August 29, 2003

Geologic Time

1. Time

a. Relative time – i.e. your father is older than you but younger than your grandparents.

b. Absolute time – radiometric dating

i. Parent Atoms

ii. Daughter Atoms

iii. Half-life

iv. Decay constant – unique to each element

2. Geologic Events

a. Deposition of sedimentary sequences

i. Principle of superposition – younger strata accumulate over older strata during deposition.

ii. Principle of original horizontality – sedimentary units are deposited in horizontal units regardless of the underlying strata.

iii. Principle of original continuity – sedimentary units are usually deposited in widespread, blanket -like deposits.

iv. Principle of faunal succession –Organisms that become extinct never reappear in the fossil record in their original forms.

b. Extrusion or intrusion of igneous rocks

i. Principle of cross cutting relationships –Intrusions are younger than the hosts they penetrate.

ii. Principle of inclusions—both igneous and sedimentary – rock fragments found in the units as inclusions are older than their hosts. (xenoliths, rip-up clasts)

c. Metamorphism

d. Deformation (folding and faulting)

e. Uplift

f. Erosion

3. Unconformities (p. 130 Fig. 8.1)

a. Angular – Rock units above and below the unconformity have different attitudes.

b. Disconformity – The rocks above and below have the same attitudes, which are different from that of the unconformity.

c. Non-conformity – Erosion surface between sedimentary rocks and older plutonic or metamorphic rocks.

4. Geologic Time Scale