Transcript Unit 10 Unit 10 - Uniformitarianism & the Age of the Earth

Parsley, Sage, Rosemary and Time
Unit 10
Unit 10 - Uniformitarianism & the Age of the Earth
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.
Unit 10
Unit 10 - Uniformitarianism & the Age of the Earth
Annual layers
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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.
Unit 10
Unit 10 - Uniformitarianism & the Age of the Earth
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.
Unit 10
Unit 10 - Uniformitarianism & the Age of the Earth
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…
Unit 10
Unit 10 - Uniformitarianism & the Age of the Earth
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.
Unit 10
Unit 10 - Uniformitarianism & the Age of the Earth
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),
…
Unit 10
Unit 10 - Uniformitarianism & the Age of the Earth
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-yearold student has lived through 0.0002 inches.
Unit 10
Unit 10 - Uniformitarianism & the Age of the Earth