Fossils. Ch. 29 and 30 Overview

Transcription

1 Ch. 29 and 30 Overview What you need to know: Chapter 29: Fossils, Relative Time, Absolute Time Chapter 30: Geologic Time Fossils Study of fossils called paleontology Original remains rare usually dead creatures decay. An example would be a mammoth carcass buried in ice. Replaced remains original material slowly replaced over time by other minerals Molds and casts see animation Trace fossils holes or imprints left behind (examples are footprints or worm burrows) Relative Time (p of your text) Relative Time: dating rocks relative to other rocks (we ll talk about absolute time later) Disconformity: original rocks eroded (still flat), then covered by new rocks Relative Time (p of your text) Unconformity: rocks are missing from the sequence like missing pages from a book Angular unconformity: original rocks tilted, eroded, then covered by new rocks Nonconformity: original rocks intruded by igneous rocks

2 Absolute Time Absolute time: the actual dates of events (or at least to an approximation) Tree Rings: Can count back in time using tree rings Varves: Layers of sediment left in bodies of water more sediment in summer, less in winter makes it apparent where new years start Radioactive dating some atoms are unstable and decay at steady rates. Geologists use this rate to tell when the rock formed. Radioactive Dating Radiocarbon dating is useful only for dating formerly living things While an organism is alive it takes in carbon -- most of it as carbon-2 but a small % is carbon-4 (a radioactive form of carbon). Carbon-4 is formed in the atmosphere by the following process (helped along by cosmic rays) As soon as the organism dies, it stops taking in carbon. The amount of carbon 2 stays the same over time, but the amount of carbon 4 decreases as it radioactively decays. 7 N + 0n C + p Radioactive Dating Carbon-4 has a half life (amount of time for half of a sample to decay) of 575 years. It decays by the following process: C 7N + -e Geologists and archeologists can use the ratio of carbon-2 to carbon-4 to determine how old an object is (less C-4 = older) For older rocks/fossils, geologists can use Uranium/Lead, Rubidium/Strontium, or Potassium/Argon dating methods. Radiometric Dating Continued Other Methods of Radioactive Dating (modified from Minnesota Geological Survey Geologic Time Parent Isotope Uranium Potassium-40 Rubidium-87 Carbon-4 Daughter Isotope Lead Argon-40 Calcium-40 Strontium-87 Nitrogen Half Life (years) 70 million to 4 (depends on Uranium isotope) Effective Range (years) 0million , million ,000 Materials used for dating Zircon, uraninite, and pitchblende Muscovite, biotite, hornblende, igneous rock Muscovite, biotite, feldspar, met./igneous rock Organic material 2

3 The Geologic Time Scale From: See also p of your text Highlights: Precambrian: first life doesn t appear until Proterozoic (simple marine organisms like algae) Cambrian: first vertebrates (fish) Ordovician: Minnesota under sea trilobites, corals, crinoids, and more Silurian: first land plants Pennsylvanian: first reptiles Mesozoic: age of dinosaurs Triassic: first conifers and cycads Jurassic: first birds, first mammals Cretaceous: first flowering plants, deciduous trees. End of cretaceous marks extinction of dinosaurs Cenozoic: age of mammals Pliocene: hominids appear Ordovician Minnesota Fossils See upcoming slides for modern day (Following images from Wikipedia.com unless otherwise noted) Trilobites: Extinct, possibly related to crabs, lobsters, etc. Anthozoans: horn coral, anemones Crinoids: sea-lillies 3

4 Brachiopods: lamp shell, rare today. 99% of the species are extinct. Look like cockles, but are not directly related. Brachiopods Cephalopods: includes cuttlefish, squid, octopus, and nautilus. The fossils found in MN most resemble the nautilus and cuttlefish Chambered Nautilus Cuttlefish Cockles Pelecypods: clams, oysters, mussels, bivalves Bryozoans: similar, but not related to coral Mussel Clams 4

5 Gastropods: sea snails 5