Geologic Time. How old is the earth? Which best describes you? One extreme view: Pres. Hugh B. Brown, GEOLOGIC TIME

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1 26 GEOLOGIC TIME Geologic Time But first, what is a day? It is a specified time period; it is an eon, a division of eternity; it is the time between two identifiable events. And each day, of whatever length, has the duration needed for its purposes....there is no revealed recitation specifying that each of the six days involved in the Creation was of the same duration.... It seems clear that the six days are one continuing period and that there is no one place where the dividing lines between successive events must of necessity be placed. ~ Bruce R. McConkie, Ensign, June 1982, p The age of the Earth Absolute dating Relative dating Geologic column Which best describes you? 1. I ve taken Exam 3 2. I m going to take it by Thursday evening 3. What Exam? How old is the earth? Many views: 1. Ancient Hindus: 1,977,949,048 yrs 2. Ancient Egyptians (LDS): 2,555,000,000 yrs 3. Archbishop James Usher: 6,000 yrs (Archbishop of Ireland, 1645) the night preceding the twenty-third day of October in the year of the Julian Calendar, 710 (4004 BC) 4. James Hutton (~1800 AD): It has no vestige of a beginning and no prospect of an end. Two extremes: the earth is very young the earth is very old The age of the earth is important to geologists because it has implications for understanding how the earth works. One extreme view: The earth is very young If so, what we observe must be explained by one or more abrupt, cataclysmic events Scientific creationism seems to claim that: 1. All things including matter, life and physical laws were created abruptly out of nothing. 2. The universe is complete and unchanging. 3. Life on earth is unique to the universe. Pres. Hugh B. Brown, 1958 But while very little was written originally on the details of the creation of the world and man s s advent upon it, it should be observed that God is the author of two accounts of creation. One is written in the Bible and amplified by modern revelation and the other was written in the strata of the earth. Each has at times been wrongly interpreted and misunderstood and they sometimes seemed to be contradictory or at variance. If you will remember that these two records have the same divine Author, you will know they cannot be fundamentally opposed, though man s interpretation of either or both may be seriously at fault.

2 Joseph Smith...and that eternity agreeable to the records found in the catacombs of Egypt, has been going on in this system almost two thousand five hundred and fifty-five five millions of years... (From a letter from W.W. Phelps to William Smith, Joseph Smith s brother. Phelps was a scribe who assisted with the translation of the Book of Abraham.) Brigham Young..In these respects we differ from the Christian world, for our religion will not clash with or contradict the facts of science in any particular. You may take geology, for instance, and it is a true science, not that I would say for a moment that all the conclusions and deductions of its professors are true, but its leading principles are; they are facts they are eternal; and to assert that the Lord made this earth out of nothing is preposterous and impossible..whether he made it in six days or in as many millions of years, is and will remain a matter of speculation in the minds of men unless He give revelation on the subject. Current View of Geologists: Some of both and a little of neither Catastrophism (occasionally)... Asteroid Collisions the formation of the Moon mass extinctions on Earth Uniformitarianism (time symmetry) (mostly) The laws that govern geologic processes are the same as they have always been. Geologists are concerned with two kinds of measurements: 1. Absolute time 2. Relative time Normal Clocks are like this Isotopic Clocks are different Estimates of the Absolute Age of the Earth Hourglass Clocks 1. Tree Rings 2. Diffusion of Water into Glass 3. Annual Sediment Layers in Glacial Lakes 4. Annual Layers of Snow and Ice in Glaciers 5. Accumulation of Cosmic Ray Damage

3 Getting a Numeric Age How does Radiometric Dating Work? Need long-lived radioactive isotopes to date Earth events. Radioactive Dating 14 C --> 14? N + e + anti-neutrino (half life of 5730 years) 40 K + e --> 40? Ar + neutrino (half life of 1.3 billion years) Many other radioactive isotopes also used: half-life 235 U (713 million years) fission track 232 Th (13.9 billion years) 238 U (4.5 billion years) Half-life If the half-life of a radioactive sample is 1 billion years and we see that 1/8 of the sample remains, how long has the sample decayed? A. 1 billion years B. 2 billion years C. 3 billion years D. 4 billion years E. 8 billion years Radiometric dating Compare what you start with to what you have left to determine age One little problem.how do you know how much you started with? There are techniques that allow you to do this but only under certain conditions. If you can figure out exactly how much daughter product there is you can determine how much you started with. Requirements for accurate radioactive dating: 1. The rock (sample) must contain enough of the radioactive material to easily and accurately measure the amount. 2. The end product must be trapped in the rock. 3. No end products can be present at the start of the clock 4. No loss or gain of either the isotope or the product since the clock started. Then... the ratio of the original isotope to the product gives the absolute date the rock formed. Only certain igneous rocks meet these criteria

