Geologic Time and

Earth History

Two Conceptions of Earth History: Catastrophism• Assumption: Great Effects Require Great Causes • Earth History Dominated by Violent Events Uniformitarianism• Assumption: We Can Use Cause And Effect to

Determine Causes of Past Events • Finding: Earth History Dominated by Small-scale

Events Typical of the Present. • Catastrophes Do Happen But Are Uncommon

Two Kinds of AgesRelative - Know Order of Events But Not

Dates• Civil War Happened Before W.W.II • Bedrock in Wisconsin Formed Before The

Glaciers Came Absolute - Know Dates• Civil War 1861-1865 • World War II 1939-1945 • Glaciers Left Wisconsin About 11,000 Years

Ago

Absolute Ages: Early Attempts

The Bible• Add up Dates in Bible • Get an Age of 4000-6000 B.C. For Earth • John Lightfoot and Bishop Ussher - 4004

B.C. (1584) • Too Short

Absolute Ages: Early AttemptsSalt in Ocean• Rivers bring dissolved solids to ocean • If we know rate salt is added, and how

much salt is in ocean, can find age of oceans.

• Gave age of about 100 million years. • Problems – Is rate at which salt is added constant? –How much salt leaves ocean?

Absolute Ages: Early Attempts

Sediment Thickness• Add up Thickest sediments for each period • Estimate rate of deposition to find age • Problem: rates of deposition very variable! • Indicated ages of at least 100 million years

Age of The Sun• If sun gets its heat from burning or other chemical

reactions, could only last 10,000 years or so. • Best 19th century guess: sun was slowly

contracting. • Problem: only 30 million years ago, sun would

have extended out to earth's orbit! • Geologists wanted more time, but you can't fight

the laws of physics... • Sun actually gets its energy from nuclear

reactions and can keep going for billions of years • The Geologists were right after all. Go Team.

Radiometric Dating: Half-Life

How Do We Measure Half-Lives of Billions of Years?

• Number of Decays/Unit Time = k * Number of Atoms

• k = Decay Constant• Leads to N = N0exp(-kt)

(This is what the exp button on your calculator is for)

• Half-Life = 0.693/k

Present Radiometric Dating MethodsCosmogenic• C-14 5700 Yr. • Be-10 2.5 M.Y. Primordial• K-Ar (K-40) 1.25 B.Y. • Rb-Sr (Rb-87) 48.8 by • U-235 704 M.Y. • Th-232 14 B.Y. • U-238 4.5 B.Y.

Primordial• Nd-Sm (Sm-147-Nd-143)

106 B.Y. • Re-187 43 B.Y. • Lu-Hf (Lu-176) 36 B.Y• Fission

The Geologic

Time Scale