Chapter 8 TIMING THE GEOLOGIC RECORD

Transcription

1 Chapter 8 Fossils (cont.): Petrified Forest, Arizona. These ancient logs are millions of years old. Their substance was completely replaced by silica, which preserved all the original details of form. [Tom Bean.] TIMING THE GEOLOGIC RECORD The angular unconformity at Siccar Point. It was one of Hutton s great inspirations. FIGURE 8.2 Layers of sedimentary rock in Marble Canyon, part of the Grand Canyon, illustrate Steno s principles. The Grand Canyon was cut by the Colorado River through what is now northern Arizona, revealing layers that record millions of years of geologic history. Stratigraphy is the study of sedimentary sequences such as this one. [Fletcher and Baylis/ Photo Researchers.] FIGURE 8.1 Fossils are traces of living organisms preserved in the geologic record. (a) Ammonite fossils, ancient examples of a large group of invertebrate organisms that are now largely extinct. Their sole representative in the modern world is the chambered nautilus. [Chip Clark.] FIGURE 8.3 Fossils can be used to correlate rock layers in different outcrops.

2 FIGURE 8.7 An angular unconformity is a surface that separates two sets of layers whose bedding planes are not parallel. This sequence of drawings shows how such a surface can form. FIGURE 8.4 Trilobites, an extinct life-form, preserved as fossils in rocks about 365 million years old found in Ontario, Canada. [William E. Ferguson.] FIGURE 8.5 An unconformity is a surface between two rock layers representing a layer that never formed or was eroded away. The type of unconformity created through uplift and erosion, followed by subsidence and another round of sedimentation, is called a disconformity. FIGURE 8.8 Cross-cutting relationships allow us to establish the relative ages of igneous intrusions or faults within a stratigraphic succession. Relative time vs. absolute time FIGURE 8.6 The Great Unconformity in the Grand Canyon, Arizona, is an angular unconformity between the horizontal Tapeats Sandstone above and the steeply dipping Precambrian Wapatai Shale below. The Wapatai Shale is part of the Precambrian Grand Canyon beds; the Tapeats Sandstone is Cambrian in age. [GeoScience Features Picture Library.] Faunal succession FIGURE 8.9 The geologic time scale, showing eras, periods, and epochs distinguished by assemblages of fossils. Boundaries of these intervals are marked by the abrupt disappearance of old life-forms and the appearance of entirely Jordan, The new Essential ones. Earth 1e The 2008 by five W. H. most Freeman and dramatic Company mass extinctions are indicated. This diagram shows only the relative ages of the intervals. Time goes left to right.

3 FIGURE 8.10 Stratigraphic sequence of the Colorado Plateau, reconstructed from strata exposed in Grand Canyon, Zion Canyon, and Bryce Canyon National Parks. [Grand Canyon: John Wang/Photo Disc/Getty Images; Zion Canyon: David Muench/CORBIS; Bryce Canyon: Tim Davis/Photo Researchers.] FIGURE 8.12 The number of radioactive atoms of any element in any mineral declines at a fixed rate over time. This rate of decay is given by the half-life of the isotope. Early Estimates of Earth s Age A typical lab where this is done million years Cooling of Earth.075-1,280 Cooling of Sun Orbital physics <10-3,700,000 Salt in ocean 25-2,350 Sediment accumulation 3-15,000 Radioactive elements 1,340-4,600 A summary of data in compiled in Table 3.1, pp of Ancient Earth, Ancient Skies. Parent isotope Daughter isotope FIGURE 8.13 The ribbon of geologic time shows the complete geologic time scale. All numbers are ages in millions of years ago. (The Tertiary and Quaternary periods are older divisions that have been largely replaced by the Paleogene and Neogene periods, but are still sometimes used by geologists.) FIGURE 8.11 The radioactive decay of rubidium to strontium. Absolute time!

4 Locations of Some of Earth s Oldest Rocks Outcrop in Jack Hills of western Australia where geologists have dated zircon grains as old as 4.4 Ga. [Bruce Watson, Rensselaer Polytechnic Institute. Inset: Zircon crystals (ZrSiO 4 ) from the Hadean eon extracted from the Jack Hills. [Photo Chd GFDL.] Earth s Oldest Rocks (4.03 Ga) (Courtesy of S.A. Bowring) Radiometric Dating Methods METHOD Potassium-Argon Rubidium-Strontium Samarium-Neodymium Lutetium-Hafnium Rhenium-Osmium Uranium-Lead Uranium-Lead Lead-Lead NUCLIDES 40 K - 40 Ar 87 Rb - 87 Sr 147 Sm Nd 176 Lu Hf 187 Re Os 235 U Pb 238 U Pb ---- HALF-LIFE (billion years) Lead-lead Ages of Some Meteorites Name Type Billion Years Allende chondrite Barwell chondrite Appley Bridge chondrite St. Severin chondrite Bovante chondrite Juvinas achondrite Moore County achondrite Nuevo Laredo achondrite Angra dos Reis achondrite Estherville stony iron 4.555

5 Chronology of the Early Solar System Globular Cluster M80 Below: A typical globular cluster as photographed by the Hubble Space Telescope. Above: Fig. 9.3, p. 186 in Ancient Earth, Ancient Skies, showing a tentative chronology of the early Solar System based on the isotopes of Pb and on extinct radioactive nuclides.