Week 9: Geochronology

the formation of rocks and landscapes, in general, were a consequence of processes that he could see happening today

“The present is key to the past.”

The earth must be very old.

Relative age
non-numerical dating (older or younger?)

original horizontality
lateral continuity
cross-cutting relations
baked contacts
fossil succession
principles of determining relative age

a surface representing a period of non deposition and possibly erosion

Angular unconformity
Angular unconformity
Types of unconformities

Angular unconformity
Angular unconformity
Type of unconformity
tilted or folded before

Type of unconformity
old igneous or metamorphic basement rock which new sedimentary layers sit upon

Type of unconformity
sea level drop > erosion of exposed surfaces > new sedimentary layers + sea level rise

Stratigraphic column
summary of sequence of sedimentary strata

Geologic column
Relative age relations
culmination of stratigraphic sequences from around the world

Numerical age
specific, numerical dating
how many years ago?

Isotopic dating
Tree rings
ways of determining numerical age

Unweathered rocks
Separation of minerals
Extracting parent and daughter isotopes
Parent-daughter ratio
What do you need to perform isotopic dating?

“Isotope clock” starts when crystals become cool enough for isotopes to be locked into the lattice
When does the “isotope clock” begin?

Closure temperature
the temperature below which isotopes are no longer free to move

No, only determines when minerals first crystallized not when minerals were deposited as sediment nor when sediment was lithified
Can you directly determine the numerical age of sedimentary rock?

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