clocks in rocks: timing the geologic record resources (by... · 2015-04-20 · chapter summary •...

Chapter Summary

• The principles of superposition and crosscutting relationships provide a basisfor establishing the relative age of a sequence of rocks at an outcrop. Usingthese two principles, geologists can order (what happened first, second, third,so on) the geologic events represented by the rocks and geologic features inone outcrop. The principle of original horizontality for sedimentary layers pro-vides a basis for identifying sequences of sedimentary rocks affected by tec-tonic forces after they were deposited.

• To reconstruct the geologic history of the Earth, geologists also need to corre-late the geologic events represented by rocks at one locality with the geologicevents represented by rocks at other localities. The principle of superposition,stratigraphic succession, fossils, and radiometric dates of rock units provide abasis for establishing how rock outcrops at different localities may be relatedto each other, even if they are 100 or 1000 miles apart.

• The Geological Time Scale is the internationally accepted reference for thesequence of events represented by Earth’s rock record. It was constructed inthe last 200 years by geologists using fossils, superposition, and cross-cuttingrelationships to establish the relative ages for thousands of rock outcropsaround the world. In the last fifty years, the Geological Time Scale has beencalibrated using radiometric methods.

• We understand Earth’s history to the degree we can place the record of geo-logic events in time. The Geological Time Scale is the accepted standard forhow geologic time is subdivided.

Learning ObjectivesIn this section we provide a sampling of possible objectives for this chapter. No class couldor should try to accomplish all of these objectives. Choose objectives based on your analysisof your class. Refer to Chapter 1: Learning Objectives—How to Define Your Goals for YourCourse in the Instructional Design section of this manual for thoughts and ideas about how togo about such an analysis.

CHAPTER 8

Clocks in Rocks:Timing the Geologic Record

97

Knowledge

• Understand that the relative age of rocks can be determined from an outcrop,using superposition, cross-cutting relationships, fossil succession, and includ-ed fragments.

• Know how the relative ages for rock outcrops at two or more locations can bedetermined using fossils and stratigraphic relationships.

• Know the major divisions of the Geological Time Scale.

• Understand the importance of the Geological Time Scale.

• Know how radiometric dating works.

Skills/Applications/Attitudes

• Be able to infer age limits for rock units in a formation using the principles ofsuperposition and cross-cutting relationships.

• Understand how the Geological Time Scale is calibrated using radiometric dating.

• Be able to use geochronological information like radiometric dates to recon-struct the geologic history represented by a simple stratigraphic sequence.

• Appreciate some of the challenges of interpreting Earth’s history given thevastness of geologic time.

• Appreciate the degree to which the theory of geological time is validated bycross-disciplinary theories and findings.

• Aware of the immense vastness of geological time and appreciative of the lim-itations the human mind encounters when intuiting such vastness.

General Education Skills

• Write a brief one-page essay titled, “Implications of the Geological Time Scalefor Human Thought.” (writing/critical thinking)

• Write a paper dealing with the implications of the Geological Time Scale formodern society.

Freshman Survival Skills

• The Geological Time Scale offers a great opportunity for students to learn andpractice a variety of memorization strategies such as the ones described in theStudent Study Guide.

Sample Lecture OutlineSample lecture outlines highlight the important topics and concepts covered in the text. Wesuggest that you customize it to your own lecture before handing it out to students. At the end

Chapter 8: Clocks in Rocks—Timing the Geologic RecordRelative vs. radiometric ages

Ordering geologic eventsCalibrating with radiometric dates

98 PART II CHAPTER 8

of each chapter outline consider adding a selection of review questions that represent a range of thinking levels.

The Stratigraphic RecordPrinciple of original horizontalityPrinciple of superpositionStratigraphic successionFossilsFormationUnconformities

disconformitynonconformityangular unconformity

Cross-cutting relationshipsSequence stratigraphy

The Geological Time ScalePhanerozoic Eon = interval of well-displayed lifeEras = stages in the development of lifeEpochs of the Tertiary and Quaternary = percent of modern species of mollusks

Correlation: establishing the time-equivalence of rocks at different locationsSequence stratigraphySeismic stratigraphy

Ordering geologic events is not enough! (the need for calibration and “absolute time”)Calibration of the Geological Time Scale with “absolute time” methodsRadiometric Time: adding dates to the time scale

