Transcript chapter 21
Diff Geo Unit 2 Chapters 21, 22, 23, and 24 CHAPTER 21 Geologic Time • Geologists use the rock record to see what happened during earth’s past • Time is divided up into units starting at 4.6 billion years ago to today • The bottom of the geologic time scale is the oldest • Eons, eras, periods, epochs • Eons- longest • Epochs- shortest Eras we need to know • • • • • Precambrian (oldest and longest) Paleozoic Mesozoic Cenozoic Don’t worry about the periods or epochs • Throughout these time periods paleontologists see an evolution of body structures & adaptations to changes in environment. Relative Dating • Relative dating allows scientists to have an idea of how old rocks are, in comparison to others • They use principles and laws to guide them Principle of Uniformatarianism • Processes that are taking place today in the natural environment were also happening long ago • Erosion • Earthquakes • Volcano eruptions • Etc. Law of original horizontality • Sedimentary rocks left undisturbed will be deposited in layers Law of superposition • In an undisturbed sequence of rock, the oldest layers will be at the bottom and younger rocks as you travel upward toward the surface Law of cross-cutting relationships • An intrusion or fault must be younger than the rock it cuts through • The cheese has to be there before you can cut it Unconformities • A break in the rock record • Disconformities- sed over sed rock (da same kind of rock) • Nonconformity- sed over non-sed rock (not da same kind of rock) • Angular unconformity- layers are at an angle and have been eroded Correlation • Using similar rock outcrops from different areas means they are similar ages • Usually contain index fossils • Organisms that lived for a small time over a wide geographic range Absolute dating • Radioactive dating • Using radioactive isotopes whose half lives are known to calculate the exact age of the rock • Half life- time it takes for half the element to decay • Organic less than 50,000 yrs uses Carbon-14 • Inorganic uses Uranium and other elements Radioactive Decay Problem • A rock contains 75% of lead-206 for every g of uranium-238. The half-life decay for uranium to turn into lead is 45,000 yrs. How old is the rock? • Hint: Start by figuring out the amount of Uranium remaining. • Dendrochronology- counting annual growth rings in trees to get exact age • Seasonal changes causes an alternation in sediment deposits called varves. Types of fossils • Original preservation (insects in amber) • All the soft and hard parts remain • Altered hard parts (petrified wood) • Minerals replace hard parts • Soft parts are gone • Molds • Outline of what once lived • Casts • Filled in mold • 3D model CHAPTER 22 PAGES 577-588 ONLY Precambrian era • Zircon found from meteorites that impacted early earth are used to date the planet (4.6by) • It is a radioactive element Earth’s heat sources • 1. Radioactive decay in the core • 2. Previous impact from asteroids and meteorites so size and temp of Earth increased • 3. Gravitational contraction • • • • Earth’s layers differentiated based on density Densest went to the core (iron) Lightest to the crust (granite) Formed the plates, which have moved throughout time due to convection currents in the mantle The Precambrian atmosphere • Initially lots of hydrogen and helium • With continual out gassing of volcanoes increased water vapor, carbon dioxide, and nitrogen emitted • Cooling of air caused the water vapor to condense, form clouds, and precipitate • Meteorite impacts • All this filled the oceans • Oxygen was emitted by uni-cellular cyanobacteria that underwent photosynthesis to create their own food • Fossilized mats of cyanobacteria called stromatolites are evidence • Iron-banded formations containing iron oxide called red beds also are evidence Later in the Precambrian • During the Proterozoic • Unicellular (prokaryotes) began to engulf each other • Gave rise to multi-cellular (eukaryotes) • Previously soft bodied organisms • Difficult to preserve • Eventually appearance of Organisms with hard parts CHAPTER 23 Paleozoic Era • Continents moving from the original land masses to different arrangements • Changes in sea level • Evidence of this shown in transgressions and regressions in the rock record • Produced most likely by glaciers melting and refreezing • Transgression • Sea level rises • Shore moves inland • Regression • Sea level falls • Shore moves out seaward • Early Paleozoic life found in Burgess Shale • Small, wild looking sea creatures • Middle Paleozoic saw the rise of vascular plants which allowed nutrients to flow through stems and leaves • Between the middle and late Paleozoic, there was a mass extinction of marine life due to global cooling • • • • Mountain building occurred Large plates combined in different ways Coal deposits formed in swampy areas Coral reefs formed • • • • • • In Late Paleozoic Sharks and ray finned fish Lobe finned fish like lungfish Ferns and seed bearing plants on land Amphibians in coal swamps Reptiles evolved amniotic egg which allowed them to live farther from water • Mass extinction due to loss of seas CHAPTER 24 Mesozoic Era • Pangaea broke up and formed the Atlantic Ocean as well as the Mid-Atlantic Ridge • Mountains formed in North America • North America developed arid climates • Universal warmer climate • Mesozoic known as the age of reptiles (ecotherms- cold blooded) • Also first small mammals, birds, and flowering plants • Phytoplankton in the oceans creates oxygen during photosynthesis • Ammonites are index fossils of this time • Angiosperms are seed bearing plants with flowers • Small mammals were endotherms (warm blooded) • Lots of large dinosaurs • Even flying dinosaurs! • Saurischia • Lizard hipped dinosaurs • Plant eaters and meat eaters • Brontosaurus • Tyrannosaurus Rex • • • • • Ornithischia Bird hipped dinosaurs Plant eaters Stegosaurs Ankylosaurs • Mass extinction at the end of the Mesozoic Era • We think it was a huge meteorite that crashed into the (now) Gulf of Mexico • Evidence • Iridium found in the layer of clay that was deposited around this time period • Radiometrically dated to 65 million years ago Cenozoic Era • Climate change caused by position of the continents • Change in ocean circulation caused ice ages • Mountain building occurred • • • • • • Cenozoic era is the age of mammals Small and later larger mammals Saber tooth cats Wooly mammoths Primates Humans • Hominoid • Larger group including the hominids • Includes great apes • • • • Hominid Bipedal Walk upright Homo sapiens