Geology of the National Parks

Wrap Up


Review the Unit 10 Introduction

You have reached the end of Unit 10! Double-check the list of requirements on the Unit 10 Introduction page and the Course Calendar to make sure you have completed all of the activities listed there.

Click here to review the Unit 10 Overview and make sure you understand all the main topics.

Review of the main topics and ideas you encountered in Unit 10.

Parsley, Sage, Rosemary and Time

  • Last chapter, we did “relative time”--which came first?
  • Now we spice it up with “absolute time”--how many years?
    • Count annual layers, for accurate estimates, for “short times” (less than about 100,000 years)
    • Calculate from recent rates and reconstructed effects, for less-accurate estimates, for short and long times (uniformitarian approach)
    • Use radiometric (radioactive) techniques, for accurate estimates, for short and long times

Annual layers

  • Overlapping-tree rings, to more than 10,000 years
  • Special-lake sediments, to more than 40,000 years
  • Ice-core layers, to more than 100,000 years
    • MANY checks, including:
    • reproducibility of counting
    • agreement with historical records (chemically fingerprinted fallout of volcanic eruptions, etc.)
    • consistency amongst ice, lakes and trees for ages of abrupt climate changes
    • agreement with radiometric, uniformitarian ages

Old as the Hills

  • Annual-layer records from geologically young materials (ice sheets, trees and lake sediments not turned to stone yet, on top of rocks) are much older than written history
  • Virtually all scientists, and most religions agree Earth looks much older than written history
  • Some religions disagree vehemently
  • Whatever the truth, the science is good

Climbing the Canyon

  • Metamorphosed old mountain range at bottom
  • Unconformity, then two miles of sediments
  • Tipped by faulting, then unconformity, then another mile of sediments with several unconformities within
  • Rocks are “normal”, with tracks, mudcracks, etc., at many different levels, fossil changes upward
  • Rim rocks slant down under Zion, which is under Bryce, which is…
  • Roughly 100 million years to deposit sediments, plus time for old metamorphics, plus erosion…

Radiometric Dating

  • Half of parent atoms decay to offspring in one half-life (easy to measure; don’t need to wait for a half-life to pass, just for a measurable change)
  • Half-life fixed by the same physics that make the sun shine and keep us from blowing up--is not a variable
  • Parent:offspring ratio plus half-life gives age
  • Requires a little care and attention
  • Agrees with written records, layer counts, uniformitarian calculations, other radiometric techniques.

Radiometric Dating Example

  • Solid potassium-40 parent included in lava flows, but gaseous argon-40 offspring escapes
  • After flow hardens, additional argon-40 produced from potassium-40 is trapped
  • 1.3-billion-year half-life
  • If you start with 400 parents, after one half-life (1.3 billion years) average 200 parents left (and 200 offspring), after second half-life (total 2.6 billion years) average 100 parents left (and 300 offspring), after third half-life (total 3.9 billion years) average 50 parents left (and 350 offspring), …

0.0002 Inches and a Cloud of Dust

  • Oldest rocks about 4 billion years old, but Earth bombarded, melted first
  • Meteorites formed with Earth; they are about 4.6 billion years old (agrees with whole-Earth estimates)
  • If 4.6 billion years is the 100-yard length of a football field, written history is about the thickness of a sheet of paper, and a 20-year-old student has lived through 0.0002 inches

Reminder - Continue to work on Exercise #5. See the Course Calendar for specific dates.

Supplemental Materials

Following are some supplementary materials for Unit 10. While you are not required to review these, you may find them interesting and possibly even helpful in preparing for the quiz!

Comments or Questions?

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