Parent vs. daughter atomsExponential decayHalf-life/rate of decayDecay schemes

uranium/leadpotassium/argonrubidium/strontiumcarbon-14

Major assumptionsDecay rate is constant and accurately knownRock or mineral has remained a “closed” systemDaughter is solely a product of the radioactive decay of the parent

Time scales of geologic processesSequence StratigraphyChemical StratigraphyPaleomagnetic StratigraphyClocking the climate system with stable and radioactive isotopes

Clocks in Rocks: Timing the Geologic Record 99

Teaching Tips Cooperative/Collaborative Exercises and In-Class ActivitiesRefer to Chapter 4: Cooperative Learning Teaching Strategies in the Instructional Design sec-tion of this manual for general ideas about conducting cooperative learning exercises in yourclassroom.

Coop Exercise 1: Hypothesis Testing, a Sense of Time, and Conversion of Units by Randall Richardson, Department of Geosciences, University of Arizona, Tucson, Arizona

Students gain valuable experience working with large numbers, significant digits, hypothesistesting, and data quality.

I break the class into groups of about fifteen. I have them calculate their ages in seconds,write it down on a piece of paper, and put it in a bag. I add my own age in seconds to the bag.We hypothesize that they can tell when my age is drawn from the bag. We pull one out. It maybe some number like 599,184,000 seconds. I ask them, “When you tell someone how old youare, how many significant digits do you use?” The typical response is something like, 'two'(in this case, the student was 19). We talk about giving your age as 19.4397 (also six signifi-cant digits), and they typically laugh. Then why is the answer given to six significant digits?“Because that's what the calculator said when I took 60*60*24*365*19.” Thus, we learnabout significant digits. Then I ask if the number drawn is mine. Most say it is not. I ask,“How come?” “Because it's too small.” I use this to help them see that they have an expect-ed result in mind when they collect the data. Then we continue pulling numbers until mine ispulled. They are all very happy when they can distinguish mine (it is at least a factor of twobigger in a first year class; if there are older students in the class we sometimes modify thehypothesis). Then we draw the rest, and in most cases there is at least one mistake. Sometimesthe mistake is enough to make it difficult to distinguish the “noisy data” from my age. Wethen talk about the quality of data, and how noise in data can lead to erroneous conclusions.

Coop Exercise 2: Building a Concrete Sense of Geologic Timeby Randall Richardson, Department of Geosciences, University of Arizona, Tucson, Arizona

This is an effective way to help elementary school to college-level students learn about geo-logic time.

One goal is for students to be able to visualize events that happened an incredibly longtime ago, such as the extinction of the dinosaurs sixty-five million years ago, and to realizethat the event occurred within less than two percent of Earth’s entire history.

After having talked about the Geological Time Scale (Precambrian: prior to 570 Ma;Paleozoic: 570–245 Ma; Mesozoic: 245–65 Ma; Cenozoic: 65 Ma–Present), I ask for two vol-


unteers from the class to hold a rope that is fifty feet long. I say that one end is the beginningof the Earth (4.6 billion years ago), and the other is today. I then give out about fourteenclothes pins and ask various students to put a clothes pin on the “time line” at various “geo-logic times.” For example, I ask them to put one where the dinosaurs died out (end of theMesozoic). They almost invariably put it much too old (65 Ma is less than two percent ofEarth history!). Then I ask them to put one on their birthday (now they laugh). Then I askthem to put one where we think hominoids (humans) evolved (~3–4 Ma), and they realize thatwe have not been here very long geologically. Then I ask them to put one at the end of thePrecambrian, where life took off in terms of the numbers of species, etc. They are amazed thatthis only represents less than fifteen percent of Earth history. Finally, I ask them to think ofthe timeline as their own age, and think about how long ago, on a comparable time scale, thedinosaurs died. It is only about two “months” ago! The exercise is very effective at lettingthem get a sense of how long geologic time is, and how “recently” some major geologicevents happened when you consider a time scale that is the age of the Earth.