4 What are we dating with these radioactive isotopes? The age of the elements? NO! We are determining the age of events Usually the last time the rock or mineral was heated above its Closure Temperature Below the closure temperature the system is closed and the isotopes can accumulate Above the closure temperature the system is open and the isotopes may escape, resetting the clock Each mineral and isotope system has a different closure temperature Closure Temperatures for some isotopic dating systems System K-Ar K-Ar Fission Track Fission Track Mineral Hornblende Biotite Zircon Apatite Closure T - ºC How old is the Earth? From physical evidence: 1. It is not infinite in age. because radioactive elements with long half-lives lives still exist in the crust. 2. It is younger than the universe-- --less than 14 billion years. 3. It is older than a few million years because radioactive elements of such short lifetimes have all disappeared. 4. It is older than the oldest dated rock-- --older than 3.8 billion years. 5. It is about the same age as the sun, moon meteorites, etc.. They are about 4.6 billion years. 6. Best estimate: ~ 4.6 billion years Geologic Time Scale compressed to one calendar year early Mar late July late Nov pools of H 2 O O 2 in plants & formed atmosphere small animals 1/1 late May Oct 25 12/15 dino s planets 1st forms multicell 11:20 pm12/31 formed of life organism 1st human Principles used to determine Relative time: 1. Original horizontality 2. Superposition 3. Cross-cutting 4. Inclusions 5. Faunal Succession Original Horizontality Rocks are deposited in horizontal layers

5 Relative Time (Determining the relative ages of events in Earth history) 1. Original Horizontality -- Sedimentary rocks were originally deposited in layers that were near horizontal. If they are now tilted, then some event must have occurred after deposition to change their orientation. Superposition Rocks on top are the youngest; oldest on the bottom Horizontal Tilted 2. Superposition -- Sedimentary rock layers are built up one upon another so that the youngest layers are on the top of the pile and the oldest layers are on the bottom. Cross Cutting Rock layers that cut across other layers must be younger than those they cut across 3. Cross-cutting Relationships -- Where one rock body or event (like a fault) cuts across another rock body, we conclude that the youngest rock or event is the one cutting across. The older rock must have been there first so that the younger one could cut through. Hydrocarbon dike cutting sandstone Granite dikes cutting darker rock

6 Fault cutting glacial deposits Inclusions Rock that is included in (surrounded by) other rock must be older than the surrounding rock Salt Lake Temple granite

7 Conglomerate 5. Faunal Succession -- In sedimentary rock layers we find a predictable succession of fossil animal and plant remains. This pattern can be used to determine the relative ages of rock layers. The Geologic Column Cenozoic Mesozoic Second Great Dying First Great Dying more complex plants and animals mammals 66 million years ago birds, mammals, flowering plants dinosaur 245 million years ago Unconformities Unfortunately, the geologic record is not usually complete. Scattered throughout the record, in any given locality, are gaps or places where the record is missing due to erosion or non-deposition. These gaps in the record are called Unconformities. Paleozoic Cambrian Explosion fish and reptiles marine invertebrates 570 million years ago Pre-Cambrian worms and soft bodied creatures primitive life First life ~3.2 billion years ago Age of the earth ~4.6 billion years Angular Unconformity 7 UPLIFT & EROSION OF CURRENT CANYON 6 History of the Grand Canyon Horizontal sediments Vishnu Schist Zoroaster Granite Tilted sediments

8 What is the Sequence of Events? E, G, L, C deposited fault H erosion, unconformity M, D, J deposited A intruded erosion, unconformity N, K, B deposited tilting, erosion, unconformity F deposited erosion of current surface