Relative Dating
Principle of superposition
Steno-
Principle of Original Horizontality
Principle of lateral continuity
Hutton
Principle of cross cutting relationships
Principle of Inclusion
Observation + common sense geologic history
William Smith: Law of faunal succession Correlation
Fossil assemblages succeed one another through time in a predictable
order. True because lifeforms have changed.
Hutton Uniformitarianism: The present is the key to the
past. Rates are different.
Radiometric Dating
Radioactivity (discovered by chance) - Henri Becquerel
Radiometric dating
Parent isotope
Daughter isotope
Half life - Is the time it takes to one half of the parent atoms to decay into the daughter elements.
close system
Complications complex history
daughter already present when rock formed
Choice of the isotopic system
Abundance of parent (measureable)
Daughter not initially present or capability to correct
Half life of parent suitable to process measured
Isotopes used: U235, U238 (700 m.y.; 4.5 b.y.)
K40 (1.3 b.y)
Rb87 (49 b.y)
C14 (5730 y) Archaeology
Radiometric and Relative ages combined
"Absolute" vs "relative" ages
Solar System = 4.5 b.y oldest cont. rocks = 4.b.y
oldest seafloor = 200 m.y.
Geologic time scale (units)
Eons > Eras > Periods > Epochs
Previous to Cambrian Precambrian (4.5 b.y.a. to 570 m.y.a.)
Phanerozoic Eon (visible life): 570 m.y.a. to Recent
Rates of geologic Processes (not required)
seafloor from spreading ridges
can be calculated
uplift in Scandinavia = 1 cm/y
erosion of rivers = load_____
drainage basin
Average erosion in N. America: tenth inch/century
Danger of extrapolation
APPENDIX A: GEOLOGIC TIME - REVIEW
1. Geologic time scale: memorize the eons, eras
and periods. Dates for the eons and eras.
2. Relative dating: principles. Correlation. Geologic history.
3. Absolute dating: definitions, basis of the methods. Restrictions.