Geologic Time Rope Table for Faculty Reference

Sample Geologic Time Cards for Students Involved in Geologic Rope Coop Exercise


ecnatsiDemiT oeG %)AM( egAtnevE

#1 Formation of Earth 4600 100.00 50.00 ft

tf 75.4400.980014kcor tsedlO 2#

#3 Earliest fossil record: algae 3500 76.09 38.54 ft

#4 Earliest shelled animals (marine) 600 13.04 6.52 ft

#5 Primitive fishes: First vertebrates 500 10.87 5.43 ft

tf 26.442.9524stnalp dnal tsriF 6#

tf 31.402.8083snaibihpma tsriF 7#

tf 66.233.5542sruasonid tsriF 8#

tf 66.233.5542slammam tsriF 9#

#10 Flowering plants and hardwoods 120 2.61 1.30 ft

#11 Extinction of the dinosaurs 65 1.41 0.71 ft

#12 Abundant placental mammals 62 1.40 0.70 ft

#13 Grazing mammals diversify 35 0.76 0.38 ft

#14 Australopithecus appears 5 0.11 0.02 ft

tf 10.030.06.1nigeb segA ecI 51#

1000.02000.010.0sdne egA ecI tsaL 61#

#1

Formation of the Earth

4600 million years

#2

Oldest Rock on Earth

4000 million years

#3Earliest fossil record:

blue-green algae

3500 million years

#4Earliest animals

with shells

600 million years


#5Primitive fishes:first vertebrates

(animals with backbones)

500 million years

#6First land plants

425 million years

#7First amphibians

380 million years

#8First dinosaurs

245 million years

#9 First mammals

245 million years

#10 Flowering plants &hardwoods appear

120 million years

#11Extinction of the

dinosaurs

65 million years

#12Placental mammalsbecome abundant

62 million years

Coop Exercise 3: Five-Minute WriteThe Five-Minute Write is done during the last five minutes of lecture. Ask students to puttheir names on a sheet of paper and then address the three questions per the overhead; seeadjacent sample. Start the next lecture by discussing the answers to some of the questions stu-dents had about the previous lecture.

Additional Ideas for Coop ExercisesReview Questions 6, 13, 17, 18, 19, and 21 in the Student Study Guide (available in the Under-

Freshman Survival Skills AssignmentThe geological time scale offers a great opportunity for students to learn and practice a vari-ety of memorization strategies such as Practice Exercises 3 and 4, provided in the StudentStudy Guide. First talk to the students a bit about memory strategies you consider useful. Youcan supplement this discussion if you like with the material from the student study and infor-mation provided below. Then assign Exercise 3 or 4 in the study guide as an extra creditassignment. Let the students know you will include some items on the exam that will requirethem to know the geological time scale.

Homework (Exercise 3): Marker Events for Geologic Time(From the Student Study Guide for Understanding Earth, Chapter 8, Practice Exercises andReview Questions, available in the e-Book.)

A. Enter each event listed below on the line in the appropriate eon box in which the eventhappened. When possible, order your listing so that the oldest is at the bottom of the listand the youngest is on top.

B. Fill in the names of eras, periods, and epochs in the correct sequence from oldest at thebottom to youngest on top; refer to both Figures 1.19 and 8.11 to complete this exercise.


#13Grazing animals

diversify

35 million years

#14Australopithecus

appears

5 million years

#15Ice Age begins

1.6 million years

#16Last Ice Age ends

10,000 years

Five-Minute Write

1. What questions do you have aboutthis lecture?

2. What did you findmost interestingabout this lecture?

3. How was this lecture relevant to you?standing Earth e-Book) require students to interpret or evaluate radiometrically dated samples

and can be used in lecture as a basis for a short Think/Pair/Share.

Significant (Marker) Events in Earth HistoryDinosaur extinction event Major phase of continent formation completedEarliest evidence of life Moon formsEnd of heavy bombardment of the Earth Nucleus-bearing cells developEvolutionary Big Bang Oxygen build-up in atmosphereHumans evolve

Notes to Students from Student Study Guide

Strategies to help you learn the Geological Time Scale

1. Marker Events are simply interesting things that happened: animals or plants thatevolved, creatures that dominated the Earth, large extinction events (see Figure 8.11).Select some marker events you already know about. For example: Can you guess oneof the periods when dinosaurs were dominant? The movie Jurassic Park has made thisan easy question. When did complex life begin? Find some other marker events of par-ticular interest to you. Maybe you are surprised at how early some events occurred; e.g.,the “earliest evidence of life.” Marker events will help you remember the GeologicalTime Scale.


