Transcript Slide 1
CHAPTER 12.1 Discovering Earth’s History Uniformitarianism = forces and processes seen today have been at work for a very long time. Relative Dating = tells the sequence of events through layers of sedimentary rock. Law of Superposition = the layer on the bottom is oldest OLD OLDER OLDEST Law of Horizontality = layers of sediment are deposited in a horizontal position. Principle of Cross-Cutting Relationships = when a fault cuts through rock layers or when magma intrudes other rocks, we can assume that the fault or the magma are younger than the rocks they affected. Inclusions = pieces of one rock unit that are contained in another Unconformity = time when deposition stopped, erosion removed rock, and then deposition resumed. Angular unconformity = unconformity involving folding or tilting of rock during the time when deposition was halted. Disconformity= unconformity involving flat erosion during the time when deposition was halted. Correlation of Strata = using multiple locations to piece together the history of a large region. Pg 341 Figure 8 Chapter 12.2 Fossils: Evidence of Past Life Fossil Formation: Unaltered Remains = remains that have not been changed over time. Ex. Teeth, bones, shells, frozen mammoth. Altered Remains = Remains that have been changed over time. Ex. Petrified wood, molds, casts, carbonized leaves and delicate animals. Trace Fossils = indirect evidence left by prehistoric life. Examples: Tracks, burrows, dung, stomach contents. Two Conditions Favoring Preservation: 1. Rapid burial 2. Possession of hard parts (shells, bones, etc.) Principle of Fossil Succession = fossil organisms succeed one another in a definite and determinable order so that any time period can be recognized by its fossil content. Index Fossil • widespread geographically • limited to a short geologic time span • occur in large numbers Using Fossils to Interpret Ancient Environments If you found… LAB: NC Fossil Hunt You might identify the ancient environment as… Chapter 12.3 Dating with Radioactivity Radioactive Decay: • Alpha Emission = element looses neutrons at a regular rate. • Beta Emission = element looses beta particles at a regular rate. • Electron Capture = an electron falls into then nucleus from the innermost Half-Life = the amount orbit. of time necessary for Animated Modes of Radioactive Decay: one half of the nuclei in http://www.wwnorton.com/college/Chemistry/gilbert/tuto rials/ch2.htm a sample to decay to its stable isotope. Tutorial on Half-Life: http://www.colorado.edu/physics/200 0/isotopes/lifetime.html Radiometric Dating = procedure using rates of radioactive decay to determine the age of rocks and minerals. Example: As uranium decays it turns into lead. This decay from uranium into lead is done at a measurable rate. Carbon-14 Dating • Carbon-14 is the radioactive isotope of Carbon. • Normally Carbon-14 is lost into the atmosphere. • We all have Carbon-14 in our bodies. • As long as you are alive, you continue to replace the Carbon-14 and the balance between Carbon-14 and Carbon stays equal. • When you die you immediately stop replacing the Carbon-14 and the balance changes. • The difference in balance between Carbon-14 and Carbon tells you how hold the item is or how long it has been dead. Carbon-14 Uranium-238 • half-life of 5730 years • half-life of 4.5 billion years • up to 75,000 years ago • extremely old samples • used by anthropologists (study human history) • used by historical zoologists (study ancient fossils) • used by geologists studying recent Earth history • used by geologists studying ancient Earth history Chapter 12.4 The Geologic Time Scale 4.56 BYA EONS – ERAS – PERIODS – EPOCH – MYA Phanerozoic – Cenozoic – Quaternary – Holocene – 0.01 Problems with the Geologic Time Scale? Sedimentary rock is made of mixtures of older rocks that have weathered and settled there. Flash: Geologic Time Scale…30:00 Flash: Boneheads…(Jurassic Park?) The age of metamorphic rock may just indicate when it metamorphosed – not when it formed.