Transcript Document
CH. 12 GEOLOGIC TIME Rock Records • Rocks record Earth’s history – Geologic events (erosion) – Changing life (extinctions) James Hutton Uniformitarianism: the forces and processes we see today are the same as the processes that have acted on the Earth’s surface in the past “The present is the key to the past” Relative Dating • tells order rocks formed (1st, 2nd, 3rd) relative to each other • Does not tell actual date • Does not tell how long ago RULES for dating rocks law of superposition: in undisturbed rock, the older rocks will be at the bottom and the youngest rocks will be at the top Principle of original horizontality: layers of sediment are deposited in a horizontal position law of cross-cutting relationships: a rock cutting through another rock is younger than the rock it has cut-cross law of included fragments or inclusions: pieces of one rock found in another rock must be older than the rock in which they are found unconformity: Long break in the geologic rock record • deposition stops, erosion occurs, then deposition resumes • 3 types Contorted rock, hard to determine relative age (Copiapo, Chile) Uncomformity • Granite formed • Granite exposed by erosion • Beds 1-3 deposited 1. angular unconformity: during long break, folding or tilting also occurred • boundary between horizontal and tilted layers of rock Angular Unconformity • Beds 1-6 deposited • Beds 1-6 tilted • Erosion • Beds 9,10 deposited 2. disconformity: two layers of rock separated by an eroded surface Disconformity • Beds 1,2 deposited • Erosion • Beds 5, 6, 7 deposited 3. nonconformity: eroded surface separates older metamorphic/intrusive rocks from younger sedimentary rocks Correlation of Rock Layers • matching up rocks of the same age in different places • creates better view of geologic time • Locally – use relative dating • Distant – use Fossils 12.2 Fossils: Evidence of Past Life fossil: remains of prehistoric life • in sedimentary rocks • tells info about past environments • correlates rocks (time) Types of Fossils (1) original remains: the actual, unchanged remains of the organism are preserved Ex: teeth, bones, shells (a) frozen in ice (b) preserved in amber (tree sap that has hardened (c) mummification (2) altered remains: soft parts of the organism have disappeared and been replaced by minerals Ex: petrified wood petrification (3) molds and casts: shells buried and dissolve by water leaving “picture” in rock of shape and surface Ex: animal shell or skeleton (4) trace fossils: indirect evidence of prehistoric life (a) footprints (b) imprint (c) tracks, trails, burrows, borings coprolites: fossilized dung or waste materials • tell about food habits T-Rex Carnivore Coprolite (8 lbs.) gastroliths: fossilized stomach stones used by animals to grind food 2 conditions that favor preservation (1) Rapid burial … protected from environment (2) hard parts … soft will decay index fossil: found exclusively in rock record of a particular geologic age (1) must be easily recognizable (2) must be widespread in occurrence (3) must be limited in time (4) must be found in large numbers 12.3 Dating with Radioactive Decay Radioactivity: when the nuclei of atoms break down b/c unstable Nucleus A B (more stable) B (more stable) half-life: time it takes for half of the atoms to decay into new stable atoms Radiometric Dating: procedure that uses radioactive elements & their half-lives to date rocks Table 1, p. 349 Uranium-238 Thorium-232 Rubidium-87 Potassium-40 Lead-206 Lead-208 Strontium-87 Argon-40 4.5 Billion 14.1 Billion 47 Billion 1.3 Billion ** Is this good for “recent events”??? Carbon dating: Carbon-14 used in radioactive dating for recent events C-14 C-12 • 1/2 life of carbon-14 is 5,730 years • Dates things up to age of 75,000 yrs old • Can only use on once living things, not rocks Geologic time scale - • summary of major events of the Earth’s history • divided by major changes in the Earth’s surface, climate and/or species Eons • • • • • • The greatest expanses of time. Divided into Phanerozoic-youngest Proterozoic Archean Hadean-oldest 45 Eras: 2nd largest segment of time (1) Precambrian (longest) (2) Paleozoic - “ancient life” (3) Mesozoic - “middle life” (4) Cenozoic - “recent life” Eons Eras Divided into Periods Divided into Epochs 48 12 1 11 2 10 9 3 8 4 7 5 6 How long did they last?? Precambrian – Paleozoic – Mesozoic – Cenozoic – How many hours on the clock? How old is Earth? How much is each hour equal to in years? Test Questions? CH 13 pg. 363 1. Which era is the longest? 2. Which era is the shortest? 3. Which era do we live in? 4. Which era did dinosaurs live in?