Eon Era Period Epoch

Phanerozoic Quaternary Holocene

Humans evolve ____________ Tertiary Pleistocene

________________________ ____________

Mesozoic ____________

Jurassic ____________

____________ ____________

____________ ____________ ____________

Pennsylvanian

____________

____________

____________

Ordovician

________________________ ____________

Proterozoic

________________________

First nucleus-bearing cells develop

Archeon

________________________

________________________

________________________

Hadean

________________________

________________________

Earth accretion begins

2. Logical Chunks. Group the information into short lists you can remember. Study thegroupings of the time scale with Figures 8.11 and 8.15 in front of you. Learn it as a seriesof short lists. Understand the following logic.

• Eons (see Figure 8.15) are the biggest time chunks. There are only four (Hadean,Archean, Proterozoic, Phanerozoic) to remember. Only the most recent(Phanerozoic eon) is broken down further. Hadean sound like Hades (hell), not abad description of the young planet with its molten surface and asteroids crashinginto it.

• Eras (see Figure 8.11) are next biggest. You only have to learn eras for thePhanerozoic eon. That’s because geological evidence is too limited to justify thedivision of earlier eons. There are only Phanerozoic eras to remember: old life,middle life and new life. Think of it that way first; you can tack on the Greekstems later (see 3 below).

• Periods (see Figure 8.11) are next. All three eras of the Phanerozoic eon are fur-ther divided into periods (no periods for earlier eons, not enough evidence).

• Epochs are the smallest divisions of geologic time. You only have to learnepochs for the most recent era (Cenozoic, or new life). All epochs of theCenozoic end in cene, for Cenozoic.

• Now that you understand the divisions return to Figure 8.15 which clarifies howthey all fit together and adds the absolute dates that have been determined byreading the radiometric rock clocks.

3. Word Stems Word stems are clues to meaning. Greek and Latin Stems are used a greatdeal by scientists. You can look them up in any good dictionary. A few helpful stems forthe Geological Time Scale include:

Eras:

Paleo- = Greek: “old”

Meso- = Greek: “middle”

Ceno- = Greek: “new,” plus zoic

-zoic = Greek: “life”

Homework (Exercise 4): Geological Time Scale MnemonicConstruct a mnemonic device for remembering the Geological Time Scale names. The first letterof each word must match the first letter of the corresponding period or epoch in the proper order.You may use your native language, but be careful not to mix up the words when you do so.

Examples (refer to Figures 8.11 and 8.15 for the Geological Time Scale):

Here’s a good mnemonic for the periods of the Geological Time Scale:

Chronically Overworked Student Decks Monotonous Physics Professor To JustifyContradictory Test Questions

Here’s a good mnemonic for the Epochs of the Cenozoic:

Please Eat Our Mushroom Pot Pie Hot

Topics for Class Discussion

• How are the rates of radiometric decay measured?

• Is the Geological Time Scale simply a geological theory? What have other disci-plines contributed to working out the Geological Time Scale? What do you thinkmight be the implications for science in general if somehow the Geological TimeScale were shown to be in error? How would this affect biology? Paleontology?


How might it impact physics? Discuss whether each of the following statementsis true or false. State your reasons for each choice.

1. (T/F) No impact on disciplines beyond geology

2. (T/F) Minor impact on other disciplines

3. (T/F) Might require rethinking of physists’ notion of radiation

4. (T/F) Might require rethinking of physics and chemistry notions concerningthe decay of radioactive elements

5. (T/F) Might require rethinking of theories in paleontology

6. (T/F) Would require a major overhaul of huge areas of scientific endeavor

7. (T/F) If possible, errors would be corrected by further calibration and theGeological Time Scale would be revised.

Provide arguments to support your reasoning.

Note to instructor: This question could also be used as the basis for a crit-ical thinking and writing assignment. The pertinent issue here is the differ-ence between a scientific theory and theory as it is used in everyday dis-course. It is important for students to understand that while all theory isopen to challenge, the likelihood that some theories will be completelyoverturned is very small. In the case of geological time you might need toexplain that while particular geotimescale dates may have to be revised, thebasis of the theory in well-worked out physics and chemistry, and extensiveinvestigations by geologists and paleontologists, makes the possibility ofever returning to the notion Earth is 6,000 years is so remote that few sci-entists would give such an idea serious thought.

• How do geologists determine a radiometric age? • How is the Geological Time Scale calibrated by radiometric dates?

• What “age” is determined when each of the following rock types is dated byradioisotopes? Discuss your answers.

