Ch 23.6: Geologic Time & Earth’s Evolution Piecing together Earth’s history Relative Dating Using laws of superposition, cross cutting, etc. to determine a sequence of events (this happened, then that) Index Fossils – constrain (pin down) a time range when a rock layer was deposited. Fossils of extinct organisms, that were wide spread, and lived during a specific period of time. Example: Trilobites
Relative Dating: Index Fossils
Ch 23.6: Geologic Time & Earth’s Evolution Piecing together Earth’s history Absolute Dating Radioactive elements in rocks, like uranium, potassium, carbon, breakdown at a known rate. If you know how much was in the initial sample … you can determine age. (Isotopic dating) How Earth was Made, Part 1: http://www.youtube.com/watch?v=-IVBfhbhU-c&feature=related
Ch 23.6: Geologic Time & Earth’s Evolution ERAS & PERIODS Why all the divisions? To manage Earth’s history, geologists have divided Earth’s history into EONS. Eons are divided into ERAS. Eras are divided into PERIODS. Periods are divided into EPOCHS. You need to know the major developments in terms of life forms, tectonics, atmosphere and climate for each ERA.
ERAS Cenozoic Mesozoic Paleozoic All time before that = Precambrian ERAS are NOT EQUAL in TIME. The Precambrian was the longest ERA. It took Earth a LONG time to be suitable for complex life.
CH 23.6 Geologic Time and Earth’s Evolution Complex multicellular life is a “recent” development.
Precambrian http://www.pbs.org/wgbh/nova/link/hist_01.html Fossils from this time are not abundant in the rock record because most organisms are “soft bodied” – no skeletons or shells.
Precambrian: 4.5 billion – 540 ma Earth starts off as MOLTEN and VOLCANIC Atmosphere = CO2, CH4(methane), H2O= Green house gases Oceans form and first continents form when Earth cools Origin of water = volcanic steam & meteorites (comets) 1st life = bacteria (single celled) 3.5 bya - use chemosynthesis (not sunlight) for energy. They can withstand extreme conditions. Cyanobacteria (blue-green algae) ~ 3 bya – take in CO2 and release O2 (photosynth) PLANET CHANGING! Temps. Decrease (take in CO2) Oxygen makes O3 = OZONE LAYER Shields surface from UV radiation (UV kills cells) O2 atmosphere– more complex organisms to develop! Simple multicellular organisms in shallow seas 1st Supercontinent (Rhodinia) MASS EXTINCTION = SNOW BALL EARTH ICE AGE! Closes PRECAMBRIAN
Paleozoic Era
Paleozoic Era: 540 – 245 mya MASS EXTINCTION = “GREAT DYING” CAMBRIAN EXPLOSION starts off ERA – “explosion” in the diversity of life forms Snowball Earth is thawed by millions of years of volcanic activity. Warm temperatures LIFE IN SHALLOW SEAS ½ through, life “invades” land (amphibians, land plants, insects) (Thank you Ozone) Early GIANT FORESTS/Swamps become our fossil fuels (Buried, but not decomposed plant and animal matter fossil fuels) Pangaea assembles Interior of continent becomes DRY and COOL(favors reptiles etc) Ends with… MASS EXTINCTION = “GREAT DYING” Volcanic activity? Lava flows in Siberia for millions of years change climate? Asteroid impact? Combination?
Mesozoic
Mesozoic Era: 245 - 65 mya AGE of DINOs WARM and subtropical environment Virtually ice-free Pangaea rifts apart 1st Flowering plants; Giant insects; birds Mammals appear (small) Ends with… MASS EXTINCTION = ASTEROID IMPACT = K-T EVENT (+ Lava flows in India) Meteoroid impact in the Gulf of Mexico
Cenozoic
Cenozoic Era: 65 mya - present AGE of Mammals ICE AGES – Glaciers advance and retreat Ocean currents change due to new continent positions; Mt. Building changes air circulation Hominids (human ancestors) – 2 mya; Modern humans – 200,000 yrs Global Mountain building - affect air circulation drier & cooler conditions Cooler & Drier Grasslands (not tropical forests) When glaciers grow … Sea level drops More land is exposed (land bridges) Ends with… ?????
Study Guide Know what units geologists use to break up Earth’s history (Eons, eras, periods, epochs) List eras in order w/ dates Major events of each era Cause & effect relationships (pink sheet questions)