GEOSC 10
Geology of the National Parks

Main Topics: Module 10

Overview of the main topics you will encounter in Module 10

 

We visit Grand Canyon National Park, Arizona in this module, so before getting to the material that is likely to be on a quiz, we’ll start with some important thoughts from President Theodore Roosevelt on the value of saving our national treasures, from his speech at the Canyon on May 6, 1903. President Roosevelt went on to protect the Canyon, first as a Game Preserve and then as a National Monument, and it was made a National Park in 2019 under President Wilson.

Leave it as it is. You cannot improve on it. The ages have been at work on it, and man can only mar it. What you can do is keep it for your children, your children’s children, and for all who come after you, as one of the great sights which every American...should see.

We have gotten past the stage, my fellow-citizens, when we are to be pardoned if we treat any part of our country as something to be skinned for two or three years for the use of the present generation, whether it is the forest, the water, the scenery. Whatever it is, handle it so that your children’s children will get the benefit of it.

Parsley, Sage, Rosemary and “Time”

  • In Module 9, 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 but still reliable and useful 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 12,000 years;
  • Special-lake sediments, to more than 45,000 years;
  • Ice-core layers, to more than 100,000 years;
    • MANY checks, including:
      • reproducibility of counting;
      • agreement with historical records (chemically fingerprinted fallout of historically dated volcanic eruptions, etc.);
      • consistency between ice, lake, and trees dates for ages of abrupt climate changes;
      • agreement with radiometric and uniformitarian ages.

Old as the Hills

  • Annual-layer records from geologically young materials (from ice sheets, trees, and lake sediments that have not turned to stone yet, on top of rocks) extend back much older than written history;
  • Data are clear that Earth looks much older than written history;
  • Most religions agree;

Rocks you see while climbing out of the Grand Canyon

  • Metamorphosed old mountain range at the bottom;
  • Eroded surface (unconformity), then two miles of sediments;
  • Tipped by faulting, then another unconformity, then another mile of sediments with several unconformities within;
  • Rocks are familiar types, with animal tracks, mud cracks, etc., at many different levels, and changes in fossil types from layer to layer going upward;
  • North Rim rocks then slant down under younger rocks at Zion, which are under Bryce rocks, which are under younger rocks, which are under prehistoric archaeological sites…
  • 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 makes the sun shine and keeps us alive;
  • Measured parent:offspring ratio today plus measured half-life give the age of sample;
  • Requires a little care and attention;
  • Agrees with written records, layer counts, uniformitarian calculations, other radiometric techniques, and more.

Radiometric Dating Example

  • Potassium-40 parent included in solidified lava flows, but gaseous argon-40 offspring escapes before liquid lava solidifies;
  • After lava solidifies, the additional argon-40 produced from decay of 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 are about 4 billion years old; Earth bombarded by meteorites and mostly melted before that;
  • Meteorites formed with Earth; they are about 4.6 billion years old, the same age estimated from radioactive dates of the Earth;
  • If 4.6 billion years is the 100-yard length of a football field, all written history is about the thickness of a sheet of paper, and a 20-year-old person has lived through 0.0002 inches.