Wrap Up

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 the 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, mud cracks, 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 the 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 sunshine 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.