A. Igneous rocksB. Metamorphic rocksC. Sedimentary rocks

Discuss your answers.

• Radioactive and other heavy elements formed by thermonuclear reactions in thecenters of stars and nova before or during the formation of our solar system. TheEarth formed from some of this matter about 4.5 to 5 billion years ago. Given thatthe radioactive elements were all formed at approximately the same time, why arethere rocks of different radiometric ages on the Earth’s crust today?

• How would you determine the accuracy of a radiometric date?

• How would you determine the precision of a radiometric date?

• How might you use tree-rings to determine the accuracy of the carbon-14 dat-ing technique?

• Precision vs. accuracy

• Age of the Earth

Historical approach

Cooling rates

Accumulation of salt in the oceans

Sedimentation rates

Modern approach

Oldest known radiometric date on a rock from Earth is 3.96 billion years


Radiometric age of meteorites average about 4.5 byrs

Radiometric age of lunar rocks

Oldest known lunar sample is 4.6 byrs

Lunar maria basalts is 3.1 to 3.8 byrs

Discussion Exercise: The Geological Time ScaleThe Geological Time Scale is a fundamental thinking tool, a keystone mime of scientific literacy.Geological time makes evolution comprehensible. It makes global climate change comprehensi-ble. It is essential to any deep understanding of the wondrous complexity of biological machineslike the echolocation of bats, the human eye and the human brain. The following discussion exer-cise may help your students to appreciate some of the basic human implications of the GeologicalTime Scale. It could also be used as the basis of a short essay assignment. The third question whichdeals with the interaction of geological time and evolution could be saved for the lecture that dealswith Chapter 11: Geobiology—Life Interacts with Earth. Note that the questions derive from theassumption that probabilities will be interpreted very differently if you live a long (geologicalscale) time than if you live a short (human scale) time.

A Touch of Science FictionHumans have evolved to live very short (geologically speaking) lifespans. This influences howwe think and make judgments. Suppose you were an alien creature from some distant planetwhere the typical lifespan is 4.5 billion years. You are a youthful member of this very long-lived species. As a matter of fact, you just celebrated your 999,978 millionth birthday and com-ing to Earth is your high school senior field trip. You have just arrived on planet Earth and arebeginning to get around and interact with its surprisingly youthful inhabitants. How might youthink and behave differently than humans.

• Would you sky dive? Would you choose to fly in an airplane? If someoneoffered you a ride in an automobile would you accept? Indeed, would you evenbe willing to walk across the street? Why might you need to be much more cau-tious than humans?

Note to instructor: It may be helpful to provide some actual risk probabilitiesfor riding in an airplane, automobile, or being hit while walking across thestreet. Then, ask students to estimate how often a creature might actually per-form an action such as flying, driving, or walking across the street over thecourse of a 4.5 billion year life. Explain further that if the odds of a disaster areonly one in a billion then that disaster will, on average, occur 4.5 times duringour alien creature’s lifespan, certainly often enough to warrant caution.

• How might it be easier for you to intuitively grasp the notion of global climatechange than it is for most humans? (Assume your planet was similar to theEarth in terms of its geological history and tectonic engine)

• Would you express great amazement at the wonders of evolution such as thehuman eye or echolocation in bats? Why might these complex designs ofnature’s evolution be less wondrous, or at least less surprising, to a creature likeyou who has lived for billions of years than they are to most humans.

Note to instructor: The idea is, of course, that 4.5 billion years is plenty of time for com-plexity to evolve once you have an organism with the information storage and processingcapacity of the human gene busily replicating itself. There are a few points you may have todevelop before students will get the idea. Consider a sequence something like the following:

1. The genetic code is digital, like that of a computer, hence, enormously powerful.

2. Passing on the instructions for complexity is well within the processing power ofour genes. The information capacity of a gene is so enormous that the entire NewTestement, if dutifully translated into digital genetic code, could be written intothe DNA of a single microscopic bacterium. Every cell of the human body


contains all of our chromosomes. In other words, each cell contains all of therecipes for creating a human infant and for driving our enormously complicatedmolecular machinery.

3. Any change, no matter how small, that favors evolution will increase an organ-ism’s chances of survival. (This point is important because people sometimesbelieve that the human eye could not have developed all at once. They need to beshown that even a simple photosensitive spot such as one finds in simple organ-isms is adaptive, that the simpler eyes of simpler organisms all work and increasechances of survival.)

