Earth Sci wk 8 – Chap 8 – Geologic TimeI. Historical geologyII. Relative DatingSome key “laws” to follow…What is an unconformity?III. Correlation of rock layersIV. Radioactivity and Radiometric DatingIf we know the half-life of an isotope, and measure the parent/daughter ratio, we can calculate the age of the sample.V. Radiometric Dating in GeologyVI. Geologic Time Scale Fig 8.15VII. Difficulties in Dating Geologic Time ScaleEarth Sci wk 8 – Chap 8 – Geologic TimeIntro – lead off with a quote from John Wesley Powell in 1869 in Grand Canyon –“canyons of the region would be a Book of Revelations in the rock-leaved Bible of Geology”- evidence for an ancient earth contained in its rocks..but lots of pages are missing- Interpreting Earth history requires detective work – study clues in rocks- geol events- changing life forms through timeEarth history requires a calendar, and this is a big contribution of geology to overall human knowledge/understanding – the geologic time scale.I. Historical geology- Thinking up to the late 1700s, “catastrophism” was main doctrineThis doctine relies on short-lived catastrophes to explain mts, canyons, features that we now believe took millions of years to form, not aday or twoCatastrophes also assumed to be onetime events, and not operating in the presentDoctrine fit into the time frame that was interpreted from the Bible by people like Bishop Ussher, respected scholar and Archbishop of England- Ussher published a work in mid-1600s that constructed a time scalegoing back to 4004 BC, widely accepted by religious leaders and scientists too- In late 1700s, James Hutton published Theory of the Earth“Uniformitarianism” was offered as an alternative view of Earth history- defined essentially as “physical, chemical, and biological processesat work today are same as those in the past” , and these processes have been at work over a long time, millions of years - commonly stated as “the present is the key to the past”Hutton argued that apparently weak, slow acting processes, given enoughtime, can produce effects like those from catastrophesEarth Sci wk8 p.2- Sir Charles Lyell – Carried on work of Hutton and theory of uniformitarianism - published Principles of Geology in 1830- subtitle says it all “Being an attempt to explain the former changes of the Earth’s surface, by reference to causes now in operation”- did better job at publicizing and explaining than did HuttonWe’ve modified our view over time, but still believe very strongly that “present is the key to the past”To believe in uniformitarianism, you need to accept that the Earth is OLD…And processes are slow…For example, fossils deposited in seabed 15,000,000 yrs ago but now uplifted to 3,000 m or 10,000 ft above sea level …works out to only 0.2 mm/year average uplift ratePoint is that it takes millions of years to build mts according to uniformitarianism..doesn’t happen overnightWe believe that things change, but not on a human scale of time…takes thousands or millions of years.II. Relative Dating - Up until recently, no means were available for absolute dating, so relative dating was all we had…placing timing of rock formation and other events in their relative position to each other (what’s older, what’syounger)Some key “laws” to follow…1. Law of Superposition (Nicholas Steno, 1669):In an undisturbed rock sequence of sedimentary and/or other surface-deposited rocks (like volcanics), oldest bed is on the bottom, youngest on top Fig 8.32. Principle of Horizontality:Most beds or layers of sediment (deposited in air or water) are deposited in an essentially horizontal attitude Fig 8.4a If beds are inclined, they have been disturbed from their original position through faulting or folding Fig 8.4bEarth Sci wk 8 p.33. Principle of cross-cutting relations: Fig 8.5Faults or intrusive magma bodies or unconformities, any feature which cuts across another , has to be Younger than the feature it cross-cutsWhat is an unconformity?It is a surface that separates rocks, sometimes wavy, sometimes planar. It records a sequence of erosion and then re-deposition of sediment- Types of unconformities:a. Angular unconformity Fig 8.7Rx underneath unconf are at some angle to the overlying bedsb. Nonconformity (“not like”) Fig 8.6Rx underneath unconf are different than those above it..usu the rx underneath are igneous or m-m c. Disconformity (little diff)Rx on either side of unconf are essen. parallel, rocks are often similar on either sideDisconf can be difficult to detect unless erosion surface cuts deeply into older rx- Example of using relative dating principles Fig 8.8Be able to figure out a diagram like this..what are the steps in the formation of this sequence??III. Correlation of rock layers- Correlation means match up or connect rock layers in terms of relative age…what is the same age layer from place to place? Fig 8.9 When well-exposed in a small area, you can connect them visually by walking along contacts, etc. Connecting across long distance (even across ocean) requires fossils.- Fossils are important to correlation because they are time indicators…same fossil lived at same time, regardless of position on Earth.Earth Sci wk 8 p.4- William Smith, canal builder in late 1700s, determined that beds containing same fossil assemblage could be traced laterally (horizontally), but beds above and below contained DIFFERENT fossil assemblages Fig 8.11Very important principle developed out of these observations:- Principle of Fossil Succession – fossil organisms succeed one another in a definite order, therefore any relative time period can be recognizedby its fossil content (Age of …Invertebrates, Fishes, etc) Fig 8.15The fossil succession shows a progression of life change through time, which we believe represents the evolution of life on EarthThe SAME succession of organisms is found, never out of order unless rx are disturbed, EVERYWHERE on EarthIV. Radioactivity and Radiometric Dating- Now possible to get absolute dates on ages by using the principleof radioactive decay.- Refresher on atomic terminology:# protons = atomic number# protons + # neutrons = mass numberIsotopes all have same # protons, but diff # neutronsExample Uranium isotopes = U-234, U-235, U-238Spontaneous breakapart of nucleus known as radioactive decaySeveral types of decay known: Fig 8.121. alpha emission (2 pro + 2 neu lost) – so mass number 4 less &