Geology 103, Spring 2006 Names:

Lab 1: The geological time scale and dating methods

The geological time scale was developed over the course of the last three centuries as a means of identifying times when distinct assemblages of organisms lived. Over the next few weeks, we will discuss how this timescale was developed and calibrated. For now, though, it is enough to notice that most timescales (such as the one attached to this handout) seem to show different periods of the Earth’s history as being approximately the same length. Let’s see how true this is. For this lab, you will need: • Some partners • One or two meter stick(s) • A length of masking tape • A calculator • A pen which will write distinctly (without smearing) on the tape • The geologic time scale handout I want you to memorize (our timescale) • The 1983/1999 geologic time scale handout (GSA timescale) Lay out a piece of masking tape down the length of a table. This will represent 4.6 billion years of Earth's history; therefore, mark one end as "0 million years (the present)" and the other end as "4600 million years (Earth's origin)". (The difficult part) Using a calculator (or by hand), mark all the numerical ages listed on our timescale proportionally on the length of tape. For instance, the boundary between the Cambrian and the Proterozoic is at 545 million years. 545/4600 = .118 = 11.8% of Earth history. Let's say your tape is 6.7 meters long. Then multiply the proportion of Earth history times the length of the tape: 0.118 ¥ 6.7 meters = 0.79 meters, so you would draw a line 0.79 meters (79 centimeters) away from the "0" end of the tape, and label one side "Cambrian" and the other side "Proterozoic". When you are done, answer the following questions: 1. What type of event do you suppose defines these boundaries between times you have just drawn? Hint: If all the species between the boundaries stayed roughly the same, then at the boundaries... 2. Why do you suppose there are different levels of divisions? In other words, how does the answer to question 1 allow for eons, eras, periods and epochs? On your tape timescale, indicate the larger groupings as best as you can (for instance, draw a large bar that connects the Cretaceous, Jurassic and Triassic periods into the Mesozoic era).

3. a. Are there any finer subdivisions of time beyond epochs? (Hint: look at the 1999 Geologic Time Scale) What are they? b. Do the periods on our timescale that don’t seem to be divided into epochs (such as the Cretaceous period) actually have epochs? If so, why didn’t I include them on our timescale? 4. The Quaternary period is recognized to be the time when more or less modern humans (that is, mammals with an upright stance and certain brain volume) were the dominant species on Earth. What percentage of all geologic time does this represent? 5. In comparison, the dinosaurs "ruled" the Earth during the Mesozoic era. What percentage of all geologic time does this represent? How does this value compare to human “dominance”? 6. Some paleontologists at the University of Chicago have proposed that the Milky Way may be the cause of the answer to question 1. Apparently, matter in the Milky Way is distributed unevenly; there are areas of dense gas and dust clouds separated by areas of lesser density. The solar system, in its orbit around the center of the Milky Way, intercepts a dense area of galactic dust on a regular basis. When we encounter such a dense area, larger bodies on the edge of the solar system (such as large comet nuclei) are perturbed and fall toward the Sun, and also the Earth, initiating the answer to question 1. During the Paleozoic era, how often (in millions of years) on average did this occur? 7. What is the average recurrence interval of the comet bombardment in the Mesozoic and Cenozoic eras (ignore the Quaternary period — we’re still in it)? Is the “recurring comet swarm” hypothesis reasonable, numerically?

8. Now look at the "1983 Geologic Time Scale/1999 Geologic Time Scale" handout. The column headings (the four across the top) are the eras. Each era is divided into periods, then further divided into epochs and, finally, subdivided into ages. In sixteen years, list three differences between the two time scales. Why, in fact, do the numbers keep changing? So how did the whole notion of figuring out the age of Earth materials start? Another handout shows the progression of thought, but roughly speaking the notion of age began with wanting to know the sequence of events. This is called “relative dating”, because you are comparing the age of one event recorded in the rocks compared to the age of a different event recorded in some other rocks; you don’t really care what the number of years ago they occurred. Questions 9 through 12 refer to the diagram below, which is a cross-section of the crust in northern Oregon. The letters refer to the events that placed those rocks there; for instance, all the unpatterned rocks (except for V) represent the deposition of sedimentary layers. X and Z represents the intrusion of dikes.

