GEOL 101 1nd Edition Lecture 17 Outline of Last Lecture I. Nonfoliated TextureII. Metamorphic EnvironmentOutline of Current Lecture I. Principles of DatingII. UnconformitiesIII. FossilsCurrent LecturePrinciples of Dating-Relative age dates—placing rocks and events in their proper formation- Before the discovery of radioactive elements, geologists in the late 1700s through the 1900s were able to construct the geologic time scale by rock characteristics.-Absolute age dates—specifying the actual number of years that have passed since the event occurred- After the discovery of radioactivity, geologists were able to assign number ages to the geologic time scale.-Law of superposition: in a sequence of sedimentary rock layers, the oldest rocks are on the bottom, and they get progressively younger as you go towards the top.-Principle of fossil succession: fossil organisms that are preserved in the rocks succeed one another in a definite and determinable order, so that any time period can be recognized by its fossil content.Unconformities-Principle of original horizontality- Layers of sediment are generally deposited in a horizontal position- Rock layers that are flat have not been disturbed-Principle of cross-cutting relationshipsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Younger features cut across older features- These include features such as igneous intrusions (large or small) and faults-Inclusions- An inclusion is a piece of rock that is enclosed within another rock- Rock containing the inclusion is younger-Unconformity- An unconformity is a break in the rock record produced by exposure and erosion and/or no deposition of rock units-Three types of unconformities:- Disconformity—An erosional surface separating two units of layered sedimentary rocks (sandstone over sandstone)- Noncomformity—An erosional surface separating igneous or metamorphic rocks below and sedimentary rocks above (sandstone/granite)- Angular unconformity—An erosional surface separating tilted or folded sedimentary rocks below and flat-lying sedimentary rocks above (shale/sandstone) Rocks are deposited horizontally Rocks are folded or tilted Folded rocks are eroded New rocks are deposited horizontally on top of folded rocksFossils-Fossil—remains or traces of prehistoric life-Types of fossils:- The remains of relatively recent organisms (teeth, bones, shells, etc.)- Entire animals, flesh included may be preserved, but are very rareo Frozen mammoth in Alaska and Siberiao Even human fossil (Otzi)- Given enough time, remains may be petrified (literally “turned into stone”)o Original bone, sell, teeth, wood that gets replaced w/minerals from the surrounding rock (usually sliced)*In 1991 a 5300 year old corpse was found in a melting glacier in the Tyrolean Alps, along with abronze ax and other artifacts. The corpse and artifacts are beautifully preserved, providing insight into the European Bronze Age culture. As they learned more about this person that was, he has been given the name Ötzi after the specific locality where he was found. (Watch this video) http://www.youtube.com/watch?v=WA3AiNup7fY-Molds and cast: the shape of the organism is preserved in the rock-Carbonization: thin residue of carbon is left behind-Tracks and Trails: animal footprints preserved in soft sediments-Burrows and resting places: traces of animal activity in soft sediments-Coprolites (fossil poo)- Gives is information about food habits or organisms-Gastroliths - Dinosaurs ingested rocks to help digest food- The dinosaurs’ stomachs essentially acted as rock tumblers, rounding and polishing the rocks- The gastroliths were either passed out of the animal’s system, or were left behind after the animal died and decayed-Conditions favoring preservation:- Rapid Burialo Scavengers and decomposers cannot destroy the remains- Possession of hard partso Shells, bones and teeth are more likely to be preserved than fleshy material-Special and uncommon cases for preservation- Amber (fossilized tree resin) – insects, spiders, and even lizards- Tarpits– Mammoths, sloths, saber-toothed cats, dire-wolfs- Ice– Wooly mammoths, mastodons, Ötzi-Recall the Principle of Fossil Succession:- Fossil organisms succeed each other in a definite and determinable order- Any period can be recognized by its fossil content-Certain fossils are much better indicators than others- Some fossils span large time periods, while others span very short time periods- Some fossils are localized in small geographic areas while others are found worldwide- The most useful fossils show time periods and are geographically widespread- These are called index fossils -Examples of index fossils:- Inoceramus– bivalve (clam) genus found world-wide that flourished in the cretaceous (modern bivalves still exist)- Mucrospirifen—brachiopod genus that flourished world-wide in the Devonian (modern brachiopods still exist, though rare)- Paradoxides– trilobite genus found world wide, and flourished in the middle Cambrian (extinct)-Correlation of rock layers- Index fossils can be used to match rocks of similar ages across different geographic regions.- This process has been used to correlate rock units across continents, supporting the theory of plate tectonics-Absolute Age Dating- Establishing numerical ages for the geologic past- Dates obtained using radioactive isotopes- Review of basic atomic structureo Atomic structure Protons – Positive charged Neutrons – Neutral charged Electrons – Negative chargedo Atomic number An element’s identifying number Equal to the number of protons in the atom’s nucleuso Mass number Sum of the number of protons and neutrons in an atom’s nucleuso Isotope Variant of an atom Differs in number of neutrons Results in a different mass number than the parent atom Parent isotope – unstable radioactive isotope Daughter product – isotopes resulting from decay of a