Day 11 - GeoTime.pdf

# Day 11 - GeoTime.pdf - Geologictime Geologictimescale...

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Unformatted text preview: Geologic time Absolute Dating Techniques Geologic time scale Relative dating methods Announcements Midterms are being marked ‐ Grades posted asap Quiz #5 now open – Due Feb 13th !!! Learning Goals • Recognize the scope of geologic time over which interior and surficial processes have occurred • Explain how we can date rocks using radioactive isotopes. Where are these isotopes found? • Discuss lines of evidence for the age of the Earth • Define the following terms: isotope, half‐life, parent isotope, daughter isotope, radioactive decay. • Describe how the geologic time scale evolved from a combination of relative and absolute dates • Apply the principles of relative dating to deciphering the geologic history of an area • List the periods of the Phanerozoic in chronological order. Clicker Question?? • Approximately how old is the Earth? • • • • • A) 45 Ga B) 4 Ma C) 400 Ma D) 4.5 Ga E) 10,000 years Age of the Earth Archbishop James Usher (1625) works backwards through scripture: The Earth was created On October 23rd, 4004 B.C. Uniformitarianists (c. 1830): measure rate of present processes to estimate how long it would take to build up the Earth’s crust in its present form. Millions of years Lord Kelvin (1866) calculated the rate of cooling from a molten body the size of the Earth 20‐40 million years old Oldest Material from Earth? ~ 4.4 Ga (Billion Years Old) Detrital Zircon (A Mineral) in Sandstone from Australia Oldest Rock from Earth? ~4.04 Ga (Billion Years Old) eg. Acasta Gniess from Great Bear Lake region, NWT On display in the Pacific Museum of Earth (go find it!) Age of the Earth? = 4.55 billion years old How old will you be after a billion heartbeats? How old will you be when you have reached a billion heartbeats? Assume 1 heartbeat per second Evidence for the Age of the Earth Dates from Meteorites? Dates from the moon? ~4.55 Ga ~4.5 Ga How do we determine ages? 1) Absolute age dating (radiometric dating) 2) Relative age dating (this is older than that) ‐ Steno’s Laws Absolute Age Dating How do we date a mineral? Radiometric Dating Provides numeric ages – specifying the actual number of years that have passed since an event occurred (also known as absolute age dating) We use decay products of radioactive elements in minerals to get at absolute ages. Dating with Radioactivity Isotopes are variants of the same parent atom. Differ in # of Neutrons Parent Isotopes are unstable > decay into daughter products Plummer et al., (2012) Figure 8.20 Dating with Radioactivity Half‐Life: The time required for half of the radioactive nuclei in a sample to decay Together with the parent/daughter ratio, half‐life (rate of decay) is used to calculate the numeric age of a sample Dating with Radioactivity The percentage of radioactive atoms that decay during one half‐life is always the same (50 percent) However, the actual number of atoms that decay continually decreases Everyone stand up! Plummer et al., (2012) Figure 8.22 Variation in Half‐Lives Plummer et al., (2012) Table 8.3 Different isotope systems have different half‐lives Some systems better suited for the study of particular materials or particular age ranges. What would you use to date the worlds oldest rocks? What would you use to date Mesoamerican artifacts? Clicker Question The world's oldest known rock is approximately 4 billion (4,000 million) years old. What are the approximate relative percentages of parent (Uranium‐235) and daughter (Lead‐207) isotopes? The half‐life of 235U = 704 million years. A) 2% parent & 98% daughter B) 12% parent & 88% daughter C)35% parent & 65% daughter D) 0.5% parent & 99.5% daughter Radiometric Dating Sources of error: • A closed system is required • If temperatures get too high, daughter products may be lost and age underestimated • To avoid potential problems only fresh, unweathered rock samples should be used What kind of rocks can be dated with Radiometric Decay? • igneous rocks (time that the magma crystallized) metamorphic rocks (time of metamorphism – not the age of the parent rock!) Activity In groups: If you dated this rock using U‐235 – Pb207 dating and found only it contained 87.5 % daughter products, how old would it be? Geologic Time Scale Formulated over time based on: observations of relative time and measure‐ ments of absolute time How Was the Geologic Time Scale Developed? • • • • Series of observations: Strata (layers) are physically continuous May be correlated over distance Correlation of fossils/fossil assemblages between layers All of the above started the development of geologic time scale These aspects of geology are addressed in the subdiscipline of stratigraphy Observations of Rock History • Contacts are the surfaces between rock types or layers – Parallel