4. Thus, the deck is stacked in favor of evolving complex functions such as beingable to see or detect via radar one’s prey and predators.

5. The vastness of geological time provides the final necessary ingredient (plenty oftime for even the normal slowly plodding variety of evolution to produce crea-tures of enormous complexity). Ask students how many organisms our billion-year-old alien might have seen come into being. The following example may helpthem appreciate the scope of our alien’s experience. Phylum Brachiopoda, theLampshells is represented by only 350 living species, but more than 26,000 fos-sil species of Brachiopoda have been described. Since Braciopods came into exis-tence in the Cambrian, only half a billion years ago, our alien, had she been liv-ing on Earth, would have been around for the arrival of all 26,000 species. Whyhave so many Brachiopods disappeared? It may be useful to show a slide ofFigure 8.11 which maps the five major extinction events that have occurred in thelast half billion years. Note that during the late Permian extinction nearly everyspecies of living brachiopod would be expected to have disappeared, andBrachiopods would have had to start radiating all over again.

The point to get to here is that the genetic machinery combined with the vastness of geologicaltime is more than sufficient to produce a virtually unlimited diversity of organisms and complexstructures. Given the Geological Time Scale and the information processing power of the gene, thehuman eye is a development to be expected. It would be no surprise at all to a creature who hadlived a billion years.

Expect students to struggle quite a bit with this question. It is not easy. As a matter of fact it issuch a difficult question that some instructors may prefer not to raise it at all. The exercise is valu-able without it, and it is probably appropriate only if you want your students to speculate deeplyabout the nature of evolution, perhaps in a special section of the course that you teach to honorsstudents, or perhaps in conjunction with Chapter 11.

If you do want to deal with the interaction of geological time with evolution, Richard DalkinsThe Blind Watchmaker is one highly recommended source. Another is the Web site of theClockoftheLongNow at http://www.longnow.org/.

“Given infinite time or infinite possibilities, anything is possible. The large numbers proverbiallyfurnished by astronomy, and the large time spans characteristic of geology combine to turn topsy-turvy our everyday estimates of what is expected and what is miraculous.” Richard Dawkins, TheBlind Watchmaker, p. 139.

Historical Criteria for the Age of the Earth• Pass out in lecture a copy of the latest Geological Time Scale published by the

Geological Society of America.

• Metaphors for geological time

Consider all of Earth’s history as the old measure of the English yard, the dis-tance from the King’s nose to the tip of his outstretched hand. Then, one strokeof a nail file on his middle finger would erase all of human history.

A roll of Scott® toilet paper contains 1000 sheets. Compressing all of Earth’shistory into one roll results in each sheet representing 4,500,000 years.


• Why do rocks on Earth have different radiometric ages?

Most radiometric ages represent the interval of time that has passed since a min-eral crystallized. The “radiometric clock” gets reset whenever minerals are heat-ed or melted, resulting in a loss or redistribution of accumulated daughter atoms.Radiometric dating works because parent and daughter atoms have significantlydifferent physical and/or chemical properties.

Teaching ResourcesStudent Study Guide Highlights (part of the Understanding Earth e-Book)

In Part I, chapters provide strategies for learning geology. Ideally, students would read these chap-ters early in the course.

Chapter 1: Brief Preview of the Student Study Guide for Understanding Earth

Chapter 2: Meet the Authors

Chapter 3: How to Be Successful in Geology

Part II, Chapter 8: Clocks in Rocks—Timing the Geologic Record

Before Lecture:

Time Management Tip

Preview Questions and Brief Answers

Vital Information from Other Chapters

During Lecture: Note-Taking Tips

After Lecture:

Check Your Notes

Intensive Study Session: Strategies for Learning the Geological Time Scale

Exam Prep:

Chapter Summary

Practice Exercises: Determining the Succession of Geologic EventsOrdering Geological EventsMarker Events for the Geological Time ScaleGeological Time Scale Mnemonic

Review questions


egAsisaBstsitneicS

Hemholtz Cooling rates and 10 to 75 myrsand Kelvin contraction of the earth/tides

Walcott Rate of sediment accumulation Approximately 100 myrs

Joly and Rate of salt accumulation 20 mys to 1.5 byrsOthers in oceans

Rutherford Radiometric decay Oldest known rock )9891( sryb 4)5091(

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