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Lecture 15

GEOL 106 Lecture Notes - Lecture 15: Radiocarbon Dating, Carbon-14

Geological Sciences and Geological Engineering
Course Code
GEOL 106
David Alan Hanes

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GEOL 106 – Class 15 02-06-
How Do We Determine Recurrence Interval? (Continued)
2. Dig trenches on active faults
Look at movements that have taken place in the past
See end of last class
Step 2: Determine absolute age for fault movement using carbon
14 dating of organic peat
Carbon 14 dating:
oTree in the swamp is constantly sucking in carbon dioxide
for photosynthesis – C14 and C12
oWhen the plant dies, it stops conducting photosynthesis
oNo longer sucking in carbon 14, and the current carbon 14
(the heavy carbon) will decay to become nitrogen 14
oOnce dead, the amount of carbon 14 in the tree decreases
at a *xed rate
oMeasure the C14/C12 ratio
oKey to do this: must know how fast the C14 turns into the
oHalf-life: length of time for ½ of radioactive element to
oHalf life for C14 = 5730 years
oEventually have so little C14 remaining it cannot be
oCan only use this to measure 10 half lives (approx. 60,000
years ago)
oUse other radiometric processes for faults in solid rock
It is a subjective determination to decide if a particular
movement event was a big earthquake or just a small one
We can turn to other evidence of big earthquake activity that is
less subjective
A: Look at human historical records
B: Dig trenches on active faults
C: Look for evidence of ancient tsunami, and of ground
subsidence, in soils
o1. Tsunami: waves pick up sand, take it inland, and dump
on the swamp (killing organic matter)
oBy looking at ancient soil records, using carbon-14 we can
date the dead plant material and determine when a
tsunami occurred, giving the age of the earthquake
oDate the layer below the sand (fossil tidal marsh)
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