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Chapter 8

TIMING THE GEOLOGIC RECORD

Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company

The angular unconformity at Siccar Point. It was one of Hutton’s great inspirations.


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.]


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.]


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 8.3 Fossils can be used to correlate rock layers in different outcrops.

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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 8.6 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.]


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 8.8 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 8.9 The geologic time scale, showing eras, periods, and epochsdistinguished by assemblages of fossils. Boundaries of these intervals are marked by the abrupt disappearance of old life-forms and the appearance of entirely new ones. The five most dramatic mass extinctions are indicated. This diagram shows only the relative ages of the intervals. Time goes left to right.

Faunal succession

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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.]


Early Estimates of Earth’s Age

Cooling of Earth .075 - 1,280Cooling of Sun 4.4 - 500Orbital physics <10 - 3,700,000Salt in ocean 25 - 2,350Sediment accumulation 3 - 15,000Radioactive elements 1,340 - 4,600

million years

A summary of data in compiled in Table 3.1, pp. 26-27 of “Ancient Earth, Ancient Skies.”


FIGURE 8.11 The radioactive decay of rubidium to strontium.

Absolute time!

Parent isotope Daughter

isotope


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.


A typical lab where this is done


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.)

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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 (ZrSiO4) from the Hadean eon extracted from the Jack Hills. [Photo Chd GFDL.]


Radiometric Dating Methods

METHOD

Potassium-Argon

Rubidium-Strontium

Samarium-Neodymium

Lutetium-Hafnium

Rhenium-Osmium

Uranium-Lead

Uranium-Lead

Lead-Lead

NUCLIDES40K - 40Ar87Rb - 87Sr147Sm - 143Nd176Lu - 176Hf187Re - 187Os235U - 207Pb238U - 206Pb

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HALF-LIFE(billion years)

1.25

48.8

106

35.9

43.0

0.704

4.47

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Locations of Some of Earth’s Oldest Rocks


Earth’s Oldest Rocks (4.03 Ga)

(Courtesy of S.A. Bowring)


Lead-lead Ages of Some Meteorites

Allende chondrite 4.553Barwell chondrite 4.559Appley Bridge chondrite 4.569St. Severin chondrite 4.543Bovante chondrite 4.510Juvinas achondrite 4.556Moore County achondrite 4.484Nuevo Laredo achondrite 4.514Angra dos Reis achondrite 4.544Estherville stony iron 4.555

Name Type Billion Years

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Chronology of the Early Solar System

Globular Cluster M80

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.

Below: A typical globular cluster as photographed by the Hubble Space Telescope.

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