contacts are said to be conformable – A type of contact called an unconformity occurs when some amount of time takes place before deposition of the next layer – Unconformities indicate past events without a rock record (a time of erosion or non‐ deposition) Relative Age Dating Clicker Question?? What came first……..the chicken or the egg? A) Chicken B) Egg C) What has this got to do with geology? How Is Relative Time Determined? • • • • Cross‐cutting relationships Inclusions Unconformities Faunal (fossil) succession “The Great Unconformity”, Grand Canyon How Is Relative Time Determined? • Original horizontality • Superposition • Lateral continuity • Cross cutting relationships The “Cold Canyon” area Steno’s Laws Plummer et al., (2012) Figure 8.1 Clicker Question?? Which rock unit is oldest? The “Cold Canyon” area e. d. c. b. a. Steno’s Laws (Principles) In an undeformed sequence of sedimentary rocks, the oldest rocks are at the bottom and youngest are at the top. Also applies to lava flows and ash beds (1) Principle of Superposition In an undeformed sequence of sedimentary rocks, the oldest rocks are at the bottom and youngest are at the top. Also applies to lava flows and ash beds (2) Principle of Lateral Continuity Layers of sediment extend laterally in all directions, they are continuous. Sediment layers themselves are not infinitely continuous, but often grade into other sediments indicating lateral changes in deposition environment By Woudloper - Own work, Public Domain, Deeper water Lower energy Finer grain sizes Shallower water Higher energy Coarser grain sizes (3) Principle of Original Horizontality Layers of sediment are generally deposited in a horizontal position. If layers are found otherwise, deformation must have occurred following deposition. (4) Principle of Cross‐Cutting Relationships If a rock unit (or fault) cuts other layers/units, the rock unit that cuts must be younger, and the layers that are cut must be older. Relative Dating Exercise: List the units in order from oldest to youngest b. gneiss a. Quartz vein b. gneiss West Vancouver, BC Unconformities Surfaces that separate younger rocks from much older rocks The hiatus in the rock record is the result of erosion (most of the time) or non‐deposition Disconformity Angular unconformity Relative Dating Exercise: 1. List the order of sedimentary formations (oldest to youngest). 2. What event(s) occurred during this time? Ant Fm. Cody, Wyoming (5) Principle of Faunal Succession Organisms have evolved through time and certain time periods can be recognized based on their fossil content. What is the sequence of events?? Using Relative Dating Principles to Determine the Geologic History of Cold Canyon Area 1. Oldest event 2. 3. 3. 4. 5. 6. 7. 8. 11. 9. 10. Cold Canyon Area Today Note that absolute ages have been determined for the igneous rocks Cold Canyon Area Today Q. What is the possible age of the Thompson River Fm.? A. Between _____ m.y. and _____ m.y. More practice?? Worksheet in CONNECT folder Optional to do, but will increase your understanding of relative time – may come in handy for future exams Worksheet will be posted after class Geologic Time Scale Formulated over time based on: observations of relative time and measure‐ ments of absolute time Geologic Time Scale Divisions of Geologic Time: Eons Eras Periods Epochs * You are expected to know the relative order of periods and the absolute age ranges of the Eras Geologic Time Scale Cambrian explosion (emergence of life forms with skeletons) Oldest fossils Oldest known rocks Age of Earth 4.55 Ga. Geologic Time Scale ‐ Homework Post on Discussion Board in Canvas – in todays lecture module Come up with a mnemonic to help you remember the order of the geological periods Don’t cheat and use someone else's, make one that’s memorable for you! EXAMPLE: Chris Ordered Some Delicious Meat Pepperoni Pizza That Just Cost Twenty Quarters Cambrian Ordovician Silurian Devonian Mississippian Pennsylvanian Permian Triassic Jurassic Cretaceous Tertiary Quaternary **NEXT CLASS: Bring an internet enabled device that can run Visible Geology** Geologic Time – another perspective Years B.P. Event Day of Date and Time Year 4.6 b.y. Earth formed 0 Jan 1, Midnight 3.6 b.y. Oldest fossils 79.3 Mar 21, 8:21 pm 242 m.y. First dinosaurs 245.8 Sept 12, 7:09 pm 66 m.y. Dinosaurs are toast 359.8 Dec 26, 6:19 pm 2 m.y. First humans 364.8 Dec 31, 8:11pm 10,000 years Ice Age ends 364.9 Dec 31, 11:59pm Geologic Time – another perspective Video Recommendation “Geologic Time” – Earth Revealed series Older video, but good information Terms to Get Comfortable With Parent Isotope Daughter Isotope Stable Isotope Radioactive decay Half‐Life Eon Era Period Epoch Next Lecture: Deformation, Structures and Mountain Building **BRING AN INTERNET ENABLED DEVICE** we will play around with ...
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