Earth Sciences 2240F/G Lecture Notes - Lecture 8: Unconformity, Relative Dating, Geologic Time Scale

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Earth Sciences 1022b Lecture Brief 2 Feb 16
Relative Time: tells us whether a rock is younger or older than another, based on:
superposition - in undeformed, horizontal sedimentary rocks, a bed is older than the
one above and younger than the one below; youngest on top, oldest on the bottom
original horizontality - layers of sediment tend to be deposited in a horizontal position
cross-cutting relations - a fault or a rock must be younger than the rock it cuts
inclusions - inclusions within a rock must be older than the rock containing them
unconformities missing rocks (due to nondeposition or erosion) within rock
sequences: angular unconformity (flat strata resting on tilted and eroded strata);
disconformity (all strata parallel, hard to see time break); nonconformity
(commonly flat sedimentary rocks resting on igneous or metamorphic rocks
following erosion of the lower rocks)
correlation - matching similar rock type, position, especially fossils in the rock record
based on fossil succession as organisms evolved through Earth’s history; with
index fossils (widespread, limited time spans) can correlate over long distances
Dating with Radioactivity: radiometric methods tell us how many years since rocks
formed this is how it was determined that Earth is ~4.5 billion years old
radioactivity radioactive parent isotopes (e.g. K 40) decay to daughter products (e.g.
Ar40) exponentially; compare amount of daughter accumulated with amount of
parent left and decay rate (half life) - indicates the time since decay started (i.e.
when the rock formed)
radiocarbon (C14) dating - for dating the last ~50,000 years, authenticity checks:
organisms absorb C14 and C12 from the atmosphere or food chain in a constant
ratio; C14 decays to N14 with a half life of 5730 years (i.e. half the amount of the
parent isotope is consumed every 5730 years) but is replaced by C14 as long as the
organism is alive
- however, after death C14 is no longer absorbed but continues decay while amount
of C12 in the fossil does not change (i.e. C14/C12 ratio steadily decreases over time)
- for dating, scientists measure the amounts of C14 and C12 in a fossil, apply the half
life, then calculate the time since the organism died (that is, since it stopped
absorbing C14) and thus the age of the sediment that contains the fossil
Geologic Time Scale: applying radiometric ages to the sequence of rock units worked
out by relative dating methods, especially correlation using index fossils scale
divided into time units (e.g. Era, Period); boundaries in years before present
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