Relative dating methods 9. Order the lettered units from oldest (least recent) to youngest (most recent).

10. The surfaces between formations P and D, and between J and T are unconformities. Name each type. Significantly, what event happened between P and D that did not occur between J and T?

11. Suppose this cross-section is exposed in deep canyon of the Deschutes River and you happen to be lucky enough to be wandering in the area. You find (oh, happy day) two fossils in the locations listed in the table. Complete the table with the aid of the charts and diagrams on the following pages. Fossil What is it? Where

was it found?

Youngest geologic period it could be from?

Oldest geologic period it could be from?

F-1

Top of layer P

F-2

Top of layer J

12. a. Which strata (write the letter, if any) must be completely Mesozoic in age? b. Which strata may be inferred to be partially Mesozoic in age? c. In order to constrain the age ranges of the rocks better, what must be done?

Of course, earth scientists were not satisfied with merely knowing the answer to the “older or younger” question; the number of years ago an event occurred really was important in the calculation of geologic and biologic process rates. Numerical (absolute) dating methods 13. a. Which rocks (write the letters) can be radiometrically dated? b. Which radiometric numerical dating methods (see text, Table 1.2, p. 15) may be appropriate for dating the dikes? Warning: not all of the methods are suitable for the type of rock a dike is. 14. The formula used to determine a numerical age for a particular isotope is:

t = 1l

ln N0

NÊ

Ë Á

ˆ

¯ ˜ , where t is the age of the sample in years, l is a constant related

to the half-life of the radioactive isotope used (l = 0.693/t1/2), N is the number of atoms of that radioactive isotope in the sample now, and N0 is the number of atoms of that radioactive isotope when the sample crystallized (this is generally calculated by measuring the amount of daughter product). For uranium-lead dating, l = 1.54 ¥ 10-10 yr-1. Calculate the numerical age for dikes X and Z and fill in the table below. (Hint: you really do need a scientific calculator for this one)

Rock sample

Number of uranium atoms originally (N0)

Number of uranium atoms in sample (N)

Calculated numerical age in years (t)

Dike X

10000

9667

Dike Z

10000

9940

15. What event do the ages in question 6 represent? 16. Are the numerical ages consistent or inconsistent with your answers to questions 3 and 4? Explain how the numbers support or do not support your fossil information. 17. Suppose another research team finds and radiometrically dates orthoclase crystals in layer Q at 315 my. Is it reasonable to conclude that layer Q is of late Paleozoic age? Short answer: NO! What is the reason for the apparently errant date (assume their lab technique was superb)? 18. If petrified wood is found in layer J, give two reasons why you can't use carbon-14 dating.

Of course, there are other uses of fossils: they can be used, with the proper dating control, to determine the age of the rocks in which they were found. This is called biostratigraphy. The diagram above shows what is called the first appearance/last appearance of various organisms (similar to the “slug diagram” in question 12). The scale to the left shows the age (in millions of years). Then the various timescale divisions are shown, and, finally, the vertical line segments show the age span in which fossils of certain organisms are found. For instance, the critter called Eusthenopteron is only found in rocks of the Frasnian age of the Late epoch of the Devonian period. 19. Fossil assemblages are the fossils one finds in a single stratum (layer) of rock. Would you expect to find Elginerpeton, Acanthostega, and Ichthyostega in a fossil assemblage? Use the diagram above to justify your answer. 20. “Reworking” is the action of tearing up previously-deposited sediments (and their fossils) and re-depositing them. Ocean waves, landslides and river channel migration are good reworking agents. Explain how reworking might be used to explain an apparent fossil assemblage of Elginerpeton, Acanthostega, and Ichthyostega.

Eon Era Period Epoch Boundary age (myr)

Holocene

Quaternary

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Cenozoic

Tertiary

Paleocene

Cretaceous

Jurassic

Mesozoic

Triassic

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Phanerozoic

Paleozoic

Cambrian

Proterozoic

Archean

Precambrian

0.01

1.8

5.3

23.8

33.7

54.8

66

144

206

248

290

323

354

417

443

490

543

2500

3800

4600

Hadean