Overview of the main topics you will encounter in Unit 10
Leave it as it is. You can not improve on it. The ages have been at work on it, and man can only mar it. What you can do is to 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.
–President Theodore Roosevelt, speech at the Grand Canyon, Arizona, on May 6, 1903
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.
–President Theodore Roosevelt, speech at the Grand Canyon, Arizona, on May 6, 1903
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 and 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, most religions agree Earth looks much older than written history;
Some religions disagree vehemently;
Whatever the Truth, the science is good.
Climbing out of the Grand 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;
North 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 give 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 before that;
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.
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