactive decay? How does radioactive decay of a parent isotope to a daughter isotope measure time? Why is it necessary that the the half-life of a radioactive clock have similar magnitude to the age of the thing you want to date? Review Questions What is the age of the Universe, our solar system, the Sun, and Earth? Who first successfully age dated the Earth and how was it done? Review Questions If absolute age dating indicates when an event happened, what information does relative age dating provide? How are the relative ages of two rocks determined, if they are separated by a great distance (i.e. they are not in contact)? Think about the oceans and continents that existed 1 billion years ago. How might they be different from today? 9